Copernicus: Sentinel-3 — Global Sea/Land Monitoring Mission including Altimetry
The Sentinel-3 (S3) mission of ESA and the EC is one of the elements of the GMES (Global Monitoring for Environment and Security) program, which responds to the requirements for operational and near-real-time monitoring of ocean, land and ice surfaces over a period of 20 years. The topography element of this mission will serve primarily the marine operational users but will also allow the monitoring of sea ice and land ice, as well as inland water surfaces, using novel observation techniques.The Sentinel-3 mission is designed as a constellation of two identical polar orbiting satellites, separated by 180º, for the provision of long-term operational marine and land monitoring services. The operational character of this mission implies a high level of availability of the data products and fast delivery time, which have been important design drivers for the mission. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)
The Sentinel-3 program represents a series of operational spacecraft over the envisioned service period to guarantee access to an uninterrupted flow of robust global data products.
Table 1: Copernicus is the new name of the former GMES program 15)
The main observation objectives of the mission are summarized in the following list:
• Ocean and land color observation data, free from sun-glint, shall have a revisit time of 4 days (2 days goal) and a quality at least equivalent to that of Meris instrument on Envisat. The actual revisit obtained over ocean at the equator (worst case) is less than 3.8 days with a single satellite and drops below 1.9 days with 2 satellites, phased 180° on the same orbital plane.
• Ocean and land surface temperature shall be acquired with at least the level of quality of AATSR on Envisat, and shall have a maximum revisit time of 4 days with dual view (high accuracy) observations and 1 day with single view. Achieved performance is shown to be significantly better, even with a single satellite (dual view: 3.5 days max, 1.8 days average).
• Surface topography observations shall primarily cover the global ocean and provide sea surface height (SSH) and significant wave height (SWH) to an accuracy and precision at least equivalent to that of RA-2 on Envisat. Additionally, Sentinel-3 shall provide surface elevation measurements -in continuity to CryoSat-2 - over ice regions covered by the selected orbit, as well as measurements of in-land water surfaces (rivers and lakes).
In addition, Sentinel-3 will provide surface vegetation products derived from synergistic and co-located measurements of optical instruments, similar to those obtained from the Vegetation instrument on SPOT, and with complete Earth coverage in 1 to 2 days.
The EU Marine Core Service (MCS) and the Land Monitoring Core Service (LMCS), together with the ESA GMES Service Element (GSE), have been consolidating those services where continuity and success depends on operational data flowing from the Sentinels.
The operational character of the mission implies a high level of availability of the data products and fast delivery time, which have been important design drivers for the mission.
Figure 1: Artist's rendition of the Sentinel-3 spacecraft (image credit: ESA)
The Sentinel-3 spacecraft is being built by TAS-F (Thales Alenia Space-France). A contract to this effect was signed on April 14, 2008. The spacecraft is 3-axis stabilized, with nominal pointing towards the local normal and yaw steering to compensate for the Earth rotation affecting the optical observations. The spacecraft has a launch mass of about 1200 kg (including margin), the height dimension is about 3.9 m. The overall power consumption is 1100 W. The design life is 7 years, with ~100 kg of hydrazine propellant for 12 years of operations, including deorbiting at the end.
AOCS (Attitude and Orbit Control Subsystem): The spacecraft is 3-axis stabilized based on the new generation of avionics for the TAS-F LEO (Low Earth Orbit) platform. The AOCS software of the GMES/Sentinel-3 project is of PROBA program heritage. NGC Aerospace Ltd (NGC) of Sherbrooke, (Québec), Canada was responsible for the design, implementation and validation of the autonomous GNC (Guidance, Navigation and Control) algorithms implemented as part of the AOCS software of PROBA-1, PROBA-2, and PROBA-V. 16)
Table 2: Overview of Sentinel-3 spacecraft parameters
Data handling architecture: The requirements for the Sentinel-3 data handling architecture call for: a) minimized development risks, b) system at minimum cost, c) operational system over 20 years. This has led to design architecture as robust as possible using a single SMU (Satellite Management Unit) computer as the platform controller, a single PDHU (Payload Data-Handling Unit) for mission data management, and to reuse existing qualified heritage. 17)
The payload accommodates 6 instruments, sources of mission data. The 3 high rate instruments provide mission data directly collected through the SpaceWire network, while the low rate instruments are acquired by the central computer for distribution through the SpaceWire network to the mass memory. The PDHU acquires and stores all mission data for latter multiplexing, formatting, encryption and encoding for download to the ground.
The payload architecture is built-up over a SpaceWire network (Figure 2) for direct collection of high rate SLSTR, OLCI and SRAL instruments and indirect collection of low rate MWR, GNSS and DORIS instrument data plus house-keeping data through the Mil-Std-1553 bus by the SMU, all data being acquired from SpaceWire links and managed by the PDHU.
The mission data budget is easily accommodated thanks to the SpaceWire performance. Each SpaceWire link being dedicated to point-to-point communication without interaction on the other links (no routing), the frequency is set according to the need plus a significant margin. The PDHU is able to handle the 4 SpaceWire sources at up to 100 Mbit/s.
All mission data sources (OLCI, SLSTR, SRAL and SMU) provide data through two cold redundant interfaces and harnesses. The PDHU, being critical as the central point of the mission data management, implements a full cross-strapping between nominal and redundant sources interfaces and its nominal and redundant sides.
Figure 2: SpaceWire architecture of the Sentinel-3 spacecraft (image credit: TAS-F)
The PDHU SpaceWire interfaces are performed thanks to a specific FPGA, the instrument’s ones are based on the ESA Atmel SMCS-332, while the SMU interfaces are implemented by an EPICA ASIC circuit developed by Thales Alenia Space.
Figure 3: Schematic view of a full cross-strap redundancy within the PDHU (image credit: TAS-F, Ref. 17)
RF communications: The S-band is used for TT&C transmissions The S-band downlink rate is 123 kbit/s or 2 Mbit/s, the uplink data rate is 64 kbit/s. The X-band provide the payload data downlink at a rate of 520 Mbit/s. An onboard data storage capacity of 300 Gbit (EOL) is provided for payload data.
Four categories of data products will be delivered: ocean color, surface topography, surface temperature (land and sea) and land. The surface topography products will be delivered with three timeliness levels: NRT (Near-Real Time, 3 hours), STC (Standard Time Critical, 1-2 days) and NTC (Non-Time Critical, 1 month). Slower products allow more accurate processing and better quality. NRT products are ingested into numerical weather prediction and seastate prediction models for quick, short term forecasts. STC products are ingested into ocean models for accurate present state estimates and forecasts. NTC products are used in all high-precision climatological applications, such as sealevel estimates.
The resulting analysis and forecast products and predictions from ocean and atmosphere adding data from other missions and in situ observations, are the key products delivered to users. They provide a robust basis for downstream value-added products and specialized user services.
Introduction of new technology: A newly developed MEMS rate sensor (gyroscope), under the name of SiREUS, will be demonstrated on the AOCS of Sentinel-3. The gyros will be used for identifying satellite motion and also to place it into a preset attitude in association with optical sensors after its separation from the launcher, for Sun and Earth acquisition. Three of the devices will fly inside an integrated gyro unit, each measuring a different axis of motion, with a backup unit ensuring system redundancy. Each unit measures 11 cm x 11 cm x 7 cm, with an overall mass of 750 grams. 18)
The SiREUS device is of SiRRS-01 heritage, a single-axis rate sensor built by AIS (Atlantic Inertial Systems Ltd., UK), which is using a ’vibrating structure gyro’, with a silicon ring fixed to a silicon structure and set vibrating by a small electric current. The SiRRS-01 MEMS gyro has been used in the automobile industry. These devices are embedded throughout modern cars: MEMS accelerometers trigger airbags, MEMS pressure sensors check tires and MEMS gyros help to prevent brakes locking and maintain traction during skids. - In a special project, ESA selected the silicon-based SiRRS-01 to have it modified for space use (and under the new name of SiREUS).
Figure 4: Photo of the MEMS rate sensor (image credit: ESA)
Launch: A launch of the Sentinel-3A spacecraft is scheduled for early 2015 on a Rockot vehicle of Eurockot Launch Services (a joint venture between Astrium, Bremen and the Khrunichev Space Center, Moscow). The launch site is the Plesetsk Cosmodrome in northern Russia. ESA awarded the contract to Eurockot Launch Services on Feb. 9, 2012. 19)
There are three spacecraft in this series: Sentinel-3A, -3B, and -3C. The second satellite is expected to be launched ~18 months after the first one.
Orbit: Frozen sun-synchronous orbit (14 +7/27 rev./day), mean altitude = 815 km, inclination = 98.6º, LTDN (Local Time on Descending Node) is at 10:00 hours. The revisit time is 27 days providing a global coverage of topography data at mesoscale.
With 1 satellite, the ground inter-track spacing at the equator is 2810 km after 1 day, 750 km after four days, and 104 km after 27 days.
For the altimetry mission, simulations show that this orbit provides an optimal compromise between spatial and temporal sampling for capturing mesoscale ocean structures, offering an improvement on SSH mapping error of up to 44% over Jason - due to improved spatial sampling (Figure )- and 8% over the Envisat 35-day orbit - due to better temporal sampling. After a complete cycle, the track spacing at the equator is approximately 100 km.
