Copernicus: Sentinel-5P

Copernicus: Sentinel-5P (Precursor - Atmospheric Monitoring Mission)

Sentinel-5P (or S-5P, or S5P) is an approved LEO pre-operational mission within the European GMES (Global Monitoring for Environment and Security) program — a collaborative effort of ESA and NSO (Netherlands Space Office). The goal is to fill the gap between the current atmospheric monitoring instruments SCIAMACHY on ESA's Envisat satellite and OMI (Ozone Monitoring Instrument) carried on NASA's Aura mission, as these instruments come to the end of their lifetimes, and the launch of the Sentinel-5 mission is planned for the timeframe 2020. Note: The Envisat mission operations ended on May 9, 2012. ^{1) 2) 3) 4)}

Copernicus is the new name of the former GMES program 5)

Copernicus is the new name of the European Commission's Earth Observation Programme, previously known as GMES (Global Monitoring for Environment and Security). The new name was announced on December 11, 2012, by EC (European Commission) Vice-President Antonio Tajani during the Competitiveness Council.

In the words of Antonio Tajani: "By changing the name from GMES to Copernicus, we are paying homage to a great European scientist and observer: Nicolaus Copernicus (1473-1543). As he was the catalyst in the 16^th century to better understand our world, so the European Earth Observation Programme gives us a thorough understanding of our changing planet, enabling concrete actions to improve the quality of life of the citizens. Copernicus has now reached maturity as a programme and all its services will enter soon into the operational phase. Thanks to greater data availability user take-up will increase, thus contributing to that growth that we so dearly need today."

The missions Sentinel-5P (LEO), Sentinel-4 (GEO) and Sentinel-5 (LEO) will be devoted to atmospheric composition monitoring for the GMES Atmosphere Service (GAS). The objective of the Sentinel-5P mission is to provide data delivery (maintain the continuity of science data) for atmospheric services between 2015-2020. The successor Sentinel-5 payload is planned to be flown on a MetOp-SG (Second Generation) mission with a launch in 2020.

At the ESA ministerial Conference in 2008 in The Hague, The Netherlands, the Sentinel-5P mission was defined in the frame of the ESA GMES Space Component Program. This program answers to a joint initiative of the EC (European Commission) and ESA on GMES.

Spacecraft

Unlike the previous missions (Sentinel-1, Sentinel-2 and Sentinel-3), the Sentinel-4 and -5 will be in the class of "hosted payload" missions embarked on meteorological satellites and will be dedicated to atmospheric composition monitoring for the Copernicus Atmospheric Service. The mission is a single payload satellite embarking TROPOMI (Tropospheric Monitoring Instrument), a pushbroom instrument with four hyperspectral channels covering the spectrum from UV to SWIR. - On Dec. 8, 2011, ESA awarded a contract to Astrium Ltd. (Stevenage, UK) to act as prime contractor for the Sentinel-5 Precursor satellite system. ^{8) 9)}

The satellite uses the AstroBus-L 250 M platform of Astrium and thus draws on the heritage from the SEOSat/Ingenio program of Spain, developed under the control of ESA, and from SPOT-6 and -7, two commercial imaging missions currently under development with Astrium internal funding. Including an ongoing export contract with Kazakhstan using this platform, Sentinel-5p is the 5th mission in the series and can rely on a robust and proven platform design. ^{10) 11)}

The mechanical platform consists of a hexagonal structure supporting the platform electrical units and the TROPOMI ICU (Instrument Control Unit), and interfacing to a standard launch vehicle interface ring.

In the baseline solution, the platform equipment is distributed over the opening side panels, thus allowing easy access during integration and in case of on-ground maintenance operations.

The platform electrical/functional allocation uses a well proven classical architecture which is currently implemented in several ESA missions as well as in national and export programs. This proven architecture allows re-use of electronic equipment from several suppliers.

The core of the platform electrical/functional architecture is the data handling housed in two physically separate units, the OBC (On-Board Computer) and the RIU (Remote Interface Unit). The OBC (LEON 3) provides the processing and housekeeping memory functions and is responsible for telemetry and telecommand (TM/TC) handling, on-board time management, system re-configuration and communication with "intelligent" platform and payload units – units which communicate via a data bus. The OBC also manages the interface with the S-band transponder, which provides the RF telemetry, telecommand and ranging link to and from the ground station.

The OBC communicates with other satellite units primarily via two independent, fully redundant MIL-STD-1553B buses. All input/output interfaces to "non-intelligent" units are managed by the RIU.

The spacecraft power conditioning functions are performed autonomously by the PCDU (Power Conditioning and Distribution Unit). For robustness, these functions are implemented without the use of software. A battery and solar array sized to satisfy the mission needs complete the power subsystem.

The thermal subsystem includes heaters that are needed to maintain the thermal environment of the platform. The thermal control loops are controlled by the CSW (Central Software) resident in the OBC.

A COTS (Commercial-off-the Shelf) monopropellant propulsion module is used for orbit maintenance, mounted in the center of the lower floor. The propulsion subsystem is a hydrazine design operating in blow-down mode with 4 x 1 N thrusters configured in two redundant pairs.

The top floor accommodates the instrument and its radiator, as well as the star trackers and the X-band and S-band communication antennas. The instrument is mounted in a canted position, such that its radiator has an unobstructed field of view.

The nominal operational scenario for the payload instrument will always be nadir-pointing in the instrument imaging mode. Measurement data is collected when the SZA (Sun-Zenith Angle) is < 92º. Sun calibration can be performed close to the northern polar region when the sun enters the FOV (Field-of-View) of the sun calibration ports. Further calibration can be performed throughout the remainder of the orbit.

The PDHT (Payload Data Handling and Transmission) subsystem consists of a PDHU (Payload Data Handling Unit) and a set of X-band transmission units. The PDHU stores and handles the data transmitted by high speed links from the instrument. PUS (Packet Utilization Standard) compliant data are sent to the transponders and transmitted to ground.

The spacecraft is 3-axis stabilized, the design provides an optional yaw steering.

The main features of the FDIR (Failure Detection, Isolation and Recovery) concept are:

• A robust and qualified design coming from a high level of reuse of the standardized operations and FDIR concept already implemented in SEOSat/Ingenio

• A hierarchical architecture (from unit level to system level) where the goal is to try to recover the observed error on the lowest possible level to maximize the system availability for nominal operations.

This FDIR design guarantees:

• A high level of autonomy for the nominal mission with extended periods of time without ground intervention

• Satellite integrity in case of any failure leading to suspend the nominal mission

• Maximizes the satellite availability and autonomy while preserving a robust and failure tolerant system

• Safe operation of the satellite in case of any credible anomaly

• Geo-location performance within requirements even after a single failure: the 3 Star Tracker Optical Heads ensure that the geo-location requirements are still met with some margin after the loss of one optical head.

EPS (Electrical Power Subsystem): Three deployable solar arrays (5.4 m²) using GaAs triple-junction solar cells, supply 1500 W of average power. The two Li-ion batteries have a capacity of 156 Ah.

RF communications: The spacecraft will be equipped with S-band and X-band communication channels for uplink commanding and housekeeping telemetry downlink and for the downlink of instrument data, respectively. The X-band payload downlink rate is 310 Mbit/s. The onboard mass memory unit has a capacity of 430 Gbit using flash memory technology.

Project Development Status

• October 13, 2017: Europe's Sentinel-5P Earth observation mission will be lofted into space on a Russian rocket from Plesetsk Cosmodrome. About 93 minutes later, the satellite – having separated from the rocket and opened its solar panels – will transmit its first signals. The transmission will indicate that all has gone well with the launch and that the satellite is ready to receive instructions. ¹³⁾

- On Earth, engineers at the ground station in Kiruna, Sweden will be watching intently, with their 15 m diameter antenna pointing at the horizon, ready to catch Sentinel-5P's signal as it rises into the sky over the country. - The Kiruna station is part of ESA's global network, and it routinely supports multiple missions such as CryoSat, Integral, the Swarm trio and Sentinel.

- At the same time, 2100 km to the south, the team at ESA's mission control center in Darmstadt, Germany, will also be watching closely, because ‘acquisition of signal' will mark the moment they assume control, sending commands and downlinking data to check on the satellite's health and status.

• October 11, 2017: ESA's air-quality mission Sentinel-5P will sift through light from the atmosphere to accomplish its ambitious monitoring goals. The Agency's optics specialists helped to verify its main TROPOMI instrument would operate as planned. ¹⁴⁾

- Sentinel-5P is the first in a series of atmospheric chemistry missions from the European Commission's Copernicus program. It carries a single high-precision optical payload called the TROPOMI (Tropospheric Monitoring Instrument), developed jointly by the Netherlands and ESA. - Its aim is to track gradual changes in the makeup of the atmosphere, providing continuity between past missions such as ESA's Envisat and NASA's Aura and Europe's future Sentinel-4 and -5.

- Orbiting at 824 km above our heads, Sentinel-5P will map a multitude of trace gases such as nitrogen dioxide, ozone, formaldehyde, sulphur dioxide, methane, carbon monoxide and aerosols – all of which affect the air we breathe and therefore our health, and our climate.

- The optimal performance of an optical instrument in space always comes down to the combination of its individual components – coatings, filters, lenses and mirrors – in the optical chain. So back during TROPOMI's development phase, ESA's Optics Laboratory tested a number of key instrument elements.

- TROPOMI works by comparing reflected light from Earth's atmosphere with direct sunlight at various wavelengths, from infrared to ultraviolet. It uses diffraction gratings to split this light, allowing it to sift out the spectral fingerprints of its target trace gases.

- The optimal performance of an optical instrument in space always comes down to the combination of its individual components – coatings, filters, lenses and mirrors – in the optical chain. So back during TROPOMI's development phase, ESA's Optics Laboratory tested a number of key instrument elements.

- One of a suite of technical labs at ESA's technical heart in the Netherlands, the Optics Lab focused on verifying controlling unwanted ‘stray light' that might leak from the diffraction gratings. Too much stray light might make trace gas detection impossible. They performed precision measurements of prototype TROPOMI gratings to ensure any stray light remained within permissible bounds.

• October 4, 2017: As preparations for the launch of Sentinel-5P continue on track, the team at Russia's Plesetsk site has bid farewell to the satellite as it was sealed from view in the Rockot fairing. ¹⁵⁾

• September 25, 2017: Engineers have been at Russia's Plesetsk launch site for a month now, ticking off the jobs on the ‘to do' list so that the Copernicus Sentinel-5P satellite is fit and ready for liftoff on 13 October. With the satellite now fuelled, the team has passed another milestone. ¹⁶⁾

• September 4, 2017: The Sentinel-5P satellite has arrived in Plesetsk in northern Russia to be prepared for liftoff on 13 October. Built to deliver global maps of air pollutants every day and in more detail than ever before, this latest Copernicus mission will set a new standard for monitoring air quality. ¹⁷⁾

- Sentinel-5P is the first Copernicus mission dedicated to monitoring our atmosphere. It follows five other Sentinel satellites already in orbit and delivering a wealth of information about our planet.

