CryoSat-2 (Earth Explorer Opportunity Mission-2)

CryoSat-2 (Earth Explorer Opportunity Mission-2)

Overview

CryoSat-2 is the follow-on Earth Explorer Opportunity Mission in ESA's Living Planet Program. It replaces CryoSat, which was selected for development in 1999 and lost as a result of launch failure on October 8, 2005.

CryoSat-2 will have the same mission objectives as the original CryoSat mission; it will monitor the thickness of land ice and sea ice and help explain the connection between the melting of the polar ice and the rise in sea levels and how this is contributing to climate change.

The original CryoSat mission was proposed by Duncan Wingham of the University College London (UCL) and an international science team. Duncan Wingham is also the mission PI. A nominal mission duration of three years is planned (excluding the commissioning and validation phases, which may last up to six months).

In Feb. 2006, ESA received the green light from its Member States to build and launch a CryoSat recovery mission, CryoSat-2, based on the same objectives as the original CryoSat mission. However, the design of the CryoSat-2 spacecrasft is being updated. The changes required to the design of CryoSat-2 were scrutinized from December 2006 to January 2007. The Δ-CDR (Critical Design Review) was completed on Feb. 1, 2007. ^{1) 2) 3) 4) 5)}

A total of 85 improvements/modifications have been approved in the design of CryoSat 2 (of which 30-40% have been small software changes that make the satellite much easier to operate).

The new key features of CryoSat-2 include the following items:

• The SIRAL-2 (SAR/Interferometric Radar Altimeter-2) design includes a full backup SIRAL system, in case the primary payload malfunctions. Once in orbit, a special algorithm will be used to convert data collected by the CryoSat-2 satellite to create more accurate ice maps. - As a result of the dual SIRAL payload and associated interfaces, and other improvements to reliability, there has been a knock-on effect to the design of the satellite. For example, the backup SIRAL system has to be kept warm while it is switched off - the additional heater power is provided by increasing the size of the satellite's battery.

• A heat-radiating panel is being added. The path of CryoSat-2's orbit means it will face extremes of temperature. The panel ensures the electronics are protected

• Solar panels on the satellite's back are being added to account for the additional power requirements. Unlike many spacecrafts, CryoSat 2 does not have solar wings.

Preparatory Campaigns

In addition to building the new satellite, a number of field experiments to support the CryoSat-2 mission, were conducted or are getting underway in the Arctic. First is the Arctic Arc Expedition, part of the IPY (International Polar Year) 2007-2008. ^{6) 7)}

- Antarctic 2008/9 CroVEx campaign in the blue ice region (see Figure 1) in December 2008: German scientists from the Technical University of Dresden and the AWI (Alfred Wegener Institute) are spending up to four months venturing out onto the vast frozen reaches of what is known as the 'blue ice' region near the Russian Novo airbase in Dronning Maud Land in Antarctica. The aim is to take very accurate measurements of the surface topography, both from the air and on the ground to contribute to the validation program for CryoSat-2. ⁸⁾

In parallel to the efforts on the ground, the Alfred Wegner Institute (AWI) will be flying their POLAR5 aircraft across the blue ice site – starting just before Christmas and finishing before the New Year. From the plane, the AWI team will collect laser and radar height measurements along the very same tracks as the ground team. To do this they are using ESA's ASIRAS (Airborne Synthetic Aperture and Interferometric Radar Altimeter System), which simulates the measurements CryoSat.

- CryoVEx (CryoSat Validation Experiment) 2008 (3-week campaign in May 2008 in the far north of Greenland and Canada). CryoVEx 2008 is a continuation of a number of earlier campaigns that focus on collecting data on the properties snow and ice over land and sea. This year's campaign is a huge logistical undertaking as airborne, helicopter and ground measurements are being taken simultaneously in three different locations - out on the floating sea-ice north of the Canadian Forces Station Alert, on the Devon ice cap in Canada and on the vast Greenland ice cap.

A Twin Otter is carrying two key instruments: ASIRAS (Airborne SAR/Interferometric Radar System), a radar altimeter that mimics the radar altimeter onboard CryoSat-2, and a laser scanner which maps the surface beneath the plan, and a helicopter with an on-board sensor that measures sea-ice thickness. ⁹⁾

- In the spring of 2007, an international team of scientists stationed in Svalbard, Norway and two polar explorers are crossing the North Pole on foot. Both teams are part of a common effort to collect vital data on the ground and from the air in support of ESA's ice mission CryoSat-2.

The expedition's two Belgian explorers, Alain Hubert and Dixie Dansercoer, 'stepped' onto the sea ice off the coast of Siberia on March 1, 2007 each pulling a 130 kg sledge holding supplies and equipment. A parallel campaign by scientists from Germany, Norway and the UK is unfolding in the extreme northern archipelago of Svalbard, Norway. As part of the CryoVEx 2007 campaign (CryoSat-2 Validation Experiment), they are spending one month making measurements of snow and ice properties along long transects that crisscross the ice sheet surface.

- As the ground experiments are carried out, measurements are also being taken from the air by the Alfred Wegner Institute (AWI), Bremerhafen, Germany. The Dornier-228 aircraft carries the ASIRAS (Airborne SAR/Interferometric Altimeter System) instrument, which is an airborne version of the radar altimeter instrument onboard CryoSat-2. By comparing the airborne data with ground measurements scientists will test and verify novel methods for retrieving ice-thickness change from the CryoSat-2 satellite mission ahead of the launch.

- ASIRAS was built by Radar Systemtechnik (RST) of Switzerland with the support of the AWI and Optimare for the implementation and operation on an aircraft. It was test flown in March 2004 over the snow and ice expanses of Svalbard.

- The CryoVEx 2006 campaign took place April/May, 2006 and consisted of coordinated airborne and ground activities in support of CryoSat-2 validation goals over three land validation sites (Devon Island in Canada, Central Greenland and Svalbard, Spitzbergen, Norway) and a series of ice experiments over Alert / Ellesmere Island, Canada.

- LaRA (Laser and Radar Altimeter) campaign in the Arctic region of Greenland and Svalbard: The D2P (Delay-Doppler Phase-monopulse Radar) instrument of JHU/APL participated in this campaign which took place in May 2002 under joint NASA/ESA sponsorship to support calibration and validation activities, and science investigations in advance of the CryoSat and ICESat missions. The D2P radar altimeter was flown aboard the NASA-P3 aircraft along with the ATM (Airborne Topographic Mapper) laser altimeters to collect simultaneous laser and radar altimeter (hence, the LaRA campaign) measurements over land and sea ice.

- CryoVEx (CryoSat Validation Experiment) campaign: As a follow-on to the LaRA campaign, the D2P system was flown again in 2003 under joint NASA/ESA sponsorship as part of the CryoVEx field campaign. As in 2002, simultaneous laser and radar altimeter measurements were collected in the Arctic.

Such painstaking ground work is necessary to be able of measuring ice thickness down to centimeter level (1-3 cm average) from space. This in turn may lead to a better understanding of the impact that changing climate is having on the polar ice fields. See also the D2P and ASIRAS descriptions on the eoPortal (along with the campaigns for the validation of the SIRAL instrument).

Many more CryoVEx campaigns were conducted throughout the CryoSat-2 mission (up to 2018) as provided in the ASIRAS file on the eoPortal.

Spacecraft

The CryoSat-2 spacecraft is being built and integrated by EADS Astrium GmbH of Friedrichshafen, Germany, as prime contractor of a consortium.

• The spacecraft structure consists of a long rectangular main platform body, surmounted by fixed solar arrays in the form of a tent.
• The spacecraft has neither deployable appendages nor any other moving parts except for thruster valves.
• The lower surface of this structure is permanently earth facing.
• All electronics are mounted on the nadir plate acting as radiator.
• The antennas used for radio communication, and the Laser Retroreflector, are mounted on this surface; an emergency antenna for command and monitoring is also fitted on top of the satellite between the solar arrays.
• The two SIRAL instrument antenna dishes are mounted on a separate rigid bench in the forward section of the S/C.
• In addition, a dedicated SIRAL radiator is mounted at the nose tip. ¹⁰⁾

The spacecraft is 3-axis stabilized. A slight nose-down attitude of the S/C (6º) is chosen (using magnetorquers and 10 mN cold-gas thrusters) to ensure minimum attitude correction due to gravity-gradient disturbances. The S/C has dimensions of 4.6 m x 2.34 m x 2.20 m. The S/C mass is about 720 kg (including 36 kg propellant), the design life is 3.5 years (goal of 5.5 years). Spacecraft power generation is provided by two triple-junction GaAs solar arrays with an efficiency of 27.5% (two oriented solar panels), each panel provides power of 850 W at normal solar incidence. A PCDU (Power Control and Distribution Unit) provides onboard distribution. The energy is stored by a lithium-ion battery (60 Ah capacity).

The pointing requirements are the main design drivers for the AOCS: ¹¹⁾

• High precision cross-track pointing knowledge of < 10 arcsec for SARIn mode (SARIn refers to the SAR interferometric mode).

• S/C attitude maintenance with a pointing accuracy of < 0.2º per axis and a pointing stability of < 0.005º for 0.5 s in the nominal Earth-pointing phase of the mission

• Provision of very low disturbances due to thruster activity to meet the very high precise orbit determination (POD) accuracy of the CryoSat orbit.

• The AOCS (Attitude Orbit and Control Subsystem) comprises the following elements:

- A cold gas system (RCS) for attitude control and orbit transfer and maintenance maneuvers, 16 attitude control thrusters (10 mN) and 4 orbit control thrusters (40 mN). Nitrogen is used as propellant (132 l tank). ¹²⁾

- A set of 3 magnetorquers is used for compensation of environmental disturbance torques in support of RCS. The MT30-2-GRC, originally developed and qualified for the GRACE mission by ZARM Technik GmbH, has been selected for CryoSat.

- A set of three star tracker heads (also a part of the payload) providing autonomous inertial attitude determination for the spacecraft. The multiple configuration makes the sensor system one-failure tolerant, except for the rare occurrence of simultaneous sun and moon blinding of two heads, to which the system software is tolerant.

Consequently, two camera heads are operated in parallel at all times to cope with sun-blinding. In its acquisition mode, which takes 2-3 seconds, the star tracker calculates a coarse attitude by matching triangle patterns of stars with patterns stored in its catalog. Subsequently, in attitude update mode it calculates precise attitude at a rate of 1.7 Hz.

The star tracker attitude serves also as reference for determining the orientation of the SIRAL interferometric baseline. The orientation of the interferometric baseline needs to be very accurately measured in-flight: small errors in knowledge of the roll-angle translate into substantial errors in the elevation of off-nadir points.

The HE-5AS star tracker of Terma A/S, originally developed and qualified for the NEMO (Navy EarthMap Observer) and FCT (Foreign Comparative Test) projects, is selected for CryoSat. It is a fully autonomous star tracker capable of delivering high-accuracy inertial attitude measurements from a lost-in-space condition with no external attitude information. The EOL performance of the star tracker is < 3.2 arcsec in the lateral axes and < 16 arcsec about the roll axis under worst-case conditions. ¹³⁾

The star tracker baffle has been designed to meet the required sun exclusion angle of 30º and the moon exclusion angle of 25º. These exclusion angles ensure together with the star tracker accommodation on the antenna bench that during the whole mission sun and moon can only blind one star tracker head at any time.

• A DORIS receiver is part of an overall system, for real-time measurements of satellite position, velocity and time. DORIS measures the Doppler frequency shifts of UHF and S-band signals transmitted by ground beacons. Its measurement accuracy is < 0.5 mm/s in radial velocity allowing an absolute determination of the orbit position with an accuracy of 2-6 cm (see DORIS and LLR description under sensor complement). -

The DORIS system comprises a network of more than 50 ground beacons, a number of receivers on several satellites in orbit and in development, and ground segment facilities. It is part of the International DORIS Service, the IDS, which also offers the possibility of precise localization of user beacons.

• CESS (Coarse Earth-Sun Sensor) of CHAMP and GRACE heritage (a patented design of Astrium GmbH). Provides attitude measurements (<5º) with respect to the sun and Earth for initial acquisition and coarse pointing. The FOV of CESS is a full spherical one, i.e. no special search maneuvers are necessary to find the Earth or the sun. Its measurement principle is based The concept is based on temperature differences measured by 6 omnidirectional arranged sensor heads (PT1000 thermistors).

• A set of three three-axis fluxgate magnetometers are used for magnetorquer control and as rate sensors. They provide a measurement range of at least ± 60.000 nT with an accuracy of better than 0.5 % full scale.

The AOCS provides high pointing accuracy (a few tens of an arcsecond), knowledge and stability in nominal Earth-pointing. It also has to perform the orbit changes between the science and validation phase orbits. The AOCS uses inertial attitude measurements from the set of 3 star tracker camera head units and DORIS real-time navigation to convert the inertial attitude into Earth referenced attitude (star sensor FOV of 22º x 22º, ).

