ASIRAS (Airborne SAR/Interferometric Radar System)
ASIRAS is an airborne SAR-altimeter instrument of ESA in preparation for CryoSat validations campaigns. The objectives are to increase the confidence level in the expected instrument performance and to validate the measurement/processing concepts prior to the CryoSat implementation and launch - and to use the instrument after the spaceborne mission launch in underflights during the commissioning phase of the CryoSat mission for calibration validation analysis. 1) 2)
The SIRAL (SAR/Interferometric Radar Altimeter) instrument of CryoSat, built by Alcatel Alenia Space, happens to be a new technology development with rather demanding observation requirements in accuracy; namely to observe ice sheet interiors and the ice sheet margins for sea ice and other topography. SIRAL is a nadir-looking radar instrument operating in Ku-band. In along-track direction, the lateral resolution is enhanced by means of Doppler filtering, while in the across-track direction, interferometric techniques are applied. While this concept had been verified by simulations, no dedicated experimental validation had been performed.
ASIRAS was built by Radar Systemtechnik (RST) of Rorschacherberg, Switzerland with the support of the Alfred Wegener Institute (AWI) and Optimare for the implementation and operation on an aircraft. The instrument design is very similar to the D2P (Delay-Doppler Phase-monopulse Radar) system, an airborne system of JHU/APL, which demonstrated the new technology in varies test flights over Greenland in the spring and summer of 2000. - However, with the D2Ps range resolution of ~40 cm, it became clear that if one could improve on this value, then it would be possible to provide a better opportunity to understand radar pulse penetration within the first few meters of snow pack and therefore provide a better tool for validating the CryoSat retrievals. 3) 4) 5) 6)
ASIRAS is essentially a Ku-band altimeter but with a high pulse repetition frequency such that it is phase sensitive and pulse-coherent. Dual receive antennas oriented adjacent to one another in across-track form a single-pass interferometer. The carrier frequency of the radar is 13.5 GHz and the bandwidth is 1 GHz. The antennas are realized as aperture coupled single linear polarized microstrip patch arrays each consisting of 8 x 32 elements with a careful amplitude taper in both coordinates.
The ASIRAS system consists of the radar altimeter instrument, the antenna module (containing 2 antennas), a control computer, and two data storage PCs. GPS-signals and information on aircraft attitude, ground speed and vertical velocity is required for accurate ASIRAS echo data post processing. The instrument transmits LFM (Linear Frequency Modulated) pulsed signals that are “fully deramped” during reception. The radiated chirped signals are internally followed by a reference signal that is delayed by the radar signal roundtrip delay time. If received signal and reference signals are mixed the output signals would consist - in case of a single radar target - of one frequency line at the frequency difference, Δf. Since the terrain surface represents a distributed but limited scattering area; hence, a limited spectrum is obtained at the output of the deramp process.
Table 1: Overview of ASIRAS instrument parameters
Figure 1: Configuration of the ASIRAS assembly (image credit: RST)
To achieve high sampling resolution of about 4.5 m in the along-track direction (with the aircraft about 1 km above the target surface), the system uses the delay-doppler concept to form beams. The echoes illuminated by these beams are corrected for the effect of slant range as the instrument passes over and samples in any given surface location (the “SAR” component of the radar). Using the difference in phase between the echoes received by the two antennas, the cross-track location of the point of closest approach can be pinpointed (the “interferometric” component of the radar). To reduce fluctuation in echoes due to thermal noise and speckle, an averaging of the echoes is required. This is known as the multi-look concept. Aircraft position and attitude are measured with a DGPS (Differential Global Positioning System) and an INS (Inertial Navigation System). 7)
Figure 2: Block diagram of the ASIRAS instrument (image credit: RST)
Although the launch of the CryoSat spacecraft failed in October 2005 (Eurockot launch failure), the scientific objectives of the CryoVEx validation experiments are still of undiminished importance. In Feb. 2006, ESA received the green light from its Member States to build and launch a CryoSat recovery mission, CryoSat-2, which is due for launch in 2009.
Figure 3: ASIRAS antenna assembly mounted underneath the cabin door (image credit: RST)
Figure 4: The ASIRAS instrument on the aircraft (image credit: RST)
CryoVEx (CryoSat Validation Experiment) campaigns
ASIRAS has been successfully flown on several measurement campaigns by the CryoSat Validation and Retrieval Team (CVRT). The instrument turned out to be a reliable “workhorse” for radar altimeter data acquisition. It has been flown on two different aircraft types and in low and in high altitude operational modes. Upgrades concerning in particular real-time display capabilities, robustness of computer equipment and reduction of recorded data volume are in progress. Plans for further campaigns up to the year 2012 exist. This may also include measurement flights for the SENTINEL mission. 8)
Table 2: CryoSat preparation campaigns with list of activities, teams and locations
A number of initial test flights with ASIRAS were conducted in the region of Bremerhaven, Germany and the North Sea during 2003. In one test, ESA was able to demonstrate pulse-to-pulse phase coherence from reflections obtained from cranes when bad weather prevented a direct corner reflector over flight.
More detailed campaign descriptions with ASIRAS participation for the CryoSat-2 mission data validation in the Arctic as well as in the Antarctic are provided in the following text, covering the period until 2018.
CryoVEx (CryoSat Validation Experiment) 2003 campaign)
In the spring 2003, an ESA/NASA funded and CVRT (CryoSat Validation and Retrieval Team) coordinated campaign saw DNSC (formerly KMS) operating their laser scanner (with auxiliary instrumentation) combined with the JHU/APL operated D2P Ku-band instrument onboard an Air Greenland Twin Otter. This campaign, known as CryoVEx 2003, covered a significant coverage of land ice in Greenland (EGIG line) and Svalbard, as well as sea ice around the Fram strait, Svalbard and north of Alert, Canada (Alert, at 82º 28' N and 62º 30' W, is the northernmost permanent settlement of the world). The D2P system employed two receive channels to allow cross-channel phase measurements. In this mode the two channels operated as a cross-track interferometer to support geo-location of the reflecting surface over sloping terrain. 9) 10) 11)
The airborne survey was performed using a chartered Air Greenland Twin Otter aircraft (registration: OY-POF), operating primarily from the military airfield at Station Nord, Greenland and the commercial airport in Longyearbyen, Svalbard.
Figure 6: The airborne survey was performed using a chartered Air Greenland Twin-Otter. Station Nord, Greenland (image credit: ESA)
In general the objectives of the CryoVEx 2003 campaign were met. Installation of the complex radar and lidar equipment in a small Twin Otter aircraft was successful, and the equipment operated with few problems, despite temperatures at –30°C and below. The actual flown patterns were close to the planned ones, despite several days during the campaign with poor weather, especially in Svalbard. Two coincident helicopter/Twin Otter flights were done, yielding useful operational experience for future CryoSat validation activities.
The primary goal of CryoVEx 2003 was to provide coincident laser and 13.6 GHz interferometric radar measurements, in order to understand the penetration of CryoSat radar signals into polar sea-ice and continental ice caps. A secondary goal was to provide coincident sea-ice measurements of the radar/laser system and a helicopter-borne electromagnetic induction system (“EM bird”) capable of direct measurement of sea-ice thickness.12)
The CRYOVEX (CryoSat Validation EXperiment) 2003 campaign was a first comprehensive Arctic Ocean airborne and surface campaign, in support of the ESA satellite CryoSat, planned for launch late 2004. The primary goal of CryoVEx-2003 was to provide coincident laser and 13.6 GHz interferometric radar measurements, in order to understand the penetration of CryoSat radar signals into polar sea-ice and continental ice caps. A secondary goal was to provide coincident sea-ice measurements of the radar/laser system and a helicopter-borne electromagnetic induction system (“EM bird”) capable of direct measurement of sea-ice thickness.
The airborne scanning laser (lidar) and radar measurements were carried out in the period April 1 to April 23, 2003 by Kort og Matrikelstyrelsen (National Survey and Cadastre Denmark; KMS) in cooperation with Applied Physics Laboratory (APL) of Johns Hopkins University, USA. APL provided and operated the D2P 13.6 GHz interferometric Doppler radar, similar to the CryoSat radar system.
Figure 7: A total of 61 hr airborne hours was flown during CryoVEx-2003, including the transits from and to the aircraft base at Kangerlussuaq, Greenland (image credit: ESA, DNSC)
The CryoVEx-2003 airborne campaign was successful. Lidar and D2P radar data were acquired over essentially all the planned tracks, and only some minor parts of tracks did not yield useful laser scanner data due to fog and low clouds. Two coincident Twin Otter/helicopter flights, where ice thickness was measured directly with helicopter EM methods and freeboard with lidar/radar, was done with some success. The second of these joint flights demonstrated that it is possible to align to such aircraft, flying at different speeds over moving ice fields, and do measurements of the same ice features. It confirms the viability of this joint flight approach for the future post-launch calibration and validation activities of CryoSat, with first activity planned for spring 2005.
The lidar, imagery and radar data collected during CryoVEx-2003 will provide important basic data for understanding the future CryoSat radar signals. All the data of CryoVEx-2003 are made available on web and DVD/CDs. The very large high-resolution lidar data are made available through set-up streams to read directly the acquired raw data.
CryoVEx 2004 campaign (spring/autumn)
During the last two weeks of March 2004, an important step forward was made in the preparations for the CryoSat Arctic validation campaigns. For the first time, the ESA ASIRAS instrument was flown and tested on an aircraft over the snow and ice expanses of Svalbard, an archipelago of islands lying in the extreme north of Europe just 12º from the North Pole.
This was immediately followed by the spring and autumn Arctic campaigns over Svalbard, Greenland and Devon Island. This logistically demanding campaign incorporated for the first time several surface teams placed at various sites conducting in-situ activities. Also the opportunity was available to test the ASIRAS radar penetration via the use of test corner reflectors (Figure 5) positioned a few meters above the snow surface and installed prior to ASIRAS over flight with accurate positioning communicated to the ASIRAS flight team.
Figure 8: CryoVEx 2004 and Bay of Bothnia 2005 campaign sites and traverses (image credit: ESA)
The specific aim of this campaign was to acquire the first data sets of the instrument over snow and ice and to make sure that the instrument could be operated under the icy temperatures and high winds prevalent in this part of the world. Coincident laser and interferometric radar altimeter measurements were taken, in order to understand the penetration of CryoSat radar signal into polar sea ice and continental ice caps and to quantify uncertainty in the CryoSat measurements.
For redundant calibration purposes, the German aircraft Polar 4 of DLR / AWI was equipped with a laser scanner (ALS), the ASIRAS instrument, a single-beam laser and two DGPS receivers. During CryoVEx 2004, two campaigns took place. Flights were performed in Svalbard across Austfonna, on the Greenland Ice Sheet along the EGIG line (central Greenland), and on the Devon ice cap (Canadian Arctic).
CryoVEx 2005 campaign
In early 2005 an extra measurement mode was added to ASIRAS allowing the combined use of ASIRAS with laser scanners, which have operating ranges below 600 m, to characterize the penetration of the radar signal. This mode (LAM-SAR) was tested in the field in March 2005 in a dedicated test in the Bay of Bothnia, Finland.