• August 30, 2017: Today, Sentinel-5P was loaded on the Antonov aircraft that will take this latest Copernicus satellite to Russia to be prepared for liftoff in October. ¹⁸⁾

- Sentinel-5P carries the state-of-the-art TROPOMI instrument to map a multitude of trace gases such as nitrogen dioxide, ozone, formaldehyde, sulphur dioxide, methane, carbon monoxide and aerosols – all of which affect the air we breathe, our health, and our climate. With a swath width of 2600 km, it will map the entire planet every day. Information from this new mission will be used through the Copernicus Atmosphere Monitoring Service for air-quality forecasts and for decision-making.

• June 22, 2017: The Copernicus Sentinel-5 Precursor (Sentinel-5P) mission is dedicated to monitoring the composition of the atmosphere. Its data will be used largely by the Copernicus Atmosphere Monitoring Service. The mission will deliver information to monitor air quality, stratospheric ozone and will also be used for climate variables monitoring, and support European policy-making. ¹⁹⁾

- The Sentinel-5P mission will be the first of a series of atmospheric chemistry missions to be launched within the European Commission's Copernicus program. With the current launch window of September 2017 and a nominal lifetime of seven years, Sentinel-5P is expected to provide continuity in the availability of global atmospheric data products between its predecessor missions, SCIAMACHY (Envisat) and OMI (Aura), and the future Sentinel-4 and -5 missions.

- Sentinel-5P products will be used by Copernicus Services, namely the Atmosphere Monitoring Service (CAMS) or the Climate Change Service (C3S). These services will transform its data into high value information (for instance, forecasts of air pollution over Europe) that can be used by decision-makers to take appropriate actions on environmental policies, from which the well-being and security of EC citizens and future generations depend.

• February 6, 2016: The launch service for ESA's Sentinel-5P satellite on the Rockot launch vehicle reached an important project milestone during this week. In the frame of the satellite's test campaign at the facilities of Intespace, Toulouse, Sentinel-5P has been mated for the first time on its dedicated launch vehicle adapter. This adapter system will attach the spacecraft to the Rockot carrier during its travel into space and will eventually release the satellite into the target orbit. ²⁰⁾

- The mating exercise, the so-called fit-check, aimed at verifying the mechanical and electrical interfaces between the Sentinel-5p satellite, built by Airbus DS in Stevenage, and the launcher hardware, manufactured by the rocketry company Khrunichev Research and Production Space Center. The purpose of a fit-check is ensuring a successful integration of the spacecraft onto Rockot at the launch complex and a check of the umbilical connections between the launcher and its payload. For the Sentinel-5P mission, the fit-check was further used to verify a customized purging system which was integrated into the adapter allowing the satellite customer to flush its contamination-sensitive instrument through the satellite-launcher interface during ground operations.

- The actual attachment of the Sentinel-5P satellite to its launch adapter is by means of a clamp band mechanism developed by Airbus Defence and Space in Madrid (formerly CASA Espacio). The clamp band is applied with high tension along the spacecraft-launch vehicle interface. The release of the satellite in space is achieved by firing pyro charges, which spontaneously open the clamp and hence allow separation. As the flawless functioning of the release is essential for a launch success, it was tested following the mating under recording the induced shock loads levels.

- Fit-check and release shock test have been conducted successfully on February 2nd and 3rd, respectively, in a remarkable team effort by Airbus Defence & Space, the Khrunichev Space Center, European Space Agency and Eurockot.

• July 24, 2015: The Sentinel-5 Precursor platform and the TROPOMI instrument have been integrated together to form the satellite which will be leaving the UK for testing. Airbus DS will deliver the spacecraft to Intespace in Toulouse, France, for final system level testing. ²¹⁾

Operational System/Service Allocations

• The Sentinel-5P satellite consists of the platform and the TROPOMI payload, the latter is supplied as CFI (Customer Furnished Item) to the spacecraft prime.

• The LEOP (Launch and Early Orbit Phase) ground station network will be used to control spacecraft after launch.

• Svalbard polar Earth station for spacecraft operations and data downlinking.

• The FOS (Flight Operations Segment) function will be performed by ESA/ESOC.

• The PDGS (Payload Data Ground Segment) function will be performed by DLR/EOC (Earth Observation Center), under contract to Astrium Ltd. This involves the development of PDGS to host the missions' ground processors and to distribute the resulting data to the user community.

Figure 17: Sentinel-5P team set-up (image credit: Astrium)

Launch

The Sentinel-5P spacecraft was launched on October 13, 2017 (09:27 GMT) on an Eurockot Rockot/Briz-KM vehicle from the Plesetsk Cosmodrome in northern Russia. The Sentinel-5P spacecraft has a launch mass of ~ 820 kg. The first stage separated 2 min 16 sec after liftoff, followed by the fairing and second stage at 3 min 3 sec and 5 min 19 sec, respectively. The upper stage then fired twice, delivering Sentinel-5P to its final orbit 79 min after liftoff. ^{22) 23) 24) 25)}

Orbit: Sun-synchronous orbit, altitude = 824 km, inclination = 98.74º, LTAN (Local Time on Ascending Node) = 13.35 hours, period = 101 minutes, the repeat cycle is 17 days (227 orbits).

A unique feature of the Sentinel-5P mission lies in the synergistic exploitation of simultaneous measurements of imager data from the VIIRS (Visible/Infrared Imager and Radiometer Suite), embarked on the Suomi NPP (NPOESS Preparatory Project) satellite of NASA/NOAA. NASA launched the NPP mission on October 28, 2011. The Sentinel-5P orbit is selected such that it trails behind Suomi NPP by 5 min in LTAN, allowing the Sentinel-5P observation swath to remain within the scene observed by Suomi NPP.

Note: As of June 2019, the previously large Sentinel5P file has been split into two files, to make the file handling manageable for all parties concerned, in particular for the user community.

• This article covers the Sentinel-5P mission plus the mission status in the period 2020

• Sentinel-5P Status and Imagery for the period 2019

• Sentinel-5P Mission Status for the period 2018-2017

Mission Status (2020 - 2022)

• September 10, 2024: The Global Methane Budget 2024 report reveals alarming increases in global methane emissions, with human activities now responsible for at least two-thirds of these emissions—a 20% rise over the past two decades, accelerating particularly in the last five years. Methane, being 28 times more effective than carbon dioxide at trapping heat over a century, has reached atmospheric concentrations of 1,923 parts per billion, the highest in 800,000 years. Despite international commitments, including the Global Methane Pledge for a 30% reduction by 2030, emissions continue to rise, especially from fossil fuels, agriculture, and waste management, posing significant challenges to climate goals. Key contributors include China, India, the USA, Brazil, and Russia, while Europe and Australia have seen reductions in methane emissions. ⁹⁴⁾

In response to the growing methane crisis, ESA is leading initiatives to improve methane monitoring and mitigation. The Copernicus Sentinel-5P satellite provides vital data for tracking global methane concentrations, aiding in identifying hotspots and verifying mitigation efforts. Projects like SMART-CH4 and AI4CH4 aim to enhance detection capabilities for methane point sources using satellite imagery and AI technologies. Additionally, ESA's Regional Carbon Cycle Assessment and Processes Phase 2 (RECCAP-2) project seeks to reconcile top-down emission estimates with traditional inventories, while the CCI Greenhouse Gases project focuses on creating long-term climate data records. Through these efforts, ESA aims to support policymakers in effectively addressing global methane emissions and mitigating climate change impacts.

• June 23, 2022: In 2020, despite the global economic slowdown caused by the COVID-19 pandemic, methane levels continued to rise significantly, as observed by data from the Copernicus Sentinel-5P satellite. Scientists from the University of Leeds used this data to pinpoint regions with large surges in methane emissions, particularly in South Sudan, Uganda, Canada, and Russia. Wetlands, which contribute significantly to methane emissions, played a major role in this increase, especially with high rainfall and dam releases affecting regions like the Sudd wetlands in South Sudan. Satellite measurements confirmed the same methane increase shown in surface measurements, reinforcing the need to monitor atmospheric changes using such technology.  ^26) 27)

- Methane, which has both natural (40%) and anthropogenic (60%) sources, is a potent greenhouse gas with a relatively short atmospheric lifetime of nine years. Reducing methane emissions could quickly mitigate climate change. Despite the pandemic, 2020 saw the largest annual increase in methane concentrations since the 1980s, a record surpassed in 2021. Sentinel-5P data and the TOMCAT model showed that wetlands were likely significant contributors to this rise. Ongoing research aims to better understand how wetlands will respond to climate changes and how methane emissions from these regions will evolve in the future.

• May 25, 2022: At ESA's Living Planet Symposium in Bonn, discussions emphasized how Earth observation technology is already playing a critical role in supporting national climate action, particularly in emissions monitoring and adaptation. Satellites like Copernicus Sentinel-5P and the upcoming CO2M mission are increasingly being used to detect greenhouse gas emissions from human activities and natural sources, offering a global view that is essential for tracking climate trends and implementing effective climate policies. These capabilities support the goals of the Paris Agreement, especially the five-yearly Global Stocktake, which reviews global progress and raises collective ambition to mitigate climate change. ²⁸⁾

- Earth observation technology also plays a key role in improving national emissions reporting, particularly in developing countries with limited in-situ measurement capabilities. ESA is pioneering new techniques, such as inverse atmospheric modeling through the RECCAP-2 project, to refine estimates of carbon surface fluxes between the atmosphere, land, and oceans. Satellites provide crucial data for developing climate adaptation strategies at the local level, such as tracking urban greening efforts to mitigate heatwaves. However, more work is needed to co-develop adaptation indicators with stakeholders to increase resilience to climate impacts.