- The RCS (Reaction Control Subsystem), developed at PolyFlex Space Ltd. (a Marotta UK Ltd. company), is a cold gas propulsion system for auxiliary attitude control (in which it provides deadband protection around the axis defined by the instantaneous geomagnetic field) and for orbit transfer and maintenance maneuvers. It has 16 attitude control thrusters of 10 mN each and 4 orbit control thrusters of nominally 40 mN each. A single high-pressure tank stores 36.2 kg of nitrogen gas at 278.6 bar. ^{14) 15)}

The CDMU (Control and Data Management Unit), consisting of a processor and a hardware-based fault detection system, handles all on-board command and control functions including telecommand decoding and the AOCS processing functions (the OBC is based on the ERC-32 microprocessor). A MIL-STD-1553B communications bus is used as payload interface (for SIRAL and DORIS). The on-board solid-state memory has a capacity of 2 x 128 Gbit.

• Experimental Rate Sensor: CryoSat-2 carries a small technology experiment as a passenger. This device is an attitude rate sensor based on MEMS (Micro-Electro-Mechanical-Systems) technology in which microelectronics and mechanical devices (in this case a sensor) are fabricated on the same substrate. The MEMS sensor detects attitude rate to provide the same function as a more traditional gyro and is based on a device widely used in in-car navigation systems. Three orthogonal MEMS sensors are mounted in the experiment, to measure 3-axis attitude rates. The unit is called MRS (MEMS Rate Sensor) in the CryoSat context. The goal is to provide a low-cost rate-sensor or gyro. The device is provided free of charge to CryoSat-2 in exchange for the flight opportunity (Ref. 10).

The measurement data are not used on-board and only sent in housekeeping telemetry to the flight control centre. Here they will be used as an additional data type in monitoring satellite dynamics during attitude transitions.

In the context of the technology program in which the MRS has been developed, it is called SiREUS-FExp, for European Silicon Rate Sensor Flight Experiment. - SiREUS is a compact and lightweight solid-state MEMS rate sensor which was developed in the context of an ESA technology technology program. The UK development team consisted of the following partner organizations: AIS (Atlantic Inertial Systems - formerly BAE Systems of Plymouth), SEA (Systems Engineering & Assessment Ltd. of Bristol), and SELEX-GALILEO a Finemeccanica owned company (formerly BAE Systems of Edinburgh). This development is based on the established BAE SYSTEMS automotive MEMS detector, however significant developments were required to meet the performance requirements while achieving compatibility of the electronics to the space environment and ensuring low recurring price. The partnership with a significant commercial provider such as AIS should be emphasized as a critical aspect of the program. ^{16) 17) 18)}

The SiREUS unit has met or exceeded the key performance requirements set at the start of the program. The unit does not contain any software; all control loops are implemented digitally in an FPGA. The SiREUS unit is fairly compact, but its size is currently dominated by analog electronics, not the MEMS. It may be cost effective to achieve a significant reduction in mass and volume, if this results in a match with many more customer requirements.

SiREUS has demonstrated that it is possible to construct multi-lateral programs to spin-in technology from non space industry organizations and to make significant improvements in the performance of the 'spin-in' technology. There are positive signs for the wider application of this technology in 'spin-off' programs. The instrument has a size of 100 mm x 100 mm x 70 mm.

Launch

The CryoSat-2 spacecraft was launched on April 8, 2010 on a Dnepr vehicle from the Baikonur Cosmodrome, Kazakhstan. The launch provider was ISC (International Space Company) Kosmotras. ^{19) 20) 21)}

Note: The technical issue with the second stage of the Dnepr rocket that delayed the launch of ESA's Earth Explorer CryoSat-2 satellite in February 2010 has now been resolved – and the new launch date of 8 April has been set. The fuel reserve problem of the second stage surfaced a week before the scheduled launch date and after the 'space head module', encasing the CryoSat-2 satellite, had been mated to the rest of the rocket in the launch silo. Consequently, the space head was returned to the integration facilities pending an investigation and new launch date. ²²⁾

The delay, from the planned launch date of Dec. 2009, is due to the limited availability of facilities at the Baikonur launch site in Kazakhstan, which is particularly busy at the moment. ²³⁾

Orbit

Non sun-synchronous circular LEO orbit, mean altitude = 717 km, inclination = 92º, nodal regression of 0.25º per day. Ground track repeat cycle: 369 days (with 30 day pseudo subcycles). This configuration allows a sufficient coverage for the polar regions.

The CryoSat mission requirements include:

• An orbit change is required during the mission with the objective to visit at least twice a validation orbit, approximately 6 km lower in altitude than the science phase orbit

• The payload must be operated in various modes, as a function of geographical region, such that the orbital operations, and data sets collected, on successive orbits are dissimilar

• The payload utilization demands very precise orbit and attitude restitution. Minimum operations of three years are required.

The CryoSat mission is aimed in part at gaining coincident coverage with the GLAS laser altimeter of the NASA ICESat mission.

The following support phases are defined:

• Commissioning phase: The nominal duration is two months. During this phase the satellite and its payload are brought into a fully operating condition in its nominal orbit.

• Science phase: This includes a long-repeat cycle [a 369-day orbit (5344 revolutions) repeat phase will be used, with a 30-day subcycle]. The science phase is the nominal operational support mode of the mission. This orbit is designed to provide very dense orbit cross-overs above 72º of latitude, for use over the ice sheets. With coverage to 88º of latitude, all but a very small area of the land and marine ice fields will be within the coverage of the satellite. In addition, the 30 day subcycle provides approximately monthly coverage of the sea ice fluctuations.

• Validation phases: The objective is to conduct calibration or validation experiments that are at a fixed locations on Earth. In these phases the satellite may be placed into a 2-day repeat orbit. A validation phase may have a duration of up to 1 month, and there may be more than one during the mission lifetime. The measurements made by the satellite mission will need to be verified by ground-based experiments.

RF communications: The S-band link is used for all TT&C communications (2 kbit/s uplink and 8 kbit/s downlink). The physical downlink operates at 16 kbit/s but carries an overhead of error correction coding. The X-band downlink (center frequency of 8.100 GHz) provides a payload transfer rate of 100 Mbit/s. All onboard data are stored in the MMFU (Mass Memory and Formatting Unit) of 2 x 128 Gbit (EOL) capacity. Data arrive at the MMFU directly from the SIRAL instrument on a pair of fast IEEE 1355 standard serial links (SpaceWire for the two high-rate interferometric data channels) and via the MIL-STD-1553 bus for the low rate data channels. Data are also transferred from the CDMU and the DORIS over the MIL-STD-1553 bus. About 320 Gbit/day of onboard source data are being generated and transmitted to the ground.

Mission Status

• June 18, 2024: The rapid warming of the Arctic is transforming its ecosystem, with thinning ice and declining snow cover allowing more light to penetrate, fueling under-ice algal blooms that form the foundation of the marine food web. Using data from ESA’s CryoSat-2, alongside Copernicus Sentinel-3 and NASA’s ICESat-2, researchers have developed models to estimate light penetration and predict algal bloom timing, revealing that blooms are starting up to 15 days earlier per decade in southern Arctic regions. Snow depth is a key factor, with heavier snow reducing light availability, while long-term satellite missions like CryoSat and upcoming collaborations such as Cryo2ice aim to refine our understanding of these changes and their broader ecological impacts. ¹⁴⁷⁾

• August 23, 2023: A new CryoSat sea level anomaly product designed to enable ocean science and the development of operational marine applications has been released. The dataset, presented in the Nature Research journal Scientific Data, provides a unique perspective on ocean surface levels, thanks to the mission’s novel orbit and its extended lifespan of more than 13 years. It was produced and verified by the UK’s National Oceanography Centre (NOC) as part of a wider ESA-backed project to validate CryoSat ocean products. The products are available for download here. ^145) 146)

• July 12, 2022: The CRYO2ICE campaign, launched in July 2020, adjusted the orbit of ESA’s CryoSat-2 to periodically align with NASA’s ICESat-2, enabling near-synchronous data collection over polar regions. By raising CryoSat-2’s orbit by 887 meters through 14 maneuvers, the satellites achieved partial parallel ground tracks every 1.33 days, with observations occurring within a few hours of each other. This alignment has improved the accuracy of sea ice thickness and land ice measurements, facilitated snow mapping, and enhanced climate models. The orbital separation between the two satellites is decreasing, reducing the time difference between their coincident observations. Current efforts focus on optimizing data collection over Antarctica, with scientists benefiting from increasingly co-spatial measurements. ²⁴⁾

• November 3, 2021: New research based on ESA’s CryoSat-2 data highlights the increasing frequency and intensity of extreme ice-melting events in Greenland over the past 40 years, raising global sea levels and heightening flood risks. Published in Nature Communications, the study reveals that meltwater runoff from Greenland has increased by 21% and become 60% more erratic, contributing 3.5 trillion tons of ice melt to the oceans in the past decade alone. Notably, 2012 and 2019 saw record-breaking ice loss, accounting for one-third of the runoff during this period. The findings underscore the role of extreme weather, such as heatwaves, in driving ice loss and emphasize the importance of emissions reductions to mitigate future impacts. CryoSat-2’s data provides crucial insights for refining climate models and predicting sea-level rise, supporting initiatives like the upcoming CRISTAL mission for continued monitoring of Earth’s ice. ^25)  26)

• June 10, 2021: Research combining data from ESA's CryoSat-2 and Envisat missions with a new snow-depth model reveals that sea ice in Arctic coastal regions is thinning 70–100% faster than previously estimated. The updated calculations, which account for climate-induced changes in snow accumulation, show accelerated ice loss in areas like the Laptev, Kara, and Chukchi seas. Thinner ice compromises the Arctic’s role in regulating Earth’s climate, affects local ecosystems, and poses risks to human activities, including shipping and resource extraction. The findings underscore the importance of improved snow and ice monitoring, a key focus of the upcoming CRISTAL mission. ^27) 28)

• June 01, 2021: New research using ESA’s CryoSat-2 mission reveals significant ice loss from mountain glaciers in the Gulf of Alaska and High Mountain Asia, with annual losses of 76 Gt and 28 Gt, respectively, between 2010 and 2019, contributing 0.26 mm per year to global sea-level rise. Utilizing advanced swath processing techniques, CryoSat overcame traditional limitations of radar altimetry in rugged terrains, providing high-resolution data that showed widespread glacier loss, except in the Karakoram-Kunlun region, where the 'Karakoram anomaly' persists. These findings highlight the importance of radar altimetry in global glacier monitoring, paving the way for future missions like CRISTAL under Europe’s Copernicus program. ^29) 30)

Figure 12: Ice loss from mountain glaciers is accelerating, with CryoSat-2 data revealing the annual losses. (video credit: Planetary Visions/ESA)

• May 21, 2021: New research using ESA’s CryoSat-2 and SMOS satellites highlights the growing influence of Atlantification, where warmer Atlantic waters are reducing Arctic sea ice regrowth during winter, particularly in the Barents and Kara Seas. This phenomenon undermines the stabilizing effect of thinner ice growing faster, making sea ice more vulnerable to summer heat and winter storms. Combining CryoSat-2's ice-thickness data with SMOS data, scientists have improved sea-ice forecasts, which are critical for industries and communities reliant on accurate predictions. Notably, the 2020-21 winter season recorded the lowest Arctic sea ice volume since 2010, with thinning ice concentrated in typically robust regions near Greenland and the Canadian Archipelago. ³¹⁾

• December 14, 2020: Using over a decade of altimetry data from ESA’s CryoSat-2, scientists have revealed a rapid cycle of drainage and recharge in subglacial lakes beneath Antarctica’s fragile Thwaites Glacier, with peak outflows reaching 500 m³/s—eight times the River Thames’ average discharge. These findings suggest that basal melting, driven by geothermal heat and friction, is significantly underestimated by models, with rates nearly 150% higher than previously thought. This hidden meltwater, which influences ice flow and ocean melting, underscores the importance of monitoring Antarctica’s subglacial hydrology to improve ice sheet projections and predict its contribution to sea-level rise, a mission that future satellites like CRISTAL aim to support. ³³⁾

• December 9, 2020: In celebration of the 200th anniversary of the discovery of Antarctica, the UK Committee for Antarctic Place-Names has honored four ESA-affiliated scientists by naming mountains, glaciers, and bays after them. These scientists—Seymour Laxon, Katharine Giles, Jonathan Bamber, and Helen Fricker—made groundbreaking contributions to polar research, particularly in satellite altimetry. Laxon and Giles' work supported the development of ESA’s CryoSat mission, which has significantly advanced our understanding of ice dynamics and sea-level rise. Fricker and Bamber's research, including efforts to align CryoSat with NASA’s ICESat-2, continues to provide critical insights into Antarctic ice shelf changes and climate impacts. These new names will be included in international maps and directories, ensuring their legacy in Antarctic exploration. ³⁴⁾

• August 3, 2020: ESA has recently adjusted the orbit of CryoSat-2 to align with NASA's ICESat-2, enabling simultaneous measurements of sea ice thickness using CryoSat's radar and ICESat's laser. This alignment will allow for the first-ever direct measurement of snow depth on top of ice, improving the accuracy of sea-ice thickness data and enhancing our understanding of how different ice and snow surfaces reflect satellite signals. The synchronized satellites will pass over the same regions every 1.5 days, providing crucial data to study sea ice dynamics, especially in Antarctica, and refine climate models. This collaboration exemplifies ESA and NASA’s combined efforts to advance climate science. ³⁵⁾