CryoVEx 2006 campaign
The campaign took place between May 11 and 15, 2006 north of Alert/Ellesmere Island. During the field campaign, a wide range of snow and ice measurements have been performed on two main study sites. Site 1 was a patch of rather level multiyear ice, while Site 2 was very level first year ice. Measurements comprised snow and ice thickness drilling, levelling of surface elevation, and snow pit studies. On each site, two corner reflectors were installed to support airborne radar altimeter measurements with ESAs ASIRAS (Airborne Synthetic-Aperture Interferometric Radar Altimeter System). Coincident flights of ASIRAS (aboard a Twin Otter of Air Greenland) and the HEM bird (helicopter) were performed on May 11 and May 12. For the Twin Otter, the coincident flight was only a small part of a much larger survey. 13)
"Having these students take part in CryoSat campaign activities is the result of a unique collaboration between ESA and the Climate Change College," said Malcolm Davidson, CryoSat Validation Manager. "The students will contribute to the fieldwork by taking and analyzing snow and ice samples on the ground along side the UK scientists already in place. Not only will the students gain first-hand knowledge of how important scientific fieldwork is carried out, but they will also gain a deeper insight into the importance of ESA's CryoSat-2 mission to better monitor and understand environmental changes, in particular, changes in ice cover in the polar regions. It should prove a wonderful experience."
Led by Dutch polar explorer Marc Cornelissen, the students from the Climate Change College will set up camp next week on the ice at point T05 on the EGIG (Expédition Glaciologique Internationale Groenland) line. With around 15 years experience exploring the Arctic, Cornelissen is passionate about the polar environment and dedicated to bringing about awareness of the environment and contributing to a better understanding of climate change. This is what led him to initiate the Climate Change College, which is sponsored by Ben & Jerry's – the ice cream company, and supported by the WWF.
The aim of the college is to give students the opportunity to experience first-hand the fragile environment of the Arctic and the skills to be 'ambassadors' with the task of inspiring businesses and the public to address the issue of climate change.
Figure 9: Greenland showing the EGIG (Expédition Glaciologique Internationale Groenland) line and site T05. This is the location where the Climate Change College students will participate in CryoVEx validation activities at the beginning of May 2006. The EGIG line crosses the central Greenland ice sheet and since it was first traversed in 1959 has been the site of various scientific surveys (image credit: ESA)
CryoVEx 2007 campaign
This mission involves an extensive ground and air campaign in preparation of the CryoSat-2 mission. 14)
The ground campaign consists of the Arctic Arc Expedition, part of the International Polar Year (IPY 2007-2008). The expedition's two Belgian explorers, Alain Hubert and Dixie Dansercoer, 'stepped' onto the sea ice off the coast of Siberia on the 1 March 2007 and have so far covered a staggering 2,500 km each pulling a 130 kg sledge holding supplies and equipment. Along the way these two intrepid explorers are contributing to the preparation of the CryoSat-2 mission by measuring snow depths at regular intervals. These data in turn will be used by scientists to assess how well snow conditions can be predicted using existing climate models as well as inputs to methods for improving the accuracy of CryoSat-2 maps of sea-ice thickness. 15)
In addition, a group of eight
scientists were transported by helicopter to the remote Austfonna ice
cap on April 12, 2007. As part of the CryoVEx 2007 campaign, they spent
one month making measurements of snow and ice properties along long
transects that crisscross the ice sheet surface.
Figure 10: The Hagen ground measurements team, led by Jon Ove Hagen, just before their helicopter airlift from Longyearbyen to the Austfonna ice cap (image credit: DLR, Irene Hajnsek)
As the ground experiments are carried out, measurements are also being taken (April 2007) from the air by the Alfred Wegner Institute (AWI). ASIRAS is flown on the Do-228 aircraft in Svalbard. 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, scheduled for launch in 2009.
CryoVEx 2008 campaign
The ESA CryoSat Validation EXperiment (CryoVEx) 2008 was carried out in April and May 2008. The airborne operations were coordinated by the National Space Institute, Danish Technical University (DTU Space) and took place in the period 15 April to 8 May. 16) 17)
The work consisted of:
• Airborne data collection with ASIRAS and laser scanner system. The operations were coordinated with ground and helicopter activities over land and sea ice in Greenland and Canada.
• Logistical support for participants in the CryoVEx 2008 experiment especially concerning transport and access to military facilities in Canadian Forces Station Alert and Thule Air Base as well as aircraft support to the UK team on the north Greenland ice sheet.
The airborne part of CryoVEx 2008 was successfully carried out by DTU Space and the gathered data sets are now secured at DTU Space on central servers backed up on magnetic tapes. A total of 72 hours were flown with the Air Greenland Twin Otter plus additional 15 hours for the transport of the UK1 team to the ice sheet. Laser scanner data has been gathered on most lines and ASIRAS data was recorded over test sites and on large parts of the other lines.
Figure 11: Airborne data collection with ASIRAS and laser scanner system was performed using a Twin-Otter (image credit: ESA)
The DTU Space operations started out on April 15 in Kangerlussuaq, Greenland, with installation of the laser scanner and ASIRAS system in the Air Greenland Twin Otter OY-POF following the same procedures as certified in 2006. Due to a minor technical problem with the Twin Otter the aircraft was not available until the 16th. This did not affect the installation since the first day was spent on retrieving the cargo with the equipment and unpacking the boxes. Assistance with the ASIRAS system was provided by Raumfahrt Systemtechnik’s engineer.
Figure 12: Flight tracks for airborne Twin Otter observations (image credit: DTU Space)
CryoVExAnt 2010 and 2011 Campaign
Following the successful CryoSat Validation Experiment Antarctica (CryoVExAnt) 2008/2009 campaign, the CryoVExAnt 2010/2011 campaign was carried out in Dronning Maud Land (Norway's territorial claim in Antarctica) from 11 November 2010 to 20 February 2011. 18)
Airborne and ground measurements were recorded in this campaign, which was the second CryoSat Cal/Val experiment supported by ESA in Antarctica. Both airborne and ground measurements were performed in the blue ice region close to Novolazarevskaya-Airbase and were carried out by scientists from TU Dresden and AWI. 19)
The key objective of the ground measurements was to provide precise ground-based information on ice surface heights in the area of investigation. To obtain surface height changes it was essential to repeat the test areas observed during CryoVExAnt 2008/2009 campaign within the blue ice region.
CryoVEx 2011 Campaign
The overall purpose of ESA’ CryoSat Validation Experiment (CryoVEx) 2011 was, for the first time, to directly validate the measurements from CryoSat-2, through a coordinated major effort involving a large group of European, Canadian and US scientist. One primary aim of the CryoVEx 2011 campaign was to provide data which could allow the optimal recommendation from the CVRT (CryoSat Validation and Retrieval Team), as to the optimal Level 1b and Level 2 retrieval algorithms, with which to reprocess the CryoSat-2 data. 20) 21)
Figure 13: Heavy sea ice conditions outside CFS Alert (left) and terminal of Austfonna ice cap (right), image credit: ESA,
This report focuses primarily on:
• Data collected with the ESA airborne ku-band interferometric radar (ASIRAS), coincident airborne laser scanner (ALS) and vertical photography to acquire data over sea- and land ice. The airborne campaign was coordinated by DTU Space using the Norlandair Air Twin Otter (TF-POF), which is the same aircraft as used in former CryoVEx campaigns, where it was owned by Air Greenland.
• Sea ice thickness data obtained with airborne electromagnetic (AEM) induction sounding conducted with a fixed-wing airplane (Polar-5, Basler BT-67) of the Alfred Wegener Institute.
The airborne work was coordinated to match CryoSat-2 ground tracks, and overflights of ground work at CryoVEx validation sites. The groundtracks including non-ESA flights are presented in Figure 14 with the validation sites marked by red circles. This includes sea ice measurements in Lincoln Sea out of Canadian base Alert, Ellesmere Island, and in the Fram Strait near research vessel RV Lance. Land ice measurements were acquired along the EGIG line across the Greenland ice sheet, together with Austfonna and Devon ice caps.
At each validation site ground teams measured ice and snow properties, and raised corner reflectors (CR) acting as a surface reference point in order to estimate the penetration depth of the ASIRAS radar. The CryoVEx 2011 campaign objectives is outlined in detail in the ESA CryoVEx 2011 Campaign Implementation Plan (editors: T. Pearson, M. Wooding).
Figure 14: Overview of the flight tracks (green lines) from the CryoVEx 2011 campaign including non-ESA flights. The five main validation sites are marked by red circles (image credit: ESA, DTU Space)
Airborne laser scanner (ALS) and radar (ASIRAS)
Due to logistics of the various field teams the CryoVEx airborne campaign was split in two operational periods. Based on experience from earlier CryoVEx campaigns the activities carried out from the Canadian base CFS (Canadian Forces Station) Alert on Ellesmere Island were planned to take place early in the season to make sure the weather was more stable (fog had delayed many flights in earlier CryoVEx campaigns from Alert) and to ensure cold conditions of the sea ice itself (target temperatures of -20ºC or below). Thus, access to Alert was acquired during operation Boxtop on April 12-19 to cover the sea ice activities out of Alert. The second part of the campaign was carried out on April 26 – May 9 to cover the sea ice work north of Svalbard, as well as the ice caps (Austfonna and Devon) and the Greenland ice sheet (EGIG line).
In the intervening period 20-25 April 2011 the Twin Otter was used in Southern Greenland for a national project, PROMICE, funded by the Danish Ministry of Climate and Energy initiated to monitor the thickness of the Greenland Ice Sheet, as an ongoing effort to assess changes in its mass budget. And to support test of ESA’s airborne ice sounding radar POLARIS.
Additionally, ASIRAS and ALS data were collected whenever possible on the transit flights, including Fram Strait and Baffin Bay to obtain sea ice information for the Greenland Bureau of Mineral and Petrol (BMP). Results of these flights are presented in Pedersen et al (2011). 22)
The installation process, planned to start on April 7, was delayed until April 9 by the re-certification of the aircraft, which was sold from Air Greenland to Norlandair, Iceland, in January 2011. The formal transfer of the aircraft to the new Iceland registration (TF-POF), was done in the very last days prior to the initiation of the CryoVEx 2011 campaign and delayed the arrival of the aircraft in Kangerlussuaq a few days.
In summary, the campaign was a success. A total of about 100 flight hours, covering about 23,500 km, was flown with ASIRAS and ALS using DTU Space charted Twin Otter. The experiment demonstrated great collaboration and timing between a large group of European, Canadian and US scientists. The effort involved airborne activities coordinated with AWI Polar-5 carrying an EM inducting sounder and ground teams on five different validation sites. In addition, collaboration with US NASA Icebridge team was done. A broad suite of data of the sea ice, Greenland ice sheet and the local ice caps Devon and Astfonna has been collected giving a unique data set to optimize CryoSat-2 Level 1b and Level 2 retrieval algorithms.
In contrast to previous validation experiments, the EM (Electromagnetic) ice thickness retrieval was conducted with a fixed-wing airplane, and not by helicopter. The use of the Polar-5 results in surveys of highly improved range and enables tandem flights with the ASIRAS Twin-Otter with very short temporal offsets over long distances, which increases the accuracy of overlapping data acquisition.
High navigational precision was obtained throughout the campaign, especially over CR sites. CR’s were all successfully hit with ASIRAS on the sea ice out of CFS Alert, the EGIG line and Devon ice cap. On Austfonna and the RV Lance validation site, none of the reflectors were observed in the ASIRAS data, at RV Lance primarily due to unknown position of the CR’s due to the large drift of the sea ice.
Based on CR analysis and comparison to coincident ALS runway overflights it is concluded that level_1B data measured with upgrade ASIRAS instrument and processed with the ASIRAS processor version ASIRAS_04_03 shows no time shifts and an overall good quality.
The ALS data is of high quality and airborne validation data exists for all the validation sites with standard deviation about 5 cm. Due to extreme cold conditions (-20 to -30ºC) and lack of proper GPS datation of the spare ALS used throughout the first part of the campaign, no ALS data are available for several of the flights out of CFS Alert, including the CryoSat-2 ground tracks on April 15 and 17 using the DTU Space system. However, data has been obtained on the main validation sites, and ALS data obtained with AWI Polar-5 is available, on request at AWI, for flight along CryoSat-2 ground track on April 15.