• March 21, 2022: Data from the TROPOMI instrument on the Copernicus Sentinel-5P satellite has been used to detect methane plumes over some of Europe's largest methane-emitting coal mines, particularly in Poland’s Upper Silesian Coal Basin. Methane, a potent greenhouse gas, is released both naturally and through human activities like fossil fuel production. Between 2018 and 2020, satellite observations revealed significant methane concentrations from underground coal mines in this region, with enhancements around 20 parts per billion above background levels. These findings align with emissions reported by the European Commission and highlight the role of coal mining in contributing to global methane emissions. Efforts to monitor and manage these emissions, including the use of high-resolution sensors like GHGSat, are critical to curbing methane output, as reducing these emissions could have an immediate impact on mitigating climate change. ²⁹⁾

• January 20, 2022: The Hunga-Tonga-Hunga-Ha'apai volcano erupted with immense force on January 15, 2022, marking the largest eruption in 30 years. The eruption triggered tsunami waves across the Pacific and caused sonic booms that were heard as far away as Alaska. It spewed ash, gas, and steam 30 km into the atmosphere, blanketing Tonga in hazardous ash and causing a major disaster. The eruption's impact reached global levels, with Copernicus Sentinel-5P satellite data showing a vast plume of sulphur dioxide spreading as far as Australia, over 7000 km from the eruption site, highlighting the far-reaching atmospheric effects. ³⁰⁾

• November 10, 2021: High-resolution satellites have detected significant methane emissions from two adjacent landfill sites near Madrid, Spain, emitting 8800 kg of methane per hour in August 2021, the highest observed in Europe. Using data from Copernicus Sentinel-5P and GHGSat, researchers identified the methane plumes, with one site alone releasing nearly 5000 kg per hour. These emissions highlight the broader issue of methane from landfills, which account for a significant portion of anthropogenic greenhouse gases. While EU regulations require landfill operators to capture methane for energy or flaring, consistent monitoring remains a challenge. GHGSat and SRON's collaboration aims to address this through advanced satellite technology, helping authorities mitigate methane emissions and track climate progress. ³¹⁾

• September 30, 2021: Australia's 2019-2020 bushfire season was unprecedented, burning 18.6 million hectares and resulting in over 715 million tons of carbon dioxide emissions, significantly more than previous estimates of 275 million tons based on fire emission inventory datasets. Two studies published in *Nature* utilized satellite data from the Tropomi instrument on the Copernicus Sentinel-5P satellite to accurately measure carbon monoxide levels as a proxy for carbon dioxide emissions. This analysis revealed that the fires released nearly double the previously reported carbon dioxide and exceeded Australia's typical annual emissions from bushfires and fossil fuels by 80%. The findings highlight the devastating impact of the fires on both the environment and air quality, emphasizing the crucial role of satellite data in understanding the complexities of climate change. ^{32) 33)}

• September 16, 2021: The Montreal Protocol, established in the late 1980s, successfully phased out ozone-depleting substances, leading to a gradual recovery of the ozone layer, which protects Earth from harmful ultraviolet radiation. The Copernicus Sentinel-5P satellite has played a crucial role in monitoring the ozone layer, providing timely data on ozone levels and the annual development of the ozone hole, which typically reaches its maximum size between mid-September and mid-October. This year's ozone hole over Antarctica was reported to be around 23 million square kilometers, larger than 75% of comparable holes since 1979, driven by a stable polar vortex that maintained cold temperatures. Future satellite missions, like the upcoming Altius mission in 2025, will enhance our understanding of ozone recovery by providing vertical profiles of ozone and other trace gases, supporting ongoing efforts to monitor atmospheric health and long-term climate trends. ³⁴⁾

• March 15, 2021: In early 2020, satellite data revealed a significant decline in air pollution, particularly nitrogen dioxide levels, coinciding with global lockdowns aimed at curbing the spread of COVID-19. The first lockdown in China set off similar measures worldwide, leading to reduced industrial activity and traffic, which were reflected in decreased air pollutants detected by the Copernicus Sentinel-5P satellite's TROPOMI instrument. However, by February 2021, as restrictions eased, nitrogen dioxide levels rebounded to pre-COVID figures in major cities like Beijing and Chongqing, with increases expected to continue in Europe and beyond. Claus Zehner, the mission manager for Sentinel-5P, noted that while weather conditions also influence air quality, the resurgence of nitrogen dioxide is primarily linked to the resumption of human activities. The satellite's advanced capabilities enable detailed monitoring of various atmospheric trace gases, underscoring its role in assessing air quality and public health. ³⁵⁾

• March 4, 2021: For the first time, scientists have utilized satellite data from the Copernicus Sentinel missions to detect individual methane plumes leaking from natural gas pipelines globally. Methane, a potent greenhouse gas with a climate impact significantly greater than carbon dioxide, is primarily emitted by the energy sector, which released over 70 million tons in 2020. A European technology start-up, Kayrros, developed a tool to accurately track these emissions using data from Sentinel-5P and Sentinel-2 missions, identifying 13 significant methane events along the Yamal-Europe pipeline and 33 along the Brotherhood pipeline between 2019 and 2020. The number of detected emission events increased by 40% in Russia from 2019, despite reduced gas exports due to the COVID-19 pandemic. Many emissions occur during routine maintenance, which could be mitigated through alternative practices. Kayrros estimated that the venting observed during this period released methane equivalent to approximately 3 million tons of carbon dioxide, highlighting the need for improved operational practices to reduce greenhouse gas emissions. The collaboration showcases how advanced satellite technology can provide critical insights for climate action. ³⁶⁾

• December 14, 2020: Scientists have successfully utilized satellite data from the Copernicus Sentinel-2 mission in conjunction with the Sentinel-5P satellite to detect individual methane emissions from space, a significant advancement in tracking this potent greenhouse gas. Methane, while less abundant than carbon dioxide, is far more effective at trapping heat and primarily originates from sources like landfills, livestock farming, and the fossil fuel industry. Previously, attributing methane emissions to their sources was challenging due to limitations in spatial resolution. However, a European technology start-up, Kayrros, developed a tool that combines data from both Sentinel satellites, enabling accurate detection and quantification of methane leaks. This capability was demonstrated in September 2020 when a leak in the Permian Basin was identified, revealing that it had begun on July 4 and emitted between five and 20 tonnes of methane per hour over its duration. This innovative approach marks a major breakthrough for methane monitoring and aligns with the European Commission's Methane strategy, facilitating ongoing surveillance of oil and gas production areas worldwide. ³⁷⁾

• November 9, 2020: For the first time, scientists have successfully utilized data from the Copernicus Sentinel-5P satellite to detect nitrogen dioxide emissions from individual ships from space, a significant advancement for monitoring maritime pollution. Maritime transport significantly impacts air quality in coastal cities, with shipping emissions estimated to cause around 400,000 premature deaths and 14 million childhood asthma cases annually. Efforts to regulate these emissions have intensified, including a global reduction in sulphur dioxide content in ship fuels since January 2020. Previously, satellite measurements required aggregation over long periods, limiting their utility for regulatory enforcement. However, an international research team has discovered patterns in ‘sun glint’ data—reflected sunlight off the ocean surface—allowing for the identification and attribution of emissions from individual ships. By combining these observations with ship location data, researchers were able to match emission plumes to specific vessels, particularly larger ships. This breakthrough opens new avenues for monitoring ship emissions and supporting environmental regulations, with future satellite missions expected to enhance the detection capabilities further. ³⁸⁾

• October 19, 2020: This year's ozone hole over Antarctica has reached its maximum size of approximately 25 million km², as measured by the Copernicus Sentinel-5P satellite, making it one of the largest and deepest in recent years. The ozone hole typically grows from August to October, with maximum size occurring between mid-September and mid-October; however, this year's hole is notable for its rapid expansion and record-low ozone values, reaching close to 100 Dobson Units. The variability in size is influenced by strong wind bands around Antarctica, which can isolate polar air masses and enhance cooling. While the ozone hole phenomenon does not directly indicate global ozone levels, it underscores the impact of chlorofluorocarbon emissions in previous decades, leading to the Montreal Protocol's establishment in 1987, which has contributed to the recovery of the ozone layer. Scientists predict that with continued reductions in ozone-depleting substances, the global ozone layer may return to its normal state by around 2050. The Sentinel-5P satellite, launched in 2017, plays a crucial role in monitoring atmospheric gases and pollutants, enhancing our understanding of ozone dynamics from space. ³⁹⁾

• September 18, 2020: Air pollution is a major environmental concern, contributing to one in eight deaths in Europe, as highlighted by a recent European Environment Agency (EEA) report. The Copernicus Sentinel-5P satellite has been instrumental in monitoring nitrogen dioxide concentrations across the continent, particularly during the COVID-19 lockdowns, which resulted in a significant reduction of air pollution—by 40-50% in southern Europe during the initial lockdowns. Research by scientists from the Royal Netherlands Meteorological Institute (KNMI) and the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) indicates that while the impact of meteorology must be considered, the satellite data aligns well with ground-based measurements, enhancing the understanding of air quality changes. Notably, nitrogen dioxide levels dropped significantly in industrialized regions during lockdowns, primarily due to reduced traffic and industrial activities, though they returned closer to pre-pandemic levels by mid-2020. The Sentinel-5P satellite, equipped with the advanced TROPOMI instrument, provides accurate and high-resolution data on air pollutants, underscoring its role in ongoing air quality assessments in Europe. ⁴⁰⁾

• September 17, 2020: The Copernicus Sentinel-5P satellite's high revisit rate, combined with GHGSat's high-resolution commercial imagery, is poised to aid landfill operators and regulators in reducing methane emissions, a significant greenhouse gas of growing concern. As the EU prepares to release a Methane Strategy targeting uncontrolled emissions from landfills, accurate measurements of methane concentrations are critical for identifying sources and implementing mitigation strategies. Collaborating with Benito Roggio Ambiental (BRA) and the Netherlands Institute for Space Research (SRON), scientists are leveraging Sentinel-5P data to guide GHGSat's high-resolution satellites, including the recently launched Iris, to measure emissions from individual waste management facilities. This collaboration aims to enhance the monitoring of methane emissions globally, enabling cost-effective inspections, emissions screening, and gas recovery opportunities. As the project progresses, BRA looks forward to integrating GHGSat's data into their operations to improve landfill gas management, while ESA supports research by making a portion of Iris's data available for studies related to waste management and other applications. ⁴¹⁾

• July 29, 2020: Monitoring methane emissions is crucial for combating climate change due to its significantly higher global warming potential compared to carbon dioxide. GHGSat, leveraging data from the Copernicus Sentinel-5P satellite, has developed its Claire satellite to map methane hotspots and has collected over 60,000 measurements of industrial facilities worldwide. While Sentinel-5P provides broad atmospheric data, GHGSat's Claire can pinpoint methane emissions at specific sites, helping industries like oil and gas, waste management, and agriculture address leaks and implement corrective actions. The Oil & Gas Climate Initiative (OGCI) has recognized the effectiveness of GHGSat's monitoring capabilities, investing in the initiative. GHGSat plans to expand its fleet with a constellation of satellites, including the upcoming Iris satellite, which will improve resolution to 25 meters, enhancing leak detection accuracy. As demand for analytics services grows, GHGSat aims to provide tailored solutions for asset managers focused on environmental, social, and governance (ESG) factors, while a partnership with ESA and the Canadian Space Agency will ensure that a portion of Iris's data is available for research purposes. ⁴²⁾

