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)
Table 1: Copernicus is the new name of the former GMES program 5)
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.
Figure 1: Artist's view of the Sentinel-5P spacecraft in orbit (image credit: ESA, Astrium) 6)
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 GMES 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. 7) 8)
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. 9)
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.
Figure 2: Fold-out illustration of the AstroBus-L elements (image credit: ESA)
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.
Figure 3: Illustration of the Sentinel-5P spacecraft (image credit: ESA, Astrium)
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.
Launch: A launch of Sentinel-5P spacecraft is planned for the summer 2015 on a Vega vehicle from Kourou (Ref. 7). The Sentinel-5P spacecraft has a launch mass of ~ 900 kg.
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.
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.
Table 2: Overview of some mission parameters
Sensor complement: (TROPOMI)
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. 10) 11) 12)
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 km2and 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 4.
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.
Figure 5: TROPOMI system breakdown and unit suppliers (image credit: Dutch Space)
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). 13) 14)
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. 15) 16)
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. 17)
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. 18)
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. 19) 20)
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 6.
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.
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. 10).
Figure 6: TROPOMI spectral window compared with GOME, SCIAMACHY and OMI (image credit: Dutch Space, TNO)
The CDR (Critical Dign Review) of the TROPOMI instrument is planned for end of 2012, early 2013. 21)
Instrument: The TROPOMI instrument is a pushbroom type imaging spectrometer (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: 22) 23) 24) 25) 26)
• 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) (Figure 7). A passive thermal radiator is used to reject heat from the system.
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 (Figure 8).
Figure 8: Projection of the spectrometer slit on-ground (image credit: Dutch Space, TNO)
Legend to Figure 8: 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 12 and 13. 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). The SWIR detector will be from Sofradir (France), it is controlled by FEE (Front-End Electronics) developed by SRON.
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.
Figure 9: TROPOMI measurement principle (image credit: KNMI) 27)
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. 28)
Figure 10: 3D view of the TROPOMI telescope (image credit: TNO)
Design with freeform mirrors: The telescope is shown in Figure 10. 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-contactMeasurement Machine for aspheric and Freeform Optics) 29) 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 O 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.
Figure 11: NANOMEFOS machine concept with long range optical probe and separate metrology frame (image credit: TNO)
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.
Figure 12: The TROPOMI external instrument configuration with view on entrance ports (image credit: Dutch Space)
Legend to Figure 12: 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 12 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.
Figure 13: The TROPOMI external instrument configuration with view on the SWIR module (image credit: Dutch Space)
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. 30)
The mass of the TROPOMI instrument is 206.6 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 #). The power consumption is: 170 W (average) and 382 W (max).
Table 3: Physical properties of the TROPOMI instrument
Achieving the performances listed in Table 4 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 4. 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.
Table 4: Performance characteristics of the TROPOMI spectral 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.
Figure 14: Illustration of the TROPOMI instrument (TNO, Dutch Space)
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. 16).
Introduction of 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. 10).
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. 31) 32) 33)
On TROPOMI the SWIR and UVN spectrometers share a common telescope. Figure 15 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.
Figure 15: Optical layout of the TROPOMI SWIR spectrometer including the IG (Immersed Grating), an AP (Anamorphic Prism), camera-objective lenses l1-l5, and detector windows (image credit: SRON, TNO, Ref. 32)
Grating design: Figure 16 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.
Figure 16: Sketch of the principle of a normal reflection grating (left) and an immersed grating (right), lithographically produced in silicon (image credit: SRON, TNO)
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. 34) 35) 36)
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 16, 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.
Figure 17: Photo of the monolithical immersed grating (left), drawing of the grating prism (right), image credit: SRON, TNO)
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 18); 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.
Figure 18: Mounting scheme of the immersed grating (image credit: SRON, TNO)
Legend to Figure 18: 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. 32).
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. 17). 37)
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.
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.
Figure 19: Photo of the EM (Engineering Model) of the TROPOMI CCD (image credit: EV2)
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 20. 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.
Figure 20: UVIS/NIR and SWIR slits projected onto the Earth surface, showing the displacement, increase with swath angle and relative curvature in the flight direction (image credit: SRON, TNO)
Legend to Figure 20: 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 21 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.
Figure 21: SNR spectrum of the UVIS spectrograph for end-of-life conditions together with requirement (image credit: SRON, TNO)
The left side of the graph (Figure 22) 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.
Figure 22: Basic pushbroom spectrograph showing how flight direction spatial information on the left is transferred to the spectral response function on the right; color separation from the grating is left out of the graph
Figure 23: Staring observation of a heterogeneous scene for the UVIS band at 500 nm; the different curves are for the different exposures where the satellite has moved (image credit: SRON, TNO)
The effect has been modelled where Figure 23 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 23, 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 24 and shows the errors are on a percent level.
Figure 24: Heterogeneous scenes errors as a function of wavelength (image credit: SRON, TNO)
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.
Figure 25: Wavelength fit errors from heterogeneous scene as compared to the reference scene; they show up because the differently shaped slit functions have effectively shifted barycenters (image credit: SRON, TNO)
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. 38) 39)
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 26.
Table 5 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.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.