The general accuracy of airborne EM data is ± 0.1 m over level sea ice. The thickness of deformed sea ice can be underestimated by as much as 60% in maximum thickness using this method. Recent (yet unpublished) studies indicate however, that the mean thickness of longer sections is comparable to the real ice thickness, because deformed features are also overestimated in lateral extent.
Upcoming CryoVEx 2012 airborne campaign will focus on long transects of CryoSat-2 ground tracks (both in SAR and SARin mode) in the Arctic Ocean north of CFS Alert and Eureka, to obtain coincident ASIRAS, ALS and AEM data. This will be followed by measurements of the Greenland ice sheet and the local ice caps, Devon and Austfonna coordinated with ground teams.
CryoVEx 2011 Alert Sea Ice Ground Campaign
This report describes the data
collected by the ground team during the Alert sea ice component of the
2011 CryoVEx experiment, operating out of Alert base, Ellesmere Island,
Canada 11-18 April 2011. The aims of the experiment were to study the
snow and ice characteristics of
Figure 15: Ground Team Operating out of Alert CFS (Canadian Forces Station) base, located in Alert, Nunavut, Canada, on the northeastern tip of Ellesmere Island (82º30'05''N 62º20'20''W), [image credit: Christian Haas (PI) & Justin Beckers, University of Alberta (UA); Seymour Laxon, Katharine Giles & Rosemary Willatt, UCL; Malcolm Davidson, ESA]
The aims of the experiment were to study the snow and ice characteristics of Arctic sea ice and its snow cover during winter and how they affected Ku-band radar penetration into the snow.
• To find three sites, two in the pack ice and one on the fast ice close to Alert, which represented as many different snow and ice types as possible.
• To deploy two corner reflectors (CRs) at each site to act as vertical height references for the airborne radar altimeter (ASIRAS) flown on the Norlandair twin otter which also operated out of Alert.
• To deploy GPS buoys beneath the CRs at the two sites in the pack ice which would transmit their locations every 15 minutes, allowing the airborne and ground teams to re-visit the ice floes for surveying at a later date.
• To place colored tarpaulins 30 m from each CR and bin bags full of snow every 10 m from the tarpaulins as visual aids to the airborne team.
Once the CRs had been over-flown by the airborne teams, the tasks when revisiting each site to survey were:
• To take a 20 x 20 m grid of snow depth measurements at 1 m intervals around each CR, to be used when analyzing the ASIRAS data to see how far into the snow pack the radar penetrated.
• To measure snow thickness, ice thickness and ice freeboard along a transect between the two CRs.
• To use the UCL (University College London) Ground Penetrating Radar (GPR) to obtain radar data including the Ku-band (at which SIRAL and ASIRAS operate, as well as the NASA radar altimeter) close to the CRs, for comparison with airborne radar altimeter data.
• To dig snow pits within the grid of snow depth measurements and record snow characteristics such as layering and grain size, density and salinity. Some of the snow pits would be where the GPR sampled for comparing the GPR return echoes to the physical snow characteristics on small scales.
Study site locations
The team deployed six CRs, two at the ‘North site’ and two at the ‘South site’, shown on Figure 16, and two at the ‘Fast site’. The North and South sites were located in the pack ice around 350 and 120 km from Alert, respectively, and accessed by Twin Otter aircraft, whilst the Fast site was located on the fast ice just a few kilometers off the coast of Alert, and accessed by skidoo.
A GPS buoy was placed directly beneath corner reflector apex, and these transmitted their locations (latitude, longitude) at 15 minute time intervals. Hand-held GPS positions were also obtained.
Figure 17: Corner reflector on wooden stand with GPS buoy beneath it (image credit: ESA)
All measurements were georeferenced by means of a handheld GPS integrated into the EM data logger, which continuously stored position and EM measurements at a sampling rate of 1 Hz. Snow thickness and surface elevation were measured next to the EM instrument with point spacings of approximately 10 steps, and the distances between measurements were calculated afterwards by means of the GPS information. The EM thickness (i.e. snow plus ice thickness) is displayed negatively, while freeboard and surface elevation are displayed positively (or snow thickness as in the case of the North Site). Freeboard was calculated by subtracting snow thickness from the measured surface elevation.
In addition to the combined ice and snow thickness and freeboard measurements along the main center line between snow grids, further ice thickness measurements were performed by dragging the EM31 parallel to the main profile. This resulted in statistically more reliable estimates of the thickness characteristics of each site, and provides insight into the across-track thickness variability which may be important for the interpretation of ASIRAS and other airborne measurements. Data were resampled to a constant point spacing of 5 m to remove biases due to variable walking speed or longer stops, during which data were continuously recorded as well.
Figure 18: Range pole with laser detector (photo R. Willatt); Tripod with rotating construction laser (photo S. Laxon)
The North site CRs were deployed on 14 April. The site was over-flown by ASIRAS on 14 April and surveyed by the ground team on 15 April. Coordinates when the CRs were deployed were:
CR1 (Orange/East) on N site: 85° 34.932 ',-69° 34.560 ' (85.582, -69.576)
General Notes on the North site
The ice was very level, very hard when drilled, and when the snow was removed the ice surface was dark and solid indicating melt ponds on surface. There was a ridge (around halfway) between the CRs – this should be visible with the laser and ASIRAS, and instruments on other aircraft. There was a mixture of soft snow and hard, crusty snow. CH drilled the ice near the plane (near NB), the ice thickness was 180 cm with soft snow on top. Figure 19 shows the layout of the North site CRs.
Figure 19: NO (North Orange, East) and NB (North Blue, West) setups, bin bags were laid at 10 m intervals, five after each tarpaulin. Tarpaulin corners 30 m from CRs. Bin bags 10 m apart. NO: Christian, Katharine, Malcolm, Rosie. NB: Seymour, Justin. The blue tarpaulin was approx. 4.5 x 6 m and the orange approx. 4.4 x 5 m (image credit: ESA)
This work was funded by the UK Natural Environment Research Council and the flights were funded by the European Space Agency, through Duncan Wingham and Malcolm Davidson.
CryoVEx 2012 Campaign
In continuation of the CryoSat Validation Experiment (CryoVEx) carried out in 2011, ESA initiated a second Arctic post-launch campaign in 2012 to further calibrate and validate CryoSat data products. The main purpose of CryoVEx 2012 was to acquire data along transects of CryoSat ground tracks through a coordinated major effort involving a large group of European, Canadian and US scientists. 24) 25)
This report focuses primarily on:
• Data collected with the ESA airborne Ku-band interferometric radar (ASIRAS), coincident airborne laser scanner (ALS) and vertical photography to acquire data over sea- and land ice along CryoSat-2 ground tracks. The airborne campaign was coordinated by DTU Space using the Norlandair Air Twin Otter (TF-POF), which is the same aircraft as used in former CryoVEx campaigns.
• Sea ice thickness data obtained with airborne electromagnetic (AEM) induction sounding conducted with a fixed-wing airplane (Polar-5, Basler BT-67) of the Alfred Wegener Insitute.
The sea ice measurements were planned to take place in the Lincoln Sea using CFS Alert as base, but also to include flights north of Svalbard and north of Station Nord, Greenland, to acquire data over different sea ice types. Part of the flights in the Lincoln Sea was coordinated with NASA P-3 carrying a variety of instruments for sea ice and snow retrievals. A special effort was made to acquire data in the “Wingham Box” off Canada, which is an area where CryoSat-2 is switched from SAR mode typically used over sea ice to the SARIn mode.
Land ice measurements were acquired over the Greenland ice sheet (the EGIG line and selected CryoSat-2 ground tracks), together with Austfonna and Devon ice caps. At Austfonna and Devon ice caps ground teams measured ice and snow properties, and raised corner reflectors acting as a surface reference point in order to estimate the penetration depth of the ASIRAS radar. Unlike previous years no ground teams were located on the Greenland ice sheet.
Figure 20: Overview of the flight tracks (red lines) from the CryoVEx 2012 airborne campaign. Dates of the respective flights are marked next to the flight lines (image credit: DTU Space)
Summary of operation
The CryoVEx 2012 airborne campaign was split into two operational periods. The sea ice activities in the Lincoln Sea were planned to take place early in the season to make sure the weather was stable (fog had delayed many flights in earlier pre-launch CryoVEx campaigns) and to ensure cold conditions of the sea ice itself (target temperatures of -20⁰C and below). Thus, access to CFS Alert and Eureka was organized on March 28 – April 6. The second part of the campaign was carried out on April 23 –May 6, mainly to cover the Greenland ice sheet, Austfonna and Devon ice caps, but also the sea ice north of Svalbard and Station Nord.
A Norlandair Twin Otter (reg: TF-POF) was chartered for the entire campaign, which is the same aircraft as used throughout previous CryoVEx campaigns. The instrument certification for the aircraft was obtained in 2006 (Hvidegaard and Stenseng, 2006). The flight altitude is typically 300 m limited by the range of the laser scanner with a nominal ground speed of 135 knots. The speed can be decreased to about 110 knots, which is necessary in connection with formation flights with AWI Polar-5 when flying the EM-bird; however the relative low speed results in an increase of the pitch by a few degrees. The aircraft is equipped with an extra ferry tank permitting longer flights (5-6 hrs), and an autopilot for better navigation accuracy. In good conditions the across-track accuracy is down to a few meters using a custom-made navigation system connected to geodetic GPS receivers.
The installation of the ASIRAS radar and laser scanner (ALS) took place in Akureyri, Iceland, and Kangerlussuaq respectively. The main installation and test of ASIRAS were performed by experienced staff from Radar System Technique (RST).
First part of the campaign was based out of CFS Alert. Five CryoSat-2 ground tracks were flown over the sea ice (March 29-April 4) and on transit flight from CFS Alert to Eureka (April 5) a CryoSat-2 ground track was flown in the “Wingham box”, which is an area where CryoSat-2 are switched from SAR mode typically used over sea ice to the SARIn mode. Parts of the flights was coordinated to be coincident with Alfred Wegener Institute Polar-5 towing an electromagnetic sounder (AEM) and NASA’s IceBridge P-3 carrying multiple sensors for sea ice and snow retrievals.
The second part of the campaign primarily covered the land ice. The EGIG line crossing the Greenland ice sheet from East to West at about 70ºN was flown on April 24 and a CryoSat-2 ground track crossing the inner parts of the Greenland ice sheet was flown on May 5 in marginal conditions. The local ice caps Austfonna and Devon were flown on April 28 and May 3, respectively. Flights on the ice caps (Devon and Austfonna) were coordinated with scientists taking measurements on the ground along CryoSat-2 ground tracks and transects of special glaciological interest. Unlike previous CryoVEx campaigns no in situ measurements were taken along the EGIG line, but airborne measurements are still important to monitor changes in the ice sheet mass balance. To acquire measurements of different sea ice types, CryoSat-2 tracks were flown north of Svalbard on April 27 and north of Station Nord on April 29.
In general, the weather was excellent and most of the transit flights were used to collect additional data. This includes:
• Flights in the Fram Strait to repeat flight tracks from 2006, 2008, and 2011, together with overflight of four upward looking sonar (ULS) buoys for validation of freeboard to thickness conversion, as part of the Greenland Climate Research Center research program.
• Flights with coincident ALS and P-band radar in Denmark Strait and the Greenland Ice Sheet.
• Measurements in Qaanaaq Fjord to support the Qaanaaq Fjord Experiment by Scottish Association for Marine Science (SAMS) and Danish Meteorological Institute (DMI).