• July 9, 2020: Every summer, the Sahara Desert sends vast amounts of dust across the Atlantic Ocean, forming the Saharan Air Layer, which peaks between late June and mid-August. This year, a particularly large and dense dust plume, referred to as ‘Godzilla,’ has been observed using data from Copernicus Sentinel satellites and ESA's Aeolus satellite. The plume is noted to be 60–70% dustier than average, making it the dustiest event recorded in two decades. Aeolus provides crucial information about the altitude and vertical extent of the dust layer, which aids air quality models in predicting its movement and effects. While this dust poses health risks and can cause air quality alerts, it also serves important ecological functions, such as supplying essential nutrients for phytoplankton and replenishing rainforest soils in the Amazon. Furthermore, the dry air from the dust layer can inhibit the development of hurricanes by disrupting the warm, humid conditions necessary for their formation. ⁴³⁾

• July 1, 2020: Between April 2019 and April 2020, concentrations of sulfur dioxide in polluted areas of India decreased by around 40%, as shown by new maps produced using data from the Copernicus Sentinel-5P satellite. This decline is largely attributed to reduced human and industrial activity during the COVID-19 lockdown, which began on March 25, 2020. Historically, India has been the world’s largest emitter of anthropogenic sulfur dioxide, mainly from fossil fuel power plants, contributing to significant air pollution and health issues. The analysis utilized data from the TROPOMI instrument on the Sentinel-5P satellite, aided by an improved algorithm from the Royal Belgian Institute for Space Aeronomy, enhancing sensitivity and accuracy in detecting emissions. The results indicate substantial drops in sulfur dioxide levels over major cities like New Delhi and near large coal-fired power plants, although some plants in northeastern states continued operations. This development promises to improve the monitoring of sulfur dioxide emissions and support efforts to verify existing emission inventories. ⁴⁴⁾

• June 11, 2020: A new online platform has been launched for tracking global air pollution, utilizing data from the Copernicus Sentinel-5P satellite to display averaged nitrogen dioxide concentrations through a 14-day moving average. This platform allows users to visualize changes in nitrogen dioxide levels over time and zoom into specific cities or regions, particularly across Europe. Nitrogen dioxide, a pollutant from power plants and vehicles, poses health risks, making such monitoring essential. The Sentinel-5P satellite, equipped with the advanced TROPOMI instrument, provides high-resolution images of atmospheric gases, facilitating accurate air quality assessments. The mapping service is part of the Sentinel-5P Product Algorithm Laboratory (S5P-PAL), funded by the European Commission, and is designed for efficient product development and global mapping. Additional functionalities, including tracking carbon monoxide and enabling time-series analysis, are in the works, with the platform also contributing to the 'Rapid Action on Coronavirus and Earth observation' (RACE) dashboard to monitor the environmental impact of COVID-19 lockdowns. ⁴⁵⁾

• June 1, 2020: GHGSat, a Canadian company, has partnered with the Sentinel-5P team at SRON Netherlands Institute for Space Research to monitor methane emissions during the COVID-19 pandemic, highlighting methane's potency as a greenhouse gas, being about 30 times more effective at trapping heat than carbon dioxide. Utilizing data from Sentinel-5P to detect global emissions and GHGSat satellites to quantify them at specific facilities, the collaboration has uncovered new methane hotspots, such as those over a coal mine in China and the Permian Basin in the U.S. Despite a reported increase in methane concentrations in 2020 compared to 2019, the results remain inconclusive due to limited observations from Sentinel-5P. The teams continue to analyze how COVID-19 impacts emissions, emphasizing the need for high-resolution measurements for accurate facility attribution. GHGSat plans to launch additional satellites to enhance monitoring capabilities, and ESA, in collaboration with the Canadian Space Agency, is offering 5% of the upcoming Iris satellite's measurement capacity for scientific research. The Sentinel-5P satellite, equipped with the TROPOMI instrument, also maps other air pollutants, further contributing to the understanding of air quality. ⁴⁶⁾

• May 4, 2020: A new tool to combat climate change, developed by the European technology start-up Kayrros, utilizes data from the Copernicus Sentinel-5P satellite to monitor and attribute methane emissions globally. Methane, the second most significant greenhouse gas, is currently increasing in the atmosphere at approximately 1% per year and has a greater heat-absorbing capability than carbon dioxide. By integrating Sentinel-5P data with ground sensor data, position tracking, and social media information, Kayrros can accurately identify the location, potency, and size of methane leaks worldwide, revealing around 100 high-volume leaks at any given time, with about half originating from oil, gas, and heavy industrial activities. This technology, highlighted in the International Energy Agency's recent Methane Tracker, enables real-time detection of emissions, offering a substantial improvement over previous engineering estimates and potentially transforming climate policy by allowing governments to establish accurate baselines for methane emissions and make informed decisions regarding energy policies. ⁴⁹⁾

• April 25, 2020: A recent study published in *Science Advances* has revealed that oil and gas operations in the U.S. Permian Basin are emitting methane at twice the rate found in 11 other major oil and gas regions, as analyzed by scientists from the Environmental Defense Fund (EDF), Harvard University, Georgia Tech, and the SRON Netherlands Institute for Space Research. Utilizing 11 months of satellite data from the European Space Agency's TROPOMI instrument, the research covered 200,000 individual readings across the 160,000 km² basin, revealing that methane losses from these operations account for 3.7% of their gas production, enough to supply 2 million U.S. households. Co-author Dr. Steven Hamburg emphasized the significant contribution of these emissions to climate change, nearly tripling the 20-year climate impact of the gas produced, and highlighted the growing capability of satellite technology in tracking emissions to provide essential data for regulators and companies to address this issue.  ^{50) 51)}

- The study also underscores the advantages of using satellites for methane measurement, as they can cover vast areas more quickly and frequently than conventional methods, offering insights into gas-producing regions that are otherwise inaccessible. The TROPOMI instrument, launched in 2017, provides higher precision and better coverage, contributing to an emerging ecosystem of methane-tracking satellites. The findings validate recent ground-based measurements by EDF's PermianMAP initiative, indicating methane escaping at a rate of 3.5% in the most productive part of the basin. High leakage rates in the Permian present opportunities for significant emissions reductions through improved infrastructure, operations, and regulation. As researchers work to automate data analysis processes, future projects like MethaneSAT are expected to deliver near-real-time data on methane emissions, enhancing the ability of stakeholders to track and compare progress in reducing emissions globally.

• April 24, 2020: Recent findings using data from the Copernicus Sentinel-5P satellite indicate that air quality in various Indian cities improved significantly during the COVID-19 lockdown, with nitrogen dioxide levels dropping by approximately 40–50%. Following the nationwide lockdown initiated on March 25, 2020, to curb the spread of the virus, essential services continued while all non-essential activities ceased, resulting in a 9.2% reduction in electricity consumption in March compared to the previous year. The satellite imagery revealed persistent high nitrogen dioxide levels in northeastern India, linked to coal-based power plants, with the Vindhyachal Super Thermal Power Station showing only a 15% reduction in emissions. Despite the lockdown, maritime traffic emissions remained consistent, highlighting the impact of human activity on air quality. As air pollution continues to pose serious health risks, the monitoring capabilities of the Copernicus Sentinel-5P satellite have become crucial for assessing environmental conditions and informing policies aimed at reducing pollution levels, especially in highly polluted areas like New Delhi. ⁵²⁾

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• April 16, 2020: Further analyses using data from the Copernicus Sentinel-5P satellite reveal a significant decline in nitrogen dioxide (NO2) concentrations across Europe, with some cities experiencing reductions of 45–50% compared to the same period last year due to COVID-19 lockdown measures. Scientists from the Royal Netherlands Meteorological Institute (KNMI) emphasize that variations in weather can significantly affect NO2 levels, making it essential to analyze data over extended periods to account for anomalies caused by daily fluctuations. Henk Eskes of KNMI noted that averaging data monthly helps clarify changes linked to human activity, although a 15% uncertainty reflects weather variability. As lockdowns continue, the KNMI plans to conduct more detailed analyses across northern Europe, leveraging air-quality models and in situ data to refine estimates of pollution impacts. ⁵³⁾

• April 06, 2020: Scientists using data from the Copernicus Sentinel-5P satellite have detected a significant reduction in ozone concentrations over the Arctic, leading to the formation of a 'mini-hole' in the ozone layer due to unusual atmospheric conditions, including freezing temperatures in the stratosphere. While ozone depletion typically occurs at both poles during winter, the current depletion over the Arctic is notably larger than in previous years, with a maximum extension of less than 1 million km², compared to the Antarctic hole, which can reach up to 25 million km². The depletion is driven by extremely cold temperatures, sunlight, wind fields, and chlorofluorocarbons (CFCs), exacerbated this year by the polar vortex trapping cold air. Although ozone levels over the Arctic have decreased to levels considered typical for an ozone hole, the region is expected to recover by mid-April 2020. This event is unprecedented in the 25 years of ozone monitoring from space by the TROPOMI instrument, which will continue to play a crucial role in tracking air quality and global ozone distribution as part of the EU's Copernicus program. ⁵⁴⁾

• March 27, 2020: New data from the Copernicus Sentinel-5P satellite reveal significant reductions in nitrogen dioxide concentrations over major European cities, including Paris, Madrid, and Rome, coinciding with strict lockdown measures implemented to curb the spread of COVID-19. Scientists from the Royal Netherlands Meteorological Institute (KNMI) have been analyzing air pollution across Europe, finding that averaging data over ten days helps account for daily weather variability and highlights the impact of human activity on emissions. To better estimate changes in emissions from transportation and industry, the KNMI team is integrating TROPOMI satellite data with atmospheric chemistry models and ground measurements. ⁵⁵⁾

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• March 19, 2020: Recent data from the Copernicus Sentinel-5P satellite reveal a significant decline in nitrogen dioxide emissions across major Chinese cities, coinciding with the nationwide lockdown implemented in response to the COVID-19 outbreak. Following the outbreak in Hubei province in late December 2019, strict measures led to the closure of factories and reduced daily activities, resulting in a 40% reduction in nitrogen dioxide levels attributed to decreased industrial activity and traffic. Additionally, the Copernicus Atmosphere Monitoring Service reported a decrease of 20-30% in surface particulate matter in February 2020 compared to previous years. As China begins to ease restrictions and reopen schools and factories, ESA officials note that the Sentinel-5P satellite provides critical insights into air quality, serving as an indicator of industrial activity and highlighting the impact of the pandemic on pollution levels. Detailed scientific analyses are underway to further quantify these observations. ⁵⁶⁾

• March 2, 2020: NASA and ESA pollution monitoring satellites have reported substantial reductions in nitrogen dioxide (NO2) levels over China, primarily linked to the economic slowdown following the COVID-19 outbreak. The decline was first observed near Wuhan after the city's transportation shutdown on January 23, 2020, and later spread nationwide as quarantine measures were implemented, marking one of the largest quarantines in history. Fei Liu, an air quality researcher at NASA, noted that this dramatic drop is unprecedented compared to past events, such as the gradual decrease observed during the 2008 economic recession or localized reductions during the Beijing Olympics. Although the Lunar New Year celebrations in late January typically result in temporary pollution decreases, NASA's preliminary analysis shows that NO2 levels in eastern and central China in 2020 were 10-30% lower than the average from 2005 to 2019, with no rebound following the holiday. This significant and sustained reduction is attributed to widespread measures aimed at minimising the spread of the virus. ⁵⁸⁾

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Sensor Complement

The Sentinel-5P mission is an atmospheric chemistry mission, providing measurements at high temporal and spatial resolution. Its payload, the TROPOMI (Tropospheric Monitoring Instrument), is being supplied as a national contribution to the GMES program by the Netherlands. The TROPOMI instrument design is of SCIAMACHY and OMI heritage; Dutch institutions provided major contributions in the development of these instruments.