• April 8, 2020: CryoSat-2 has become an important mission for understanding climate change impacts on polar ice. With its innovative radar altimeter, CryoSat measures ice height variations, providing data on the thickness and volume of both land-based ice sheets and sea ice. Its high-precision measurements have enhanced climate models, revealing how Earth's ice is declining, particularly in the Arctic, and aiding in predicting future climate change. Beyond its primary mission, CryoSat has contributed to studying mountain glaciers, lake and river water levels, and even marine gravity, while laying the groundwork for future missions like CRISTAL to continue monitoring polar ice variability. ³⁶⁾

• January 27, 2020: CryoSat-2 has provided new insights into the complex ice loss patterns of Pine Island Glacier, a major contributor to sea-level rise in Antarctica. While earlier models offered conflicting projections about the glacier's future mass loss, CryoSat's data revealed a significant decrease in the glacier's ice loss rate since 2007, suggesting a slower contribution to sea-level rise than some models predicted. This new understanding helps refine future projections, indicating that while the glacier will continue to lose mass, the rate of loss may not increase as dramatically as previously thought. These findings are crucial for improving climate models and predicting the glacier's long-term impact on sea-level rise. ³⁷⁾

• December 13, 2019: CryoSat-2 has provided data on the state of Antarctic ice shelves, particularly in mapping changes in the seaward edges. Using a novel approach called "elevation edge," scientists have created a time series of calving front positions for the Filchner-Ronne ice shelf, revealing significant annual advances and occasional iceberg calving events. This data, combined with ice velocity information from Sentinel-1, enhances understanding of ice shelf dynamics, including ice thickness changes and calving rates. The ability to regularly monitor ice shelf changes is vital for assessing the stability of the Antarctic ice sheet and its contribution to sea-level rise, with future missions like CRISTAL expected to expand these capabilities. ³⁸⁾

• August 5, 2019: CryoSat-2 has been used to measure the thickness of lake ice in the Arctic and sub-Arctic regions, providing new insights into climate change. Scientists applied the satellite’s radar altimeter to monitor the ice thickness on the Great Slave and Great Bear Lakes in Canada, revealing seasonal changes and ice volume variability. This method, which involves measuring the radar reflections from the ice surface and its bottom, was validated with drill-hole measurements and can be applied to smaller lakes. The findings contribute to understanding the role of ice in local and global climate systems, with potential applications in monitoring lake water levels and volumes. ^{40) 41)}

• July 24, 2019: Scientists have used data from the CryoSat-2 mission, along with other Earth-observing satellites, to create the first hybrid bathymetry map of the Arctic Ocean, revealing previously unknown details of the ocean floor. By combining satellite-derived gravity measurements with existing ship-based data, the new map improves our understanding of Arctic seafloor features, which are crucial for studying ocean dynamics, tides, and ensuring maritime safety. This hybrid map, developed by DTU Space, fills gaps in the previously sparse bathymetric data, offering a more complete and accurate picture of the Arctic Ocean’s bathymetry and advancing scientific knowledge in this remote region. ^42) 43)

• July 11, 2019: The Arctide2017 tidal atlas, developed using ESA’s CryoSat-2 and Envisat satellite data, provides a high-resolution model of Arctic Ocean tides, addressing gaps in in situ observations due to the region's challenging conditions. By combining satellite altimetry and advanced modeling techniques, this atlas improves tidal predictions, particularly in the Arctic where sea ice, sparse observations, and complex bathymetry complicate traditional measurements. The model enhances understanding of both linear and non-linear tidal components, offering more accurate tidal data for maritime activities, environmental monitoring, and scientific research in the Arctic Ocean. ^{44) 45)}

• May 14, 2019: At the Living Planet Symposium in Italy, scientists from the University of Edinburgh presented a novel 3D dataset of Antarctica, created using data from ESA's CryoSat-2 mission. By processing CryoSat-2’s radar altimeter measurements in a new way, they produced detailed maps of surface elevation and rates of change for the Antarctic ice sheets, which help improve understanding of how glaciers and ice caps are responding to climate change. This technique, previously applied to Greenland, is crucial for predicting future sea-level rise. The resulting CryoTop datasets, including elevation models and change maps, are available for download, offering valuable resources for further research on polar ice dynamics. ^{46) 47)}

• January 23, 2019: The ongoing US government shutdown is affecting services that provide NOAA’s input data products used in CryoSat’s ice and ocean processing chains. As a result, CryoSat L1b and L2 products with validity starting from December 21, 2018, may experience degraded quality, particularly in geophysical corrections and derived parameters such as sea ice concentration and the GDP+ wet tropospheric correction. Users will be notified later about the extent of the impact once the US services resume normal operations. ⁴⁸⁾

• September 15, 2018: After eight years in orbit, CryoSat-2 remains in goodcondition, with funding approved until December 2019 and a proposal to extend it through 2021. While the power subsystem was switched to backup in 2013, all other subsystems are functioning on their primary hardware, and the satellite has enough consumables for operation until at least 2025. Monthly orbit maintenance maneuvers are performed, along with periodic Collision Avoidance Maneuvers due to space debris threats. The mission has exceeded expectations, providing essential data on sea-ice volume and ice sheet elevation changes, contributing to climate change studies. CryoSat-2 continues to face new scientific challenges, such as understanding snow loading in Arctic sea ice, while leveraging synergies with missions like ICESat-2 and Sentinel for ongoing and future observations. ⁴⁹⁾

• May 11, 2018: CryoSat-2 has produced the most accurate 3D map of Antarctica's ice sheet and floating ice shelves to date, with a new digital elevation model offering twice the resolution of the previous version from 2017. Covering an additional 350,000 km² and sampling every kilometer, this model includes five million more data points, representing 95% of the continent. The improved topography will aid in predicting how Antarctica's ice sheet will respond to climate change, providing valuable insights for scientific fieldwork, ice sheet modeling, and assessing potential sea level rise. ⁵⁰⁾

• May 2, 2018: The CryoSat-2 mission has expanded its capabilities by providing detailed insights into the retreat of mountain glaciers, particularly in Patagonia, where glaciers are melting faster than anywhere else. Using a new technique called swath processing, CryoSat-2's radar altimeter has mapped glacier height changes with unprecedented detail. This technique has revealed significant ice loss between 2011 and 2017, particularly from glaciers like Jorge Montt and Upsala, which lost several gigatonnes of ice per year. Overall, the Patagonian ice fields lost over 21 gigatonnes of ice annually, contributing to sea level rise and marking a 24% increase in ice loss compared to previous years. ⁵¹⁾

• April 3 2018: ESA's CryoSat-2 mission has revealed significant ice loss in Antarctica, with 1463 km² of underwater ice disappearing between 2010 and 2017, primarily due to warm ocean water melting ice at the grounding lines of glaciers. This marks the first complete map of grounding-line motion along 16,000 km of coastline, showing that retreat is particularly pronounced in West Antarctica, where more than 20% of the ice sheet has retreated faster than during the last ice age. CryoSat-2's ability to detect grounding-line movement from space has provided valuable insights into ice-sheet dynamics, including unexpected behavior, such as the halted retreat of the Pine Island Glacier. These findings underscore the complexity of ice-sheet instability and highlight CryoSat's continued importance in polar science. ^53) 54)

• October 11, 2017: ESA's CryoSat-2 and Copernicus Sentinel-1 missions have uncovered the hidden impact of deep canyons on Antarctica's ice shelves, particularly under the Dotson ice shelf in West Antarctica. These inverted canyons, likely formed by warm ocean water circulating under the shelves, are causing accelerated melting and weakening the ice. The channels, which have been deepening by 7 meters per year, contribute significantly to ice loss, with the Dotson ice shelf alone releasing 40 billion tonnes of freshwater annually. This process, visible through changes in surface elevation and ice velocity data, is threatening the stability of the ice sheet, potentially leading to faster ice flow and higher sea-level rise. The discoveries offer new insights into the ongoing changes in Antarctica, thanks to satellite monitoring. ⁵⁷⁾

• July 5, 2017: ESA's CryoSat-2 and Copernicus Sentinel-1 missions are closely monitoring the Larsen C ice shelf, where a deep crack is poised to release one of the largest icebergs on record. The iceberg, estimated to be 190 meters thick and containing 1155 km³ of ice, will be tracked as it drifts away from the shelf. CryoSat-2 has already provided critical data on its size and depth, while Sentinel-1 monitors the crack's progression. Once calved, the iceberg could pose a maritime hazard as it may travel northward, potentially impacting shipping routes like the Drake Passage. These missions work together to track the iceberg and better understand the dynamics of ice loss in Antarctica. ⁵⁸⁾

• March 24, 2017: The CryoSat-2 mission has provided 250 million measurements over six years to create a detailed 3D digital elevation model of Antarctica, shown in Figure 33. Including data on ice elevation, this model allows scientists to distinguish between topographic changes and ice motion, aiding in studies of ice-sheet dynamics, sea-level rise projections, and ice loss through melting and iceberg calving. Soon to be freely available, the model has a wide range of applications, from fieldwork planning to refining projections of future sea-level rise. ⁶⁰⁾

• March 20, 2017: The CryoVEx (CryoSat Validation Experiment) campaign, one of the largest Arctic expeditions organized by ESA, is underway, involving over ten global agencies and research institutes. Scientists are conducting fieldwork in extreme Arctic conditions to validate and refine CryoSat-2's ice thickness measurements, which have been crucial for understanding the changing volume of polar ice. The campaign also aims to test new radar altimeter technology that could further improve future satellite missions. In addition to this, CryoSat-2 continues to provide vital data on Arctic ice changes, which are increasingly significant in global climate discussions and adaptation strategies. ⁶¹⁾ 

• February 8, 2017: ESA's CryoSat-2 mission has revealed the drainage of subglacial lakes beneath the Thwaites Glacier into the Amundsen Sea, marking the largest such outflow reported in this part of West Antarctica. The event, which occurred between 2013 and 2014, caused the ice surface to subside by several meters and accelerated glacier movement by about 10%, contributing an additional 3.5 km³ of freshwater to the sea. This discovery provides crucial insights into how water beneath glaciers affects ice dynamics, helping scientists understand glacier flow and predict future changes in the ice sheet, which has implications for sea level rise. The findings highlight CryoSat-2's enhanced capability to monitor such events, aided by new processing techniques and the support of the Copernicus Sentinel-1 mission. ^{63) 64)}

• December 16, 2016: ESA's SMOS and CryoSat-2 missions, though designed for different purposes, are being used together to provide a clearer picture of Arctic sea ice. SMOS, with its radiometer, captures data on sea ice thickness, particularly for thinner ice, while CryoSat-2’s radar altimeter measures the thickness of thicker ice. By combining the daily, coarser data from SMOS with the high-resolution, monthly coverage of CryoSat-2, scientists have improved the accuracy of sea-ice thickness maps and forecasts. This collaboration enhances our understanding of Arctic ice dynamics and supports applications like marine traffic routing and climate change studies, with data extending back to 2010. ⁶⁶⁾

• November 30, 2016: ESA's CryoSat-2 satellite has revealed that the Arctic sea ice volume for November is among the lowest on record, matching the lows of 2011 and 2012, with early winter growth about 10% lower than usual. While the ice was thicker than in 2011, with an average thickness of 116 cm at the end of summer, the rate of ice growth in November has been slower than expected, possibly due to warmer-than-usual temperatures. CryoSat-2's precise measurements of ice thickness provide valuable data for monitoring climate change and supporting maritime operations in the Arctic. CPOM will release a full assessment of 2016 sea ice conditions soon. ⁶⁷⁾

• July 26, 2016: The CryoSat-2 satellite is enhancing the measurement of sea levels, especially in challenging coastal zones with rugged terrain, where traditional radar altimeters struggle to provide accurate data. By using its specialized radar altimeter, CryoSat-2 delivers highly precise measurements, even near coastlines like Norway's, where it closely matches tide gauge data with only a 7 cm discrepancy, compared to 10–15 cm from conventional altimeters. This improved accuracy is crucial for understanding regional sea level rise and its impacts on coastal communities. The data from CryoSat-2 is also paving the way for similar advancements with the Sentinel-3 mission, which carries a similar altimeter. ⁶⁸⁾

• July 21, 2016: After six years in orbit, ESA's CryoSat-2 satellite is performing well, with all subsystems remaining in prime condition except for the power subsystem, which switched to its redundant side in 2013. The satellite's operations are funded until February 2017, with a planned extension into the next phase of the Earth Observation Envelope Program (EOEP-5). Onboard consumables are expected to last until 2025, and the startracker software has recently been upgraded for improved performance. The CryoSat ground segment has evolved to meet the growing demands of scientific communities, and the satellite's user base has more than tripled, reflecting its broad scientific impact. ⁶⁹⁾