• Measurements of Kongsvegen glacier in Svalbard and additional flights over the Greenland Ice Sheet.
Calibration flights of the instruments over buildings and runways were performed whenever possible. Corner reflectors were erected by the end of the runway in CFS Alert and along flight tracks on Austfonna and Devon ice caps to be used as reference point to estimate penetration depth and potential time shifts of the ASIRAS radar.
The DTU Space part of the CryoVEx 2012 campaign ended officially on May 6 in Kangerlussuaq after 112 airborne hours.
A total of 16 CryoSat-2 ground tracks were flown, covering distances from 81-523 km. Over the sea ice most tracks were measured both ways, in order to obtain a precise estimate of the ice drift. Whenever possible the tracks were timed to match the CryoSat-2 passage times, however this was hampered by limited airport opening hours, e.g. CFS Alert.
The CryoVEx 2012 campaign was a success and the CryoVEx team now has a collection of unique measurements to analyze.
Between the CryoVEx campaigns the aircraft was used to support ESA’s IceSAR campaign. The overall objective of the IceSAR 2012 campaign is to demonstrate and document the potential of the BIOMASS satellite mission to monitor ice motion and subsurface structure using an airborne version of the P-band radar (POLARIS). The IceSAR campaign were split into three periods; A test flight in Iceland on March 20-23, flights over the Greenland Ice Sheet from Kangerlussuaq on April 20-22, which were repeated on May 8-9 to simulate repeat tracks of the BIOMASS satellite. An overview of the IceSAR campaign is included in the day-2-day description, as some of the installation took place coincident with the CryoVEx 2012 campaign.
The airborne team consisted of Henriette Skourup (HSK), Rene Forsberg (RF), Arne V. Olesen (AVO), Sine M. Hvidegaard (SMH), Indriði Einarsson (IE), Johan Nilsson (JN) assisted by Harald Lentz (HL) from Radar System Technique (RST) during the ASIRAS installation. Robert Ricker (RR) from Alfred Wegener Institute participated during the flights at Svalbard to be trained in operation of the ASIRAS radar. Jørgen Dall (JD) and Anders Kusk (AK) were responsible for the IceSAR program.
Overview of acquired data
Data acquisition of the various instruments was acquired where feasible, considering the limited height range of the ALS system and the weather.
All the ASIRAS data was acquired in Low Altitude Mode (LAM) with low along-track resolution (LAMa). This allows flight at an altitude of 300 m, which is within the operational range of the ALS system and a relative low data volume of about 28 GB per hour. A total 1.86 TB raw ASIRAS data were collected during the CryoVEx 2012 campaign. The data was stored on hard discs as ASIRAS level 0 raw data in the modified compressed format (Cullen, 2010) and has been shipped to the Alfred Wegener Institute (AWI) for further processing.
In general, the ALS worked excellent. At extreme low temperatures (below -20ºC), experienced through the first part of the campaign, moisture on the inside of the instrument prevented the laser to see through the instrument window. Total blocking of the laser signal was only an issue during take-off most likely due to the extreme temperature decrease caused by the acceleration of the aircraft, which caused the moisture to freeze. To circumvent the laser to lock on the frozen instrument window, the ALS was switched to measure the “last laser pulse”. It took about 30-45 minutes after take-off to heat the system to obtain full scan width. Data was only lost on the flight from Station Nord to CFS Alert on March 28 (DOY 88) and the first 20 minutes of the flight on March 29 (DOY 89) due to freezing of the moisture. After the end of the campaign the ALS was shipped to Riegl to be dried out.
The data volume obtained by the ALS is about 250-300 MB an hour which is a relative small amount, when compared to the ASIRAS data volume. During the campaign a total of 26.2 GB ALS data were acquired.
The airborne GPS units logged data internal in the receivers (AIR1, AIR2 and AIR3) during flight, which were downloaded upon landing on laptop PCs. The Novatel GPS was dedicated to support ASIRAS and was not part of the logging system. GPS files were recovered for all receivers at all flights, except on May 1 (DOY 122), where AIR3 was started 20 min. after take-off, as the antenna cable had been unplugged during the reconfiguration of the OxTS INS antenna setup.
Both INS systems logged continuously throughout the campaign and no problems were observed with the systems. Due to operator handling, logging of the Honeywell INS was started late on March 26 (DOY 86) and the OxTS INS was stopped during the flight on March 28 (DOY 88).
Vertical photography was collected during flights primarily to support the analysis of ALS data over sea ice. Pictures were acquired every 3-4 seconds for most flights by nadir and slant-looking photography.
All data are stored on external hard discs, as well as the DTU Space servers with tape backup system.
Figure 21: Photo of Norlandair Twin Otter (TF-POF) at CFS Alert (image credit: M.Davidson, ESA)
Figure 22: Processed ALS elevations wrt WGS-84 reference ellipsoid. Missing sections are mainly due to low clouds and fog (image credit: DTU Space, ESA)
Figure 23: Example of slant-looking image taken out of rear starboard window using Canon 60D. Full resolution image (upper) and sample in full zoom (lower), image credit: DTU Space, Technical University of Denmark, ESA
In total, eight CryoSat-2 underflights were performed over sea ice, as outlined in Figure 19. Data was acquired in the Arctic Ocean north of Alert, north of Station Nord and north of Svalbard. These three areas represent different sea ice types and settings, with very rough ice north of Greenland and thinner ice north of Svalbard. The track marked by red is data acquired in the “Wingham box” (confined by 80-85ºN and 100-140ºW), where SIRAL is switched to SARIn mode.
For satellite underflights, timing is crucial especially over drifting sea ice. This was unfortunately difficult due to limited opening hours of the airports. To account for the ice drift between data acquisition of flights and CryoSat-2 passages, most of the tracks were measured twice. In addition, information of the drift was kindly prepared from repeated SAR images from Envisat and distributed to the involved field teams by R. Saldo (DTU Space).
Figure 24: Flight tracks from underflights of CryoSat-2 in the Arctic Ocean (image credit: DTU Space, ESA)
In summary, the CryoVEx 2012 airborne campaign has been a success. In general, the weather was excellent, which allows data acquisition from all validation sites as well as most transit flights. Coincident ASIRAS, ALS and photography have been gathered along 16 CryoSat-2 ground tracks covering different sea ice conditions, parts of the Greenland ice sheet, as well as the local ice caps Devon and Austfonna. Over the sea ice most tracks were measured twice (both out- and inbound), in order to obtain a precise estimate of the ice drift. Whenever possible the tracks were timed to match the CryoSat-2 passage times, however this was hampered by limited airport opening hours, e.g. at CFS Alert.
Three coincident sea ice flights along CryoSat-2 ground tracks with Alfred Wegener Institute’s aircraft Polar-5 towing an electromagnetic (AEM) sounder to measure the ice draft, were organized. Two of them took place in the Lincoln Sea out of CFS Alert, while the last flight was acquired in the “Wingham Box”, where CryoSat-2 is switched to SARIn mode. These data sets together with coincident ASIRAS and ALS are very important for sea ice freeboard to thickness conversion in the CryoSat-2 validation.
Two CryoSat-2 underflights near coincident with NASA’s Operation IceBridge aircraft P-3, equipped with multiple sensors for sea ice and snow retrievals, were performed in the Lincoln Sea. Unfortunately, it was not possible to align all three aircraft, due to technical issues with the AEM and a limited time schedule.
Flights on the ice caps (Devon and Austfonna) were coordinated with scientists taking measurements of snow and ice properties on the ground along CryoSat-2 ground tracks and transects of special glaciological interest. In addition the ground team erected several corner reflectors along the validation lines. The reflectors are used as a reference point to validate the penetration of the radar signal in the upper layers of the ice cap, and to check the timing of the ASIRAS radar. Unlike previous CryoVEx campaigns no in situ measurements were taken along the EGIG line, but airborne measurements were acquired, as they are still important to monitor changes in the ice sheet mass balance.
The ASIRAS and ALS instruments worked without any major problems. Based on CR analysis and comparison to coincident ALS runway overflights it is concluded that ASIRAS level_1B data processed with the ASIRAS processor version ASIRAS_04_03 shows no datation errors and an overall good quality. The ALS data is likewise of high quality with standard deviation of less than 8 cm at existing cross-over points.
CryoVEx AEM (Airborne Electromagnetic) 2014 Campaign
ESA operates the CryoSat mission which is dedicated to the measurement of sea ice thickness in the Arctic. Numerous CryoSat Validation Experiments (CryoVEx) have been performed over the past several years. York University has led the CryoVEx 2014 field campaign, also referred to as CryoVEx AEM, to obtain ground-truth data from Arctic sea ice to validate the satellite measurements. 26)
CryoVEx 2014 was carried out in March and April 2014, a large, international field campaign in the Arctic to obtain these validation measurements. It included airborne ice thickness surveys with York University's airborne electromagnetic (AEM) ice thickness profiler, and in-situ measurements on ice floes visited by aircraft.
The CryoVEx 2014 campaign performed measurements over the Beaufort Sea and Arctic Ocean north of Canada (Ellesmere Island) and Greenland. Airborne surveys and ground-based snow and ice measurements were performed. In the Beaufort Sea the ground-based measurements were carried out at two ice camps operated by the University of Washington for the Office of Naval Research Marginal Ice Zone project.
Seven AEM surveys were carried out between 18 and 31 March.
Due to a severe blizzard on 26/27 March, major damage occurred on the EM Bird. Most components could be successfully replaced. However, the on-board GPS system had failed and could not be replaced or repaired. Therefore only positional data from a hand held GPS inside the aircraft cabin is available.
All measurements were carefully coordinated with the DTU ASIRAS flights and the NASA Operation IceBridge team. Planning of ground-measurements was closely coordinated with Jackie Richter-Menge [CRREL (Cold Regions Research and Engineering Laboratory), USA] and Sinead Farrell [UMD (University of Maryland)]. The airborne measurements reported here were carried out by Anne Bublitz (YU) and Alec Casey (YU/UoA). Ground measurements were performed by Christian Haas [YU (York University) , Toronto, Canada], Justin Beckers (YU/UoA), Bruce Elder (CRREL), and Christopher Hiemstra (CRREL). 27)
The CryoVEx 2014 field campaign was very successful and met all major objectives outlined in ESA’s CryoSat Validation Implementation Plan (CIP).
CryoVEx2014 has been carried out in March and April 2014, a large, international field campaign in the Arctic to obtain these validation measurements. The campaign involved researchers from Canada, Denmark, the US, the UK, and the Netherlands. It included airborne ice thickness surveys with York University’s airborne electromagnetic ice thickness profiler, and in-situ measurements on ice floes visited by aircraft. The campaign was conducted from various locations in Canada (Northwest Territories and Nunavut) and Greenland.
Figure 25: CryoVEx 2014 main camp north of Greenland (image credit: YU)
ESA has provided funds for the charter of two aircraft between March 15 and April 5, 2014, a ski-equipped Twin Otter and a Basler BT67. The aircraft were operated by Kenn Borek Air Ltd. from Calgary, AB. Additional funds for small equipment purchases, shipping, travel, and other logistics services required to perform the project were provided as well.
CryoVEx 2014 was performed in close collaboration with DTU and NASA, who operated the ASIRAS and Airborne Laser Scanner, or conducted the Operation IceBridge project, respectively. Three researchers from the University College London (UCL) also participated in the ground measurements north of Greenland to preform Ground-Penetrating-Radar measurements. All those activities are reported elsewhere.