The Sentinel-3 mission poses the most demanding POD (Precise Orbit Determination) requirements, specially in the radial component, not only in post-processing on-ground, but also in real-time. This level of accuracy requires dual-frequency receivers. The main objective of the mission is the observation with a radar altimeter of sea surface topography and sea ice measurements (see columns 3, 4, 5 in Table 3).
The second satellite will be placed in the same orbit with an offset of 180º, such that the ground tracks of a complete cycle fall exactly in the middle of the ground tracks of the first satellite.
With two satellites flying simultaneously, the following coverage will be achieved (Ref. 11):
- Global Ocean color data is recorded with OLCI and SLSTR in less than 1.9 days at the equator, and in less than 1.4 days at latitudes higher than 30º, ignoring cloud effects.
- Global Land color data is recorded with OLCI and SLSTR in less than 1.1 days at the equator, and less than 0.9 days in latitudes higher than 30º.
- Global Surface temperature data is recorded in less than 0.9 days at the equator and in less than 0.8 days in latitudes higher than 30º.
- Continuous altimetry observations where global coverage is achieved after completion of the reference ground track of 27 days.
Figure 5: Sentinel-3 spacecraft with payload layout (image credit: ESA)
Figure 6: Alternate view of the Sentinel-3 spacecraft and the accommodation of the payload (image credit: ESA)
Sensor compliment (optical payload, topographic payload)
In the context of GMES (Global Monitoring for Environment and Security), the objectives of the Sentinel-3 mission, driven by ESA and the user community, encompass the commitment to consistent, long-term collection of remotely sensed data of uniform quality in the areas of sea / land topography and ocean color. Measurements over oceans will be provided jointly with other operational missions, such as the Jason series, to contribute to the realization of a permanent Global Ocean Observing System (GOOS). Regarding ice, it is foreseen to monitor land ice (also denoted as ice sheet) including ice margins and sea ice. At last, measurements over rivers and lakes will help in the water level monitoring of spots of interest throughout the world.
Sentinel-3 will support primarily services related to the marine environment, such as maritime safety services that need ocean surface-wave information, ocean-current forecasting services that need surface-temperature information, and sea-water quality and pollution monitoring services that require advanced ocean color products from both the open ocean and coastal areas. Sentinel-3 will also serve numerous land, atmospheric and cryospheric application areas such as land-use change monitoring, forest cover mapping and fire detection. 21)
Figure 7: FOVs (Field of Views) of the Sentinel-3 instruments (image credit: ESA)
Optical payload (OLCI, SLSTR)
The primary mission objective of the optical payload is to ensure the continuation of the successful Envisat observations of MERIS for ocean color and land cover and AATSR for sea surface temperature. In addition, due to the overlapping field of view from both optical sensors, new applications will emerge from the combined exploitation of all spectral channels.
OLCI (Ocean and Land Color Instrument):
OCLI is a medium resolution pushbroom imaging spectrometer of MERIS heritage, flown on Envisat, but with a slightly modified observation geometry: the FOV (Field of View) is tilted towards the west (~ 12º away from the sun), minimizing the sun-glint effect over the ocean and offering a wider effective swath (~ 1300 km, overall FOV of 68.6º). The sampling distance is 1.2 km over the open ocean and 0.3 km for coastal zone and land observations. The instrument mass is ~ 150 kg; it is being designed and developed at Thales Alenia Space España.
The FOV of OLCI is divided between five cameras on a common structure with the calibration assembly. Each camera has an optical grating to provide the minimum baseline of 16 spectral bands required by the mission together with the potential for optional bands for improved atmospheric corrections.
Figure 8: Schematic view of the OCLI instrument (image credit: ESA)
Each camera is constituted of a Scrambling Window Element to comply with the polarization requirement, a COS (Camera Optical Sub-assembly) for the spectral splitting of the different wavelengths, a FPA (Focal Plane Assembly) with a CCD for the signal detection and a VAM (Video Acquisition Module) for the monitoring of the analog signal. The optical sub-assembly of each camera includes its own grating and provides the 21 spectral bands required by the mission in the range 0.4-1.0 µm. 25)
The control of the instrument assembly is realized by a CEU (Common Electronic Unit), which assumes the function of instrument control, power distribution and digital processing.
A calibration assembly, including a rotation wheel with five different functions for normal viewing, dark current, spectral and radiometric calibrations insures the calibration of the instrument.
Figure 9: Schematic view of the OCLI observation geometry with the 5 camera assembly (image credit: ESA)
Compared to ENVISAT, the following improvements have been implemented:
• Along-track SAR capability for coastal zones, inland water and sea-ice altimetry
• Off-nadir tilted field of view for OLCI cameras to minimize sun-glint contamination (i.e. loss of ocean color data)
• New spectral channels in OLCI and SLSTR allowing improved retrieval of geophysical products and detection of active fire
• Synergy products (e.g. vegetation) based on combination of OLCI and SLSTR data (OLCI swath fully covered by SLSTR swath).
SLSTR (Sea and Land Surface Temperature Radiometer):
SLSTR is an upgraded and advanced version of the AATSR instrument on Envisat, offering a wider swath which completely overlaps the OLCI swath, as required to produce accurate vegetation products. The SLSTR is designed for ocean and land-surface temperature observations. Unlike AATSR, SLSTR has a double-scanning mechanism, yielding a much wider swath stretching almost from horizon to horizon. The OLCI and SLSTR swaths are overlapping broadly, yielding extra information. SLSTR has a wide nadir view and a narrow oblique view. 26) 27) 28)
Selex Galileo of Finmeccanica signed a contract with Thales Alenia Space, to supply the SLSTR instrument. Overall, the SLSTR team involves some 20 European companies or institutions, referred to as “SLSTR consortium” (among them RAL (Rutherford Appleton Laboratory), Jena-Optronik, TAS-F, ABSL, ESA/ESTEC), for the development of this rather complex payload.
The instrument design follows the dual view concept of the ATSR series with some notable improvements. An increased swath width in both nadir and oblique views (1400 and 740 km) provides measurements at global coverage of Sea and Land Surface Temperature (SST/LST) with daily revisit times, which is useful for climate and meteorology (1 km spatial resolution).
Improved day-time cloud screening and other atmospheric products will be possible from the increased spatial resolution (0.5 km) of the VIS and SWIR channels and additional SWIR channels at 1.375 µm and at 2.25 µm. Two additional channels using dedicated detector and electronics elements are also included for high temperature events monitoring (1 km spatial resolution).
The two Earth viewing swaths are generated using two telescopes and scan mirrors that are optically combined by means of a switching mirror at the entrance of a common FPA (Focal Plane Assembly). The eleven spectral channels (3 VIS, 3 SWIR, 2 MWIR, 3 TIR) are split within the FPA using a series of dichroics. The SWIR, MWIR and TIR optics/detectors are cooled down to 80 K with an active cryocooler, while the VIS detectors work at a stabilized uncooled temperature.
The SST and LST data generated from SLSTR observations are intended to ensure continuity of the data sets started in 1991 by the ATSR series. The plan is for the Sentinel 3 Mission to operate with two satellites operating concurrently for 20 years when fully implemented, although initially a single SLSTR will provide equivalent or a better performance (Table 4) when compared to its predecessors in the following ways:
- Increase of the dual view swath width from 500 to 744 Km centered on the subsatellite track (Figure 10) gives a mean global coverage revisit time at the equator of 1.9 days (1 spacecraft) or 0.9 days (2 spacecraft)
- Enlarged single view swath width of 1400 km provides a mean global coverage revisit time at the equator of 1 day (1 spacecraft) or half a day (2 spacecraft)
- A nadir on-ground resolution of 0.5 km at nadir (instead of 1 km) for all VIS (S1-S3) and SWIR (S4-S6) channels. Radiance measurements from these channels are used for both land & clouds daytime observations
- Two added channels in the SWIR band to allow improved cloud and aerosol detection to give more accurate SST/LST retrievals
- Two dedicated channels for fire and high temperature events monitoring at 1 km resolution
- A mission design lifetime of 7.5 years which is higher compared to the earlier instruments.
High SNR (600) for the VIS channels at Earth albedo signals (30%) and low NEDT (< 30 mK) for the TIR channels is achieved via the use of a very efficient detector technology (TIA Si for VIS at 260 K, PV-CTIA for SWIR, and PC for TIR at 80 K). The polarization sensitivity is less than 5% EOL (2% BOL) thanks to the use of high reflecting/transparent optical coatings within the channel passbands.
Table 4: Comparison of SLSTR and AATSR performances
Table 5: EOL predicted radiometric resolution performance
Legend to Table 5: λ = center wavelength, Δλ = spectral width, Albedo Ref (Top of Atmosphere Reflectance), T (Top Of Atmosphere Brightness Temperature), SNR (Signal-To-Noise Ratio), NEΔT (Noise Equivalent Temperature Difference).
In the SLSTR viewing geometry, two SLSTR dedicated telescopes and scan mirrors generate the Earth view swaths (Figure 10) as portions of two conical CCW (Counter-Clock-Wise) scans; in between these observation views, SLSTR acquires calibration data with the BBs (Black Bodies) and the VISCAL (Visible Calibration unit).