• April 21, 2016: Sea ice physicists from the AWI/Helmholtz Center for Polar and Marine Research have projected that the Arctic sea ice cover this summer could shrink to record lows, similar to the 2012 minimum. Their assessment, based on CryoSat-2 satellite data and autonomous snow buoy measurements, shows that the ice was exceptionally thin in the summer of 2015, with little new ice forming during the previous winter. Despite warm winter temperatures in the Arctic, the ice did not melt but grew slowly. In regions like the Beaufort Gyre and south of Spitsbergen, the ice is thinner than usual, and large amounts of thick ice will be carried away by currents, contributing to further sea ice loss. If unfavorable weather conditions persist, 2016 could see another record low for Arctic sea ice. ^70) 71)

• March 2016: A group of 179 researchers has urgently called for a successor to the ageing CryoSat-2 mission, which has been crucial for monitoring polar ice sheets and sea ice. Launched in 2010 with an expected lifespan of just over three years, CryoSat-2's performance has far exceeded expectations, providing key insights into Arctic sea-ice thickness and volume, as well as Antarctic and Greenland ice sheet dynamics. While it still has fuel for the early 2020s, the satellite is at risk of failure, and a replacement is needed to continue critical climate monitoring. Although ESA is not responsible for funding such a mission, scientists are advocating for a successor within the Copernicus program, noting that existing platforms like Sentinel-3 lack the capabilities to fully replicate CryoSat-2’s work. A timely replacement is seen as essential to avoid gaps in data and ensure ongoing, accurate sea-level rise projections. ⁷²⁾

• December 11, 2015: Satellite technology has greatly advanced our understanding of Earth's climate, especially in tracking polar ice changes, which are crucial indicators of climate change. While early polar explorations in the 1800s offered limited insight, modern satellites like CryoSat-2 have provided vital data on Arctic sea ice and ice sheets, helping inform climate agreements like COP21. The Arctic is particularly sensitive to warming, and its shrinking sea ice affects global heat transport, ocean circulation, and the Earth's albedo. CryoSat-2, launched in 2010, has been instrumental in measuring ice thickness and volume, offering essential data to understand the ongoing impact of climate change on polar regions and sea-level rise. ⁷³⁾

• July 20, 2015: ESA’s CryoSat-2 satellite data reveals that the volume of Arctic sea ice increased by 41% following the cool summer of 2013, suggesting that Arctic ice is more sensitive to summer melting than winter cooling. A study using 88 million ice thickness measurements between 2010 and 2014 showed a 14% reduction in ice volume from 2010 to 2012, but a significant recovery in 2013 due to cooler temperatures that reduced melting. While the findings highlight the potential for recovery when the melting season is shortened, the short duration of the CryoSat-2 record limits long-term trend analysis. The data will help improve climate models and assist Arctic maritime navigation. ^74) 75)

• May 22, 2015: ESA's CryoSat-2 mission has detected an acceleration in ice loss along the Southern Antarctic Peninsula, a previously stable region. Starting in 2009, multiple glaciers began shedding ice into the ocean at a rate of about 60 km³ per year, contributing around 300 km³ of water over the past six years. This makes the region one of the largest contributors to sea-level rise in Antarctica. The rapid ice loss is linked to ice-shelf thinning and subsurface glacier melting, triggered by warming oceans, rather than changes in snowfall or air temperature. The study, using five years of CryoSat-2 data, highlights the unexpected shift in the region's ice dynamics. ⁷⁶⁾

• April 17, 2015: ESA's CryoSat-2 mission, marking its five-year anniversary in 2015, has become the first satellite to provide near-real-time data on Arctic sea-ice thickness, aiding maritime activities such as shipping, tourism, and Arctic exploration. The mission now delivers complete sea-ice thickness maps within two days, made possible through specialist data processing by the UK’s CPOM. This rapid access to data is crucial for safe operations in the Arctic and for advancing scientific research on climate change. Notably, recent measurements show that sea ice around Norway’s Svalbard Archipelago has significantly thinned, reflecting a warming trend in the Barents Sea. ^{77) 78) 79)}

• January 23, 2015: Recent satellite data from Sentinel-1A and CryoSat-2 have revealed rapid ice loss on the Austfonna ice cap in Norway's Svalbard archipelago, with parts of the ice thinning by over 50 meters since 2012. Led by the University of Leeds' Centre for Polar Observation and Modelling, the study documented a dramatic acceleration in ice discharge and thinning in the previously slow-moving sector of the ice cap, with thinning rates exceeding 25 meters per year. Over the past two decades, the ice flow velocity has increased 45-fold. This rapid change, potentially triggered by rising ocean temperatures, underscores the importance of long-term satellite observations for understanding and monitoring climate-related ice loss and its contribution to sea-level rise. ⁸⁰⁾

• December 15, 2014: CryoSat-2 has provided this year’s map of autumn sea-ice thickness in the Arctic, showing a slight decrease in ice volume from 10,900 km³ last year to 10,200 km³. Despite this small drop, the volume remains the second-highest since 2010, with a relatively stable five-year average. However, this does not suggest a reversal of the long-term decline in Arctic sea ice. The data, processed by researchers at the Centre for Polar Observation and Modelling (CPOM) at UCL, can now also assist with navigation in the northern coastal waters of Alaska. ⁸²⁾

• November 2014: The CryoSat-2 mission has recently been extended to the end of 2016. ⁸³⁾

• October 3, 2014: ESA's CryoSat-2 mission, primarily designed to measure ice elevation, has also contributed to creating a detailed gravity map of the ocean, revealing previously uncharted underwater features like seamounts, ridges, and deep ocean structures. By using radar altimetry to measure sea-surface height, CryoSat-2 has helped produce a new, highly accurate marine gravity map in collaboration with NASA’s Jason-1 satellite. This map, twice as precise as the previous version, offers new insights into seafloor processes, such as seafloor spreading, and exposes hidden geological connections, like those between South America and Africa, as well as buried ridges in the Gulf of Mexico. This map will also aid in improving seafloor depth estimates in the largely unexplored areas of the ocean. ^84) 85)

• August 20, 2014: ESA's CryoSat-2 mission has provided new data on the changing ice volumes of Greenland and Antarctica, revealing a combined annual ice loss of approximately 500 km³. A study by the Alfred Wegener Institute, based on three years of CryoSat-2 data, generated new digital elevation models (DEMs) and volume change estimates for both ice sheets, with accuracy comparable to previous satellite-based measurements. The results show a threefold increase in volume loss from West Antarctica, and a significant volume loss from Greenland, particularly along its west and southeast coasts. Greenland's contribution to the total ice volume loss is nearly 75%. These findings, based on 68.5 million measurements, are the most comprehensive to date from a single satellite mission ^86) 87)

• May 19, 2014: ESA’s CryoSat-2 satellite has revealed that the Antarctic ice sheet is now losing 159 billion tons of ice annually, double the previous rate of loss. This acceleration, particularly in West Antarctica's Amundsen Sea sector, contributes significantly to global sea-level rise, adding 0.45 mm annually to the global sea level. The ice loss in this region, which includes 134 billion tons annually from West Antarctica, 3 billion from East Antarctica, and 23 billion from the Antarctic Peninsula, is primarily driven by glaciers thinning and retreating. This ongoing imbalance in ice loss is compounded by findings that glaciers in the Amundsen Sea sector have passed an irreversible point of decline, accelerating their contribution to sea-level rise. The results, supported by over 40 years of radar observations, signal a need to revise sea-level rise predictions. ^{88) 89) 90) 91)}

• April 2014: After completing four years of successful operations, CryoSat-2 has proven to be highly reliable, with its performance being closely monitored for signs of aging. The spacecraft has completed over 20,000 orbits and, thanks to good power and fuel margins, is expected to continue beyond its planned end in 2016. The mission has implemented measures like collision avoidance maneuvers to handle space debris risks and has supported calibration campaigns for its interferometer performance. CryoSat-2 also plays a key role in supporting scientific campaigns like IceBridge and CryoVEx, ensuring minimal data loss. Improvements to its Mass Memory and Formatting Unit (MMFU) storage capacity and updates to its Star Tracker software are also in progress to enhance data handling and mission longevity. The satellite's data continues to serve a wide range of scientific communities, contributing to studies in sea-ice, land-ice, marine gravity, meteorology, and hydrology. ⁹²⁾

• March 26, 2014: Global sea levels are rising by approximately 3 mm per year due to the combined effects of melting glaciers and ice sheets, along with the thermal expansion of seawater caused by rising temperatures. This poses significant risks to low-lying coastal areas. Identifying the specific contributions of these factors is a complex task, requiring tracking water in all forms across the Earth system. Satellites, including ESA’s CryoSat-2, play a crucial role in this effort by providing data to map changes in ice mass and quantify their contribution to sea-level rise. Under ESA’s Climate Change Initiative, scientists from various fields are collaborating to balance the sea-level budget by integrating observations of oceans, land, atmosphere, and cryosphere. ⁹³⁾

• February 2014: CryoSat-2 completed its first three years of operations on Nov. 19, 2013 when the spacecraft was declared operational. It continues to work flawlessly, acquiring and generating science data systematically, to measure the variation of sea-ice mass floating in the Arctic and trend of land-ice volume over Greenland and Antarctica. An issue affecting the onboard power system forced operations to fall back to the redundant system with little impact on the science retrieval. ⁹⁴⁾

• February 2014: Regarding collision avoidance, CryoSat-2 experienced seven conjunctions within 300 m in 2013, two required evasive maneuvers: ⁹⁵⁾

- Oct. 11, 2013 (JSpOC alert): conjunction at ~340 m (53 m radial)

- Oct. 15, 2015 (JSpOC alert): conjunction at ~205 m (200 m radial).

• January 2014: Measurements from CryoSat-2 near the center of Antarctica revealed an unusual diamond ring pattern of alternating high and low ice sheet elevations (Figure 52), visible where the satellite's northbound and southbound orbits cross. Initially suspected to be a technical issue, scientists determined that this pattern is an artefact caused by the interaction of CryoSat-2’s polarized radar signal with surface features shaped by persistent Antarctic winds. These erosional and depositional features modify the radar returns, producing the observed effect. Since the pattern remains stable over time, corrections can be applied to ensure the accuracy of CryoSat’s data. This finding not only enhances the precision of past and future measurements but also improves understanding of how radar waves interact with wind-affected ice surfaces. ⁹⁶⁾

• December 16, 2013: Measurements from CryoSat-2 show that the volume of Arctic sea ice has significantly increased this autumn. In October 2013, CryoSat-2 measured about 9000 km³ of sea ice – a notable increase compared to 6000 km³ in October 2012. This year’s multi-year ice is now on average about 20%, or around 30 cm, thicker than last year. While this increase in ice volume is welcome news, it does not indicate a reversal in the long-term trend. - It’s estimated that there was around 20 000 km³ of Arctic sea ice each October in the early 1980s, and so today’s minimum still ranks among the lowest of the past 30 years. ⁹⁷⁾

• December 11, 2013: Three years of observations by ESA’s CryoSat-2 satellite show that the West Antarctic Ice Sheet is losing over 150 km³ of ice each year – considerably more than when last surveyed. The imbalance in West Antarctica continues to be dominated by ice losses from glaciers flowing into the Amundsen Sea. The ice thinning continues to be most pronounced along fast-flowing ice streams of this sector and their tributaries, with thinning rates of between 4–8 m per year near to the grounding lines – where the ice streams lift up off the land and begin to float out over the ocean – of the Pine Island, Thwaites and Smith Glaciers. ⁹⁸⁾

- The melting of ice sheets that blanket Antarctica and Greenland is a major contributor to global sea-level rise. An international team of polar scientists had recently concluded that West Antarctica caused global sea levels to rise by 0.28 mm each year between 2005 and 2010, based on observations from 10 different satellite missions. But the latest research from CryoSat-2 suggests, that the sea level contribution from this area is now 15% higher.