Figure 26: Map of CryoVEx 2014 study area showing location of land bases and ice camps, ice thickness survey profiles with mean thickness of 20 km flight segments, and ground and air team travel route (image credit: YU)
A Basler BT67 aircraft (C-GCKB) was used for ice thickness surveying and to transport passengers and equipment between the different locations. Note that the provisions for the AEM system, in particular the winch (see below) could remain in place, thus facilitating quick transition between ferry and survey flights. The EM Bird (see below) was stowed inside the cabin with all other equipment for the ferry flights.
From March 16 to March 22 the Basler was based in Inuvik, carrying our ice thickness surveys. On March 23, it transferred to Resolute Bay. En-route it stopped in Sachs Harbor to pick up personnel and equipment from the CryoVEx/ONR ice camp activity. On March 24 it flew to Qaanaaq in Greenland to pick up four more personnel, the UCL team members and their equipment and Emil Nilsen from DTU. On March 25, the Basler brought all 10 scientists and equipment from Qaanaaq to Station Nord, and continued to Alert on the same day with just the two AEM surveying scientists and equipment. It returned from Alert to Station Nord on March 29, performing an ice thickness survey of the CryoVEx ice camp en-route. On April 2, the Basler returned to Resolute Bay, with six scientists and two ice camp managers.
Between March 24 and April 3, an additional ski-equipped Twin Otter (C-GKGB) was chartered to carry the two ice camp managers and all camping equipment from Resolute Bay to Station Nord. It reached Station Nord on March 25 after a two day journey with strong head winds, requiring an overnight stop at Eureka and a fuelling stop at Alert. The Twin Otter was generally based at Station Nord overnight, and supported the ice camp work during daily flights from Station Nord.
After a blizzard from March 26 to the early hours of March 28, the Twin Otter established a runway at the location of the main ice camp, and left four people to set up the ice camp. On March 29, four more people were brought to the ice camp. On March 30, the Twin Otter brought its own fuel to the main camp to refuel there and carry on to the northern site, with 5 people on board.
Four people were removed from the ice camp on April 1, followed by the remaining party of four and all equipment on April 2. All personnel and equipment returned to Resolute Bay on the same day with the Basler, while the Twin Otter returned on April 3, 2014.
Methods and instrumentation
AEM thickness sounder: Airborne ice thickness measurements were performed with an airborne electromagnetic (AEM) sensor (EM Bird) developed and owned by York University, towed below a Basler BT-67 airplane operated by Kenn Borek Air Ltd from Calgary AB.
The retrieval of sea ice thickness with airborne EM is based on the contrast of electrical conductivity between sea water and sea ice, which can be sensed by low-frequency electromagnetic fields. Because the airplane is a significant conductor, the EM-Bird has to be lowered 70-80 m below the aircraft using a winch installed in the cabin. It is operated 15-20 m above the ice surface. By means of the EM measurements the height of the EM Bird above the water surface which coincides with the bottom of the ice is determined. The height of the EM Bird above the ice surface is measured with a single-beam Riegl LD90 laser altimeter. Therefore, AEM sea ice thickness retrievals represent the total, snow-plusice thickness.
EM measurements are prone to temperature-related electronic drift. To correct for this drift, the EM Bird is lifted in regular intervals of between 10 and 20 minutes to altitudes higher than 80 m above sea level for zero-level measurements. During these ascends and following descends, no ice thickness data is available.
The general accuracy of airborne EM data is ±0.1 m over level sea ice. However, due to the footprint size of the method of more than 3 times the flying altitude the maximum thickness of ridges can be underestimated by as much as 60%. Recent (yet unpublished) studies indicate however, that the mean thickness of longer sections is comparable to the real ice thickness, because deformed features are also overestimated in lateral extent.
The position and ellipsoidal height of the EM Bird is measured with an on-board, Novatel OEM-2 geodetic differential GPS system (DGPS). Note that the data streams of the different sensors deliver data at different sampling frequencies. These are 10 Hz for the EM measurements, 100 Hz for the laser measurements, and 2 Hz for the DGPS measurements.
Extensive ground-based measurements of snow and ice thickness and freeboard were performed at four sites at various ice camps. These measurements were performed in close collaboration with Bruce Elder and Christopher Hiemstra from CRREL. Measurements included the following:
• Drill-hole survey of a line profile up to 2 km long. Measurements were performed every 50 m with a Kovacs ice auger. At each drill-hole, ice thickness, snow thickness, and freeboard or draft were measured, providing information about ice thickness and isostatic conditions.
• Snow pit studies of snow stratification and density at the same 50 m points, using standard snow sampling tools.
• Snow thickness and snow freeboard survey along the same line, with point spacings of 5 m. Snow thickness was measured with a so-called Magnaprobe, a computerized, snow ruler probe connected with a GPS receiver and data logger. Snow freeboard was measured with a Leica Rugby rotating construction laser and associated laser receiver and range pole. We also attempted to gather 2D snow freeboard information with a Leica terrestrial laser scanner. Unfortunately too cold air temperatures severely limited the operability of the scanner such that only few scans could be obtained at the Beaufort Sea ice camp.
• Electromagnetic ice thickness measurements along the same line, using portable Geonics EM31 and GSSI EMP400 conductivity sensors. Unfortunately most sensors malfunctioned and only one short profile could be obtained at the Greenland ice camp.
• Installation of two corner reflectors on each line, near the center of the drill profiles, spaced 300-400 m apart. Corner reflectors were located within the coarse and fine resolution snow grids (see below).
• Installation of metocean SVP-B drifting buoys with GPS receivers to monitor the drift of the ice for later correction of other GPS referenced measurements. GPS data were transmitted in realtime to our project partners, such that precise aircraft overflights of the profiles could accurately be planned.
• Snow grid surveys of a 60 m x 400 m snow grid centered about the long drill-hole profile. These were measured with Magnaprobes, with 5 m x 5 m point spacing.
• Nested dense snow grid surveys of 40 m x 40-60 m with point spacings of 1 m x 1 m. These were centered about the corner reflectors.
• Lines and corner reflectors were further marked with orange garbage bags well visible for the pilots overflying the lines. These helped to optimize alignment between overflights and the profiles.
Figure 27: Ground measurements at Camp 2 in the Beaufort Sea. Photo shows Eastern corner reflector, Magnaprobe snow thickness measurement, row of orange garbage bags (background), and terrestrial laser scanner (image credit: YU)
Seven AEM surveys were carried out between March 18 and 31. Figure 28 shows their regional coverage and mean ice thicknesses, and compares the flight tracks with those of the ASIRAS/ALS and IceBridge overflights. Table 1 summarizes flight dates and regions, and the main purpose and coordination of the flights. Overall data quality was excellent.
Due to a severe blizzard on March 26/27, major damage occurred on the EM Bird. Most components could be successfully replaced such that the next survey could already be performed on March 29. However, the on-board GPS system had failed and could not be replaced or repaired. Therefore only positional data from a handheld GPS inside the aircraft cabin is available. These have been merged with the EM data to georeferenced them. However, their accuracy and synchronization is reduced, leading to uncertainties of approximately 60 m (corresponding to flight distance during 1 s) in the collocation of AEM, ASIRAS, and ground measurements for flights on and after March 29. These time shifts can be corrected by manually shifting the profile data to best correlate with the in-situ or other airborne measurements.
Figure 28: Map of all AEM ice thickness surveys, and ASIRAS and Operation IceBridge (OIB) flight tracks. Background map is ASCAT radar backscatter information on March 26, 2014, obtained from BYU (image credit: YU)
CryoVEx 2016 (Spring) Campaign
The 2016 airborne campaign was conducted during 4-16 April 2016, with the ESA airborne Ku-band interferometric radar (ASIRAS), coincident airborne laser scanner (ALS) and vertical photography. It was partly funded by ESA CryoSat-2 Validation Experiment (CryoVEx) and partly by EU FP7 project on Ice, Climate, Economics – Arctic Research on Change (ICE-ARC). As the same aircraft and instrument installation were used for both campaigns this report includes both the CryoVEx and ICE-ARC campaign data. 28)
The ESA CryoVEx 2016 campaign was primarily carried out to follow up on a recommendation given within ESA CryoVal Land Ice project (2014-2015), where it was found that the traditional under-flights of the CryoSat-2 satellite were inadequate. This is primarily due to uncertainties in the radar-echo location (POCA) due to topography. To account for this effect, the 2016 ESA CryoVEx airborne campaign was aimed at flying dense grids of parallel lines at Austfonna ice cap along CryoSat-2 ground tracks, to cover a broad range of possible POCA locations from different retrackers (CryoVal-LI D4).
The ICE-ARC campaign was mainly used to repeat some previously flown sea ice flights into the Wandel Sea and the Fram Strait, which had partly failed in 2015 due to problems with the ALS logging system. The opportunity was taken to make the first Sentinel-3A under-flight over sea ice in Fram Strait.
The CryoVEx/ICE-ARC 2016 campaign was a success and the processed data is of high quality. Preliminary comparisons between CryoSat-2 and ALS data at Austfonna ice cap, proves the concept of using a gridded ALS data set of lines parallel to CryoSat-2 ground tracks in areas with varying topography. First direct comparisons of Sentinel-3A and ALS also show consistent results.
The 2016 airborne campaign was conducted during April 4-16, 2016, with the ESA airborne Ku-band interferometric radar (ASIRAS), coincident airborne laser scanner (ALS) and vertical photography. It was partly funded by ESA CryoSat-2 Validation Experiment (CryoVEx) and partly by EU FP7 project on Ice, Climate, Economics – Arctic Research on Change (ICE-ARC). As the same aircraft and instrument installation were used for both campaigns this report includes both the CryoVEx and ICE-ARC campaign data. Below is given an overview of which data belongs to which project. The mobilization costs were shared equally between the projects. The campaign was coordinated by National Space Institute, Technical University of Denmark (DTU Space) using a Twin Otter (reg. TF-POF) chartered from Norlandair, Iceland. 29)
The ESA CryoVEx 2016 was primarily carried out to follow up on a recommendation given within ESA CryoVal Land Ice project (2014-2015), where it was found that the traditional under-flights of the CryoSat-2 satellite were inadequate. This is primarily due to uncertainties in the radar-echo location (POCA) due to topography. To account for this effect, the 2016 ESA CryoVEx airborne campaign was aimed at flying dense grids of parallel lines at Austfonna ice cap along CryoSat-2 ground tracks, to cover a broad range of possible POCA locations from different retrackers (CryoVal-LI D4).
The ICE-ARC campaign was mainly used to repeat some previously flown sea ice flights into the Wandel Sea and the Fram Strait, which had partly failed in 2015 due to problems with the ALS logging system. The opportunity was taken to make the first Sentinel-3A under-flight over sea ice in Fram Strait.
The primary objectives achieved during the campaign:
• Land ice validation of CryoSat-2 – Austfonna ice cap, Svalbard to follow up on ESA CryoVal-LI recommendations (ESA CryoVEX).
• First Sentinel-3 underflights over sea ice in Fram Strait (EU FP7 ICE-ARC).
• Monitoring sea ice thickness north of Greenland and Fram Strait, repeat lines (EU FP7 ICE-ARC).
• Overflight of upward looking sonars moored in Fram Strait to support CryoSat-2 sea ice freeboard-to-thickness conversion (EU FP7 ICE-ARC).
• Repeated flights from earlier campaigns, to monitor the interaction between the ice shelf and the buttressing sea ice in the Nioghalvfjerdsfjorden glacier complex (EU FP7 ICE-ARC).
The campaign was based out of Station Nord (STN), Northeast Greenland, and Longyearbyen (LYR), Svalbard, Norway.