OMI was launched in 2004 on NASA's Aura spacecraft and SCIAMACHY in 2002 on ESA's Envisat mission. Both instruments are very successful. Since OMI started observing the atmosphere, its service has never been interrupted. SCIAMACHY, OMI and TROPOMI are passive sun backscatter spectrographs using the ultraviolet-to-SWIR wavelengths. SCIAMACHY uses a scanning concept and linear detector arrays, and covers almost the entire Solar irradiance spectrum from 240 to 2400 nm. OMI is scaled down in terms of wavelength range (270 –500 nm) but uses a staring pushbroom concept. This concept measures all ground pixels in the swath simultaneously and therefore allows a much improved spatial resolution. ^{61) 62) 63)}

TROPOMI takes the best of the two by combining the large wavelength range of SCIAMACHY (albeit with some gaps) and with OMI's staring concept. The full advantage of staring concept is taken by reducing the ground pixel size to 7 x 7 km² and on top of that making the instrument suitable for very dark scenes (albedo 2 – 5 %). This means that the instrument etendue is improved by more than an order of magnitude. This allows for unprecedented observations of sources and sinks of air quality, and climate related gases and aerosols. The spatial resolution results in a high fraction of cloud-free observations and is combined with a wide swath of 104° (about 2600 km on ground) to allow daily coverage of the complete Earth with sub-city resolution, as illustrated by Figure 52.

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The basic TROPOMI applications are:

• Monitoring changes in the atmospheric composition (e.g. ozone (O₃), nitrogen dioxide (NO₂), sulphur dioxide (SO₂), carbon monoxide (CO), methane (CH₄), formaldehyde (CH₂O), and the properties of aerosols and clouds at high temporal (daily) resolution.

• Troposphere variability.

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TROPOMI (Tropospheric Monitoring Instrument)

TROPOMI is an advanced nadir-viewing imaging absorption spectrometer, a DOAS (Differential Optical Absorption Spectrometer) instrument, to provide data on atmospheric trace gases and aerosols impacting air quality and climate. The instrument is being co-funded by the Dutch Ministry of Economic Affairs and ESA. ESA signed an agreement with the Netherlands in July 2009; the instrument development is led by Dutch Space, Leiden, The Netherlands, as prime contractor (Ref. 1). ^{64) 65)}

TROPOMI is a collaboration between KNMI (Royal Netherlands Meteorological Institute), SRON (Space Research Organization Netherlands), TNO (Netherlands Organization for Applied Scientific Research), and Dutch Space, on behalf of NSO (Netherlands Space Office). KNMI (PI) and SRON (co-PI) are responsible for the scientific management and the data products of the project. Dutch Space is the principal contractor for the construction of the instrument. The TROPOMI development is jointly funded by NSO and ESA; both agencies cover the programmatic aspects of TROPOMI. ^{66) 67) 68)}

NSO responsibility is the development, procurement, calibration, in-orbit commissioning of TROPOMI, and the generation of Level-1B data. ESA is responsible of the procurement of the satellite, the ground segment, the launch and in-orbit commissioning. The implementation of the Sentinel-5P mission is performed by a ESA/NSO Joint Project Team (JPT).

The instrument development passed its Instrument-PDR review (IPDR) in May 2011. The IPDR was conducted as a top down review and subsystem PDRs followed in the remainder of 2011. ⁶⁹⁾

In May 2014, the TROPOMI instrument for ESA's Sentinel 5-precursor satellite, is in its final stages of integration now and many hardware results are becoming available. This concerns at this moment the SWIR, UVIS and NIR spectrometers and all flight detectors and soon the remaining UV spectrometer. ⁷⁰⁾

Background

TROPOMI is the most recent in a series of UV-VIS-NIR-SWIR sun backscatter hyper spectral instruments that measure the atmospheric composition. These instruments measure with or without polarization the combination of Earth and sun spectra. From their ratio, the reflectance spectra, absorptions taking place in the Earth atmosphere are derived. Concentrations of trace gases can be determined because these gases have very specific wavelength-dependent absorption features (Figure 54). Other products, like aerosols, clouds and surface properties, have broader absorption structures and can be derived after accurate radiometric calibration (Ref. 68).

After NASA's EOS-Aura satellite, carrying the OMI instrument, and ESA's Envisat satellite, carrying the SCIAMACHY instrument, no other instrumentation was planned in space with comparable capabilities as OMI and SCIAMACHY until the launch of the GMES Sentinel 5 mission in 2020. - This means that from ultimately 2014 onward, a data gap will exist in measuring the troposphere from space. The GOME-2 and IASI instruments on MetOp will not be able to cover this gap, due to their limited spatial resolution and lack of CH₄ and CO measurements with good sensitivity down to the Earth's surface. For these reasons, the TROPOMI instrument has been defined as the successor of OMI and SCIAMACHY and bridge the time period from 2015 on Sentinel-5P until the GMES instrumentation on Sentinel-5. ⁷¹⁾

The TROPOMI mission objective is to measure the troposphere for scientific research, and in support of services to society, down to the Earth's surface, with sufficiently high spatio-temporal resolution to quantify anthropogenic and natural emissions and atmospheric life cycles of trace gases (O₃, CO, HCHO, and SO₂) and two major greenhouse gases (tropospheric O₃ and methane (CH₄)). In addition, aerosol particles will be monitored, which impact on air quality and climate forcing from the regional to the global scale. ^{72) 73)}

Derived from the overall mission objective, the TROPOMI science objectives are:

• To better constrain the strength, evolution, and spatio-temporal variability of the sources of trace gases and aerosols impacting air quality and climate.

• To improve upon the attribution of climate forcing by a better understanding of the processes controlling the lifetime and distribution of methane, tropospheric ozone, and aerosols.

• To better estimate long-term trends in the troposphere, related to air quality and climate from the regional to the global scale, and provide boundary conditions for assessing local and regional air quality.

• To develop and improve air quality model processes and data assimilation in support of operational services, including air quality forecasting and protocol monitoring.

Besides filling the gap, TROPOMI combines the strengths of SCIAMACHY, OMI, and state of the art technology to provide observations with performances that cannot be met with today's instruments in space. Performance of current in-orbit instruments will be surpassed in terms of sensitivity, spectral resolution, spatial resolution and temporal resolution. However, TROPOMI will observe a smaller part of the spectrum compared with SCIAMACHY as is shown in Figure 54.

Programmatic aspects: The schedule of the Sentinel-5P program is very compact with respect to similar traditional programs. To achieve this schedule and to reduce costs of the instrument development, measures are taken in the development process of TROPOMI. These measures are, amongst others: reducing the number of requirements, reducing the number of documents to be generated, using ECSS's as guidelines rather than applicable documents, an efficient decision making process and applying the LightSat approach defined by ESA. The LightSat approach relaxes product assurance requirements and allows higher risk in some areas. On the satellite level, parallel procurement of the spacecraft platform and the instrument is applied to achieve the compact schedule. This requires flexibility in the development processes of the instrument and the spacecraft.

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The development of the TROPOMI instrument was started long before the development of the spacecraft platform was selected. Of course, this is normal procedure for a complex payload that needs to be designed compared to the spacecraft platform with a standard bus architecture. This is one of the reasons that the spacecraft selection took place in a late stage of the development of TROPOMI (Ref. 61).

The CDR (Critical Dign Review) of the TROPOMI instrument is planned for end of 2012, early 2013. ⁷⁴⁾

Instrument: The TROPOMI instrument is a pushbroom type imaging spectrometer (use of 2D detector technology) that covers a spectral range from ultraviolet to visible and selected bands in near-infrared, referred to as UVN (UV-VIS-NIR), and SWIR (Short Wave Infrared) around 2.3 µm. The relevant subsystems are: ^{75) 76) 77) 78) 79) 80)}

• Instrument Telescope

• Instrument Calibration Unit

• UVN spectrometer, funded by NSO

• SWIR spectrometer

• ICU (Instrument Control Unit)

• TSS (Telescope Support Structure)

• RC (Radiant Cooler)

• GSE (Ground Support Equipment), funded by ESA.

The instrument is mounted on a TSS (Telescope Support Structure) which in turn in mounted onto the spacecraft (S/C). A passive thermal radiator is used to reject heat from the system.

The main building blocks of TROPOMI are the following (Figure 55):

• UVN (UV-VIS-NIR) module, with telescope, three spectrograph bands and the calibration module

• SWIR module, with SWIR spectrograph

• ICU, the control unit and electrical spacecraft interface

• Thermal radiator, the passive detector and SWIR module cooler

Part of the spectrographs are the detector modules with 2-dimensional detector (CCD for the UVN and CMOS for the SWIR).

The UVN module consists of the telescope – which is shared by the UVN and the SWIR – and the 3 UVN spectrometer channels (UV, UVIS and NIR) each equipped with individual detector units. The telescope has a very wide FOV of 108º. A polarization scrambler is placed in the optical path to make the measurements insensitive to the polarization state of the incoming light. The light from the telescope is separated in the flight direction by a reflective slit. This means that the UV and SWIR channels will see a slightly shifted part of the Earth than the UVIS and NIR channels.

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Legend to Figure 56: The so-called spatial smile is caused by off-axis mirrors in the telescope. The NIR (and UVIS) channel use a common slit, while the SWIR (and UV1) channels are in-field separated by ~1º in the flight direction.

CU (Calibration Unit): The CU includes the following:

• Two sun diffusers; one for regular use, one as a backup

• WLS (White Light Source); PRNU (Photo Response Non-Uniformity) calibration and on-ground health checks

• A LED (Light-Emitting Diode) to monitor the short term variation in the output of the WLS

• For the SWIR channel, a number of laser diodes are placed in the CU, in order to be able to monitor the instrument spectral response function.