• December 9, 2013: During the major North Sea storm of December 5-6, 2013, CryoSat-2 measured the resulting storm surge as high waters passed through the Kattegat sea between Denmark and Sweden, providing data that closely matched storm surge model predictions. Designed to measure sea-ice thickness, CryoSat-2's radar altimeter has proven valuable for monitoring sea levels, even in coastal areas where traditional altimeters struggled due to land interference. These measurements enhance storm surge models and can be used in near-real-time forecasting, as demonstrated by the eSurge project, which leverages satellite data to improve predictions during extreme weather events. ⁹⁹⁾

• September 2013: Since its launch in 2010, ESA's CryoSat-2 has delivered three years of continuous Arctic sea-ice thickness measurements, from October 2010 to April 2013, revealing a consistent thinning trend. By combining these data with observations of ice extent, scientists can quantify ice loss, track seasonal changes, and identify long-term trends. With the satellite in excellent health, CryoSat-2 is expected to continue its mission of monitoring diminishing Arctic sea ice until at least 2017. ¹⁰⁰⁾

• July 02, 2013: ESA is reporting that CryoSat-2 has found a vast crater in Antarctica’s icy surface. Scientists believe the crater was left behind when a lake lying under about 3 km of ice suddenly drained. Far below the thick ice sheet that covers Antarctica, there are lakes of fresh water without a direct connection to the ocean. These lakes are of great interest to scientists who are trying to understand water transport and ice dynamics beneath the frozen Antarctic surface – but this information is not easy to obtain. By combining new measurements acquired by CryoSat-2 with older data from NASA’s ICESat satellite, the Cryosat team has mapped the large crater left behind by a lake, and even determined the scale of the flood that formed it. ¹⁰¹⁾

^- The CryoSat science team analyzed the data acquired by the SIRAL (SAR Interferometer Radar Altimeter) instrument on CryoSat-2 and demonstrated its novel capability to track topographic features on the Antarctic Ice Sheet. The perimeter and depth of a 260 km² surface depression was mapped above an Antarctic SGL (Subglacial Lake) and, in combination with ICESat laser altimetry data,the decadal changes were charted in SGL volume. During 2007-2008, between 4.9 and 6.4 km³ of water drained from the SGL, and the peak discharge exceeded 160 m³/s. The flood was twice as large as any previously recorded, and equivalent to ~ 10 % of the meltwater generated annually beneath the ice sheet. - The ice surface has since uplifted at a rate of 5.6 ± 2.8 m/yr. Our study demonstrates the ability of CryoSat-2 to provide detailed maps of ice sheet topography, its potential to accurately measure SGL drainage events, and the contribution it can make to understanding water flow beneath Antarctica. ¹⁰²⁾

• April 30, 2013: CryoSat-2 has been in orbit for three years, completing over 15,000 orbits with its spacecraft and payload operating nominally. The mission has proven highly reliable, with no major anomalies since May 2011. To maximize the availability of its SIRAL radar for scientific observations, the Flight Control Team carefully schedules orbit maintenance and other operations to avoid interruptions, especially during key campaigns like NASA’s IceBridge and ESA’s CryoVEx. ^{103) 104)}

• February 14, 2013: An international team of scientists using new measurements from Europe’s ice mission has discovered that the volume of Arctic sea ice has declined by 36% during autumn and 9% during winter between 2003 and 2012. A team of scientists led by University College London has now generated estimates of the sea-ice volume for the 2010–11 and 2011–12 winters over the Arctic basin using data from ESA’s CryoSat-2 satellite. This study has confirmed, for the first time, that the decline in sea ice coverage in the polar region has been accompanied by a substantial decline in ice volume. Since 2008, the Arctic has lost about 4300 km³ of ice during the autumn period and about 1500 km³ in winter. The team confirmed CryoSat-2 estimates using independent ground and airborne measurements carried out by ESA and international scientists during the last two years in the polar region, as well as by comparing measurements from NASA’s Operation IceBridge. ^{105) 106) 107)}

• December 2012: ESA’s ice mission is now giving scientists a closer look at oceans, coastal areas, inland water bodies and even land, reaching above and beyond its original objectives. The satellite’s radar altimeter not only detects tiny variations in the height of the ice, it also measures sea level and the sea ice’s height above water to derive sea-ice thickness with an unprecedented accuracy. At a higher precision than previous altimeters, CryoSat’s measurements of sea level are improving the quality of the model forecasts. Small, local phenomena in the ocean surface like eddies can be detected and analyzed. Taking CryoSat a step further, scientists have now discovered that the altimetry readings have the potential to map sea level closer to the coast, and even greater capabilities to profile land surfaces and inland water targets such as small lakes, rivers and their intricate tributaries. ¹⁰⁸⁾

• May 28, 2012: The CryoSat-2 spacecraft and its payload are operating nominally in 2012. While the main objective of the CryoSat-2 mission is to measure the thickness of polar sea ice and monitor changes in the ice sheets that blanket Greenland and Antarctica, the radar altimeter, SIRAL (SAR Interferometer Radar Altimeter), is not only able to detect tiny variations in the height of the ice but it can also measure sea level. Recent studies at the Scripps Institution of Oceanography in San Diego, USA, found that the range precision of CryoSat-2 is at least 1.4 times better than the US's GEOSAT or ESA's ERS-1. They estimate that this improved range precision combined with three or more years of ocean mapping will result in global seafloor topography – bathymetry – that is 2–4 times more accurate than measurements currently available. ¹⁰⁹⁾

- Most satellite radar altimeters, such as the one on the joint CNES/NASA/Eumetsat/NOAA Jason-2, follow repeated ground-tracks every 10 days to monitor the changes in ocean topography associated with ocean currents and tides. On the other hand, the 369-day repeat cycle of CryoSat-2 provides a dense mapping of the global ocean surface at a track spacing of over 4 km. Three to four years of data from CryoSat can be averaged to reduce the ‘noise’ due to currents and tides and better chart the permanent topography related to marine gravity.

• April 2012: After nearly a year and a half of operations, CryoSat has yielded its first seasonal variation map of Arctic sea-ice thickness (Figure 88). Results from ESA’s ice mission were presented at the Royal Society in London as part of the events celebrating the 50th anniversary of the UK in space. ¹¹⁰⁾

• February 2012: Ocean measurements from ESA’s CryoSat-2 mission are being exploited by the French space agency, CNES, to provide global ocean observation products in near-real time. Understanding sea-surface currents is important for marine industries and protecting ocean environments. ¹¹²⁾

• January 2012: Although the primary objective of CryoSat-2 was to measure the thickness of ice, fast data delivery was not initially intended. The CryoSat team has changed this to demonstrate that CryoSat-2 can deliver marine information in near-real time from most of its orbits around Earth. Up to now, this new product called 'fast delivery mode' has only been provided to organizations such as the US NOAA (National Ocean and Atmospheric Organization). This is about to change: marine information is expected to be available systematically to all users from February 2012 onwards. ¹¹³⁾

- At NOAA’s LSA (Laboratory for Satellite Altimetry), the CryoSat-2 data are processed to estimate wind speed and wave height, which are then provided to forecasters at NOAA’s NCEPs (National Centers for Environmental Predication). LSA combines CryoSat-2 data with information from other organizations such as CNES of France, the ECMWF (European Centre for Medium-Range Weather Forecasts) and NASA. This processing takes a matter of only three days. NOAA delivers these data to ocean modelers and forecasters worldwide. For example, Australia’s Integrated Marine Observing System now uses CryoSat observations of sea level to monitor surface currents.

• December 2011: A team of Australian and German scientists from the University of Tasmania, the Australian Antarctic Division and the Alfred Wegner Institute (AWI, Bremerhaven) have just finished the first leg a remarkable measurement campaign. The campaign is being carried out in East Antarctica around Law Dome and the Totten Glacier. Law Dome is relatively stable but features steep surface slopes and Totten Glacier is changing rapidly – so both offer ideal locations for validating CryoSat-2 data. The campaign involves taking measurements from a Polar-6 aircraft. It carries the ASIRAS radar, which mimics CryoSat’s SIRAL. Ground-truth measurements are also collected for comparison. The skidoos drag GPS to map the height of the ice, which are later compared to the aircraft and satellite measurements. The skidoo team gathered ground measurement over about 250 km transects. ¹¹⁴⁾

• June 21, 2011: The first map of sea-ice thickness from ESA’s CryoSat-2 mission was revealed at the Paris Air and Space Show. This new information is set to change our understanding of the complex relationship between ice and climate. CryoSat-2 has spent the last seven months delivering precise measurements to study changes in the thickness of Earth’s ice. CryoSat-2’s detailed data have been used to generate this map of sea-ice thickness in the Arctic (Figure 60). Data from January and February of 2011 have been used to show the thickness of the ice as it approaches its annual maximum. Thanks to CryoSat-2’s orbit, ice thickness close to the North Pole can be seen for the first time. ¹¹⁵⁾

- A new map of Antarctica has also been created showing the height of the ice sheet (Figure 61). This is just a preliminary result, because more data are needed to exploit the full capabilities of CryoSat-2. Nevertheless, the extra coverage that CryoSat-2 offers near the poles can be demonstrated: parts of Antarctica can now be seen for the first time from space. In addition, detail of edges of the ice sheet where it meets the ocean can now closely be monitored thanks to CryoSat’s sophisticated radar techniques. This is important because this is where changes are occuring.

• April 19, 2011: ESA and NASA collaborated on a month-long Arctic campaign to validate CryoSat-2’s measurements, ensuring its accuracy in monitoring polar ice thickness. The campaign involved coordinated efforts between ground teams, airborne surveys, and CryoSat-2 overpasses, targeting key regions such as Greenland, Svalbard, and northern Canada. On April 17, CryoSat-2’s ground track was closely aligned with NASA's P-3 aircraft and other airborne platforms equipped with advanced instruments, allowing for direct comparison of ice and snow data. These efforts were crucial for verifying CryoSat-2’s ability to monitor dynamic changes at the margins of ice sheets, where the satellite’s advanced radar techniques excel in capturing intricate ice-ocean interactions. ^{116) 117) 118) 119)}

- NASA’s IceBridge mission complemented CryoSat-2’s observations by conducting over 200 hours of airborne surveys, focusing on rapidly changing features of Arctic and Antarctic ice. This combined dataset, which integrates CryoSat-2’s satellite measurements with detailed airborne and ground observations, provides a robust foundation for studying ice dynamics and their role in climate change and sea-level rise. By comparing CryoSat-2 data with in situ and airborne measurements, scientists can enhance the mission's contribution to understanding the polar regions’ evolving ice cover.

• February 1, 2011: ESA announced at the CryoSat Validation Workshop (Frascati, Italy, February 1-3, 2011) the release of the CryoSat-2 ice data. This means that the international science community will have free and easy access to all of the measurements from CryoSat-2. This will amount to a unique dataset to determine the impact climate change is having on Earth's ice fields. ¹²⁰⁾

• November 19, 2010: The CryoSat-2 mission was declared operational The results of the intense commissioning phase were presented to more than 80 scientists and engineers from ESA, industry and universities at the CryoSat-2 Commissioning Results Review, held in Noordwijk, the Netherlands, on 22 October. ^{121) 122)}

• July 2010: The project is already releasing access data to about 150 scientists of around 40 research institutes (users outside the project team) as part of the calibration and validation procedure. The intent is to help ensure that these measurements meet the mission's exacting standards before the data are released to the wider scientific community later this year. ¹²³⁾

• April 23, 2010: On April 20, 2010, as part of NASA’s Operation IceBridge, a DC-8 aircraft equipped with laser altimeters and imaging systems conducted a 670 km underflight along CryoSat-2’s orbital path over the Arctic Ocean. This mission, designated Sea Ice 07, provided an early opportunity to validate CryoSat-2’s sea-ice measurements. Flying at an altitude of 7,000 meters, the DC-8 collected detailed elevation maps using instruments such as the ATM (Airborne Topographic Mapper), while CryoSat-2’s SIRAL radar operated in SAR mode to capture corresponding data. The campaign, a collaboration between NASA and ESA, helped ensure the accuracy of CryoSat-2’s early observations. ^124) 125)

• April 11, 2010: The LEOP (Launch and Early Orbit Phase) was formally ended. The spacecraft is in excellent condition. Later on April 11, SIRAL (Synthetic Aperture Interferometric Radar Altimeter) was switched on for the first time and started gathering the first radar echo data. CryoSat-2 delivered its first data just hours after ground controllers switched on the satellite's sophisticated radar instrument for the first time. ¹²⁶⁾

Sensor Complement

SIRAL (SAR Interferometer Radar Altimeter)

SIRAL is the primary instrument of the mission, designed and developed for ESA by Thales Alenia Space (formerly Alcatel Alenia Space), France. SIRAL is of Poseidon-2 heritage flown on the Jason-1 mission. The objective is to observe ice sheet interiors, the ice sheet margins, for sea ice and other topography. ¹²⁷⁾

The SIRAL-2 design is based on existing equipment, but with several major enhancements designed to overcome difficulties associated with measuring ice surfaces. It works by bouncing a radar pulse off the ground and studying the echoes from the Earth's surface. By knowing the position of the spacecraft - achieved with an onboard ranging instrument called DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) - the signal return time will reveal the surface altitude. Correct antenna orientation is vital for this and is maintained using a trio of star trackers.

The design of SIRAL for the CryoSat-2 mission was made completely redundant. ¹²⁸⁾

The SIRAL instrument design makes use of the DDA (Delay Doppler Altimeter) concept representing a new technology introduction into spaceborne altimetry and permitting that detailed views of irregular sloping edges of land ice, as well as non-homogenous ocean ice, can also be obtained. The new features of SIRAL have been demonstrated with the airborne D2P (Delay-Doppler Phase-monopulse Radar) of JHU/APL, first test flights were conducted in 2000 (for a DDA concept introduction see the last chapter of the description).

The SIRAL design features two receiving antennas forming an interferometer in the cross-track direction with a baseline of 1.2 m (support for SARIn mode). In addition, the return signal in along-track direction is processed to construct a synthetic aperture for enhanced ground resolution. The instrument is a Ku-band radar altimeter which uses the full-deramp range compression technique of conventional altimeters (conventional single frequency pulse-limited altimeter). However, it introduces two features that make it different from previous spaceborne altimeter implementations: ^{129) 130) 131) 132)}

• The instrument has two parabolic antennas (including pulse-to-pulse phase coherence) and two receive chains, permitting an interferometric mode of operation (and interferometric signal processing).