Figure 10: Near nadir (left) and backward inclined (right) views of the scanning mirror geometry (image credit: SLSTR consortium)
The SST/LST measurement accuracy is obtained by means of a high accurate calibration of the three infrared channels (S7-S8-S9), which are used for the correction of the water vapor atmospheric absorption (split window during day and triple window during night), and the observation of the same on-ground pixel by means of two atmospheric path views for the correction of aerosols effects (Figure 11).
Two different synchronized conical scanners are used to optimize the IR radiometric accuracy, allowing constant optical area beam and incidence angle (23.5º) for all scan points (both scene and BBs), low polarization effects and frequent views of BBs (every scan) with the same Earth observation geometry.
The conical scan, inherited from AATSR (Advanced Along Track Scanning Radiometer), is of fundamental importance for TIR (Thermal Infrared) radiometric accuracy as each scanner uses a constant optical area at a fixed angle. Optimizing incidence angles and reflectivities provides low polarization. Both scans are performed by means of a mirror inclined at θ=23.5º with respect to the rotation axis with an half cone angle of β=47º: the inclined view rotation axis is pointed to nadir direction while the near-nadir view rotation axis is backward inclined of γ=41º (Figure 11).
All on-ground pixels are viewed with an OZA (Observation Zenith Angle) less than 55º, so limiting the radiance variations with sea emissivity changes due to salinity, temperature and wind speed and permitting emissivity modelling only as a function of OZA.
A path length ratio between the two views of 1.54 is achieved for a good optimization of the SST retrieval algorithm. Two detector pixels (IR channels) simultaneously view 2 km along-track in nadir view, to ensure adequate signal integration and also reducing the scan speed with respect to previous AATSR thereby allowing heritage qualifications for the scanners’ bearing operational lifetime requirement of 7.5 years.
Figure 11: Near nadir and backward inclined views scanning mirror geometry (image credit: SLSTR consortium)
The SLSTR instrument comprises two physical units, that are integrated separately to the platform:
- SLOSU (SLSTR Optical Scanning Unit), simply referred to as OSU, is mainly composed by OME (Opto-Mechanical Enclosure) together with thermal radiators, FEE (Front End Electronics), the cooling system, and DA (Detection Assembly)
- SLCPE (SLSTR Control and Processor Electronics), simply called CPE, controls all subsystems and manages the data interface with the satellite.
For each view, IR and VIS radiant energy is reflected by a scan mirror mounted on a scan mechanism towards a paraboloid mirror. The energy is then focused and reflected into a common FPA, with a flip mirror mechanism switching from one view to the other. A fast switching flip-mirror alternates the optical beams from the two scanners so that they superimpose at prime focus and acquire signals from both Earth views (nadir and oblique) and the on-board calibrators (BBs and VISCAL). To increase lifetime, each scanner has a period of 300 ms, a factor of two slower than its predecessor AATSR.
This technology is housed in the OME, which mounts the two telescopes, the flip mirrors, the calibrator units (BBs and VCU), and the SUE (Scan Unit Electronics). The OME feeds the optical beams into the FPA (Focal Plane Assembly) which spectrally separates eleven channels (3 VIS, 3 SWIR, 2 MIR, 3 TIR) with dichroics. There are nine DUs (Detector Units), each one is equipped with a precision filter to define its spectral response. The IR channels’ optics/detectors are cooled to 80 K by an active cryocooler that has vibration compensation. The separated housed visible channels need to be run at a stable ambient temperature.
Figure 12: Functional block diagram of SLSTR (image credit: SLSTR consortium)
Figure 13: Illustration of the SLSTR instrument (image credit: SLSTR consortium)
OSU (Optical Scanning Unit) technologies:
1) General design considerations for the OME (Opto-Mechanical Enclosure): Two conical scanners provide the two swaths by using rotating mirrors inclined at 23.5º with respect to its rotation axis. The oblique swath (740 km) is obtained with the scanner rotation axis pointed versus nadir, while for the near nadir one (1400 km), the scanner rotation axis is inclined backward at 41º. In this way, the same on ground swath of 740 km can be observed with two observation zenith angles (< 55º) with a minimum atmospheric path length ratio of 1.54 (Figure 14).
Figure 14: SLSTR swath configuration – the near nadir swath is red, while the oblique swath is shown in green (image credit: SLSTR consortium)
The OME layout for the scanners has been carefully optimized. Placing the two blackbodies (one hot at ~300 K, and another cold at ~265 K) in front of all other optical elements at the intersections of the two scanned cones (Figure 16 and Figure 17) affords both scanners frequent, continuous and consistent IR in-flight calibration.
The radiation from the earth or calibration targets is reflected via the two scanners and focused into a common focal plane. This plane at the telescope prime focus is an intermediate focus in the system since the FPA re-images it on to the detectors. The two reflective telescopes are realized with single off-axis paraboloid mirrors and use folding mirrors to feed a small, fast and precise FMD (Flip Mirror Device) as shown in Figure 15).
The FMD steps a highly reflective 13 mm diameter mirror about an attrition-free rotation axis, driven by a limited angle torque motor and a limited angular range optical encoder. Optical design optimization has permitted the tilt angle to be limited to < ±9.5º, so the SLSTR achieves the required flex-pivot lifetime. The FMD has a steady state angular stability accuracy of 10-15 arcsec.
Figure 15: Beam Path on Centre Plane (left image) and the location of the FMD combining oblique (blue) and nadir (green) beams coming from the right (image credit: SLSTR consortium)
Figure 16: Bended boundary line between green (OB) and red (NA) scan cone showing potential calibration source positions. Black bodies have to be positioned on this line to be seen by both scanners (image credit: SLSTR consortium)
Figure 17: Sectioned SLSTR side-view showing both scanners looking into the hot blackbody (image credit: SLSTR consortium)
SLSTR has a VISCAL (Visible Calibration unit) which is illuminated by the sun for ~1 minute each orbit. Besides the IR blackbodies already mentioned, the OME relays photons from the VISCAL via both scanners. The optical beams have been geometrically accommodated, but viewing them also has to be integrated in the timing duty cycle of the scanners. An optimized FMD switching time of 34 ms has been chosen, combining a minimized stepping angle with this minimizes drive thermal dissipation.
What the 34 ms does, means that calibration sources will be observed via both views only every two scans or 600 ms rather than every scan. A timing diagram for is shown in Figure 18, where the upper (magenta) and lower (blue) lines represent the oblique and nadir view scanner observations, while the middle brown line represents the position of the flip mirror and its transitions between the two views.
Figure 18: Combined Scanning Scheme over two scans showing the operation of the flipping device (image credit: SLSTR consortium)
2) Switching device FMD (Flip Mirror Device): The FMD (Figure 19) has been implemented by using components with limited development cycles:
- A conventional glass based flat mirror with 13 mm free aperture
- Attrition free axis bearing implemented by means of flexural pivots with infinite life time for max excursion angle of 15º and a powerless 0º position
- A limited angular range high resolution (19 bit) and accurate (< 15 arcsec) optical encoder
- An efficient LAT motor,
- A transition time between both operation positions separated by 19 of 34 ms
- 4 x switching operations per 600 ms
- More than 2 billion operations life-time.
Figure 19: Illustration of the FMD shown with the external encoder electronics (image credit: SLSTR consortium)
The final components selection includes a LAT-Motor actuator with an inertia of 1.46 x 10-6 kgm2 providing a torque of 36 mNm.
3) Scanning devices: To meet the improved GSD and mission performance requirements of the GMES program, the SLSTR scanning mechanism accuracy has been specified to be of the order of a quarter of 1 arcmin.
SLSTR has two scanners, referred to as SMU (SLSTR Scanning Mechanism), and one unit for their synchronization the SUE (Scan Unit Electronics). This synchronization is performed relative to an instrument clock for each 1 km pixel. An optical encoder with 21 bit resolution, a repeatability of 2 arcsec and a maximum absolute position error of about 6 arcsec had been selected.
Figure 20: Illustration of the SMU device (image credit: SLSTR consortium)
The primary performance requirement for the SMUs is to function in a system that defines the angle of mirrors via which SLSTR optically scans, positioning the pixels to the required accuracy. For vector components across the rotational axis, this limits run-out and lubrication track roughness. Along the scan swathes or around the SMU axis, accurate rotation angles are required at the time when each pixel is acquired.
In the SLSTR design, the SMUs drive the control position. This represents a considerably improved implementation over the AASTR design, in which the SMUs were driven at a constant angular velocity, and the swath pixel data was sampled and selected just on a time delay from a synchronization pulse. - For every SLSTR 'pix10sync signal', the position is being measured, implemented in a 100 Hz cut-off frequency closed loop circuit and transmitted to the ground for geolocation referencing. These measures offer a much more robust system in the event of some bearing torque degradation. Small scan mirror angle errors due to electronic noise or mechanical torques, either in the bearings or in external sources, can be corrected in part by means of the real-time positional control loop, the actual angles can then be used for later ground processing.
The maximum control loop pointing error is 5 arcsec. In case this tolerance will be exceeded, a flag will be set and transmitted with the telemetry data package of each scan.
The selected implementation for the SMU control system is an FPGA (Floating Point Gate Array) with an oscillator frequency of 40 MHz. A cascaded control scheme (Figure 21) is used with three levels:
- Current control loop (inner control loop) including the motor current commutation
- Speed control loop
- Position control loop (outer control loop).