Besides the sun, a WLS, SLS (Spectral Light Source), common LEDs and channel specific LEDs are used for calibration purposes in eclipse. The WLS which is implemented using a halogen light bulb, since it provides a broad spectral range. The LEDs, positioned close to the WLS, are used to analyze the small WLS degradation. The WLS and the LEDs calibration light will pass the spectrograph. Therefore, channel LEDs are positioned close to the detectors that are used to be able to distinguish the degradations of the optical components and the detectors. The fifth calibration source is the SLS that is implemented using temperature-controlled Laser Diodes. This calibration source is located in the CU and is solely used for in flight calibration of the SWIR channel. The Laser Diodes have a very narrow spectrum that will be shifted by varying the temperature of the Laser Diodes.

The general instrument layout is shown in Figures 62 and 63. The UVN (UV-VIS-NIR) module contains the UVN spectrometer bands, the telescope and the calibration unit. The UVN module is accommodated on the UVN-OBM (Optical Bench Module). The SWIR spectrograph has its own module for thermal reasons. Since a shared telescope is used, the light for the SWIR channel is guided by relay optics in the UVN-OBM to the SWIR module.

All detectors are optimized for the light that they will detect. The UVIS and NIR detectors have a graded anti-reflective coating, in order to reduce stray light and decrease interference effects in the silicon. The SWIR optics and detector need to be cooled down to ~200 K and 140 K, respectively, to achieve the required performance. The UVN detectors operate at 210 K and 220 K, and the UVN-OBM is maintained at room temperature. The two-stage RC (Radiant Cooler) enables cooling of the optical and electrical components. Thermal busses consisting of heat pipes and flex links form the thermal interfaces with the radiant cooler. The radiant cooler is equipped with a large door that blocks irradiance from Earth (not shown in the figures). This cooler door must be stowed to fit inside the launcher fairing. Once Sentinel-5P is in orbit, the cooler door will be opened after one month. This delay prevents that the cooler areas will be contaminated by outgassing particles from the spacecraft and other instrument units by keeping these cooler surfaces warm.

The UVN module is developed by Dutch Space and TNO. The SWIR module is the cooled module containing the SWIR spectrograph and is developed by SSTL in the UK. The multilayer optical coatings are developed at CILAS Etablissement de Marseille (France).

SWIR Detector/FEE Subsystem

The SWIR detector is from Sofradir (France), it is controlled by FEE (Front-End Electronics) developed by SRON. The SWIR spectrum is projected onto a Sofradir-developed 2D detector array consisting of 1000 spectral pixels and 256 spatial pixels on a 30 µm pitch, the Saturn geometry. The detector consists of an HgCdTe-based photo-sensitive layer on top of an CdZnTe substrate with a protective layer, also acting as a reflection-limiting layer. This stack is hybridized by indium bump bonds to a silicon ROIC (Read-Out Integrated Circuit) where the signal charge is converted into a voltage by CTIA (Capacitive Trans-impedance Amplifiers). Upon read out, the signal of each individual pixel is clocked simultaneously onto a sample-and-hold circuit. Signals are subsequently clocked onto 4 parallel video output lines and amplified and digitized by the FEE (Ref. 70).

The SWIR detector is mounted in a hermetically-closed package with an anti-reflection coated silicon window. The detector package is mounted onto a molybdenum base plate, attached to an aluminum cold finger providing the connection to the cold-stage radiator (Figure 57). The operational temperature of the SWIR detector is 140 K. The cold detector is connected to the 200 K SWIR spectrometer by a thin-walled titanium double cone. This construction is very stable, closed for EMI (Electro-Magnetic Interference) and provides thermal insulation. An anti-reflection coated silicon window just before the SWIR detector prevents any water vapor from freezing onto the cold detector window. The small volume around the detector is vented though the cold finger and through a PTFE (PolyTetraFluoroEthylene) tube to space. This prevents any water vapor from reaching the cold detector from this side. The measures to prevent water from reaching the detector are based on lessons learned from the EnviSat/Sciamachy SWIR channels.

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All electrical signals between the cold detector and the warm front-end electronics are carried by a multi-layer kapton-copper flex link. Both outer surfaces of the flex link contain a copper mesh to provide protection against EMI.

The room-temperature FEE powers and commands the SWIR detector. It amplifies the four analog video-output signals of the detector and provides digitization by using four 14 bit ADCs (Maxwell 9240LP, with the AD9240 chip of Analog Devices inside). Digital detector data and house-keeping data are relayed to the ICU (Instrument Control Unit) using the ChannelLink protocol. All communication with the ICU as well as all coordination inside the FEE is performed by a RTAX2000S FPGA.

The SWIR detector is read with a pixel speed of 800 kHz, leading to a frame-read time of 82 ms, and a maximum frame rate of 12 Hz. This is much lower than the detector capabilities, but sufficient for the application. The FEE controls the detector temperature by reading the two internal T-sensors of the Saturn detector and a PID algorithm in the FPGA. Thin-film heaters are mounted at the back of the molybdenum plate. A typical detector temperature stability obtained is 6 mK rms.

The combination of the TROPOMI SWIR detector and its FEE has been thoroughly characterized. Only the memory between two consecutive readings is with 1.6 - 1.9 % out of spec, leading to a data correction. Where this correction is not complete an effective smear of information from one ground pixel to the next has to be accepted. The detector performance on sensitivity, dark current, noise, and their uniformities are better than specified, as well as the linearity and the number of dead pixels. Overall, the performance of the SWIR detector and its FEE is considered very good for the TROPOMI application.

SWIR module: The SWIR module on TROPOMI, designed and developed by SSTL, is a pushbroom grating spectrometer operating between 2305-2385 nm. The system has a spatial resolution of approximately 7 x 7 km² at nadir and covers a swath of 2670 km. The SWIR module will be used to measure the concentration of methane and carbon monoxide in the Earth's atmosphere providing global daily coverage from a sun-synchronous orbit.

The optical design consists of a telescope, slit, collimator, immersed grating, anamorphic prism and an imaging lens. The SWIR telescope forms an image of ground on the SWIR Spectrometer slit. The slit acts as a spatial filter selecting a strip of ground as an input to the rest of the spectrometer. A collimator takes light from the slit and creates collimated beams as an input to the immersed grating. The immersed grating disperses the collimated beams in the along track direction and the anamorphic prism and imaging lens form a dispersed image of the slit on the detector. A band pass filter is manufactured as part of the slit prism and restricts light that can reach the detector from outside the operational waveband.

The SWIR immersed grating is supplied by SRON and the SWIR detector is supplied by Sofradir. The SWIR optics are mounted in housings off a common optical bench. The optical bench is mounted off the TROPOMI telescope support structure with bipods; the bipods form a pseudoisostatic mount. The optical bench is a two piece aluminum alloy sandwich plate. Optical mounts are mounted on both sides of the optical bench. The optical bench provides a stiff framework and attachment points for mounts to be structurally connected. Aluminum alloy is selected in order to minimize thermal gradients. The optical housings are manufactured in titanium and flexure supports are used to accommodate for the difference in CTE (Coefficient of Thermal Expansion) between the housings and the optical bench. The assembled SWIR module is shown in Figure 58.

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The ICU is the main electronics unit including clock sequencers for detector readout and is developed by RUAG in Sweden; the TSS (Astrium Germany) is the structure carrying the UVN with telescope and the SWIR modules.

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Telescope design: TROPOMI is a pushbroom instrument imaging a very wide field of view on Earth on a rectangular slit. The slit is relayed to four spectrometers for four different channels (UV, UVIS, NIR, and SWIR). In one direction, the spatial information is resolved over the long direction of the slit. This direction is referred to as the swath direction. In the other direction, spatial information is resolved by ‘sweeping' over the Earth surface. This is the flight direction. ⁸²⁾

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Design with freeform mirrors: The telescope is shown in Figure 60. Light from Earth passes the entrance pupil and is reflected by a concave primary mirror that forms an intermediate focus. The intermediate focus is imaged by a second concave mirror on the spectrometer entrance slit. The entrance pupil is imaged in the focal plane of the second mirror, where the physical pupil stop is located. This has the advantage that the light beams leave the telescope nearly parallel (i.e. the image is telecentric), which eases the design of the spectrometers, keeping the dimensions small. Near the pupil stop, a polarization scrambler is placed to make the telescope polarization independent. In the vicinity of the intermediate focus, two field limiting apertures are present: one limits the field in the swath direction and functions as an actual field limiter. The other limits the field in the perpendicular direction (referred to as flight direction). The latter functions as a baffling aperture.

The telescope is an almost perfect f-q system: the angle in the swath direction in the entrance pupil depends linearly on the position in the slit. In the flight direction, the angle in the entrance pupil depends quadratically on the position in the slit. This latter effect is called ‘smile', after the shape of the field of view in the entrance pupil.

Diamond turning: The freeform mirrors cannot be manufactured using conventional tools. They are both non-spherical and have no axis of symmetry. The sag of the non-rotational symmetric terms varies on the order of 1 mm. In addition to the requirements on resolution, which dictates the form and the tolerances on the surface shape, other issues to deal with are throughput and stray light, dictating requirements on reflectivity and roughness, respectively.

Measurement: Suitable absolute metrology is a key ingredient in the freeform production chain. Specifically for freeform measurement, the project developed a unique absolute metrology tool called NANOMEFOS (Non-contact Measurement Machine for aspheric and Freeform Optics) ⁸³⁾ that has the capability of non-contact measuring surfaces with an uncertainty of better than 15 nm rms. It is fast, universal, and can accommodate large work pieces. Typical sampling speed is as high as several tens of thousands of points per minute. Its measurement volume is ∅ 500 mm x 100 mm. The NANOMEFOS machine scans the surface with an optical probe, and therefore has variable point spacing. The sampling point distance is in practice limited by the measurement time. For measuring form, ~0.1 to 1 mm is usually applied, but also line scans with mm point spacing can be applied, thus giving the possibility to perform measurements over a very large spatial frequency range.

The measurement concept resembles a giant CD-player (Fig. 6). The product is mounted on a spindle, rotating at typical speed of a few rpm around a q axis. As the product rotates, the non-contact optical distance probe moves in radial and vertical (RZ) direction. Mounted on a rotation axis Ψ, it is continuously being positioned perpendicular to the best fit (rotationally symmetric) aspheric fit of the product. The probe follows focus with an additional stage with a range of 5 mm. Thus, NANOMEFOS is able to measure any freeform surface with a that has up to 5 mm (PV) maximum deviation with respect to the best fit aspheric (convex or concave) surface.