• SIRAL operates at high PRF (Pulse Repetition Frequency), ensuring coherent along-track sampling for aperture synthesis (PRF>Doppler bandwidth). The distinguishing feature of SIRAL compared to conventional altimeter instruments (generally with pulse intervals of about 500 µs) is that it sends bursts of pulses separated by intervals of only 50 µs. Though the return echoes are correlated, the bursts are instead treated using “aperture synthesis” data processing techniques.

The instrument consists of three major subsystems, two of these are in discrete electronic boxes:

• DPU (Digital Processing Unit), it serves all digital altimeter functions, including the digital chirp generation, the full sequencing functions of the altimeter, and the receive and processing functions of the echo

• RFU (Radio Frequency Unit). It contains all analog IF and RF electronics and a solid-state power amplifier with an RF peak power of 25 W.

• The antenna subsystem consists of two Cassegrain antennas, mounted side-by-side and forming the interferometric cross-track. Both antennas are identical; one is used to transmit and receive, whereas the other antenna is used to receive echoes (bistatic configuration). The primary super-elliptic reflectors are about 1.1 m x 1.2 m in size. They are supported by a composite sandwich plate. A high thermoelastic stability is needed to meet the interferometric instrument performance.

The science requirements demand of CryoSat to measure variations in ice thickness of perennial sea and land ice fields to the limit allowed by natural variability, on spatial scales varying over three orders of magnitude. The natural variability of sea and land ice depends on fluctuations in the supply of mass by the atmosphere and ocean, and snow and ice density. The precisions of the measurements are expressed in terms of cm of yearly ice equivalent thickness variations. These are:

• Arctic sea-ice: 1.6 cm/year vertical measurement accuracy at 10⁵ km² scale (equivalent to 300 km x 300 km cells). Temporal sampling: 1 month

• Land ice (small scale): 3.3 cm/year at 10⁴ km² (equivalent to 100 km x 100 km cells). Temporal sampling : 1 year

• Land-ice (large scale): 0.17 cm/year over 13.8 x 10⁶ km² (about the area of Antarctica). Temporal sampling: 1 year.

The monitoring of the interferometric behavior of the receive chains is ensured by a dedicated interferometric calibration mode that can be used as an operational mode. An additional calibration mode permits the measurement of the amplitude/phase distortions of each receive chain. By using an internal frequency synthesizer, this measurement can be done for several frequencies inside the IF bandwidth. In addition, different gain settings can be used, which makes it possible to accurately determine the gain of the receiver. Either of the receive chain (chain 1 or 2) may be selected in LRM or in SAR support modes. This in-flight capability increases the knowledge of the instrument contribution on the echo measurement.

The chirp generator is composed of a digital pulse generation section, operating with a sampling rate of 160 MHz, followed by an analog multiplier section expanding the pulse bandwidth by a factor of 16, up to 350 MHz. This configuration ensures the pulse-to-pulse coherence required for the SAR modes. Parameters: chirp frequency = 4.08 GHz; bandwidth = 350 MHz; signal duration = 51 µs; SNR=30 dB.

The SSPA (Solid-State Power Amplifier) is composed of four parallel hybrid amplifiers in PHEMT technology. Parameters: frequency = 13.575 GHz; peak power >25 W; gain = 9 dB; amplitude ripple = 0.2 dB in 350 MHz; efficiency = 24%. - The FFT (Fast Fourier Transform) module, needed for the on-board tracking algorithm to estimate range and gain commands, makes use of the existing FFT module of POSEIDON 2.

SIRAL Operational Modes

SIRAL provides the following operational modes for different observational support types. The complex waveform data stream from the CryoSat altimeter requires a sophisticated processing scheme in particular for exploiting the synthetic aperture and interferometry techniques over ocean and ice surfaces.

1) LRM (Low Resolution Mode) operation support: LRM uses a single receive channel and low PRF for conventional pulse-limited operation for ice sheet interiors/open oceans. The transmitted pulse length and the transmitted bandwidth are set to the same value as that for Envisat in a similar mode (51 s, 320 MHz). The PRF is kept constant over the orbit at a value around 2 kHz to ensure the decorrelation of received echoes. The averaging for tracking and ground processing is performed after the FFT (Fast Fourier Transform).

The LRM mode is useful over surfaces where the topography is homogeneous, at least as large as the antenna footprint of about 15 km. The altimeter echoes have a predictable shape and the mean surface level of this area can be derived by an appropriate model.

2) SARM (Synthetic Aperture Radar Mode) support mode (also referred to as advanced SAR mode): SARM uses a single channel and a high PRF. Closed burst timing is employed to ensure a high along-track resolution. The PRF is chosen higher than the Doppler bandwidth over the half-power beamwidth to avoid aliasing in the ground processing of the data (the PRF is about 10 times higher than that of LRM to ensure coherence between the echoes of successive pulses). Bursts of 64 pulses at a PRF of 18.5 kHz with a burst repetition frequency of 85 Hz are transmitted.

In SARM, the resolution of the radar is improved in the along-track direction. This is achieved by exploiting the Doppler properties of the echoes as they cross the antenna beamwidth. The result is equivalent to decomposing the main antenna beam into a set of 64 narrower synthetic beams in the along-track direction. The footprints of the different sub-beams over a flat surface are adjacent rectangular areas, about 250 m wide in along-track and as large as the antenna's cross-track footprint (up to 15 km). Hence, a larger number of independent measurements are available over a given area; this property is used to enhance the accuracy of the measurements over sea ice. The echoes are transmitted to the ground segment in the time domain, prior to any averaging. Hence, the data rate in SARM is significantly higher than that for LRM.

3) SARIn (SAR Interferometric) support mode. The objective is to provide improved elevation estimates over variable topography. This mode is used mainly over ice sheet margins with high surface slopes. Both receive channels are operating simultaneously at high PRF to ensure the availability of a high cross-track resolution used for ice sheet margins and coastal areas (accurate determination of the arrival direction of the echoes in along-track and in cross-track). This is needed to derive the height of the surface from the range measurement of the radar. Narrow-band tracking pulses, transmitted in-between successive wide-band measurement bursts are used in this range-tracking concept to cope with abrupt height variations.

In the SARIn mode, the addition of the interferometric feature to the SAR further improves the echo localization capabilities, as the cross-track direction angle of the echoes can be determined. This is achieved by comparing the phase of one receive channel with respect to the other.

The innovative technical features of SIRAL are:

• The capability to operate in all measurement modes

• Digital chirp generation with pulse-to-pulse coherence for Doppler processing

• Solid State Power Amplifier (SSPA) in Ku-band with high performance (25 W),

• Dual antennas forming an interferometer, mounted on an optical bench together with star-tracker heads, ensuring the accurate knowledge and stability of the interferometric baseline orientation

• Two receive chains matched together with very low distortions.

The novel feature of SIRAL, as compared with conventional altimeters, is the capability to locate a resolution cell in the 3 dimensional space. The SIRAL concept is based on a Ku-band nadir-looking radar which can be operated in the conventional mode over oceans. Over terrain (ice or land) the “advanced SAR mode” uses Doppler filtering for the enhancement of the along-track resolution. A second antenna and receiving channel provides a second take of the scene which is used for surface height retrieval as it is usually done with SAR interferometry.

Operations principle: Conventional radar altimeters send pulses with a long interval : about 500 µs. SIRAL sends a burst of pulses with an interval of only 50 µs between them. The returning echoes are thus correlated, and by treating the whole burst of pulses in one operation, the data processor can separate the echo into strips arranged across the track by exploiting the slight frequency shifts (caused by the Doppler effect) in the forward- and aft-looking parts of the beam. The strips laid down by successive bursts can therefore be superimposed on each other and averaged to reduce noise. This mode of operation is called the SARM (Synthetic Aperture Radar Mode). ¹³³⁾

In interferometric mode (SARIn), a second receiving antenna is activated to measure the arrival angle. It enables to receive some radar echos coming from a point not directly located beneath the satellite. The difference in the path-length time of the radar echos is tiny between radar echos on the track and radar echoes out of the track. The measure of the angle between the baseline joining the antennas and the echo direction is essential and must be very accurate. The baseline orientation is so operated by three star trackers.

The Cryosat-2 mission is the first altimeter mission to operate the SARM (SAR mode), next to the LRM (Low Resolution Mode), in the SIRAL (SAT Interferometer Radar Altimeter) instrument.

DORIS (Doppler Orbitography and Radiopositioning Integration by Satellite)

DORIS measures the Doppler frequency shifts of both VHF and S-Band signals transmitted by ground beacons. Its measurement accuracy is better than 0.5 mm/s in radial velocity allowing an absolute determination of the orbit position with an accuracy of 2-6 cm. DORIS is an uplink radio frequency tracking system based on the Doppler principle. The CNES instrument provides accurate measurements for a precise orbit determination. Knowledge of the orbit is essential for exploitation of the altimeter data and the overall performance. The onboard receiver measures the Doppler shift of uplink beacons in two frequencies (2.03625 GHz for Doppler measurement and 401.25 MHz for the ionospheric correction) which are transmitted continuously by the ground stations. One measurement is used to determine the radial velocity between spacecraft and beacon, the other to eliminate errors due to ionospheric propagation delays. The 401.25 MHz frequency is also used for measurements of time-tagging and auxiliary data transmission. The DORIS instrument comprises:

• A fixed omni-directional dual-frequency antenna

• A receiver performing the Doppler measurements every ten seconds. The nominal mode of operation is an autonomously programmed mode in which the receiver tracks the beacon signals according to information provided by the navigation software (DIODE) based on an on-board table of beacon data.

• An USO (Ultra Stable Oscillator) delivering the reference frequency with a stability of 5 x 10^-13 over a period of 10 to 100 s.

The mass of DORIS is 15 kg (including the antenna of 160 mm diameter). The instrument requires 20 W of power, the data rate is 4 kbit/s.

The following DORIS services are used for CryoSat operations:

- Real-time orbit determination for spacecraft attitude and orbit control (on-board)

- Provision of a precise time reference based on TAI (International Atomic Time); in addition a precise 10 MHz reference signal is used (on-board)

- Provision of on-ground POD (Precise Orbit Determination) and ionospheric modelling.

The entire DORIS system comprises a network of more than 50 ground beacons, a number of receivers on several satellites in orbit and in development, and ground segment facilities. It is part of IDS (International DORIS Service), which also offers the possibility of precise localization of user-beacons.

LRR (Laser Retroreflector)

LRR is a passive optical device. The objective is to use LRR as an additional tool and backup for precise orbit determination with the aid of the international laser tracking network. LRR is accommodated in the nadir plate of the spacecraft, its FOV of ±57.6º is suitable for range measurements above 20º elevation angles at all azimuths from the ground. For any aspect angle the predicted rms target error is below 6 mm.

Introduction of the DDA (Delay Doppler Altimetry) technology into the SIRAL design

The concept of DDA, initially proposed by Keith Raney of JHU/APL (Johns Hopkins University/Applied Physics Laboratory), represents a new technology introduction with the potential to greatly increase the value of observations from satellite radar altimetry. The DDA scheme takes advantage of the Doppler shift of the pulse frequency in the along-track direction to allow for an increase in pulse repetition frequency and a subdivision of the illuminated area along-track into discrete Doppler bins to provide a dramatic improvement in efficiency and precision. ^{134) 135)}

A conventional pulse-limited altimeter independently averages many radar pulses as the spacecraft moves along its track during the averaging time window and its illuminated area becomes defocused with increasing significant wave height. The relatively slow repetition of pulses and the impact of the waves limit the available resolution of the instrument.

The DDA (Delay-Doppler Altimeter) differs from a conventional radar altimeter concept in that it exploits coherent processing of groups of transmitted pulses and the full Doppler bandwidth is exploited to make the most efficient use of the power reflected from Earth's surface. While the conventional altimeter technique is to measure the distance between the satellite and the mean ocean surface, the DDA method differs from those instruments in two ways: ^{136) 137) 138) 139)}

- Pulse-to-pulse coherence and full Doppler processing to allow for measurement of the along-track position of the range measurement

- Use of two antennas and two receiver channels that allow for measurement of the across-track angle of the range measurement.

This is a significant improvement over conventional Doppler beam sharpening. To exploit this full bandwidth, the range variation that exists across the Doppler bins is removed as part of the data processing. The reflected pulses from a given area of the observed surface are integrated over the entire time that the target area is within the radar beamwidth. As a result, much more of the reflected energy is captured and a smaller transmitted power is required to obtain a given level of performance.