Figure 21: Control loop scheme for the scan motors (image credit: SLSTR consortium)
These loops work with different sampling frequencies. The current control works with 27.5 kHz, while the outer loop is fixed at the frequency generated from the PIX10SYNC impulse each 82 µs. The maximum PWM resolution is limited to 10 bit.
The Scanning mirror will be manufactured from beryllium alloy to provide a low overall mass and inertia as well as sufficient stiffness and thermal stability. It is mounted to the shaft via an angular adapter and a centering shim. Using a mechanism shaft of very high accuracy, the angular adapter can provide the required value and accuracy of the scan cone angle. The centering shim allows the dismounting of the mirror to the adapter without alignment loss.
The scan mirror I/F has a stress relieved mechanical design to prevent the mirror surface from disturbances arising from mounting forces (Figure 22).
Figure 22: Structure of the scan mirror assembly (image credit: SLSTR consortium)
FPA (Focal plane Assembly) technologies:
1) General design considerations for the FPA: The FPA has an enclosure composed of a base-plate, a cylinder and an aluminum dome (Figure 23). The base-plate has an input field-limiting aperture, dichroics for spectral channels separation and lens optics to focus the beam onto each detector units. Note that the two scans are combined time multiplexed before they reach the FPA aperture.
Figure 23: Illustration of the FPA structure (image credit: SLSTR consortium)
The IR (S7, S8, S9) and SWIR (S4, S5, S6) channels are implemented on either side of the cryo-optical bench, cooled to 80 K (Figure 24), while the VIS (S1, S2, S3) channels are disposed outside in a separate enclosure at about 260 K. All S4-S9 optics, the detectors and the baseplate are cooled down to 80 K by a cryocooler with good established space heritage, life-time and reduced vibrations.
The VIS beam is sent to a VIS box (containing all VIS dichroics, optics and detectors) which is controlled to a temperature of 260 K and is situated in the upper part of the FPA.
Figure 24: FPA IR optical bench showing IR optics, detector and cables (image credit: SLSTR consortium)
2) Detection module: The detection module is formed by 9 detectors optimized for low photon flux and high temporal response. Three custom photovoltaic (PV) silicon detectors cover the VIS and NIR bands (0.4 µm to 1.0 µm), four HgCdTe PV detectors are used on the SWIR/MWIR band (1 µm to 4.4 µm) and two photoconductive (PC) HgCdTe detectors provide coverage for wavelengths beyond 10 µm.
The Si detectors are mated to companion TIA (Trans-Impedance Amplifier) arranged in a particular configuration to achieve high bandwidth with very high feedback impedance. The HgCdTe PV detectors are wire-bonded to readout integrated circuits (ROIC) that provide integration and multiplexing. The CAIA (Capacitive Trans-Impedance Amplifier) readout cells provide customized integrating capacitance for each band with high efficiency at low photon fluxes and high bandwidth. The HgCdTe PC detectors are designed for high responsivity and detectivity and are technologically critical, requiring a material cut-off wavelength close to 14 µm at 90 K (S9). They are mounted inside the DA (Detector Assembly) to operate, respectively, at about 260-270 K (Si) and 80 K (HgCdTe), and are connected to the FEE (Front End Electronics) through a combination of flexible (inside the DA) and standard (outside the DA) cables for a total length of 1.5 m.
To cover the visible bands, the PV Si detectors are realized with two different epitaxial layers and three anti-reflection coatings optimized in the region of interest in order to achieve a global quantum efficiency of 80%.
Each device is composed by four 185 x 205 µm elements, separated by a narrow guard ring to reduce dark current and improve the MTF characteristics. A TIA feedback resistance, in the order of 240 MΩ, is needed to meet the severe performance requirement causing non-trivial difficulties in achieving electrical bandwidth higher than 15 kHz.
The SWIR/MWIR detectors operate with low photo-generated currents at nominal dwell time of 40 µs. With these levels of photon fluxes, the classical direct injection (DI) structure does not operate, because of its electrical bandwidth and injection efficiency. These detectors are based on CTIA technology to enhance the electrical bandwidth of the ROIC input stage while its capacitance is designed to comply with the expected radiance for each band to minimize the readout noise that is the main contribution to the noise figure at low signal levels. The S4-S6 detectors are formed by two columns of four 100 x 100 µm elements to have off-chip oversampling of the observed scene, while the S7 detector is formed by single column of 200 x 200 µm elements.
Additional elements for the F1 fire detector are mounted inside S7; these are arranged in a column of four pixels with an active area of 25 x 100 µm to reduce the photo-generated current. These elements are in fact bonded to the ROIC in a BDI ( Buffered Direct Injection) technology of the input stage, to comply with 500 K target temperature. The 1 km response is obtained by integrating the reduced FOV signal for the dwell time, because the fire detector elements are mounted perpendicular to the along-scan direction. The BDI technique was chosen to implement the large capacitance needed to store the signal generated by high fluxes at high temporal variations but it is also able to handle the calibration signal from on-board 300 K BB at long observation periods with sufficient SNR.
The global quantum efficiency of the SWIR/MWIR devices is 80%, including the narrow band filter, with saturation charges and readout noises, respectively, of 0.6 M to 4 M and 150 to 350 electrons, depending on band, together with linearity of 1%. The global quantum efficiency of the MWIR fire elements is 70%, including the narrow band filter, with saturation charge and readout noise, respectively, of 20 M and 550 electrons, together with linearity of 1%. The photodiodes, the ROICs and the temperature sensors are mounted on the fan-out ceramics with thick film conductors as shown in Figure 25.
Figure 25: Illustration of the SWIR/MWIR 3D model assembly (image credit: SLSTR consortium)
The TIR detectors use the bulk photoconductor concept with particular layout to increase their responsivity. Although the increasing of the bias current would appear to be a straightforward way to improve the detector sensitivity, it involves also the increasing of the noise. Indeed, low current operation provides optimum SNR and an improved response spatial uniformity.
A responsivity as large as 150 KV/W is required to minimize the noise coming from the downstream electronics and in particular to the voltage noise of the OpAmp used for the high gain preamplifier. The resistor values of the PC elements are low and hence, the time constant is of few µs, due to the recombination time of the photo-generated carriers. The detectors contain two pixels, and each pixel is made with pairs of elements, one active and the other blind. The advantage of this arrangement is that the two parts are balanced when there is no radiation falling on the active part. This measure reduces the offset considerably as well as the thermal and ageing effects, since the signals from the active and blind part are differentially amplified. The global quantum efficiency of the LWIR (Long Wave Infrared) S9 elements is 70%, including the narrow band filter, with a detectivity of about 2.5 x 1011 Jones at 80 K, a linearity of 1%, and an element matching of 2%. Note: the 'Jones' is a unit of specific detectivity. - All detectors are in compliance with the SLSTR radiation requirements.
The SLSTR FPA requires special care in the routing, shielding and grounding of very low noise PC signals in the presence of high level PV signals. The FPA signals are received by the FEE (Front-End Electronics) in radiation-tolerant and redundant design with free fault-propagation circuits interfacing the detectors. The FEE operation is controlled by FPGA allowing some flexibility in the acquisition characteristics via ground commands. The main demands of the detection system are:
- Same temporal-spatial response for the VIS/SWIR, TIR and fire channels
- Extremely low thermal variation of the gain and offset
- Very low noise.
3) FEE (Front End Electronics): The FEE is subdivided into a common and a cold-redundant part as depicted in Figure 26.
The former (in red) includes the functions strictly associated with DU like bias and p/s voltages, differential receivers of the analog signals and temperature sensor switching. It is formed by nine separated and independent sections having dedicated protections to avoid failure propagation.
The latter comprises the functions associated to the video processing of the analog signals, including amplification, filtering, offset correction, sampling and/or integration up to the A/D conversion, the generation and distribution of the timing signals, the HK (Housekeeping) signal acquisition, the power supply filtering, the LVDS interface for the synchronization signals, and the SpaceWire interface for command/telemetry. To manage the 20 analog chains, the FEE architecture uses 10 radiation hardened ADC RHF1401, working in parallel and interfacing to the FPGA RTAX2000 though 10 private buses.
Figure 26: Block diagram of the FEE (image credit: SLSTR consortium)
Three types of analog processing are employed for the VIS, SWIR/MWIR, and the TIR signals. The implementation of the F2 channel is of some interest; it is derived from the S8 chain after the preamplification and offset correction as shown in Figure 27.
Figure 27: Concept of the S8 and F2 integrating stages (image credit: SLSTR consortium)
Each PC signal is generated by a bridge configuration, using the active and blind elements whose bias currents can be adjusted by resistors, to reduce the dc offset when blanked. It is amplified by using two stages with an intermediate offset correction feature for a total gain of about 300. The PC preamplifier uses the RH1028 with excellent noise performance having a voltage noise less than the noise of a 50 Ω resistor. However, careful attention has to be paid to limit the effect of the bias noise.
Multiple acquisitions with the decimation technique are used to reduce the ADC (Analog Digital Converter) noise to about 100 µV rms (~ 1 LSB of 14 bit ADC) with each data coded in a 16 bit string. In this way the ADC noise contribution to the overall noise is further reduced allowing fixed amplification, because each channel noise is dominated by the corresponding detector noise.