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For TROPOMI a pushbroom telescope was designed that combines a very high resolution of better than 0.1° with a large FOV of 108° and a f/9 x f/10 aperture. Applying fully freeform surfaces, the telescope could be realized using no more than two mirrors. The improvement over predecessor OMI would not have been feasible without the freeform design.

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Legend to Figure 62: The SWIR module is located at the right and the UVN in the center; the telescope with the 108º wide FOV (Field-of View) angle is in front as well as the sun viewing port.

Figure 62 shows how the UVN module and telescope and the SWIR module are mounted on a common base plate. The telescope and UVN module have a common structure and the light from the telescope is fed into the SWIR module via relay optics.

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All four TROPOMI detectors have their own read-out and control modules that have the functions of detector readout, analog-to-digital conversion, and detector thermal control. The UVN detectors are back-illuminated CCD detectors read out by Detector Modules (UVN-DEMs) that share the same design. The SWIR channel employs a CMOS detector and has a dedicated Front End Electronics module (SWIR-FEE) for detector readout and detector thermal control.

The UVN-DEMs and SWIR modules are all FPGA-based modules that are powered, controlled, and read out by the ICU (Instrument Control Unit) positioned inside the spacecraft. The ICU takes care of processing the data and forwarding the data to the spacecraft mass memory. The electrical interfaces used to transfer the science data are 140 Mbit/s Channel Link interfaces between the detectors modules and the ICU and an 80 Mbit/s SpaceWire interface between the ICU and spacecraft mass memory. - Besides controlling the detector modules, reading-out, processing and forwarding science data, the ICU provides other functions to the TROPOMI instrument. These functions are; providing thermal control to all instrument units, controlling the calibration light sources and mechanisms, and providing engineering data for instrument health status. ⁸⁴⁾

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The mass of the TROPOMI instrument is 220 kg. This is fairly low taking into account the instruments capability; this has been made possible by the SWIR design using a specially developed silicon immersed grating (Figure 65). The power consumption is: 170 W (average) and 382 W (max).

Achieving the performances listed in Table 5 in combination with the coregistration (the same viewing direction for all wavelengths of the given detector rows) and SNR (Signal-to-Noise Ratio) requirements, the alignment and thermal control of the optical elements are key requirements. The TSS (Telescope Support Structure) uses a 10 cm thick honeycomb slab with a 3 mm face-sheets base plate that ensures proper mounting and alignment of the units and serves as mechanical interface with the spacecraft.

Spectral characteristics: The spectral properties of each of the spectrometers are shown in Table 5. The spatial sampling is 7 km x 7 km with the exception of bands 1 and 6. Band 1 has a larger ground pixel to allow good SNR given the low radiances for these wavelengths. Band 6 is used to obtain the most important cloud products and is read at higher spatial resolution to have as good as possible coregistration of these cloud products and the other bands.

The UVIS and NIR bands make use of the same spectrograph slit whereas the UV and SWIR have separate slits. This allows having a wider slit for the UV to compensate for the lower radiance for these wavelengths and for the SWIR it allows to have the slit included in the cooled SWIR module. The different slits result in slightly different viewing angles in the flight direction.

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The DOAS (Differential Optical Absorption Spectrometer) radiometry of Earth's atmosphere trace gases is the new frontier of remote sensing. To achieve the required instrument performance it is necessary to have very stable spectrometers with high spectral resolution and high SNR. TROPOMI is paving the way not only for future high end instruments that will be embarked on the next ESA Sentinel 5 mission, but it is also the opportunity to build a solid technology platform to be used as stepping stone for future instruments. While high end applications as Sentinel 5 will further push the limit of the technology, a number of simpler and more affordable, but still meaningful, instruments could be designed using the technology platform developed for TROPOMI. The TROPOMI technology platform encompasses materials, manufacturing processes, metrology, calibration, and equally important, a tight cooperation between the engineering and science teams (Ref. 67).

Innovative Technology

The most important innovation in the TROPOMI-SWIR band is the silicon immersed grating developed by SRON, together with TNO. Compared to normal gratings, the silicon immersed grating works more efficient, yielding to a smaller grating and a much smaller SWIR module in terms of volume and mass. For the TROPOMI SWIR module this innovation enabled a volume reduction of almost a factor 40 (Ref. 61).

By letting the infrared light first pass through a medium with refractive index n before it is dispersed by the grating from inside the medium, the grating works n times more efficient than the traditional version. This trick allows the project to make the grating n times smaller and the total instrument n³ times smaller. SRON, together with TNO, have developed silicon immersed gratings with a refractive index of 3.42. This has yielded a huge, almost forty-fold, volume reduction for the spectrometer. - Immersion means that diffraction takes place inside the medium, in our case silicon. The high refractive index of the silicon medium boosts the resolution and the dispersion. Ultimate control over the groove geometry yields high efficiency and polarization control. Together, these aspects lead to a huge reduction in spectrometer volume. This has opened new avenues for the design of spectrometers operating in the short-wave-infrared wavelength band. Immersed grating technology for space application was initially developed by SRON and TNO for the short-wave-infrared channel of TROPOMI, built under the responsibility of SSTL. ^{85) 86) 87)}

On TROPOMI the SWIR and UVN spectrometers share a common telescope. Figure 66 shows the layout of the SWIR spectrometer. A relayed image of the TROPOMI telescope input pupil is provided at the interface to the SWIR module. A SWIR telescope comprising a silicon germanium doublet forms an image of the ground on a SWIR slit. The slit is manufactured on a silicon prism using photo lithographic methods. A second silicon germanium doublet collimates light from the slit into the IG (Immersed Grating). A five element imaging lens (l1 - l5), comprising silicon and germanium elements forms a spectrally dispersed image of the slit on a MCT detector. An AP (Anamorphic Prism) is included between the immersed grating and the imaging lens and this provides fine alignment adjustment for coregistration requirements.

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Grating design: Figure 67 shows the IG schematically, with the incoming rays in black and dispersed rays in blue and red. The grating surface shows the characteristic V-grooves (not to scale) that arise from our etching technique; grooves are etched along a specific crystallographic direction of the mono-crystalline silicon grating material. This method results in a controlled blaze angle close to 55°, and smooth groove surfaces. The grating period is 2500 nm corresponding to 400 lines/mm. The flat parts in the plane of the grating, between the grooves are 800 nm wide. The grating is used in order six. The grating facet has a reflective coating. The angle between the grating entrance facet and grating facet is ~60º. The incoming beam is at normal incidence with the entrance facet.

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Diffraction grating: The stringent requirements on both the imaging properties and the quality of the spectra translate to a high-tech grating scheme. Hence, a novel diffraction grating scheme was developed at SRON for the SWIR band based on lithographical techniques and anisotropic etching in silicon. In the design, the dispersion and resolution is increased by a factor of 3.4 with respect to conventional gratings; the grating is developed in an immersion, such that diffraction takes place inside the silicon grating material. By lithographic patterning and anisotropic etching of the mono-crystalline silicon the line spacing and blaze angle can be precisely controlled. ^{88) 89) 90)}

The grating has a line spacing of 2.5 µm and is operated in sixth order. We show that an efficiency of 60% is reached on a 50 x 60 mm² grating surface. The test results with numerical calculations for grating efficiency for both polarizations are compared and were found in good agreement.

This novel approach has a fourfold benefit over conventional mechanical ruling of gratings:

1) First the gratings are lithographically produced and thus benefit from state-of-the art methods, materials and equipment from the semiconductor industry.

2) Secondly, using anisotropic etching along preferred axes in the silicon crystal arbitrary blaze angles can be obtained enabling optimization of the diffraction efficiency.

3) Thirdly, the etched reflecting facets are very smooth suppressing stray light.

4) A fourth and decisive improvement over traditional gratings is that silicon gratings can be illuminated from inside the medium, or "in immersion", for wavelengths above 1.2 µm for which silicon is transparent.

The resolution of a grating scales with its size, relative to the wavelength. By illuminating the grating from the inside, as illustrated in Figure 68, the wavelength is reduced by the index of refraction of the medium n. Therefore, immersed gratings of high index materials can be made smaller than conventional gratings. The volume gain of the complete spectrometer can be up to n-cubed, implying a huge cost reduction for many applications, in particular for space applications. These advantages make the immersed gratings "enabling technology" for future scientific space missions.

The grating, selected for the TROPOMI SWIR spectrometer, has a line spacing of 2500 nm and a 54.7º blaze angle. The total grating area is 50 mm high and 60 mm in width. An efficiency of 60% is obtained.

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Mounting the grating: The immersed grating is a single crystal of silicon with a grating surface etched onto one face. The operational temperature of the optical bench is 200 K. The immersed grating is mounted in a monolithic titanium alloy structure (Figure 69); it is held into the structure by epoxy adhesive (Masterbond EP21TCHT-1). The epoxy adhesive is contained in recesses within invar buttons to control the bond line thickness. Invar is used at the adhesive interface as its CTE (Coefficient of Thermal Expansion) is a good match to that of the silicon of the immersed grating over the required temperature range. This will limit the stress induced birefringence and also ensures that stresses in the adhesive are kept to a minimum. Due to the CTE difference between titanium alloy and the silicon prism, flexure sections are required in the mounting structure to compensate for displacements at operating temperature. The invar buttons are therefore mounted into flexure arms which feature a thin blade section to compensate for displacements across the prism and folded flexure spring sections to compensate for further displacements.

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Legend to Figure 69: The immersed grating prism (purple, on left side image) is mounted in a monolithic titanium alloy structure (grey); it is held into the structure by epoxy adhesive. Invar is used at the adhesive interface. The right side image is a zoom of the mounting structure showing an invar button (brown) mounted in a flexure spring section of the titanium structure.

The manufacturing and test of the IGs was completed in July 2012. The FM and spare gratings are fully compliant with the optomechanical specifications. The wave front error is 0.6 µm rms and can be reduced to 0.3 µm rms with focus correction (Ref. 86).

Detector development: The UVN detector developed for TROPOMI is a back illuminated 1024 x 1024 pixel frame transfer CCD with a pixel pitch of 26 µm. The device is developed by e2V in the UK and has different coatings for the different wavelength bands to allow maximizing the quantum efficiency and minimizing interference structures for the NIR (Ref. 69). ⁹¹⁾

The device is operated in non-inverted mode (NIMO). Despite the higher intrinsic dark current (surface dark current is not suppressed) this has a number of advantages. The first is that it allows using the full pixel full well instead of being limited by the so-called ellipsoid effect present in inverted mode (IMO). This effect is an ellipsoid shaped noisy structure occurring when pixels are filled more than typically 50%. Such a reduction of the pixel full well is not acceptable in view of the already high pixel readout rate of 5 MHz. A further advantage of NIMO is the lower power dissipation. This allows obtaining a lower operating temperature with the same cooling power and thereby repairs much of the increase in dark current. Since the largest contribution to the dark current will come from the surface of the CCD, the contribution from RTS (Random Telegraph Signal) will be much lower. In addition the lower temperature will not only decrease the bulk dark current but also the RTS, both in amplitude and in time scale. At the proposed operating temperature the time scale of any RTS will probably be long enough such that any RTS that may be present can be corrected for.