The DDA concept (Figure 72b) retains the inherent advantages of a pulse-limited altimeter with its spherical wavefront always providing a nadir component, thus avoiding instrument nadir-pointing errors. In addition, the DDA exploits the faster pulse repetition frequency by binning the Doppler frequency shifts in the along-track direction. These bins appear as narrow strips orthogonal to the satellite ground track. As the DDA moves along its path, the leading edge Doppler bin illuminated during the first pulse becomes the second Doppler bin during the next pulse and receives a second “look” by the instrument. This process repeats as long as the bin remains within the DDA footprint. Each pulse defines a new leading edge Doppler bin, re-samples each bin within the footprint, and integrates the retrievals as the satellite moves along its track. Since each bin is sampled many times, the samples can be coherently processed and the higher pulse repetition frequency provides for a higher resolution footprint along-track that is independent of the significant wave height. For example, a 30 Hz altimeter pulse provides a signal integration length that results in widths of the Doppler bins as narrow as 250 m.

The DDA technology provides several advantages over conventional altimetry. The sea surface height precision available from this type of instrument is approximately twice that of existing sensors. Simulations of the associated signal processing concepts have produced 0.5 cm precision in a calm sea, with precision remaining better than 1.0 cm even in significant wave heights as great as 4 m. The DDA technique is much less sensitive to errors induced by ocean waves. For a calm sea, DDA and conventional altimetry experience comparable levels of random noise; however, as the waves grow, a conventional altimeter experiences a dramatic noise level increase. With the coherent processing of the DDA, only a slight increase in random noise with wave height is experienced. This makes the DDA particularly well suited for geodetic applications where the random error due to ocean waves is the dominant error source. Wind speed and wave height retrievals from the DDA have twice the precision of current sensors.

Another advantage of DDA is the ability to sample the coastal ocean where conventional altimeters experience signal contamination from land. As the spacecraft approaches or departs a coastline where the angle of intersection with the satellite ground track is nearly orthogonal, on board processing can identify individual Doppler bins close to the coast and continue to sample it as the satellite passes over the boundary.

From a system architecture perspective, the efficiency of the DDA provides for less transmitted power by the instrument and the potential for smaller and lighter spacecraft components - and thus a less costly mission - when compared to conventional altimeters with a similar design life.

There are also some data processing consequences with regard to SIRAL data which is based on the precise wavenumber domain approach. ^{140) 141) 142)}

NOC Sea Level Anomaly (NOCSLA) Gridded Product

The new NOC Sea Level Anomaly (NOCSLA) gridded product is based on high-quality geophysical ocean products processed from SIRAL observations. This daily ¼° sea level anomaly product covers non-coastal oceans – from 60° north to 60° south – between January 2011 to October 2020.
To establish the scientific validity of NOCSLA, the product was compared against data delivered by a variety of space-borne sensors as part of a two-pronged verification process.
Firstly, NOCSLA was contrasted with observations from the international altimetry mission Jason-3, as well as with a sea level product produced by ESA’s Climate Change Initiative, and a sea surface height product produced by the Copernicus Climate Change Service (C3S).
Secondly, oceanic case studies were completed using NOCSLA and the results were compared with those from other data sources and models.

One study focused on assessing the product’s performance in observing events related to El Niño using NOAA’s ENSO (El Niño and the Southern Oscillation) index.
In another case study, NOCSLA data covering the Indian Ocean were used to investigate the occurrence of Rossby waves, also known as planetary waves, that form naturally in rotating fluids. These waves form as a result of Earth’s rotation. Rossby wave speeds determined by NOCSLA were in excellent agreement with a comparison dataset from ESA’s Climate Change Initiative.
Across both prongs of the verification process, NOCSLA outputs aligned well with ocean products from a variety of data sources, demonstrating the suitability of NOCSLA to enable a range of investigations into ocean processes.
Chris Banks, satellite oceanographer at NOC and lead author of the study, said: “Sea level anomaly products based on satellite data – such as the C3S product we employed in the NOCSLA validation – are already widely used.
“NOCSLA is unique because it is based on a single mission with a novel orbit, rather than a traditional repeat orbit every 10-30 days, and a long-term timeseries, creating a benefit for researchers and other data users.” 
 

Ground Segment

The CryoSat mission will be operated from ESA/ESOC, Darmstadt, Germany. The Kiruna ground station in Sweden functions as the prime command and data acquisition facility. The payload data segment (data processing, archiving and distribution) function is also located at the Kiruna station. The ground segment makes use of the existing infrastructure. All user interfaces are coordinated via ESA/ESRIN with dissemination of data from Kiruna.

An important aspect of the CryoSat-2 ground segment is that it had been designed for operations with a low level of manpower. Furthermore, remote operations and troubleshooting can be performed on all systems located in Kiruna.

The major elements of the ground segment are:

• RPF (Reference Planning Facility): responsible for the planning of the payload and the satellite resources verification.

• FOS (Flight Operations Segment): responsible for the telecommand scheduling, satellite command and control, and telemetry acquisition.

• PDS (Payload Data Segment): responsible of scientific data processing, archiving and distribution

• MF (Monitoring Facility): responsible for providing measures of the performance of the system, and in particular of the instruments.

• Complementary supporting ESA elements, shared with other missions:

- USF (User Services Facility)

- LTA (Long-Term Archive)

• Other elements outside ESA:

- DORIS control and processing centre (SSALTO) which provides precise orbits

- Auxiliary data providers (e.g. meteorological data)

- SLR (Satellite Laser Ranging) stations.

• User community, calibration, validation and retrieval.

Permanent Calibration Station for Altimeters in Crete with Microwave Transponder

The Technical University of Crete (TUC) is installing a new permanent microwave transponder ground infrastructure on the Island of Crete, Greece, to serve as an alternative and independent technique for the calibration of, mainly, European altimetric missions. The facility was initially planned as a calibration site for the Sentinel-3 in the south west of Crete, Greece, using the developed transponder. However, this ground infrastructure, along with other permanent facilities in Crete, may also be used for the calibration of other Ku-band altimetric missions such Jason-2, Cryosat-2, etc. ¹⁴³⁾

The idea for incorporating land based transponders was initially introduced in 2000. ¹⁴⁴⁾ A microwave transponder is an electronic equipment which receives the pulsed radar signal, transmitted by the altimeter of the over-passing satellite and actively amplifies and retransmits the signal towards the spacecraft, where it is recorded. The time delay of the signal is measured, from which the absolute range between the transponder and the satellite can be deduced. The main advantage of this technique, compared to the conventional sea-surface calibration, stands for the fact that no ocean dynamics errors are involved in satellite altimeter’s calibration.

However, in the past, only few transponders have been built and implemented for this reason. The ESA premises in Svalbard, Norway host a transponder developed by RAL, UK in 1987 that has been used mainly for the Cryosat-2 calibration. The Gavdos Island Cal/Val facility in Greece hosted the Austrian Academy of Sciences transponder and that transponder has been effectively used for the calibration of Envisat and Jason-2 missions. There has been another transponder placed in Rome, Italy which was used for the Envisat sigma-0 calibration.

TUC Transponder

In 2011, the Geodesy and Geomatics Engineering Laboratory at the Technical University of Crete in Greece developed a new Ku-band microwave transponder. The TUC transponder is mobile, allowing calibration at different locations but also modular for operating in other frequencies, provided that some parts are modified. It is capable of recording the incoming and outgoing signals, while it can be controlled and operated remotely. The transponder frequency has been selected to be compatible with past, current and future European as well as international altimetry missions that operate in this microwave range (i.e., Jason series, Cryosat-2, Sentinel-3). Additionally, it is equipped with a GPS (Global Navigation Satellite System) receiver and appropriate meteorological sensors to provide precise time-tagging, as well as the atmospheric delay corrections during transponder calibration. This is of importance for the accurate determination of the altimetric range because the atmosphere affects the altimetric measurements. Furthermore, this prototype transponder is the only microwave transponder that incorporates circularly polarized antennas. The latter, allows performing calibration experiments on different satellite missions at the same location, approaching from different directions, providing that the satellite ground track is in a range of 3-5 km away from the transponder location.

The TUC transponder has been characterized for 4 months (March-July 2012) at the CPTR (Compact Payload Test Range) facilities in ESA/ESTEC, the Netherlands.

The transponder has already been used for the calibration of several Cryosat-2 passes (10-May, 8-June and 3-August 2013) over the SLR2 (Satellite Laser Ranging 2) site (35° 32.084' N, 24° 04.061' E) in North West Crete, Greece, and a clear response has been captured on the satellite’s data (Figure 75).

A TUC transponder site has been selected on Crete Island which represents a triple cross-over point between Sentinel-3A, -3B and Jason-2&3 (and also Jason-CS, as it will most likely fly over the same Jason-series tracks). This criterion was used to finally define and freeze the ground tracks for Sentinel-3 mission.

The CDN2 (35° 20.729' N, 23° 46.577'E) site is exactly under Jason, 100 m east of Sentinel-3A and 300 m west of the Sentinel-3B ground tracks. The CNES team will verify the satellite signal observed using Jason-2 around the CDN2 candidate.

The instruments at the CDN2 site will be protected using either weather-proof boxes or a container with appropriate covers to avoid/reduce any satellite echoes by their metallic parts. Figure 76 illustrates an indicative spatial distribution of the necessary and ancillary instrumentation to be constructed at the CDN2 Sentinel-3 altimeter calibration site. Besides the instrumentation and infrastructure, the preparatory steps taken for the establishment of the CDN2 site involve also the development of appropriate software for data archival and transmission and for the determination of the transponder’s precise positioning.

The Sentinel-3 altimeter calibration site is expected to be fully operational in early 2014, that is about one year prior to the Sentinel-3A launch. During this period, calibration campaigns for the Jason-2 and Cryosat-2 altimetry mission will be performed to test the transponder’s operational capabilities in real-field conditions. These campaigns will aim at: a) delivering altimeter calibration values for these satellites, b) getting familiarized with the remote operation procedures to be followed, and 3) identifying potential upgrades necessary for improving the transponder’s performance.

The transponder is to be upgraded, improved, and characterized before its final deployment and support for Sentinel-3A commissioning phase in 2015.

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25) ”Meltwater runoff from Greenland becoming more erratic,” ESA Applications, 3 November 2021, URL: https://www.esa.int/Applications/Observing_the_Earth/CryoSat/Meltwater_runoff_from_Greenland_becoming_more_erratic

26) Thomas Slater, Andrew Shepherd, Malcolm McMillan, Amber Leeson, Lin Gilbert, Alan Muir, Peter Kuipers Munneke, Brice Noël, Xavier Fettweis, Michiel van den Broeke & Kate Briggs, ”Increased variability in Greenland Ice Sheet runoff from satellite observations,” Nature Communications, Vol. 12, Article Number: 6069, Published: 1 November 2021, https://doi.org/10.1038/s41467-021-26229-4, URL: https://www.nature.com/articles/s41467-021-26229-4.pdf

27) ”Arctic coastal sea ice thinning twice as fast than thought,” ESA Applications / Observing the Earth / CryoSat, 10 June 2021, URL: https://www.esa.int/Applications/Observing_the_Earth/CryoSat/Arctic_coastal_sea_ice_thinning_twice_as_fast_than_thought

28) Robbie D. C. Mallett, Julienne C. Stroeve, Michel Tsamados, Jack C. Landy, Rosemary Willatt, Vishnu Nandan, and Glen E. Liston, ”Faster decline and higher variability in the sea ice thickness of the marginal Arctic seas when accounting for dynamic snow cover,” The Cryosphere, Vol. 15, pp: 2429-2450, Published: 04 June 2021, https://doi.org/10.5194/tc-15-2429-2021

29) ”CryoSat reveals ice loss from glaciers in Alaska and Asia,” ESA Applications, 01 June 2021, URL: http://www.esa.int/Applications/Observing_the_Earth/CryoSat/CryoSat_reveals_ice_loss_from_glaciers_in_Alaska_and_Asia

30) Livia Jakob, Noel Gourmelen, Martin Ewart, and Stephen Plummer, ”Spatially and temporally resolved ice loss in High Mountain Asia and the Gulf of Alaska observed by CryoSat-2 swath altimetry between 2010 and 2019,” The Cryosphere, Volume 15, pp: 1845–1862, 2021, Published: 14 April 2021, https://doi.org/10.5194/tc-15-1845-2021, , URL: https://tc.copernicus.org/articles/15/1845/2021/tc-15-1845-2021.pdf

31) ”Arctic sea ice succumbs to Atlantification,” ESA Applications, 21 May 2021, URL: https://www.esa.int/Applications/Observing_the_Earth/CryoSat/Arctic_sea_ice_succumbs_to_Atlantification

32) ”Winners presented with ESA-EGU Excellence award,” ESA Applications, 19 April 2021, URL: https://www.esa.int/Applications/Observing_the_Earth/Winners_presented_with_ESA-EGU_Excellence_award

33) ”CryoSat reveals surprising ebb and flow of subglacial lakes,” ESA Applications, 14 December 2020, URL: http://www.esa.int/Applications/Observing_the_Earth/CryoSat/CryoSat_reveals_surprising_ebb_and_flow_of_subglacial_lakes

34) ”Places in Antarctica named in honor of ice scientists,” ESA / Applications / Observing the Earth / CryoSat, 09 December 2020, URL: https://www.esa.int/Applications/Observing_the_Earth/CryoSat/Places_in_Antarctica_named_in_honour_of_ice_scientists