SLSTR budget: The SLSTR instrument has a nominal mass of 140 kg (160 kg max) and the nominal power consumption of 155 W.
Topography payload: (SRAL, MWR, GNSS receiver, DORIS, LRR)
The objective of the topography mission is to provide measurements over the open Ocean, coastal zones, ice sheets, rivers and lakes. Measurements over open oceans will contribute jointly with other operational missions to the realization of a permanent Global Ocean Observing System (GOOS). The main parameters measured over the open sea are SSH (Sea Surface Height) and SWH (Significant Wave Height) allowing to retrieve sea surface wind speed.
The science goals of the topography mission can be summarized as: 29)
• to continue and extend the current set of altimetry measurements at least at the level of quality of the Envisat RA (Radar Altimeter)
• to provide along-track SAR processing to improve acquisitions for coastal zones, in-land water and sea-ice topography
• to provide open loop tracking through an onboard stored DEM (Digital Elevation Model) to improve acquisitions over inhomogeneous or rough topography.
All altimetry products will be delivered as NRT (Near-Real- Time) within 3 hours after acquisition with an orbit estimate from the GNSS receivers. STC (Standard Time Critical) and NTC (Non Time Critical) products will improve orbit estimates with complementary information from DORIS and the laser reflector.
SRAL (SAR Radar Altimeter):
SRAL is a redundant dual-frequency (C-band + Ku-band) nadir-looking altimeter instrument, and the core instrument of the topographic payload. The overall objectives are to provide altimetric data (basic measurements of surface heights, sea wave heights and sea wind speed) relative to a precise reference frame. SRAL has a strong heritage of the instrument techniques implemented for the Poseidon-3 altimeter on Jason-2 (launch June 20, 2008), SIRAL (SAR Interferometer Radar Altimeter) on CryoSat-2 (launch April 8, 2010), and AltiKa (Altimeter in Ka-band) on the SARAL mission of ISRO and CNES (launch 2012). The SRAL instrument is being developed at TAS (Thales Alenia Space) of Toulouse, France. 30) 31) 32) 33)
The SRAL radar uses a linearly frequency-modulated pulse (chirp) and the pulse compression is carried out on-board by means of the deramp technique. The main frequency used for surface height measurements is the Ku-band (13.575 GHz, bandwidth=350 MHz), whereas the C-band frequency (5.41 GHz, bandwidth=320 MHz) is used for the ionospheric corrections. The frequency plan is compliant with the ITU (International Telecommunication Union) regulations. A 50 ms pulse duration for both frequencies has been sized as a trade-off result between a high BT product and the timing constraints of the burst pattern of the SAR mode.
Figure 28: Comparison between Jason (black) and Sentinel-3 (purple) ground tracks for a complete cycle (image credit; ESA)
Figure 29: The measurement principle of the topography payload (image credit: ESA)
Figure 30: A diagram of the corrections applicable to the altimeter range measurement and the contributions to the height of the instantaneous sea surface above a reference earth ellipsoid (image credit: Gary M. Mineart) 34)
The SRAL altimeter instrument is made of one nadir looking antenna subsystem which is externally mounted on the satellite +Zs panel and central electronic chains composed each of a DPU (Digital Processing Unit) and a RFU (Radio Frequency Unit). The central electronic chains are mounted inside the satellite on the -YS panel and are treated according to a cold redundancy scheme.
Figure 31: SRAL accommodation on the Sentinel -3 spacecraft (image credit: ESA)
SRAL modes of operation:
The SRAL instrument includes measurement modes, calibration modes and support modes. The measurement modes are composed of two radar modes associated to two tracking modes. The two radar modes are the following:
• LRM (Low Resolution Mode). It refers to the conventional altimeter pulse-limited resolution mode (so far, the LRM mode is being used on all altimetry missions). It consists of regular emission/reception sequences at a fixed PRF (Pulse Repetition Frequency) of around 1920 Hz leading to an ambiguity rank of 10.
• SARM (SAR Mode): This is a high along-track resolution mode composed of bursts of Ku-band pulses.
These modes are associated to two tracking modes which consist of the following:
- Closed-loop mode: refers autonomous positioning of the range window (ensures autonomous tracking of the range and gain by means of tracking loop devices implemented in the instrument).
- Open-loop mode: refers to the positioning of the range window based on a-priori knowledge of the terrain height from existing high-resolution global digital elevation models.
The open-loop is intended to be used instead of the more conventional closed-loop tracking over some surfaces, to improve the acquisitions over inhomogeneous or rough topography. While in open-loop, the setting of the tracking window of the altimeter is driven by predetermined commands, stored on board, combined with real-time navigation information available from the GNSS receiver. The main advantage is that the measurements are continuous, avoiding the data gaps typical of closed-loop tracking, which has problems in tracking the rapid topographic changes at coastal margins and in mountainous regions.
Table 6: Summary of SRAL support modes
Figure 32: Comparison of a conventional pulse-limited radar altimeter’s (a) illumination geometry (side view) and footprint (plan view) and (b) impulse response, with a delay/Doppler altimeter’s (c) illumination geometry and footprint and (d) impulse response (image credit: JHU/APL) 35)
Figure 33: Instrument architecture of SRAL (image credit: TAS, ESA)
The SRAL instrument generates either C-band or Ku-band pulses in order to simplify the hardware design. However, the periodic emission of elementary patterns (1 C-band pulse is surrounded by 6 Ku-band pulses, denoted by 3Ku/1C/3Ku) ensures a sufficient correction of the ionosphere bias (Figure 34).
Figure 34: The LRM transmit/receive pattern scheme (image credit: ESA)
After de-ramping and digital processing, the echo received from each pulse is sampled on 128 points corresponding to a 60 m range window. The C- and Ku-band echoes are submitted each to a FFT (Fast Fourier Transform) to return to the time domain after deramp. Then, C- and Ku-band echoes are accumulated separately over a 50 ms cycle corresponding to an accumulation of 84 Ku-band pulses and 14 C-band pulses over that cycle.
SARM (SAR Mode): The implementation of a nadir SAR mode provides an enhanced along-track (azimuth) resolution (~ 300 m) w.r.t. the LRM mode. This feature allows to acquire height measurements over along-track sliced areas sampled at the 300 m resolution. It is of prime interest to discriminate finely sea/ice transitions, sea/land transitions in a coastal area or inland water areas.
The SAR mode consists of periodical emissions/receptions of bursts composed of 64 Ku-band pulses surrounded by 2 C-band pulses (Figure 35) again for ionosphere delay correction. The 64 Ku-band pulses are generated coherently within a burst to carry out azimuth resolution enhancement on a burst basis by means of Doppler filtering. The burst emission / reception cycle is completed before the next burst cycle. The burst cycle duration is about 12.5 ms in such a way that a 4-burst cycle is equal to the LRM cycle of 50 ms. The PRF within a burst is around 18 kHz.
Figure 35: The SAR burst pattern scheme (image credit: ESA)
Figure 36: Final shape of resolution cells in SAR mode (image credit: ESA)
Calibration mode: Two specific calibration modes have been designed to refresh the calibration parameters required for ground processing and to monitor the good health of the instrument in flight configuration. The CAL-1 mode allows to calibrate the internal impulse responses (range and azimuth impulse responses in C- and Ku-band) whereas the CAL-2 mode allows to calibrate the gain profile of the range window by averaging thermal noise measured at each C- and Ku-band antenna port.
Table 7: Some SRAL instrument parameters
The “dual-like” features of the SRAL instrument (dual frequency, dual radar mode, dual tracking mode) make it possible to acquire very accurate topography data over all types of surfaces covered by the Sentinel-3 mission. And the “dual” central electronic chain ensures a high degree of reliability.
SRAL antenna: The antenna is made up of a 1.20 m parabolic reflector with a C/Ku dual frequency feed horn placed in a centered configuration at a focal length of about 430 mm. The feed is supported by 3 struts separated by a 120º angle: Two of them are doubled to improve the sidelobe ratio performance and the third one supports the Ku-band waveguide. It must be pointed out that the position of the strut ends on the reflector to match the reflector brackets position in order to improve the mechanical robustness of the antenna.
The antenna provides a minimum gain of 41.5 dBi in Ku-Band and 31.6 dBi in C-band at bore sight in the signal bandwidths. The side-lobe level is lower than –18 dB in Ku-band in order to minimize the Range Ambiguity Ratio.
The SRAL antenna is manufactured by MDA (MacDonald Dettwiler and Associates Ltd.), Richmond, BC, Canada. The center-fed linearly-polarized antenna has a mass of < 7 kg. 36)
Figure 37: Photo of the SRAL EM antenna (image credit: TAS, ESA)
RFU (Radio Frequency Unit): The RFU equipment (Figure 38) is made up of slices which are stacked together except the C- and Ku-band duplexers that will be fixed independently on the satellite panel. The RFU up-converts chirp signals from 50 MHz to C- and Ku-band and provides an output power of 38 dBm in Ku-band and 43 dBm in C-band. The up -conversion stage also includes an expansion of the chirp bandwidth by a factor of 16. Received echoes in C- and Ku-band are deramped down to 100 MHz. The deramp output produces a useful signal bandwidth of 2.86 MHz which is then processed by the DPU.