A Norlandair Twin Otter (reg: TF-POF), which is the same aircraft as used throughout previous CryoVEx campaigns, was chartered for the entire campaign. The instrument certification for the aircraft was obtained in 2006 (Hvidegaard and Stenseng, 2006). The flight altitude is typically 300 m above ground level, limited by the range of the laser scanner, and the nominal ground speed is 135 knots. The aircraft is equipped with an extra ferry tank permitting longer flights (5-6 hrs), and an autopilot for better navigation accuracy. In good conditions the across-track accuracy is down to a few meters using a custom-made navigation system connected to geodetic GPS receivers. Due to shared logistics of the Twin Otter with Danish company Polar Logistics Group ApS (POLOG), the installation and de-installation of the ASIRAS radar and laser scanner (ALS) took place in Lufttransport AS hangar in LYR and was performed by DTU Space personnel.
Figure 29: Overview of the flight tracks (blue lines) from the CryoVEx/ICE-ARC 2016 airborne campaign. Dates of the respective flights are marked next to the flight lines. Yellow star marks the location of the NPI Fram Strait ULS buoys (image credit: DTU Space)
As the first part of the campaign was aimed at measuring sea ice in the proximity of the Station Nord, North-East Greenland the ferry flight was used to seek the opportunity of the first under-flight of ESA Sentinel-3A SAR altimeter only 51 days after the launch. At Station Nord the weather conditions proved optimal for aerial surveying, despite low temperatures (down to -35ºC). The low temperatures resulted in long start-up time for the ALS as fog was frozen on the inside of the instrument window during take-off, preventing the laser to penetrate through the window, see Section 4. Despite the difficulties with the start-up of the scanner, all planned sea-ice flights north of Greenland and in the Fram Strait were surveyed between April 6 and April 10. Unfortunately a planned coincident flight with NASA Operation IceBridge (OIB) was not performed due to unexpected aircraft maintenance of the OIB aircraft.
During the second part of the campaign, the weather presented itself more challenging with Austfonna ice cap covered in low clouds. This together with strong winds postponed the survey flights at Austfonna to the very last days of operations. On April 15 and 16, two dense grids were flown along two CryoSat-2 tracks, during two long flights.
The airborne team consisted of Henriette Skourup (HSK), Louise Sandberg Sørensen (SLSS), and Sebastian B. Simonsen (SSIM). Calibration flights of the instruments over buildings and runways were performed whenever possible. The CryoVEx/ICE-ARC 2016 campaign ended on April 16 where the equipment was uninstalled in Longyearbyen.
The CryoVEx/ICE-ARC 2016 campaign was a success and the scientific community now has another unique collection of measurements to analyze as an extension to the data time series from the previous campaigns.
Hardware installation: The installation of the ASIRAS system was identical to the setup used throughout the previous CryoVEx campaigns. To support the ASIRAS system a Novatel GPS DL-V3 was kindly loaned from the Alfred Wegener Institute (AWI). The ALS equipment was of type Riegl LMS Q-240i-60. To prevent malfunction of the ALS during the extreme low temperatures (-25C and below) in the first part of the campaign, the ALS was wrapped with external heater pads. In addition, an external heater fan as well as an electrical heater, were installed in the instrument bay in the rear baggage compartment of the aircraft.
In addition, three geodetic dual-frequency GPS receivers were mounted for precise aircraft positioning. The receivers (AIR1, AIR2 and AIR3) were connected to two separate GPS antennas (“front” and “rear”) through antenna beam splitters. The GPS antennas are permanently installed on TF-POF. Receiver types, antenna information, as well as logging rates for the GPS receivers are given below:
• AIR1 Receiver type Javad Delta front antenna logging rate 1 Hz
• AIR2 Receiver type Javad Delta rear antenna logging rate 2 Hz
• AIR3 Receiver type Javad Delta front antenna logging rate 1 Hz
The higher logging rate for AIR2 was chosen to obtain a higher precision for the on-board navigation system.
To record the attitude (pitch, roll and heading) of the aircraft, two inertial navigation systems (INS) were used. The primary unit is a medium grade INS of type Honeywell H-764G. This unit collects data both in a free-inertial and a GPS-aided mode at 50 Hz. Specified accuracy levels in roll and pitch are better than 0.1º, and usual accuracy is higher than this. A backup INS is provided by an OxTS Inertial+2 integrated GPS-INS unit, with a nominal similar accuracy as the H-764G. The Honeywell INS was connected to the front GPS antenna. During most of the campaign the OxTS used dual antenna setup with the rear GPS antenna as primary antenna.
Figure 30: Overview of instrument setup in the TF-POF Twin Otter aircraft (image credit: DTU Space)
Data from the various instruments were acquired where feasible, considering the limited height range of the ALS system and the weather. All the ASIRAS data were acquired in Low Altitude Mode (LAM) with low along-track resolution (LAMa). This allows flight at an altitude of 300 m, which is within the operational range of the ALS system and a relative low data volume of about 28 GB per hour. A total of 604 TB raw ASIRAS data were collected during the CryoVEx/ICE-ARC 2016 campaign. The data were stored on hard discs as ASIRAS level 0 raw data in the modified compressed format (Cullen, 2010). The ASIRAS system performed well during the campaign only using PC1, due to a malfunction of PC2 detected during the first test flight in Longyearbyen.
In general, the ALS worked well. At low temperatures (below -25ºC) encountered at Station Nord, icing of the instrument window during take-off and steep climbs/descends prevented the laser to see through the instrument window. Partly blocking of the laser signals was apparent for the first hour of operation resulting in no surface return or a narrow scan width. Slow climbs during take-off reduced the icing on the scanner window. The actual loss of data was limited since most of the flights included some ferry flight to the designated survey areas. To circumvent the laser to lock on the frozen instrument window, the ALS was switched to measure the “last laser pulse”. The data volume obtained by the ALS is about 250-300 MB per hour, which is a relative small amount, when compared to the ASIRAS data volume. During the campaign a total of 7.6 GB ALS data were acquired.
The airborne GPS units logged data internally in the receivers (AIR1, AIR2 and AIR3) during flight, which were downloaded upon landing on laptop PCs. The Novatel GPS was dedicated to support ASIRAS and was not part of the logging system. GPS files were recovered for all receivers at all flights.
Both INS systems logged continuously throughout the campaign and no problems were observed with the systems.
Vertical photography was collected during sea ice flights. Pictures were acquired every 2 seconds for most flights by nadir-looking photography. Due to problems with the data system running the uEye webcam, this camera was only used on the first flight from Longyearbyen to Station Nord. The GoPro camera recorded nadir-photography from the remaining sea ice flights.
As an opportunity Sentinel-3A (S3A) under-flight over sea ice in Fram Strait was acquired on April 6 along orbit 712. The satellite passed above the aircraft 16:30 UTC at position 81° 24.43’N and 04° 50.03’W. At the passage time only ASIRAS was recording, due to low thick clouds, which appeared on the track 5 minutes before the satellite passage. This direct under-flight of the S3A over sea ice is to our knowledge the first successful under-flight for this satellite.
On transit flight from Station Nord to Longyearbyen, April 10, a planned CryoSat-2 under-flight (orbit 31841) was cancelled due to low clouds in the area north of Svalbard, and instead a second S3A under-flight was prioritized. The flight followed S3A ground track orbit 769 with passage time of the satellite at 16:27 UTC at position 81° 20.24’N 01° 07.00’E. The weather for this flight was excellent and we only encountered low clouds when approaching Svalbard.
ESA has confirmed that S3A operated in SAR-mode on April 6 and provided the respective level 1b products (i.e. waveforms) so that a first comparison with ALS results could be performed. On the flight on April 10 S3A was switched to Low resolution mode (LRM). A preliminary inter-comparison study of S3A and ALS data from underflight on April 6 was presented at AGU fall meeting 2016 by Di Bella et al. (2016). A short update of the presentation is given below. The S3A waveforms have been classified using their pulse peakiness as being generated by reflections from sea ice or leads. While sea ice waveforms have been retracked using a threshold algorithm (TFMRA50%), lead elevations are estimated using a Gaussian retracker.
In summary, the CryoVEx/ICE-ARC 2016 airborne campaign has been a success. In general, the weather was good in Greenland, which allowed data acquisition from all planned flights, and partially good in Svalbard allowing 2 out of 3 planned flights to be carried out. Thus, airborne measurements with the ASIRAS and coincident airborne laser scanner (ALS) have been collected to acquire gridded data over Austfonna ice cap in Svalbard for CryoSat-2 validation, first direct Sentinel-3A underflight over sea ice in the Fram Strait and additionally to repeat some previously flown sea tracks in Fram Strait and the Wandel Sea. Vertical photography was acquired during sea ice flights to support analysis of ASIRAS and ALS observations.
The ASIRAS and ALS instruments worked without any major problems. ASIRAS was only operated in LAM-mode, due to a malfunction of PC2. Comparison of ASIRAS to coincident ALS runway overflights concluded that ASIRAS level-1b processed with the ASIRAS processor version ASIRAS_04_03 shows overall good quality. The ALS data is likewise of high quality with mean difference less than 3 cm and standard deviation less than 10 cm at existing cross-over points. The actual loss of ALS data, due to freezing of moisture on the ALS window in extreme cold conditions (<-25°C) experienced at Station Nord was limited to ferry flights to the designated survey areas.
Preliminary comparisons between CryoSat-2 and ALS data at Austfonna ice cap, proves the concept of using a gridded ALS data set of lines parallel to CryoSat-2 ground tracks in areas with varying topography. First comparisons of Sentinel-3A and ALS also show consistent results, with some unexplained high freeboard values in the Sentinel-3A retracked data potentially originating from off-nadir icebergs.
CryoVEx KAREN 2016 Fall Campaign
The main objectives of the campaign were to: 30)
• Fly new Ka-band radar technology radar altimeter, named KAREN (Ka-band SARIn altimeter). KAREN is a modified version of the operational airborne side-looking interferometric radar developed and operated by the Dutch company MetaSensing.
• Collect simultaneous Ku-band radar altimeter data with ASIRAS and ALS data, to study penetration of radar signals into the ice sheet
• Repeat of the earlier flown EGIG-line to detect ice sheet changes
• Surface measurements (shallow ice cores and ice densities) on the ice sheet inland of Ilulissat at T1, T4 and T5 on EGIG-line and in the Ilulissat glacier region using helicopter
• Investigate dependencies of SARIn phase information from coincident measurements of KAREN and ASIRAS in high-altitude mode.
Airborne data was acquired over the Greenland Ice Sheet along the EGIG-line at different altitudes representing both the ablation, percolation and the dry snow zone. This data set along with the in-situ observations of density profiles from shallow ice cores and SnowMicroPen adds valuable information of the seasonal behavior of the radar signals, which has so far only been obtained in the spring during previous CryoVEx campaigns.
The horizontal location of the KAREN and ASIRAS reference points from the post-processed data are within the expected accuracies of their physical displacement. Comparison of phases of coincident SARIn acquisition from KAREN and ASIRAS data show no direct dependencies. However, the phases seem to be anti-correlated, which is currently being investigated.
As expected KAREN showed primarily surface scattering with little penetration, whereas ASIRAS showed primarily surface scattering in the percolation zone and penetration down to about 15 m in the dry snow zone with several visible melt layers.