35) ”CryoSat taken to new heights for ice science,” ESA Applications, 03 August 2020, URL: https://www.esa.int/Applications/Observing_the_Earth/CryoSat/CryoSat_taken_to_new_heights_for_ice_science

36) ”CryoSat still cool at 10,” ESA / Applications / Observing the Earth / CryoSat, 8 April 2020, URL: http://www.esa.int/Applications/Observing_the_Earth/CryoSat/CryoSat_still_cool_at_10

37) ”CryoSat sheds new light on Antarctica’s biggest glacier,” ESA / Applications / Observing the Earth / CryoSat, 27 January 2020, URL: http://www.esa.int/Applications/Observing_the_Earth/CryoSat/CryoSat_sheds_new_light_on_Antarctica_s_biggest_glacier

38) ”CryoSat maps ice shelf on the move,” ESA / Applications / Observing the Earth / CryoSat, 13 December 2019, URL: http://www.esa.int/Applications/Observing_the_Earth/CryoSat/CryoSat_maps_ice_shelf_on_the_move

39) Jan Wuite, Thomas Nagler, Noel Gourmelen, Maria Jose Escorihuela, Anna E. Hogg and Mark R. Drinkwater, ”Sub-Annual Calving Front Migration, Area Change and Calving Rates from Swath Mode CryoSat-2 Altimetry, on Filchner-Ronne Ice Shelf, Antarctica,” Remote Sensing, Published 23 November 2019, Volume 11, Issue 23, 2761, https://doi.org/10.3390/rs11232761

40) ”CryoSat conquers ice on Arctic lakes,” ESA, 5 August 2019, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/CryoSat/CryoSat_conquers_ice_on_Arctic_lakes

41) Justin F. Beckers, J. Alec Casey, Christian Haas, ”Retrievals of Lake Ice Thickness From Great Slave Lake and Great Bear Lake Using CryoSat-2,” IEEE Transactions on Geoscience and Remote Sensing, Volume 55, Issue: 7, July 2017, URL:  https://web.archive.org/web/20221218192024/http://ieeexplore.ieee.org/document/7891554/

42) ”Expanding our knowledge of Arctic Ocean bathymetry,” ESA, 24 July 2019, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/CryoSat/Expanding_our_knowledge_of_Arctic_Ocean_bathymetry

43) Adili Abulaitijiang, Ole Baltazar Andersen, David Sandwell, ”Improved Arctic Ocean Bathymetry derived from DTU17 Gravity model,” AGU 100 (Advancing Earth and Space Science), First published: 08 July 2019, https://doi.org/10.1029/2018EA000502

44) ”Modelling tides in the Arctic Ocean,” ESA, 11 July 2019, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/CryoSat/Modelling_tides_in_the_Arctic_Ocean

45) M. Cancet, O. B. Andersen, F. Lyard, D. Cotton, J. Benveniste, ”Arctide2017, a high-resolution regional tidal model in the Arctic Ocean,” Advances in Space Research, Volume 62, Issue 6, 15 September 2018, Pages 1324-1343, URL: https://tinyurl.com/y2axpw8c

46) ”Antarctica detailed in 3D,” ESA, 14 May 2019, URL: http://www.esa.int/spaceinimages/Images/2019/05/Antarctica_detailed_in_3D

47) N. Gourmelen,M. J. Escorihuela, A. Shepherd, L. Foresta, A. Muir, A. Garcia-Mondéjar, M. Roca, S. G. Baker, M. R. Drinkwater, ”CryoSat-2 swath interferometric altimetry for mapping ice elevation and elevation change,” Advances in Space Research, Volume 62, Issue 6, 15 September 2018, Pages 1226-1242, https://doi.org/10.1016/j.asr.2017.11.014, URL: https://tinyurl.com/y63k74v2

48) ”US government shutdown effects on CryoSat geophysical corrections and parameters,” ESA, 23 January 2019, URL:  https://web.archive.org/web/20191227133848/https://earth.esa.int/web/guest/missions/esa-operational-eo-missions/cryosat/news/-/article/us-government-shutdown-effects-on-cryosat-geophysical-corrections-and-parameters

49) T. Parrinello, A. Shepherd, J. Bouffard, S. Badessi, T. Casal, M. Davidson, M. Fornari, E. Maestroni, M. Scagliola, ”CryoSat: ESA’s ice mission – Eight years in space,” Advances in Space Research, Vol. 62, Issue 6, 15 September 2018, pp: 1178–1190, https://doi.org/10.1016/j.asr.2018.04.014, Published by Elsevier Ltd.,URL: http://tinyurl.com/y2uzf5xr

50) ”New view of Antarctica in 3D,” ESA, 11 May 2018, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/CryoSat/New_view_of_Antarctica_in_3D

51) ”CryoSat reveals retreat of Patagonian glaciers,” ESA, 2 May 2018, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/CryoSat/CryoSat_reveals_retreat_of_Patagonian_glaciers

52) L. Foresta, N. Gourmelen, F. Weissgerber, P. Nienow, J. J. Williams, A. Shepherd, M. R. Drinkwater, S. Plummer, ”Heterogeneous and rapid ice loss over the Patagonian Ice Fields revealed by CryoSat-2 swath radar altimetry,” Remote Sensing of Environment, Available online 20 April 2018, https://doi.org/10.1016/j.rse.2018.03.041

53) ”Antarctica loses grip,” ESA, 3 April 2018, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/CryoSat/Antarctica_loses_grip

54) Hannes Konrad, Andrew Shepherd, Lin Gilbert, Anna E. Hogg, Malcolm McMillan, Alan Muir, Thomas Slater, ”Net retreat of Antarctic glacier grounding lines,” Nature Geoscience, 2018, URL: https://www.nature.com/articles/s41561-018-0082-z.epdf?author_access_token=HCt_-ggOqYI1iPWrsR-BINRgN0jAjWel9jnR3ZoTv0PwzmpCvh7bKPjXpWeaTWIEiwEEdPz4GfJiBeHy6NOuuZ3WoFwUDLq8F36txXpSfoAD5yEjM0CMICYGZHTlKXPqt6QGyeIMDNsHH0PlKmlOjQ%3D%3D

55) ”Shifting grounding lines,” ESA, 3 April 2018, URL: http://m.esa.int/spaceinimages/Images/2018/04/Shifting_grounding_lines

56) ”Effect of grounding line on surface,” ESA, 3 April 2018, URL: http://m.esa.int/spaceinimages/Images/2018/04/Effect_of_grounding_line_on_surface

57) ”Secrets of hidden ice canyons revealed,” ESA, 11 Oct. 2017, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/CryoSat/Secrets_of_hidden_ice_canyons_revealed

58) ”Giant iceberg in the making,”ESA, July 5, 2017, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/CryoSat/Giant_iceberg_in_the_making

59) ”Historical iceberg tracks,” ESA,5 July 2017, URL: http://m.esa.int/spaceinimages/Images/2002/07/Historical_iceberg_tracks

60) ”CryoSat reveals Antarctica in 3D,” ESA, March 24, 2017, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/CryoSat/CryoSat_reveals_Antarctica_in_3D

61) ”To the Arctic for CryoSat and beyond,” ESA, March 20, 2017, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/CryoSat/To_the_Arctic_for_CryoSat_and_beyond

62) ”Two frequencies on one plane,” ESA, March 20, 2017, URL: http://m.esa.int/spaceinimages/Images/2017/03/Two_frequencies_on_one_plane

63) ”CryoSat reveals lake outbursts beneath Antarctic ice,” ESA, Feb. 8, 2017, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/CryoSat/CryoSat_reveals_lake_outbursts_beneath_Antarctic_ice

64) Benjamin E. Smith, Noel Gourmelen, Alexander Huth, Ian Joughin, ”Connected subglacial lake drainage beneath Thwaites Glacier, West Antarctica,” The Cryosphere, Vol. 11, pp: 451-467, doi:10.5194/tc-11-451-2017, published on Feb. 8, 2017, URL: http://www.the-cryosphere.net/11/451/2017/tc-11-451-2017.pdf

65) ”Glacier speed West Antarctica,” ESA, released Feb. 8, 2017, URL: http://m.esa.int/spaceinimages/Images/2017/02/Glacier_speed_West_Antarctica

66) ”Satellite cousins have ice covered,” ESA, Dec. 16, 2016, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/SMOS/Satellite_cousins_have_ice_covered

67) ”Arctic freeze slows down,” ESA, Nov. 30, 2016, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/CryoSat/Arctic_freeze_slows_down

68) ”CryoSat sets new standard for measuring sea levels,” ESA, July 26, 2016, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/CryoSat/CryoSat_sets_new_standard_for_measuring_sea_levels

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70) ”The Arctic is facing a decline in sea ice that might equal the negative record of 2012 — Data collected by the CryoSat-2 satellite reveal large amounts of thin ice that are unlikely to survive the summer,” AWI Press Release, April 21, 2016, URL: http://www.awi.de/nc/en/about-us/service/press/press-release/der-arktis-droht-ein-meereisverlust-wie-im-negativrekordjahr-2012.html

71) EGU General Assembly 2016, Vienna Austria, April 17-22, 2016

72) Jonathan Amos, ”Call for dedicated polar Sentinel satellite,” BBC News, March 24, 2016, URL: http://www.bbc.com/news/science-environment-35886817

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77) “Fast access to CryoSat’s Arctic ice measurements now available,” ESA, April 17, 2015, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/CryoSat/Fast_access_to_CryoSat_s_Arctic_ice_measurements_now_available

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80) Satellites catch Austafonna shedding ice,” ESA, Jan. 23, 2015, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Satellites_catch_Austfonna_shedding_ice

81) Malcolm McMillan, Andrew Shepherd, Noel Gourmelen, Amaury Dehecq, Amber Leeson, Andrew Ridout, Thomas Flament, Anna Hogg, Lin Gilbert, Toby Benham, Michiel van den Broeke, Julian A. Dowdeswell, Xavier Fettweis, Brice Noël, Tazio Strozzi, “Rapid dynamic activation of a marine-based Arctic ice cap,” Geophysical Research Letters, Volume 41, Issue 24, pp: 8902–8909, 28 December 2014, DOI: 10.1002/2014GL062255

82) CryoSat-2 extends its reach on the Arctic,” ESA, Dec. 15, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/CryoSat/CryoSat_extends_its_reach_on_the_Arctic

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85) David T. Sandwell, R. Dietmar Müller, Walter H. F. Smith, Emmanuel Garcia, Richard Francis, “New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure,” Science, October 3, 2014, Vol. 346 No 6205, pp: 65-67, DOI: 10.1126/science.1258213

86) “Greenland ice-sheet height,” ESA, Aug. 20, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/CryoSat/Ice_sheet_highs_lows_and_loss

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88) “CryoSat finds sharp increase in Antarctica's ice losses,” ESA, May 19, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/CryoSat/CryoSat_finds_sharp_increase_in_Antarctica_s_ice_losses

89) Malcolm McMillan, Andrew Shepherd, Aud Sundal, Kate Briggs, Alan Muir, Andrew Ridout, Anna Hogg, Duncan Wingham, “Increased ice losses from Antarctica detected by CryoSat-2,” Geophysical Research Letters, 2014, DOI: 10.1002/2014GL060111

90) E. Rignot, J. Mouginot, M. Morlighem, H. Seroussi, B. Scheuchl, “Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith and Kohler glaciers, West Antarctica from 1992 to 2011,” Geophysical Research Letters, manuscript accepted May 12, 2014, DOI: 10.1002/2014GL060140, URL: http://onlinelibrary.wiley.com/doi/10.1002/2014GL060140/pdf

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93) “Balancing the sea-level budget,” ESA, March 26, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Space_for_our_climate/Balancing_the_sea-level_budget

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97) “Arctic Sea Ice up from record low,” ESA, Dec. 16, 2013, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/CryoSat/Arctic_sea_ice_up_from_record_low

98) Antarctica's ice loss on the rise,” ESA, Dec. 11, 2013, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/CryoSat/Antarctica_s_ice_loss_on_the_rise

99) “CryoSat measures European storm surge,” ESA, Dec. 9, 2013, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/CryoSat/CryoSat_measures_European_storm_surge

100) “New dimensions on Ice,” ESA, Sept. 11, 2013, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Living_Planet_Symposium_2013/New_dimensions_on_ice

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102) Malcolm McMillan, Hugh Corr, Andrew Shepherd, Andrew Ridout, Seymour Laxon, Robert Cullen, “Three-dimensional mapping by CryoSat-2 of subglacial lake volume changes,” Geophysical Research Letters, June 2013, accepted article, published online, DOI: 10.1002/grl.50689

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106) “CryoSat reveals major loss of Arctic sea ice,” UKSA, Feb. 14, 2013, URL: http://www.bis.gov.uk/ukspaceagency/news-and-events/2013/Feb/cryosat-reveals-major-loss-of-arctic-sea-ice

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113) “CryoSat ice satellite rides new waves,” ESA, Dec. 22, 2011, URL: http://www.esa.int/esaEO/SEMI2KBX9WG_index_0.html

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