The device has metal buttresses to have the line transfer time as low as possible to minimize exposure smear. Having metal buttresses means that with today's technologies, the project is bound to 2 phase parallel clocking. This results in a lower pixel full well, as compared to 4 phase clocking, but this was acceptable in view of the lower development risk.

First test results with a front illuminated breadboard detectors show that most performances are as expected. There is no sign of RTS pixels in the test devices and the dark current is better than anticipated.

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For the SWIR range, TROPOMI uses the off-the-shelf Sofradir SATURN detector. This is a HgCdTe-based CMOS detector with 1000 x 256 pixels of 30 µm pitch. The detector is, apart from the number of pixels, similar to the MARS detector which was used successfully in a SWIR spectrograph breadboard.

Operational flexibility: TROPOMI is a very flexible instrument in terms of the readout of its detectors. The most important instrument settings are as follows.

• To avoid saturation in the detectors, there are up to 25 detector readouts during the spatial sampling satellite travel distance. TROPOMI allows users to set the exposure times with step size 1 ms for the UVN and 200 µs for the SWIR and the number of exposures to be co-added into the spatial sampling distance in the flight direction.

• The UVN module CCDs bin a programmable number of pixels to have the wanted sampling in the swath direction, nominally 4 detector pixels are binned to have a 7 km resolution at nadir. Since the sampling measured on ground increases with the swath angle, it is possible to have lower binning factors towards the extreme swath angles.

• The UVN module CCDs allow binning groups of pixel rows below and above the illuminated regions to have stray light estimates and also to bin covered rows on top and the bottom of the CCDs for exposure smear and dark current. Gains for these rows are selected separately to allow fair SNRs (Signal-to-Noise Ratios).

• For each UVN band, it is possible to select the CCD output amplifier gain and the ADC (Analog Digital Converter) gain, separately for each row.

The exposure time settings are to be used to optimize the SNRs for different latitudes and for special cases such as ozone hole conditions. Since the exposure times for all bands have to fit into the same satellite travel distance, it is also possible to adjust the exposure co-addition time and thereby the spatial sampling in the flight direction.

The flexibility in selecting the exposure times introduces in turn a risk of EMC effects in the detector readout. This risk is minimized by including ADCs in the detector proximity electronics, thereby having digital signals in the harness between detector modules and electronics unit. On the other hand, the detectors are read at a frequency of 5 MHz which is high enough to be cautious.

This risk is mitigated by synchronizing the different detectors. This is achieved by implementation of a few simple rules.

• During a frame transfer of any UVN detector, there shall be no readout of any other detector

• During a line transfer (of the storage section into the register) of any UVN detector, there shall be no readout of any other detector.

• Frame transfers, line transfers and readout shall not be interrupted.

It is possible to switch the synchronization off in case the EMC risk does not show up in later hardware phases.

Co-registration: Co-registration means that all wavelengths of a given detector row have the same viewing directions, both in the across-flight and in the flight direction. Coregistration is important because level 1-2 product retrieval algorithms assume all wavelengths in the Level 1 product observe the same air mass. There are a number of hardware effects that impact the coregistration performance.

In the flight direction, the different slits for UV, UVIS/NIR and SWIR lead to a swath dependent offset as shown in Figure 71. The effect for the UV is similar but this is not so critical as this band observes the upper atmosphere with few clouds and scene variation.

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Legend to Figure 71: The two bottom curves mark the start and end of the slit projection for the UVIS/NIR and the two upper curves for the SWIR.

In the swath direction and within detector bands, there are the cushion-shaped distortions related to using gratings. These are minimized as much as possible in the optical design but there will be remnants due to manufacturing tolerances. - In the swath direction the most difficult effects are between the detector bands, as these require subdetector pixel accuracies in detector mounting and optical element alignment and the accuracies include orbital effects due to the changing thermal environment.

The most critical coregistration performance is between the NIR and the SWIR and the NIR and the UVIS bands because the NIR band yields the cloud product needed to obtain accurate air mass estimates for the trace gas products from the SWIR and UVIS bands.

Hence, the project downlinks the NIR data at improved spatial sampling and interpolate the NIR data towards the SWIR and the UVIS viewing. In the swath direction, this is possible by reducing the detector pixel binning from 4 to 2 and thereby have a spatial sampling of 3.5 km. In the flight direction, the co-addition time is reduced by a factor 3 and thereby the spatial sampling is about 2.3 km. Given the relevant resolutions, this is sufficient for interpolation. Interpolation is seen as an effective correction of the inter-channel co-registration errors. The accuracy of the knowledge of the pointing difference between channels and its stability form now the most important remaining error. The stability of the co-registration during flight is estimated to be within 10% of a ground pixel which is sufficiently small.

Heterogeneous scenes: Trace gas products from instruments such as TROPOMI are normally derived from reflectance spectra, the ratio of Earth radiance and sun irradiance measurements. The absorption signatures in these spectra can be small, e.g. in the order of a per cent in the case of minor absorbing gases. Therefore, if an accuracy in the product of a few per cent is wanted, then the reflectance spectra need to be free from any distortion on the 10^-3 to 10^-4 level and with a very accurate wavelength definition in the order of 1/100 of a spectral sampling distance.

There are several mechanisms introducing such distortions or features:

• Sun measurements use a diffuser to convert irradiance into radiance entering the telescope; because of the good spectral and spatial resolution coherence effects will show up as seemingly random spikes or features; the Earth spectra do not use a diffuser and therefore the features are present in the reflectance spectra.

• The polarization scrambler consists of a stack of four birefringent wedges and result in wavelength and viewing angle dependent modulations of the signals; these modulations vary in the Earth measurements with polarization but they are constant in the sun measurements as this is not polarized.

• Varying pixel-to-pixel variation in the detector sensitivity (PRNU) in combination with a varying scene and pixel binning leads to small errors in Earth radiance measurements; sun measurements do not have the effect because the sun is spatially constant via the diffuser.

• Non-uniform illumination of the slit in the across slit direction leads to distortion of the slit function and effectively in a wavelength shift and radiometric effect.

The latter is the heterogeneous scenes effect and is the topic of this section. Figure 72 shows the basics of an OMI-type spectrograph, showing the transfer of flight direction spatial information towards slit illumination and from there onto the spectral direction of the 2D detector.

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The left side of the graph (Figure 73) shows light beams from different directions entering the telescope and being imaged on different edges of the slit. The right size of the graph shows a spectrograph where the beams are projected on different locations of the detector. If the scene is constant, this leads to the desired slit function, or SRF (Spectral Radiance Function), imaged onto the detector. If the illumination of the slit is not uniform, the beams have different radiance content and cause a distortion of the slit function. This distortion causes a radiometric error, and, because the barycenter of the slit function is changed, to a wavelength error.

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The effect has been modelled where Figure 74 shows the different parts of the scene observed for the different exposures within a co-addition or dwell time. The dwell time was chosen such that it includes a major change in the scene. Following the graphs in Figure 74, an additional convolution was applied to include the motion of the satellite and the curves were converted to wavelength scale to obtain the slit functions or SRFs. This conversion is directly from the fact that a half a ground pixel on Earth (3.5 km) is imaged onto 3 detector pixels and the same 3 detector pixels represent the oversampled spectral resolution.

The slit functions are used to compute from high resolution scene spectra the pixel content for each detector pixel and this allows to compute an error by comparing the result with that of an averaged constant scene. The result is shown in Figure 75 and shows the errors are on a percent level.

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The errors can be seen as a shift in the barycenter's of the slit functions and can therefore also be expressed as wavelength errors. This is shown in Fig. 12 where the wavelength errors are shows from fitting the Fraunhofer structures in the spectra in a number of predefined wavelength windows. The heterogeneous scene has errors of about 0.015 nm.

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Ground Segment

The ground segment main elements are the FOS (Flight Operations Segment) located at ESOC in Darmstadt and the PDGS (Payload Data-processing Ground Segment) and Mission Planning Facility, located at DLR in Oberpfaffenhofen, both in Germany. Their tasks are the commanding, tracking and monitoring of the spacecraft as well as the acquisition, processing, archiving and dissemination of science data, respectively. The PDGS will, in particular, host the Level 0-1b & Level 2 processing facility which will generate routine Level 0 / 1b and Level 2 data products. The Level 0-1b software is provided by KNMI to be installed in the PDGS. The Level 2 products are a joint procurement by ESA and NSO and is coordinated by KNMI. The Level 2 products are developed by KNMI, SRON, DLR and BIRA. ^{92) 93)}

The envisaged near-real-time dissemination scheme for Level 1b and 2 data products implies that the recorded science telemetry is downlinked at least once per orbit. This will be accomplished by use of high latitude X-band stations in the Svalbard region of Spitzbergen, Norway. A schematic view of the primary elements of the ground segment is given in Figure 76.

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Table 7 lists the level 2 data products of Sentinel 5. With the exception of the CO₂, all products are targeted by the Sentinel 5P TROPOMI instrument.

The DLR German Remote Sensing Data Center (DFD) in Oberpfaffenhofen developed and integrated the entire payload ground segment, including data reception, processing, archiving and distribution. Operation of the payload ground segment as part of the ESA ground segment also belongs to the remit of the German Remote Sensing Data Center. The data processors that convert the measurement data into geophysical data products were developed by the DLR Remote Sensing Technology Institute, the University of Bremen and the Max Planck Institute for Chemistry in Mainz as part of a European consortium.

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29) "Methane detected over Poland's coal mines," ESA Applications, 21 March 2022, URL: https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-5P/Methane_detected_over_Poland_s_coal_mines

30) "Sulphur dioxide from Tonga eruption spreads over Australia," ESA Applications, 20 January 2020, URL: https://www.esa.int/ESA_Multimedia/Images/2022/01/Sulphur_dioxide_from_Tonga_eruption_spreads_over_Australia

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86) Aaldert van Amerongen, Hélène Krol, Catherine Grèzes-Besset, Tonny Coppens, Ianjit Bhatti, Dan Lobb, Bram Hardenbol, Ruud Hoogeveen, "State of the art in silicon immersed gratings for space," Proceedings of the ICSO (International Conference on Space Optics), Ajaccio, Corse, France, Oct. 9-12, 2012, paper: ICSO-070, URL: http://congrex.nl/icso/2012/papers/FP_ICSO-070.pdf

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