The ESA CryoVEx/KAREN campaign 2016 is the first combined Ka- and Ku-band airborne campaign in Greenland to exploit the dual-frequency concept for future polar satellite missions. The primary measurements were obtained over the Greenland Ice Sheet along the EGIG-line on October 25 and 27, 2016. The airborne team covered all different glacial facies including the ablation, the percolation, and the dry snow zone. The in situ team supported by helicopter took measurements of snow density profiles from shallow ice cores and SFL SnowMicroPen at EGIG-line points T1, T4 and T5, and could not make it further inland due to a persistent layer of low clouds east of T5. 31)
Summary of operation
The airborne campaign was conducted in the period October 21-29, 2016. An overview of the flight tracks, together with markings of the in situ sites, is presented in Figure 1. The instrument installation and test flights took place in Akureyri (AEY), Iceland, followed by survey flights along the EGIG-line based out of Ilulissat (JAV). Unfortunately, the weather did not allow any surveying on the transit flights between Akureyri and Ilulissat, except one test of the Ka-band radar in the Denmark Strait at high altitude ~3,500 m above ground.
A Norlandair Twin Otter (reg: TF-POF), which is the same aircraft as used throughout previous CryoVEx campaigns, was chartered for the airborne part of the campaign. In order to adapt the Kaband radar the whole in the back of the aircraft had been enlarged prior to the campaign. The general instrument certification for the aircraft was obtained in 2006 (Hvidegaard and Stenseng, 2006). The flight altitude during survey is typically 300 m agl (above ground level), limited by the range of the laser scanner, and the nominal ground speed is 135 knots. The aircraft is equipped with an extra ferry tank permitting longer flights (5-6 hrs), and an autopilot for better navigation accuracy. In good conditions the across-track accuracy is down to a few meters using a custom-made navigation system connected to geodetic GPS receivers.
An Air Greenland helicopter AS350 was chartered to support the in situ team permitting landings on the ice sheet. A fuel cache of 2 drums was positioned at glacier lodge Eqi north of Ilulissat prior to the flights for maximal range of the helicopter along EGIG-line.
Figure 31: Overview of the flight tracks (black lines) from the CryoVEx/KAREN 2016 campaign. The star marks the location of the in situ site October 25 and the circle marks their location October 27 (image credit: DTU Space)
The CryoVEx/KAREN 2016 overall aim was to sample different snow conditions on the Greenland ice sheet along the EGIG-line to document the different glacial facies, i.e. the dry snow zone, the percolation zone, and the ablation zone (Figure 32). Studies of radar backscatter from high-resolution SAR images from Sentinel-1 prior to the campaign, shows that in 2016 the inland limit of melt seems to have reached its maximum between 19th and 26th of July in between EGIG-line points T12 and T21. Thus, the highest priority field site was to reach as far inland on the Greenland ice sheet as possible and preferable reach T21 located at 43°W, or as close to T21 as the helicopter range allowed given the weather conditions and final weight carried on the day. The second flight was planned to be the shortest flight to site T1, and the final flight to visit site T12. However, due to persistent low clouds in the area east of 47.25°W and strong winds the point furthest inland ended up being T5 on April 25, followed by T1 and T4 on April 27.
Three science experiments were conducted at each in situ site:
• Shallow firn/ice cores to directly measure the density of the top 3-5 meters of snow
• Upper layer density profiles by use of SnowMicroPen (SMP)
• Metal surface reflectors to calibrate the snow surface in the dual frequency Ku- and Ka-band airborne measurements.
The airborne activities were collocated with the ground work, and field site T5 was overflown on April 25, followed by T1 and T4 on April 27. As the airborne survey were based out of Ilulissat, each of the two survey flights to the EGIG-line were started and concluded with a survey of the outermost of the Jakobshavn Isbræ. Climbing the ice sheet from Jakobshavn isbræ, the survey passed the Swiss Camp, which when it was established in the early 1990’es where located on the Equilibrium-Line Altitude (ELA), however at present the camp is in the ablation area. The turn-around point of survey flight on April 25 was at T9 due to persistent low clouds east of T9. On the route to T9 the ground site at T5 was overflown four times from different directions. The flight on April 27 passed the ablation and percolation zones, and turned around deep into the dry snow zone about 55 nm west of T41 at 42.5°W, and thereby documenting the autumn conditions of all glacial facies encountered on the Greenland ice sheet. The in situ site at T1 was overflown from 2 directions. With the arrival of low clouds at about 48°W the aircraft ascended to measure in high-altitude mode with the KAREN and ASIRAS systems. Further inland at about 45°W the cloud cover disappeared, which demonstrated the flexibility of using a ski-equipped Twin Otter in the future for landings on the ice cap supporting the in situ work, instead of a helicopter.
Following the CryoVEx/KAREN campaign October 30 - November 12, the Twin Otter was used to support a team from University of Kansas in testing of low-frequency radar for measuring the ice thickness of Jakobshavn glacier. Logistics and expert knowledge was supported by DTU Space (Arne Vestergaard Olesen) and the DTU Space near-infrared ALS remained in the aircraft for surface reference. Three flight lines following Jakobshavn glacier was repeated multiple times November 7-12.
The hardware installation in the Twin Otter (TF-POF) consisted of the following instruments:
• MetaSensing Ka-band radar altimeter KAREN
• ESA Ku-band interferometric radar ASIRAS
• DTU Space Airborne Laser Scanner (ALS) of the type Riegl LMS Q-240i-60
• Four geodetic dual-frequency GPS receivers of type Javad Delta (AIR1-4), where AIR4 was dedicated to support ASIRAS time tagging
• An Inertial navigation system (INS) of the type Honeywell H-764G
• An Inertial navigation system (INS) of the type iMAR RQH (test and backup)
• A NovaTel integrated GPS-INS system of type SPAN-CPT to support KAREN (used as backup).
CryoVEx KAREN 2017 Campaign
The main objectives of the campaign were to: 32)
• To fly dual frequency (Ka/Ku-band) radar altimeters together with laser to study penetration depths in support of future satellite missions
• Coordinated sea ice underflights of CryoSat, Sentinel-3 and SARAL/AltiKa in different locations
• Coincident flights with PanArcMIP EC/AWI, NASA Operation IceBridge and GLISTN (Glacier and Ice Surface Topography Interferometer)
• Large-scale in situ work on sea ice (Lincoln Sea and Cambridge Bay) and land ice (EGIG-line).
First results of KAREN and ASIRAS differences over sea ice show about half the snow depth when compared to large scale in situ measurements taken in the Lincoln Sea. This is however, based on a simple TRMFA 50% for both radars, and needs further evaluation. Published results of the in situ data show overall good agreement with snow depths obtained using Warren climatology for this particular period and location. Quicklook data of snow depth obtained with NASA Operation IceBridge snow radar show results between in situ and KAREN-ASIRAS.
Over the EGIG line the in situ firn cores show good correlation with RACMO firn density model. Based on pRES and ASIRAS data the upper annual layers can be identified in the dry snow zone T19 and beyond with expected volume scattering down to 6 m at T19.
The ESA CryoVEx/KAREN 2017 Arctic campaign is a very ambitious airborne and in situ campaign using combined Ka-and Ku-band airborne sensors (KAREN and ASIRAS) to validate CryoSat-2, Sentinel-3 and SARAL/AltiKa altimeter missions and to exploit the dual-frequency concept for future polar satellite missions. The concept builds upon the experience gained during ESA CryoVEx/KAREN 2016 fall campaign, which were the first ESA campaign to explore the dual-frequency airborne setup. 33)
The 2017 airborne campaign was an ESA CryoSat-2 Validation Experiment (CryoVEx) primarily for CryoSat-2 validation, but also Sentinel-3 and SARAL/AltiKa, to monitor sea ice and land ice in the Arctic. The CryoVEx/KAREN campaign also built upon the existing EU FP7 project ICE-ARC campaign. The airborne campaign was coordinated by National Space Institute, Technical University of Denmark (DTU Space), in cooperation with York University (YU) and Environment Canada(EC),and took place in spring 2017.
The campaign was divided in two separate campaigns referred to as phase 1 and phase 2. This was done first of all to ease the load on CFS Alert, but also to secure coordinated laser and radar flights with PanArcMIP EC/AWI Polar-5 and NASA Operation IceBridge.
Phase 1 took place March 20-31 and involved operations with ESA’s Ku-band radar (ASIRAS), Dutch Company MetaSensing’s Ka-band radar (KAREN) and laser scanner using chartered Twin Otter (reg. no.: TF-POF) from Norlandair, Iceland. The campaign covered sea ice flights in the Arctic Ocean and Baffin Bay, together with land ice flights over Devon icecap and the Greenland Ice Sheet. This part included a coordinated sea ice flight involving for the first time four aircrafts; ESA CryoVEx Twin Otter, PanArcMIP EC/AWI Polar-5, NASA Operation IceBridge and NASA wide-swath topographic mapper (GLISTN), carrying a suite of instruments to monitor the snow and sea ice along a CryoSat-2 ground track. In addition, flights were coordinated with large scale in situ work along the EGIG line of the Greenland Ice Sheet lead by University of Leeds.
Phase 2 took place April 5-23 and involved flights over sea ice and land ice from Cambridge Bay, CFS Alert, and Svalbard/Station Nord, as well as an intensive large scale sea ice in situ program with multiple landings between 82-86°N on sea ice along CryoSat-2 tracks north of CFS Alert. Two aircraft were used during this phase. A Twin Otter from Kenn Borek Air, Calgary, was chartered for sea ice landings, and a British Antarctic Survey Twin Otter (VP-FAZ) was used to collect combined Ka-band radar (MetaSensing KAREN), laser and radiometer measurements, but also acting as safety back-up for the sea ice landings. This phase also included coordinated flights with Cambridge Bay Snow Experiment (CamBay SnowExp) and Austfonna ice cap in Svalbard.
Figure 33: Overview of the flight tracks from the CryoVEx/ICE-ARC 2017 airborne campaign, phase 1 (red) and phase 2 (yellow). Satellite underflights and in situ sites have been marked (image credit: DTU Space, ESA)
Summary of operation
The CryoVEx/ICE-ARC 2017 airborne campaign was split into two operational periods. The first part, referred to as phase 1, basically circumnavigated Greenland north of 70°N, see red flight lines in Figure 33. The second part, referred to as phase 2, started out in Cambridge Bay, Nunavut, Canada and proceeded via CFS Alert to Svalbard, Norway, see yellow flight lines in Figure 33. A total of scientific flight hours were about 60 hours for each phase of the campaign.
The Norlandair Twin Otter (reg: TF-POF), which is the same aircraft as used throughout previous CryoVEx campaigns, was chartered for phase 1. The instrument installation and test flights took place in Akureyri, Iceland, March 15-18, following the general instrument certification for the aircraft obtained in 2006 (Hvidegaard and Stenseng, 2006). The exceptional setup was that for the first time four aircraft were jointly underflying a Cryosat-2 ground track, carrying a variety of instruments to compliment the dual-frequency setup. This was done in the Lincoln Sea on March 24, and included Norlandair Twin Otter with Ka-band radar (KAREN), Ku-band (ASIRAS) and Airborne Laser Scanner (ALS), AWI Polar-5 towing an electromagnetic sensor to measure the sea ice thickness, Operation IceBridge P3 with ATM and snow radar, and NASA wide-swath topographic mapper (GLISTN). The flight program was very tight, leaving no space for delays. Luckily, the weather was favorable and all planned flights were achieved within the estimated time.
The primary aim of phase 2 was to support the in situ work out of CFS Alert, both by taking coincident measurements, but also to act as safety backup for the in situ team aircraft (Ken Borek Twin Otter) which was used for multiple landings on the drifting sea ice to transport the in situ group to the different sites. Phase 2 was supported by the British Antarctic Survey Twin Otter (VP-FAZ), which was used for similar measurements in spring 2015 (Hvidegaard et al., 2017).