Figure 38: Illustration of the RFU device (image credit: TAS, ESA)
DPU (Data Processing Unit): The DPU is a rack of 6 boards plus an interconnection board. Its main functions are:
• Generation of a chirp signal centered at 50 MHz at PRF (Pulse Repetition Frequency) rate
• Processing of deramped echoes including digitization, I/Q demodulation, FFT and echo accumulation
• Transmission of science data on SpaceWire link
• Echo processing (range and tracking) for the closed loop mode operation
• Storage and management of on-board Digital Elevation Model for Open-Loop tracking
• Management of 1553 TM/TC interface with the platform
Figure 39: Illustration of the DPU device (image credit: TAS, ESA)
MWR (Microwave Radiometer):
MWR is a nadir looking sounder, operating at 23.8 and 36.5 GHz (K/Ka-band) covering a bandwidth of 200 MHz in each channel. The objective is to provide water vapor and cloud water contents in the field of view of the altimeter, necessary to compensate for the propagation delay induced by these atmospheric components and affecting the radar measurements. Such corrections are only possible over the ocean, where the background noise is stable and can be quantified either by the 3rd (optional) radiometer channel, or derived from the altimeter measurements of the backscattered power. Alternatively, over ice and land surfaces where MWR data cannot be used, wet troposphere corrections will be derived based on global meteorological data and dedicated models.
The MWR instrument is being developed by EADS CASA Espacio (ECE) under contract with Thales Alenia Space France (TAS-F) and ESA. MWR measures the thermal radiation emitted by Earth (brightness temperature). The received signal is proportional to the abundance of the atmospheric component emitted at the observed frequency and the sea-surface reflectivity. This information reveals the delay added to the altimeter pulses by moisture in the troposphere. 37) 38) 39)
The MWR instrument is comprised of the following part elements: antenna assembly, REU (Radiometer Electronics Units), the main structure and the thermal control hardware. Both K-band and Ka-band channels are fully redundant, except for the antenna assembly, with cold redundancy without cross-strapping.
Figure 40: MWR block diagram (image credit: ESA)
Table 8: Main characteristics of the MWR instrument
Conceptually, the MWR is a balanced Dicke radiometer for brightness temperatures below the Dicke load temperature. The balancing is achieved by means of a noise injection circuit. For brightness temperatures higher that the Dicke load temperature a conventional Dicke mode is used. The radiometer employs a single offset reflector of 60 cm in diameter and two separate feeds for the two channels. Calibration is achieved through a dedicated horn antenna pointing at the cold sky.
Figure 41: Block diagram of the detailed functional architecture of the MWR instrument (image credit: ESA)
The REU (Radiometer Electronics Unit) consists of the RFFE (Radio Frequency Front End) and the RPM (Radiometer Processing Module). The RFFE is located as close as possible to the measurement feeds to optimize the length of the waveguides and thus the radiometric performance. It contains the amplifiers, switches (calibration and redundancy) and other performance determining elements. The RPM contains the thermal control, the RFFE control, the noise injection loop, the power supplies (also for the RFFE) and provides the electronic interface to the platform. The REU includes a mode to blank the receiver inputs when the radar altimeter emits its pulses to avoid potential disturbances. This mode is accessible by ground commands.
Antenna assembly: The antenna assembly consists of the main nadir-looking reflector that has a diameter of 60 cm, the two measurement feeds and a calibration feed-horn, also known as sky horn. The main antenna is a single offset reflector fed by one horn for each channel. The calibration antenna is based on a wide band single corrugated horn and a frequency diplexer. Both antennae receive a single linear polarization and their signals are routed towards the receivers by means of separate waveguides.
The antenna assembly receives the noise temperature emitted by the objects within the antenna field of view. Discrimination between the different measurement frequencies is done by using different feed horns, each covering a separate frequency band. A separate sky measurement is provided by means of a dedicated sky horn. In this way, the satellite can continue the regular nadir measurements without the need of any maneuvers to turn over the satellite to look at the cold sky. The different frequencies received in the sky horn, are separated by a wave guide diplexer. The signals received by the feeds are guided towards the receiver electronics by means of waveguides. The physical temperatures of the different sections of the antenna assembly are measured and sent to the RPM (Radiometer Processing Module) of the REU.
The RFFE (Radio Frequency Front End) is manufactured by Thales Alenia Space Italia (TAS-I) and is implemented in a very compact design of 200 mm x 290 mm x 120 mm, a total mass of 5 kg, and 10 W power consumption.. Each RFFE channel holds a MSA (Microwave Switching Assembly), a redundant receiver (Rx) and a NS (Noise Source). The RPM is comprised of the FPGA-based radiometer control and acquisition and the DC/DC converters. It provides the electrical interface of the MWR to the Sentinel-3 SMU.
Figure 42: Illustration of the RFFE instrument (left), outline of the RPM (right) image credit: TAS-I)
The MSA includes the Quad-switch, made of 4 ferrite switches, which is in charge of the selection of the nominal/redundant and observation/calibration paths. A redundant driver provides the required pulsed current to the switches so that one of the following two states is selected:
1) Nominal → Observation / Redundant → Calibration, or
2) Nominal → Calibration / Redundant → Observation.
The RF part of the Quad-switch is the only non-redundant section of the RFFE, but its reliability is estimated to be better than 0.99924 (11.56 FIT). The MSA is completed with a waveguide coupler for the noise injection and the Dicke switch with a 25 dB matched load. Measurements data from the EQM model shows a remarkable performance in terms of return loss (> 25 dB), isolation (> 40 dB) and insertion loss (< 1 dB) for the MSA, with switching times lower than 2.5 µs and low power consumption.
The NS contains a single noise diode and two cascaded PIN switches, providing high isolation (> 80 dB) in the off state. Additional attenuators are included inside the NS to tune the ENR (Excess Noise Ratio) at the output of the Dicke switch to be around 0 dB. Fine tuning can be also achieved by adjusting the current to the noise diode. The Rx is a superheterodyne receiver; its first element is the LNA, which provides low noise figure and permits good instrument sensitivity even with a SSB (Single Side-Band) architecture. The RF signal is down-converted to an intermediate frequency of 2.5 GHz, which is common to both frequency modules. The local oscillator is based on an X-band DRO and frequency multiplier, while the bandwidth selection is achieved by means of a microstrip band-pass filter centered at the IF frequency. A distributed Rx gain of some 65 dB allows the input noise power to be detected through linear behavior of all Rx components. The square law detector diode is followed by a 4th order Butterworth filter with a cut-off frequency of 50 kHz. The LPF is optimized taking into account the sampling frequency of the ADC (100 kHz) and the offset produced in the retrieval of Ta when the filter’s group delay is increased.
Figure 43: Conceptual view of the MWR instrument and accommodation on the Sentinel-3 spacecraft (image credit: ESA)
Instrument operation: The MWR is a digitally controlled NIR (Noise Injection Radiometer) instrument. It measures the brightness temperature of the observed scene (the surface of the Earth through the atmosphere, as the instrument is pointing to nadir in its nominal attitude). The noise captured by the RF receiver at both operating frequencies (23.8 and 36.5 GHz) is compared to a very stable Dicke load. This load produces a noise power equivalent to its physical temperature, which is precisely known, and to the receiver bandwidth. The NIR principle reduces the effect of receiver gain and offset instabilities and has successfully been implemented in the frame of the SMOS mission. Every Dicke period is divided into three phases (Figure 44):
- The first half of the Dicke cycle contains two phases: First, a precisely known noise contribution is added to the scene measurement. Second, only the noise power from the scene is received.
- During the second half of the Dicke cycle the noise of the Dicke load is measured.
The working principle is based on the balance between the total amount of power received in the first and second half of the Dicke cycle. This is achieved by varying the duty cycle of the noise injection during the first half of the Dicke cycle. - Then, the injected pulse length is integrated along several Dicke cycles. With this approach, the performance of the instrument is independent of variations of the receiver gain, as all its contributions are cancelled in the balancing process. As a result, the brightness temperature captured by the antenna (Ta) can be derived from known parameters:
Ta = Tref - η Tn (1)
Tref being the physical temperature of the Dicke load, Tn the temperature of the noise injected into the system, and η the length of the noise injection (normalized to half a Dicke cycle; equation (1) means that noise has been injected during the whole first half of the Dicke cycle). Equation (1) can only be used in case Ta is below Tref. If this condition is not met (for very hot scenes, or for a cold receiver), the instrument loses its balanced condition and works in a DNB (Dicke Non-Balanced) mode, in which η goes to 0 and the antenna temperature is retrieved from voltage difference (Ve) between the power integration of the first and the second half of the Dicke cycle:
Ve = G x (Ta-Tref) ⇒ Ta = Ve/G + Tref (2)
where G is the gain of the receiver that is also periodically calibrated. Equations (1) and (2) are the so-called NIR and DNB equations. Both η and Ve are integrated over 12 Dicke cycles, and then included in the science TM packets.
Figure 44: Power detected along one Dicke cycle (image credit: ESA)
A dual-frequency instrument based on GPS constellation - and optionally on Galileo. The objective is to provide data for precise orbit determination (POD), established after ground processing. In addition, the GNSS receiver will provide real-time navigation bulletins periodically, as required by the open loop tracking mode of the altimeter, with an accuracy of about 3 m rms. This information is used to control SRAL's open-loop tracking and for Sentinel-3 navigation. Ground processing yields the altitude to an accuracy of < 8 cm within 3 hours for operational applications, and 2 cm after some days of refinement.
The 11 kg GNSS receiver can track up to 12 satellites at the same time. The signals transmitted by the navigation satellites are also disturbed by the ionosphere. The effect is corrected by comparing two signals at different frequencies within 1160-1590 MHz.
LRR (Laser Retroreflector):
LRR is a passive device, composed of a set of corner cubes (mass of 1 kg). The LRR is mounted on the Earth panel of the spacecraft. Its purpose is to enable the accurate localization of the satellite from the ground, through laser ranging techniques. A network of laser ground stations (SLR) will be used for this purpose and their measurements will contribute to refining and validating the POD solutions derived from GNSS data.
Sentinel-3 (S3) Ground Segment:
The main objective of the Sentinel-3 mission is the provision of ocean observation data in routine, long term and continuous fashion with a consistent quality and a very high level of availability. In addition, the mission will be designed to generate land optical observation products, ice topography, vegetation and land hydrology products. The different elements composing the Sentinel-3 system and the related interfaces for the provision of the operational marine and land services are shown Figure 45. 40)
In the frame of the Cooperation Agreement on the GMES space component, ESA and EUMETSAT have established a specific implementing arrangement concerning the cooperation on Sentinel-3. On this basis, following completion of spacecraft in-orbit commissioning, EUMETSAT will be responsible for the operation of the mission covering the products for the marine user community and the monitoring and control of the spacecraft in routine phase.
Figure 45: Overview of the Sentinel-3 system (image credit: ESA, EUMETSAT)
The GMES Sentinel-3 ground segment is in charge of the overall commanding and monitoring of the spacecraft constellation (2 S/C) as well as the acquisition, processing and dissemination of their observational data. The two primary components of the ground segment are the FOS (Flight Operation System) and the PDGS (Payload Data Ground Segment).
The different elements composing the Sentinel-3 system and the related interfaces for the provision of the operational marine and land services include (Figure 45) :
• The Sentinel-3 satellite(s), which produce and downlink the observation data
• The Sentinel-3 Ground Segment, which acquires the observation data and produce the operational products
• Interfaces to:
- The S3-Mission Management charged with the overall responsibility for the routine mission of the Sentinel-3 system
- DAS (Data Access Ground Segment) as the gateway between the users and the Sentinel-3 Ground Segment. The DAS forwards users’ requests to the PDGS and delivers the related data products generated by the PDGS. The DAS is capable to manage users’s communities, providing access to different classes of Users, with different priorities, specific rules, access privilege and security limitations
- SSALTO (Segment Sol multimissions d'ALTimétrie, d'Orbitographie et de localisation précise) operated under responsibility of CNES. SSALTO is a multi-mission ground segment which interfaces with the FOS to provide command information for DORIS and it will interface PDGS to receive DORIS mission raw data and to provide auxiliary data for PDGS POD (Precise Orbit Determination)
- Expert Teams as a privileged set of users which support the Satellite Commissioning phase and for this reason interface with the PDGS.
FOS (Flight Operation System):
The FOS main responsibility encompasses the spacecraft monitoring and control, including execution of all platform activities and the commanding of the payload schedules.
• The Ground Station and Communications Network performing telemetry, telecommand and tracking operations within the S-band frequency. The S-band ground station used throughout all mission phases will be the ESA Kiruna terminal (complemented by the Svalbard and Troll as backup stations).
• FOCC (Flight Operations Control Center), including:
- The Mission Control System, supporting hardware and software Telecommand coding and transfer, HKTM (Housekeeping Telemetry) data archiving and processing tasks essential for controlling the mission, as well as all FOCC external interfaces
- The Mission Planning System (part of the Mission Control System), supporting command request handling and the planning and scheduling of spacecraft/payload operations
- The Spacecraft Simulator, supporting procedure validation, operator training and the simulation campaign before each major phase of the missions
- The Flight Dynamics System, supporting all activities related to attitude and orbit determination and prediction, preparation of slew and orbit maneuvers, spacecraft dynamics evaluation and navigation.
• A General Purpose Communication Network, providing the services for exchanging data with any other external system during all mission phases.
Two instances of the FOS exist in the Sentinel-3 system. Up to end of commissioning of the space segment (i.e. launch+5 months), the FOS instance based at ESA/ESOC is responsible for flight operations, following hand-over the responsibility for flight operations will be with the FOS instance based at EUMETSAT.
To optimize the use of resources, reduce the development risk, and to meet the challenging schedule, it has been agreed to adopt a specific cooperation approach in relation to the FOS implementation, IVV (Integration, Verification and Validation) and operations. The concept relies on:
- The use of the same facilities for the FOS main components, including Mission Control System, Spacecraft Simulator, Flight Dynamics specific modules
- The development under a Joint Team of common Operations Preparation (OPSPREP) products, including Flight Operations Procedures and Spacecraft Operations Database
- The provision by ESA/ESOC of specific services to EUMETSAT, including the TT&C ground stations, the back-up Control Center, the debris collision prediction, and operations support during all mission phases.
During all mission phases, ESA/ESOC will provide a set of specific services in support of the EUMETSAT FOS. This includes the use of the S-band ground station passes needed for satellite control. This service will rely on the ESTRACK Kiruna site as primary station, with Svalbard and Troll acting as back-up in support of contingency or special operations. The baseline is that two S-band passes per day are provided to EUMETSAT, which is compatible with the Sentinels operations concept. The S-band Services will cover a mechanism for provision of additional S-band passes upon EUMETSAT request.
Figure 46: Mission context showing the data distribution for an operational ocean forecasting system (image credit: ESA)
PDGS (Payload Data Ground Segment):
The access to the Sentinel data will be provided through a dedicated Ground Segment infrastructure where the PDGS is one building block. For Sentinel 3, this is currently implemented by ESA in conjunction with EUMETSAT. TelespazioVega Deutschland is leading the Core PDGS implementation, with ACS, Werum, and Telespazio Italy as partners. 41)
The PDGS is primarily in charge of receiving and processing the Sentinel-3 instrument payload data, including HKTM data; ensuring that satellite tasking is performed according to the overall GMES user requirements and satellite capabilities; guaranteeing that suitable Sentinel-3 products meeting the expected quality and timeliness constraints are available to the GMES Users; and of all necessary support activities.
For the Sentinel 3 mission, the following product delivery timeliness are foreseen:
• NRT (Near Real Time) products made available to the users in less than 3 hours after sensing.
• STC (Short Time Critical) products made available to the users in less than 48 hours after sensing.
• NTC (Non-Time Critical) products made available less than 1 month after sensing.
As per mission requirements and on-board instruments, the processing is broadly grouped into Land data processing and Marine data processing.
Distributed Centers: The Sentinel-3 PDGS functions are distributed across a number of centers and locations, as illustrated in Figure 47.
Figure 47: Sentinel-3 PDGS configuration (image credit: ESA, EUMETSAT)
• A primary CGS (Core Ground Station) at Svalbard, providing X–band service, and where the acquisition and ingestion function is deployed, together with the NRT processing chains for the Land products.
• A backup CGS that provides a backup X-band service covering for planned unavailability of the prime CGS X-band service.
• A Land PAC (Processing and Archiving Center) for the NTC production of the OLCI products.
• A Land PAC for the STC/NTC production of the SRAL and MWR products.
• A MPC (Mission Performance Center) hosting the Mission Performance Monitoring function for Land products
• A Marine Center, hosting the NRT and STC/NTC production for all Marine products, as well as their own Mission Performance Monitoring function.
Specific platforms are deployed at the centers to support different activities: operational platform for the routine operations, reference platform for validation, reprocessing platform to support reprocessing campaigns, and development platforms to support maintenance and evolution of the system.
Automation: The Sentinel-3 PDGS operational concept is driven by the requirement of largely automated operation with no routine manual interaction required for nominal processing. All PDGS elements are autonomously running in nominal operations during time periods where an operator is not available. This is achieved by:
• a data driven approach implemented across the whole PDGS, whereby software components are activated on the availability of the data they require as input
• machine-to-machine interfaces between components for propagating configuration and workflow changes.
For the routine operation (i.e. systematic processing, circulation, archiving, etc.), immediate operator intervention is only required in case of critical failures, that are identified by the monitoring component. A notification-based approach allows the system to execute the nominal workflows based on the availability or change of input data without explicit user intervention.
Nominal operations are completely automatic and can be managed without operator intervention.
NRT (Near Real Time) system: One of the key requirements driving the design of the Sentinel-3 PDGS is the capability to acquire, process, and make the data available to the users in Near Real Time, i.e. within 3 hours from sensing.
In the Sentinel-3 mission all the data downlinked by the satellite is processed in near real time, producing a complete set of Level 0, Level 1 and Level 2 products. Level 1 and Level 2 products are directly accessible by the end users, while the access to the Level 0 products is limited to special users.
The deployment of components across different ground stations adds a constraint to the architecture, as:
• Raw data is acquired in the CGS Center
• Land NRT products are processed in the CGS Center
• Marine NRT products are processed in the Marine PDGS Center, starting from the same raw data used in the Land case.