The instrument installation took place March 27-April 3 in Springbanks airport, Calgary, Canada. April 4-5 the aircraft was transferred to Cambridge Bay, Nunavut, Canada, where a large in situ program “CamBay SnowExp” to measure the snow and ice properties along a SARAL/AltiKa ground track was setup with UK, US and Canadian scientists. During phase 2 a total of four CryoSat-2, one SARAL/AltiKa and one Sentinel-3 underflight over sea ice was measured. In addition, a pattern of parallel lines, with a similar angle as ascending CryoSat-2 tracks were flown over Austfonna similar to CryoVEx 2016 spring campaign (Skourup et al., 2016). In addition, about 5 flight hours were dedicated to make a tight grid over land in Cambridge Bay area to validate a high-resolution DEM from NASA. In general, the weather was favorable. However, some delays were introduced due to waiting for weather and de-mounting of wheel-skies, which only could be performed in Resolute Bay. The second phase of the campaign was followed by an airborne ESA campaign to test a novel “cold atom” gravimeter in an aircraft in Iceland in cooperation with ONERA, France.
The flight altitude during survey is typically 300 m agl, limited by the range of the laser scanner, and the nominal ground speed is 135 knots. The aircraft is equipped with an extra ferry tank permitting longer flights (5-6 hrs), and an autopilot for better navigation accuracy. In good conditions the across-track accuracy is down to a few meters using a custom-made navigation system connected to geodetic GPS receivers. Calibration flights of the instruments over buildings and runways were performed whenever possible.
CryoVEx KAREN Antarctica 2017-2018 Campaign
ESA’s CryoVEx/Karen 2017-18 campaign took place in Antarctica in from Dec 2017 to Jan 2018. The campaign was composed of an airborne and in-situ campaign and acquired extensive data sets of scanning lidar, Ku- and Ka-band nadir-looking radar, and auxiliary imagery for validation of the ESA CryoSat-2 satellite (Ku-band radar altimetry) and the French-Indian AltiKa mission (Ka-band radar altimetry). 34)
The campaign was also designed to acquire data for studying the radar penetration of Ku- and Ka-band radar in snow and firn over the Antarctica sea ice and ice sheet. The airborne radar and laser survey was supported by 2 ground teams, one that took shallow cores on the southern part of the Antarctic Peninsula and the other collected sea ice thickness and snow depth in the Weddell Sea.
Estimated freeboard (elevation above local sea level) from radar Ku (ASIRAS), Radar Ka (KAREN) and ALS (near infrared laser) was calculated. The L1b radar products were retracked (using TFRMA 50%) to give elevations similar to the ALS product. Results show that the freeboard from ALS provides the highest freeboard values, ASIRAS the lowest and KAREN is in between.
Freeboard differences for a single flight compared with snow thickness observations from ASPeCt (Antarctic Sea ice Processes and Climate) observational data (Worby et al. 2008) spanning The ASPeCt dataset spanning from 1981-2005 showed that the observed snow depth during ASPeCt is generally lower than the ALS/ASIRAS difference.
The ESA CryoVEx-Karen airborne satellite validation campaign in Antarctica was successfully carried out in the period 20 Dec 2017 to 16 Jan 2018, taking advantage of several logistics opportunities for support from the British Antarctic Survey (availability of aircraft and pilot support and also, Rothera Station support for the team, and support for in-situ measurements both over land ice and sea ice). The field campaign was set up at relatively short notice, and set up along the same principles and instrumentation as in earlier arctic campaigns (scanning lidar for measurement of precise snow and ice heights, Ku-band (13 GHz) radar ASIRAS as a proxy for CryoSat-2 measurements, and a recent developed Metasensing Ka-band radar altimeter (38 GHz), providing an airborne equivalent of the AltiKa radar (36 GHz). 35)
Figure 34: Antarctic field campaign with combined airborne Ku/Ka-band radar and laser altimeters, together with extensive in situ measurements over sea- and land ice (image credit: DTU Space, ESA)
The campaign flight program was set up to attempt to fly several near-realtime underflights of both the CryoSat-2 and the AltiKa radar altimetry missions, both over sea ice in the Weddel Sea, over the Ronne and George VI ice shelves, as well as further inland over Antarctic ice sheet regions. This was a challenging planning effort due to the day-to-day changes in orbit ground tracks, which meant that underflight opportunities had to be continuously re-evaluated, to allow for changing weather and logistics constraints. A total of ca. 60 flight hours was flown, sampling a large variety of cryosphere regions from Antarctic sea ice, ice shelves, nunatak ice cap zones on the Antarctic Peninsula, and deep interior high-elevation ice sheet regions around the Ellesworth Mountains, the highest mountains in Antarctica.
The new data from CryoVEx 2017/18 will allow an enhanced quantification of radar signal penetration into the Antarctic cryosphere, and thus potentially improved measurement of ongoing ice sheet, ice shelf and sea ice changes, as well as provide a demonstration experiment for a future multi-frequency radar altimetry mission, such as the Copernicus High Priority Candidate Mission CRISTAL. The airborne flight tracks are shown in Figure 35.
The airborne measurement program was complemented by an ice cap /ice shelf University of Leeds/BAS (British Antarctic Survey) field team, carrying out in-situ measurements on ice sheet and ice shelf regions in the southern Antarctic Peninsula region in the days Jan 12-16, 2018, using a 2nd BAS Twin-Otter for logistics support. The in-situ data acquired included snow properties and shallow ice core measurements at 8 sites, coordinated with airborne overflights, and coincident with CryoSat-2 orbit tracks. The location of the in-situ sites is shown in Figure 2, along with the near-coincidental CryoSat tracks.
Another UK team, led by Rachel Tilling (University College London), was doing sea ice measurements from the BAS icebreaker R/V “Ernest Shackleton”. Ice thickness, snow depth, and broadband radar measurements were carried out on an opportunity basis in the Weddell Sea, as part of the BAS resupply of Halley Station and fuel depots on the Ronne-Filchner ice Shelf. Coordinated airborne overflights were done at two in-situ data locations (near the ice shelf front at Three Ronne Depot (TRD), and over a major ice-floe about 200 km to the NE of TRD, marked with “ES” in Figure 35). The sea ice overflights consisted of a local pattern along predetermined in-situ data lines. Logistics allowed a visit of the airborne survey crew to meet visit the R/V Shackleton at the TRD, facilitating the flight/in-situ coordination.
The primary objectives achieved during the campaign were:
• To fly dual frequency (Ka/Ku-band) radar altimeters together with laser to study penetration depths and key properties of Antarctic conditions in support of future satellite missions
• Coordinated land sea ice underflights of CryoSat-2 and SARAL/AltiKa in different locations
• Coincident flights with in situ observations on sites on both land and sea coordinated with ground team
Figure 36: Left: Location of the in-situ ice cap (T2, T4, T6, T8) and ice shelf (T1, T3, T5, T7) UL shallow drilling locations, overlaid by airborne tracks(white); topography in colors. Right: Offloading drilling equipment at the Stange Ice Shelf in-situ site (T3), image Credit: DTU Space, ESA
Figure 37: Left: R/V Ernest Shackleton at Three Ronne Depot (Alex Coccia, Metasensing, in front). Right: R/W Shackleton moored at large ice floe in the central Weddell Sea (photos: Dave Landy, BAS)
Summary of operation
The CryoVEx 2017-18 campaign was started ahead of the first field crew arriving at Rothera on Dec 20, by Russell Ladkin, BAS aircraft technician. The airborne lidar and radar equipment was installed in the BAS Twin-Otter aircraft VP-FAZ, taking advantage of existing BAS hardware (VP-FAZ has been used regularly for other science flights, including first BAS/DTU cooperative flights with lidar as early as in 2010). An additional Twin-Otter aircraft, VP-FBL, was used to support the deployment of the UL/BAS field team in the southern Peninsula region, operating out of the BAS SkyBlu summer camp, a highaltitude blue ice runway, where wheel landings with the larger BAS Dash-7 support aircraft are possible.
The operations, given the logistics and remote operations under challenging Antarctic conditions, were actually quite close to the original time plan [RD1], although the actual layout of airborne flight tracks is quite different.
The flight altitude during the CryoVEx survey flights was typically 300 m agl, limited by the range of the laser scanner, and with a nominal ground speed is 135 knots. The aircraft was equipped with an extra ferry tank permitting longer flights, and an autopilot for better navigation accuracy (in good conditions the across-track accuracy is down to a few meters using a DTU custom-made navigation system connected to geodetic GPS receivers). Calibration flights of the instruments over buildings and runways were performed at Rothera (with GPS survey ground truth), as well as over the Sky Blue blue ice airstrip (snowmobile GPS survey).
The airborne CryoVEx 2017-18 science team consisted of Arne V. Olesen and Rene Forsberg (DTU Space) and Alex Coccia (MetaSensing), with ESA technical officer Tânia Casal participating on behalf of ESA. Russell Ladkin (British Antarctic Survey) assisted with the instrument installation, and the skills of BAS pilot David Landy was instrumental for the success of the campaign.
The land ice ground team consisted of Andy Shepard, Adrianos Lemos, and Anna Hogg (University of Leeds). The in-situ sea ice team Rachel Tilling (University of Leeds) and ph.d. student Isabel Nias (University of Bristol). The land ice ground team was part of the CryoVEx team, whereas the sea ice ground team was a British funded project, using the logistical support from R/V Shackleton resupply cruise in the Weddell Sea.
Figure 38: Top: BAS main base Rothera, one of only 3 hard-rock runways in Antarctica. Hanger at lower left (image credit: BAS)
Figure 39: Overview of instrument setup in the VP-FAZ Twin Otter aircraft (image credit: DTU Space, ESA)
Overview of acquired data
Data from the various instruments were acquired where feasible, considering the limited range of the ALS system and the weather.
The sampling frequency of the KAREN sensor was set to 25 MHz corresponding to one sampling each ~30cm on the ground. The high sampling frequency results in a large amount of 360 GB of raw data per hour, plus a few MB of navigation data (dedicated GPS/IMU module). Acquisitions have been manually started and stopped by the operator according to the area which was flown; data sets of typically 10-30 minutes duration have been logged. At the end of the campaign the raw data amount is ~10 TB. During the flight and at the end of each acquisition day a quick data look was performed on randomly selected dataset to assess the quality and eventually adjusting the Pulse Repetition Frequency (PRF) according to the flight altitude. Data was primarily gathered at low altitude consistent with the range of the ALS but also High Altitude Mode (HAM) was tested on selected flight lines.
ASIRAS data was manly acquired in Low Altitude Mode (LAM) with low along-track resolution (LAMa) but also a few selected lines were observed in HAM after initial mal-fuction of PC2 was repaired. One of the embedded ASIRAS computers (“PC2”) was non-functional upon arrival in Antarctica, which ruled out any data collection in HAM (SarIn) mode; it was only repaired partially after consultations with RST mid-campaign, but PC2 continued to be unstable until the last flights. LAMa allows flight at an altitude of 300 m, which is within the operational range of the ALS system and a relative low data volume of about 28 GB per hour. A total of about 1 TB raw ASIRAS data was collected during the campaign. The data were stored on hard discs as ASIRAS level 0 raw data in the modified compressed format (Cullen, 2010).
The lidar, GPS and Honeywell IMU instrumentation worked flawlessly during the campaign, while the radars and nadir looking cameras had several problems. For the cameras, data were only collected where it made sense (e.g., over sea ice and mountains, but not over featureless ice sheet and shelf surfaces.
The data volume obtained by the ALS is about 250-300 MB per hour, which is a relatively small amount, when compared to the ASIRAS data volume. During the campaign a total of 32.8? GB ALS data was acquired.
The airborne GPS units logged data internally in the receivers during flight, which were downloaded upon landing on laptop PCs.
Overall, more than 10 TB of data were collected. Nearly all the satellite underflights had all three airborne data types (lidar, Ku- and Ka-band radar) collected.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).