In order to cope with the NRT timeliness requirements and the processing deployment constraints, the design of the acquisition and ingestion functionality has been built focussing on two key concepts:
• data "granules" pipelined processing
• NRT data circulation.
According to the mission requirements, the data flowing from the satellites to the acquisition ground stations is not processed as a single monolithic entity after the completion of the passes. The chosen approach has been to divide the input data stream in size-optimized granules while the acquisition is on- going.
Marine Center: The Marine Center implements the processing, archiving, and dissemination for all marine products and all levels of processing, for NRT as well as NTC/STC timeliness. It relies on the following functions:
The Processing function applies all the necessary data processing levels, starting from the reconstructed payload science data (Instrument Source Packets) to produce Level 0 to the algorithms and formatting techniques to generate higher level products. The processing function is capable of producing the desired products in a systematic way. It is essentially composed of a processing management layer, controlling the data processors required for each level of processing and each timeliness.
The Long Term and Short Term Archives ensure the long-term and short-term storage of the payload data products and of the auxiliary data according to the longterm/short-term storage policy. These functions include all operations to be put in place to store and to circulate the data within the PDGS and to ensure their integrity according to the applicable requirements. They also include the inventory (at the appropriate level) of the stored data to enable their retrieval by the PDGS for internal purposes and by the PDGS users.
Figure 48: Schematic view of the Marine Center functions (image credit: ESA, EUMETSAT)
EUMETCast is the EUMETSAT generic multi-mission dissemination system based on the standard DVB-S (Digital Video Broadcast-Satellite) multicast technology. EUMETCast is the main dissemination mechanism for distribution of data to EUMETSAT’s end users (Ref. 41).
Sen3Exp (Sentinel-3 Experiment) campaign:
ESA conducted the Sen3Exp airborne campaign in June and July 2009. The campaign started in Barrax, La Mancha, Spain. An aircraft operated by the Spanish National Institute for Aerospace Technology (INTA), equipped with three hyperspectral imaging spectrometers, made two flights over the area. Meanwhile, satellite data were acquired by Envisat’s MERIS and AATSR instrument and by the CHRIS (Compact High Resolution Imaging Spectrometer) instrument aboard ESA’s PROBA-1 satellite. At the same time, ground teams, under the direction of Prof. Jose Moreno from the University of Valencia, made atmospheric radiometric and biophysical measurements. 42)
Figure 49: Photo of the CASA aircraft instrumentation (image credit: ESA)
Legend to Figure 49: Hyperspectral imaging spectrometers were installed on board INTA’s CASA-212-200 aircraft in support of the Sen3Exp airborne campaign. The AHS (Airborne Hyperspectral System) occupies the left-hand port; the CASI-1500i (Compact Airborne Spectrographic Imager) on the left and the SASI-600 (Shortwave Infrared Airborne Spectrographic Imager) occupy the right-hand port.
The campaign then moved to Pisa in Italy, from where a pine forest at San Rossore could be reached. At San Rossore, Prof. Federico Magnani from the University of Bologna oversaw the week-long ground measurement program. The dataset was again complemented with MERIS, AATSR and CHRIS satellite data.
In July, activities focused on the marine environment where measurements were taken at two oceanic sites: the Boussole monitoring buoy in the Ligurian Sea and the Aqua Alta Oceanographic Tower (AAOT) in the Adriatic Sea, close to Venice. Both sites have played an important role in supporting ocean color algorithm development and product validation for many years.
Figure 50: Photo of the AAOT (Aqua Alta Oceanographic Tower) in the northern Adriatic Sea (image credit: ESA)
Boussole typifies the global ocean, where the measured signal is determined solely by the absorption of phytoplankton. AAOT is in an area where there is both open ocean water and also water that is optically complex because phytoplankton, suspended sediments and colored dissolved organic matter also affect the measured signal. Such water can be found in all coastal regions and represents a challenge to analyze and interpret the data from spaceborne measurements.
A unique, comprehensive and valuable dataset has been created that will significantly support the development of the Sentinel-3 mission.
Permanent calibration station for altimeters in Crete with microwave transponder
The Technical University of Crete (TUC) is installing a new permanent microwave transponder ground infrastructure on the Island of Crete, Greece, to serve as an alternative and independent technique for the calibration of, mainly, European altimetric missions. The facility was initially planned as a calibration site for the Sentinel-3 in the south west of Crete, Greece, using the developed transponder. However, this ground infrastructure, along with other permanent facilities in Crete, may also be used for the calibration of other Ku-band altimetric missions such Jason-2, Cryosat-2, etc. 43)
The idea for incorporating land based transponders was initially introduced in 2000. 44) A microwave transponder is an electronic equipment which receives the pulsed radar signal, transmitted by the altimeter of the over-passing satellite and actively amplifies and retransmits the signal towards the spacecraft, where it is recorded. The time delay of the signal is measured, from which the absolute range between the transponder and the satellite can be deduced. The main advantage of this technique, compared to the conventional sea-surface calibration, stands for the fact that no ocean dynamics errors are involved in satellite altimeter’s calibration.
However, in the past, only few transponders have been built and implemented for this reason. The ESA premises in Svalbard, Norway host a transponder developed by RAL, UK in 1987 that has been used mainly for the Cryosat-2 calibration. The Gavdos Island Cal/Val facility in Greece hosted the Austrian Academy of Sciences transponder and that transponder has been effectively used for the calibration of Envisat and Jason-2 missions. There has been another transponder placed in Rome, Italy which was used for the Envisat sigma-0 calibration.
In 2011, the Geodesy and Geomatics Engineering Laboratory at the Technical University of Crete in Greece developed a new Ku-band microwave transponder. The TUC transponder is mobile, allowing calibration at different locations but also modular for operating in other frequencies, provided that some parts are modified. It is capable of recording the incoming and outgoing signals, while it can be controlled and operated remotely. The transponder frequency has been selected to be compatible with past, current and future European as well as international altimetry missions that operate in this microwave range (i.e., Jason series, Cryosat-2, Sentinel-3). Additionally, it is equipped with a GPS (Global Navigation Satellite System) receiver and appropriate meteorological sensors to provide precise time-tagging, as well as the atmospheric delay corrections during transponder calibration. This is of importance for the accurate determination of the altimetric range because the atmosphere affects the altimetric measurements. Furthermore, this prototype transponder is the only microwave transponder that incorporates circularly polarized antennas. The latter, allows performing calibration experiments on different satellite missions at the same location, approaching from different directions, providing that the satellite ground track is in a range of 3-5 km away from the transponder location.
Table 9: TUC’s transponder radio frequency characteristics
The TUC transponder has been characterized for 4 months (March-July 2012) at the CPTR (Compact Payload Test Range) facilities in ESA/ESTEC, the Netherlands.
The transponder has already been used for the calibration of several Cryosat-2 passes (10-May, 8-June and 3-August 2013) over the SLR2 (Satellite Laser Ranging 2) site (35° 32.084' N, 24° 04.061' E) in North West Crete, Greece, and a clear response has been captured on the satellite’s data (Figure 51).
Figure 51: Cryosat-2 SAR raw waveforms using the transponder at the SLR2 site in Crete, Greece, on May 10, 2013 (image credit: TUC)
A TUC transponder site has been selected on Crete Island which represents a triple cross-over point between Sentinel-3A, -3B and Jason-2&3 (and also Jason-CS, as it will most likely fly over the same Jason-series tracks). This criterion was used to finally define and freeze the ground tracks for Sentinel-3 mission.
The CDN2 (35° 20.729' N, 23° 46.577'E) site is exactly under Jason, 100 m east of Sentinel-3A and 300 m west of the Sentinel-3B ground tracks. The CNES team will verify the satellite signal observed using Jason-2 around the CDN2 candidate.
Figure 52: A triple cross-over point for Sentinel-3A (red), -3B (purple) and Jason series (yellow) exist at the CDN2 site in western Crete (image credit: TUC)
The instruments at the CDN2 site will be protected using either weather-proof boxes or a container with appropriate covers to avoid/reduce any satellite echoes by their metallic parts. Figure 53 illustrates an indicative spatial distribution of the necessary and ancillary instrumentation to be constructed at the CDN2 Sentinel-3 altimeter calibration site. Besides the instrumentation and infrastructure, the preparatory steps taken for the establishment of the CDN2 site involve also the development of appropriate software for data archival and transmission and for the determination of the transponder’s precise positioning.
Figure 53: General infrastructure layout of the CDN2 facility (image credit: TUC)
The Sentinel-3 altimeter calibration site is expected to be fully operational in early 2014, that is about one year prior to the Sentinel-3A launch. During this period, calibration campaigns for the Jason-2 and Cryosat-2 altimetry mission will be performed to test the transponder’s operational capabilities in real-field conditions. These campaigns will aim at: a) delivering altimeter calibration values for these satellites, b) getting familiarized with the remote operation procedures to be followed, and 3) identifying potential upgrades necessary for improving the transponder’s performance.
The transponder is to be upgraded, improved, and characterized before its final deployment and support for Sentinel-3A commissioning phase in 2015.
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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.
The Sentinel series:
Provides data continuity for:
Validation provided by: