Meteosat Third Generation
MTG (Meteosat Third Generation)
MTG is the next-generation European operational geostationary meteorological satellite system - a collaborative EUMETSAT/ESA program. Preparatory activities for the EUMETSAT MTG series started in late 2000 in cooperation with ESA, following the decision of the EUMETSAT Council to proceed with a Post-MSG User Consultation Process. The process is aimed at capturing the foreseeable needs of users of EUMETSAT's satellite data in the timeframe of 2015-2035. Typical development cycles of complex space systems are on the order of a decade or more. The current MSG (Meteosat Second Generation) system is expected to deliver observations and services until at least 2018.
MTG will continue services beyond this date and address future challenges in weather forecasting and other services for European citizens, such as improved air quality or UV-radiation warnings, as well as climate and atmospheric chemistry monitoring. 1) 2) 3) 4) 5) 6) 7) 8) 9)
In October 2008, an agreement was signed regulating the respective roles, responsibilities and financial commitments of the two organizations concerning future phases of the MTG program as well as the approval of the MTG payload complement:
• EUMETSAT will specify and consolidate the end-user requirements, the overall mission requirements, the space-to-ground requirements, and the ground segment requirements. In addition, EUMETSAT will be responsible for the overall mission and system engineering and ground segment design and development. Further along, it will fund the procurement of the recurring satellites, the launch services and launch and early orbit phases, and also execute commissioning and operations.
• ESA is responsible for the development and implementation of the space segment technologies and the first MTG twin satellites. ESA is funding the related cost, apart from 30% to be contributed by EUMETSAT. ESA will procure all recurring satellites as part of the EUMETSAT development and operations program.
The MTG series spacecraft and sensor complement are specified to be operated in-orbit for 20 years, compared to 15 years for MSG.
In March 2010, the MTG program took another step towards full approval with the EUMETSAT special Council, by accepting the MTG End-User Requirements Document which defines not only the deliverables to the user community, but also the duration of the operational service - at least 20 years for the imagery mission and at least 15 1/2 years for the sounding mission - the number of satellites, and the satellites in-orbit lifetime. 10)
• On February 25, 2011, the full MTG program entered into force. The consolidation of the full MTG program, as now approved, resulted from a long and intensive process through which the initial studies to define user needs for the post-MSG (Meteosat Second Generation) era led to the establishment of end user requirements for the new geostationary program, followed by the early definition phases of MTG until the end of 2007, when dedicated MTG activities started at EUMETSAT under the framework of the MTG Preparatory Program. 11)
- The SRR (System Requirements Review) for the space segment was completed in April 2011. This is being followed by preparation of the PDR (Preliminary Design Review) for late 2011.
Table 1: MTG overall timeline
More than 50 leading experts in a variety of disciplines were involved, representing operational and research organizations from Europe, the United States and other international partners, as well as WMO (World Meteorological Organization). The definition of the requirements was driven by the EUMETSAT customers’ long-term strategic objectives, namely the most business-critical improvements to meteorological and environmental services to be achieved in the 2015-2025 timeframe. The main customers include National Meteorological Services and other operational organizations from EUMETSAT Member States, the ECMWF (European Centre for Medium-Range Weather Forecasts) and EUMETNET. Identification of candidate observing techniques was also performed, together with the preliminary assessment of their capabilities and suitability to satisfy customer needs. 12) 13) 14) 15)
Based on these assessments, five candidate observation missions were identified for MTG: 16)
- HRFI (High Resolution Fast Imagery) mission. For MTG, the most stringent requirement is the absolute geolocation knowledge error needed for the HRFI imaging mission: the Earth location of acquired samples needs to be determined with accuracy (knowledge) better than 250 m at SSP (Subsatellite Point) (i.e. 7 µrad for a sample distance of 0.5 km) with a 68.26 % confidence level over the HRFI coverage.
Further, the timeliness requirements are also very demanding. For the HRFI imagery level 1b data need to be delivered within 150 seconds of their acquisition. This sets constraints on the number of observations that can be accumulated and used in the image navigation processing. Therefore, efficient algorithms for the processing of image and on-board data are requested.
- FDHSI (Full Disk High Spectral Imagery) mission
- IRS (Infrared Sounding) mission. For the IRS sounder, the requirement is more relaxed, and amounts to 800 m at SSP (i.e. 22 µrad) and at 68.26% confidence over a LAC (Local Area Coverage), equivalent to a quarter of the Earth. However the sounder has a demanding pointing stability requirement of 300 m at SSP and at a 68.26% confidence level, over the dwell time of about 9 seconds.
- LI (Lightning Imagery) mission
- UVS (UV-VIS Sounding) mission.
Regarding the MTG missions, the satellite availability shall be at least 96% calculated on an annual basis for the duration of the satellite nominal operational life. From this outage, 1% is allocated to unscheduled outages (e.g. safe mode) and 3% to scheduled outages (e.g. station keeping maneuver and operations like satellite decontamination). Therefore disruptions due to orbit control maneuvers must be minimized, using appropriate station keeping strategies and satellite performances.
INR (Image Navigation and Registration) requirements: An INR system concept, generally applicable to all MTG instruments, but exemplified here for the FCI only, has been developed during the Phase A study. The concept has some affinity to the GOES-N INR system (Ref. 16).
Figure 1: INR and FD (Flight Dynamics) system architecture design (image credit: EUMETSAT)
The INR system shown in Figure 1 has been chosen for the simulation to prove the feasibility to MTG geolocation requirements. On the satellite, a combination of star tracker and gyroscope are the essential elements to determine the space bus attitude. The ST (Star Tracker) determines an absolute pointing reference while the gyroscope records relative changes in the accelerations. The star tracker uses an on-board star catalog, which is uploaded to the spacecraft (about every six months). Star tracker and gyro data are inputs to the Attitude Determination unit which calculates the necessary updates of the attitude. A control signal is sent to the reaction wheels (RW) to point the spacecraft towards the center of the Earth and correct for bus disturbances. The bandwidth of the control signal is typically significantly smaller than the bandwidth of the signals coming from the ST/gyro unit, which is providing typically gyroscope measurements of some tenth of Hertz.
The basic orbit and attitude estimation accuracy performance that can be obtained with standard tracking system and on-board attitude sensors is insufficient for meeting the image navigation requirements of MTG. The performance of this basic approach is therefore complemented by a simultaneous INR processing based on instrument observations. The derived performance estimates from simulations indicate that the combined approach is sufficient for fulfilling the MTG INR and operational requirements.
Figure 2: Orbit determination strategy with and without INR (image credit: EUMETSAT)
Following Phase-A studies and consolidation of MTG mission definition, a baseline for the EUMETSAT/ESA Phase-B activities and for the preparation of the EUMETSAT full MTG Program Proposal was agreed composed of the Flexible Combined Imager (FDHSI and HRFI missions), the IRS (Infrared Sounder) mission, the LI (Lightning Imager) mission and the accommodation of GMES Sentinel 4 instrument (UVS mission) to be provided by ESA.
The MTG satellite system concept is based on a twin configuration of 3-axis stabilized satellites:
• MTG-I (MTG-Imaging mission satellite)
• MTG-S (MTG- Sounding mission satellite)
The target date for the operational deployment of the MTG space segment element, ensuring continuity of the the imagery mission, is now the end of 2018. This requires a launch in early 2018 with a one year commissioning phase anticipated for the the new MTG system, including ground and space segments. Figure 3 shows the baseline MTG deployment strategy. This has a direct impact on the envisaged overall mission development and deployment schedule. High mission and satellite reliability and availability are required combined with the nominal and extended mission lifetime for both the MTG-I and MTG-S satellites. 17)
EUMETSAT, in coordination with ESA and industry, have established priorities vis-à-vis of the overall mission. The highest priority being for the Imaging missions (HRFI + FDHSI), which will provide continuity to the current MSG mission.
The EUMETSAT MTG program includes overall system activities, development of the ground segment, procurement of the recurrent satellites. The imagery mission will consist 4 MTG-I satellites, whereas the sounding mission will be fulfilled with 2 MTG-S satellites.
It is planned to use a common bus design for the MTG-I and MTG-S satellites with local adaptations as required per mission. It is expected that the following bus subsystems will be largely common for both satellite types:
- Structure (separate mechanical interfaces are foreseen for the two payload complements)
- Thermal (conventional thermal control design envisaged)
- Attitude and Orbit Control (similar design approach for both satellites)
- Propulsion (based on a chemical propulsion system)
- Electrical Power (being modular to adapt the power needs as required per satellite type)
- Command and Data Handling (similar design envisaged for both satellites with adaptation per satellite)
- Telemetry, Tracking and Command (heritage of existing conventional S-band systems).
Legend to Figure 3: The figure shows the anticipated overlap coverage between the MSG (Meteosat Second Generation) and MTG (Meteosat Third Generation).
Following on from MSG (Meteosat Second Generation), MTG is a cooperative venture between EUMETSAT and ESA, and will ensure continuity of high-resolution meteorological data to beyond 2037. The cooperation on meteorological missions between EUMETSAT and ESA is a success story that started with the first Meteosat satellite in 1977 and continues today with the MSG series and the polar-orbiting MetOp series.
In February 2010, ESA selected a consortium led by TAS (Thales Alenia Space) of France and Italy and OHB Technology of Germany with its subsidiary Kayser Threde (KT) to build Europe's next-generation meteorological satellites. The six MTG spacecraft - four imaging and two equipped with sounders - will operate from GEO (Geostationary Earth Orbit). However, the decision does not mean an immediate contract for the winning team. Instead, ESA will enter into final negotiations with TAS and OHB to settle open issues. 19) 20) 21)
On Feb. 24, 2012, ESA and TAS signed the MTG contract. Thales Alenia Space leads the industrial consortium that is building the MTG family. Along with being the prime contractor, TAS is responsible for the MTG-I imaging satellite, including the primary payload, the FCI (Flexible Combined Imager). Bremen-based OHB is responsible for the MTG-S satellites and the provision of the common six satellite platforms, supported by Astrium GmbH as the System Architect. The IRS (Infrared Sounder), to be flown on MTG-S, will be developed by Kayser Threde. 22)
Table 2: Summary of key requirements for MTG missions 23)
Figure 4: Overview of the MTG industrial core team (image credit: ESA, EUMETSAT, Ref. 18)
Table 3: Overview of main technical challenges of the MTG series (Ref. 21)
Figure 5: Artist's rendition of the MTG-I series spacecraft (image credit: ESA)
Structure: A two-winged satellite configuration is selected, which minimizes the solar momentum build-up and therefore the reaction wheels off-loading frequency and its associated mission outage. A yaw flip is performed around each equinox, to ensure that the satellite –Y side is always protected from sun illumination, which is beneficial to the thermal control of the instruments and AOCS fine sensors.
AOCS (Attitude and Orbit Control Subsystem): The AOCS supports a “fine pointing mode” to provide high pointing stability. Attitude determination is based on high performances gyrometer and multi-head star trackers necessary to ensure an accurate and stable Earth pointing. To make the most of their performances, the AOCS fine sensors have been accommodated inside the platform towards its cold side. This configuration is indeed the only one which provides sufficient protection with respect to the varying sun illumination conditions, and minimizes therefore the attitude restitution errors at sensors level. Actuation is provided by 5 reaction wheels, enabling the avoidance of zero crossing and the related outage. The reaction wheels are off-loaded by the thrusters. The reaction wheels are also in charge of the compensation of the disturbances induced by the instruments mechanisms (FCI or IRS). Microvibrations are minimized by implementing isolating devices for the main disturbances which are the reaction wheels and the instruments cryocoolers.
EPS (Electrical Power Subsystem): The EPS architecture relies on a regulated 50 V bus with current limiter protections. Power conditioning and battery management are autonomous and based on dedicated hardware functions. A PCDU (Power Conditioning and Distribution Unit) provides all the satellite units with the necessary equipment regulated lines and thermal heater distribution.
Avionics: The MTG spacecraft are based on a central avionics architecture, with central OBSW (OnBoard Software) in charge of all satellite subsystems and instrument high level management. The SMU (Satellite Management Unit) is at the core of the MTG spacecraft, and provides the processing resources and the interfaces to all platform units and the bus management. It is also the master of spacecraft autonomy and FDIR (Failure Detection, Isolation and Recovery) functions.
The satellite OBSW is based on a modular and hierarchic architecture, providing standard “bus” interfaces to all applications. System management (modes transitions, FDIR recovery actions) is parametrized by a system database and based on On-Board Procedures or action sequences.
The on-board architecture relies on 3 main communication buses:
• 1 MIL-STD-1553 dedicated to platform units
• 1 MIL-STD-1553 for the command and control of instrumentation
• A high rate SpaceWire (SpW) network dedicated to payload data and high rate payload telemetry.
The architecture furthermore ensures the modularity and parallel development of instruments and platform modules, through the following payload related functions:
• The ICU (Instrument Control Units), which are in charge of the FCI and IRS (most complex instruments) management and data-processing. The ICU is part of each instrument.
• The PDD (Payload Data-Downlink), which is in charge of the payload data acquisition and high rate downlink in Ka-band.
• A Common MTG Packet User Standard (MTG tailoring), which is implemented in the platform and instrument software.
Figure 6: General overview of the MTG-I (top) and MTG-S spacecraft (bottom), image credit: TAS
MTG SpaceWire architecture:
The MTG satellites accommodate, respectively, the FCI imager, LI imager and the DCP digital transponder for the Imager S/C (or MTG-I), and the IRS and UVN sounders for the Sounder S/C (or MTG-S), over a payload SpaceWire network for mission data distribution and instrument’s configuration with a total high rate telemetry of 295 Mbit/s (Imager S/C) and 557Mbit/s (Sounder S/C) after RS (Reed Solomon) concatenated encoding and encryption. 24) 25)
The payload data network is built around a DDU (Data Distribution Unit) that implements SpaceWire (SpW) routers for 3 instruments (FCI, LI, DCP or IRS1, IRS2, UVN) communication and one SMU (Satellite Management Unit) computer for INR auxiliary data collection and network management. The network supports full cross-strapping between each terminal (instrument’s and SMU) and DDU leading to a total of 16 terminal ports based on to independent nominal and redundant DDU SpaceWire-10X routers.
Figure 7: On-board data handling architecture of the MTG-I spacecraft (image credit: TAS)
Figure 8: On-board data handling architecture of the MTG-S spacecraft (image credit: TAS)
The network architecture is identical for both imager and sounder configurations. The network is running at 200 Mbit/s on all links providing large margins. The large margin vs data distribution and the asynchronous behavior of the SpW link, allow to accommodate variable instrument’s data flow according to their operational modes. For example the UVN instrument provides 40 Mbit/s in normal mode or 125 Mbit/s in commissioning mode.
The SpaceWire time code distribution is not used for payload synchronization due to the instrument heritage: a classical OBT (On-Board Time) associated to a PPS (Precise Positioning Service) pulse is broadcast through the payload Mil-Std-1553 command control bus.
Thanks to the implementation of SpW routers, the full-duplex capability of the SpW is used for command/control messages required to configure quickly the instruments without outage: large configuration tables are loaded from SMU mass memory into the instruments through SpW links; i.e. 135 Mbit of data are transferred between 2 consecutive image acquisitions.
All messages are formatted with ECSS (European Cooperation for Space Standards) PUS (Packet Utilization Standard) and distributed over the SpW network with the ECSS SpW CCSDS transfer protocol, using the user field for identifying the virtual channel for the high telemetry destination.
Figure 9: Schematic view of the DDU (Data Distribution Unit) within the on-board SpW configuration (image credit: TAS)
The MTG satellites are the first space mission in a TAS contract for implementing and using the complete SpW network capability with a full cross-strapping redundancy with SpW-10X routers and full duplex used for command/control configuration messages with large tables (Ref. 24).
RF communications: Use of S-band for TT&C functions. The S-band subsystem consists of redundant transponders and two hemispherical TT&C antennas. The payload data downlink uses the DDU (Data Distribution Unit), which gathers the data to be transmitted to ground, a transmitter and Ka-band antenna. The latter is folded at launch and then deployed once in orbit. It is mounted on a two-axis pointing mechanism so as to ensure accurate pointing of the spot beam towards the MTG ground station before and after the yaw flip, and when the satellite is repositioned on the geostationary arc.
House keeping telemetry is downloaded in real-time through both S-band and Ka-band links, whereas the mission telemetry is downlinked in Ka-band only.
Propulsion module: The propulsion module is accommodated on the platform central tube and shears webs. The MON (monopropellant) and MMH (Monomethylhydrazine) tanks are inside the tube. Further subsystems are: Helium tanks, the thrusters and LAE (Liquid Apogee Engine), the associated tubing, valves and pyros.
Astrium has developed a Unified Propulsion System (UPS), which is adapted for the MTG satellites and offers a complete pre-integrated drive system with 16 (sixteen) 10 N thrusters for orbit and attitude control, and one 400 N apogee engine, all fuelled from two propellant tanks (925 l) with hydrazine (MMH) and nitrogen tetroxide (NTO). After its release from the launch rocket, the 400 N apogee engine will propel the satellite from GTO to GEO . At this stage, the majority of the fuel, around 80% of the total, will have been expended. The remaining 20% of the fuel will serve the satellite’s 16 thrusters to maintain its exact orbit path for the 13-year mission, and for any adjustments required. 26)
Table 4: Some parameters of the MTG series spacecraft
Launch: A launch of the first MTG-I spacecraft is planned for early 2018 (Ref. 11).
Orbit: Geostationary orbit at an altitude of ~35,786 km, the nominal longitude of MTG-I-1 is 0º.
Sensor complement: (FCI, LI, IRS, UVN, GEOSAR, DCS)
The payload for MTG-S series is comprised of two instruments: the IRS (Infrared Sounder), and UVN (Ultraviolet Visible Near-infrared) Sounder. The UVN sounder will be provided by ESA as part of the Copernicus program and is referred to as the “Sentinel-4 GEO component mission”.Hence, a separate file, Sentinel-4, is provided on the eoPortal which describes only the UVN Sounder.
FCI (Flexible Combined Imager):
FCI is the follow-on instrument of SEVERI (Spinning Enhanced Visible and Infrared Imager) heritage flown on the MSG series missions of EUMETSAT which started with the launch of MSG-1 (Meteosat-8) on Aug. 28, 2002. Originally, there were two successor instruments defined for MTG - namely HRFI (High Resolution Fast Imagery) and FDHSI (Full Disk High Spectral Resolution Imagery) - whose requirements were eventually combined into one instrument named FCI (Flexible Combined Imager). This implied also some descoping actions of the requirements: 30)
- Drop of the capability of targeted observations (selectable area 6º x 6º)
- Reduction of FCI channels to 16 - originally considered: 28
- Redefinition of FCI channels more in line with current MSG SEVIRI performances
- Relaxation of longest wavelength channels (13.3 µm) compatible with more mature detectors technologies - originally considered: up to 14.9 µm.
The key requirements for FCI spatial and temporal data resolutions remain unchanged, namely:
• Spatial resolution: 0.5 / 1.0 km (solar channels) and 1.0 / 2.0 km (thermal channels)
- For imagery on local scales (1/4 of full disk) the spatial resolution is 0.5 km with 4 channels at high spatial resolution 0.5 km (2 solar), and 1.0 km (2 thermal channels)
- For full disk imagery the spatial resolution is 1 km (8 solar channels) and 2 km (8 thermal channels)
• Temporal resolution: BRC (Basic Repeat Cycle) = 10 minutes for a full disk image; BRC = 2.5 minutes for local scale (Europe / North Atlantic) retrievals.
The relaxations adopted by the MMT (MTG Mission Team) result in a system:
- which is less risky
- and has a more efficient development - by still enabling to fulfil the user’s needs.
The FCI instrument will outperform the SEVERI observations on cloud, aerosol, moisture and fire detection by adding new channels and by improving temporal-, spatial-, and radiometric resolution of the data.
Table 5: Overview of band specification/requirements for the FCI imager (Ref. 17)
The channels VIS 0.6, NIR 2.2, IR 3.8 and IR 10.5 are delivered in FDHSI spatial sampling and HRFI spatial sampling configurations. The spatial sampling and spectral requirements for the HRFI sampling configuration are indicated by #1. The fire application channels are marked with #2. All other channels are delivered in FDHSI sampling configuration. In total, up to 22 image colors and sampling configurations could be delivered by each image cycle, covering all the needs.
In the nominal imaging mode, the FDHSI covers the full Earth disk with a 10 minutes BRC (Baseline Repeat Cycle). RSS (Rapid Scan Services) are provided thanks to coverage equivalent to BRC/n referred to as LAC (Local Area Coverage). The LAC coverage can be variably placed anywhere over the Earth (this shall be taken into account for the scan mechanism qualification).
In particular for the FCI coverage (Figure 10), will be used for the RSS. The images will be delivered with the following considerations:
- Imagery data will now be delivered to the users at level 1c, e.g. after image rectification
- Co-registration error is specified at level 1b as knowledge; the end performances are to be determined after image rectification
- In-flight absolute and relative image geometric quality of FCI will be assessed after image rectification, so that high accuracy landmark processing can be fully applied
- The image quality process will be applied also for the IRS data, only for in-flight performance verification. However, the sounding data will be delivered to the users before rectification.
Figure 10: Example of RSS (Rapid Scanning Services) for the FCI (image credit; ESA, EUMETSAT)
One of the challenging requirements of the MTG-I remains the ICRA (Inter-Channel Co-registration Accuracy). The contributors to the ICRA characterization include at instrument level:
- Detectors (spatial response uniformity of each pixel, pitch uniformity, dispersion and alignment)
- Optics (relay optics magnification, field distortion, alignment, stray light)
- Scan mechanism (actual scan rate and direction, scan rate stability)
- Integration (alignment)
- Satellite (launch stability, thermal stability, ageing, pointing stability in terms of drift and micro- vibrations, master-clock jitter).
FCI instrument: The FCI instrument is composed of the following elements Ref. 21):
• FCI-TA (Flexible Combined Imager-Telescope Assembly). TAS is prime contractor for the instrument, Kayser Threde is responsible for the procurement of the FCI-TA:
• The spectral Separation & Detection Assembly
• The FCI Electronics.
Figure 11: Overview of the main components of the FCI instrument (image credit: TAS)
Figure 12: Main components of the FCI-TA (image credit: Kayser Threde)
The FCI instrument parameters are : (Ref. 29)
• Volume: 1.57 m x 1.72 m x 2.2 m
• Mass: < 394 kg
• Power: < 495 W (max)
• Data rate: < 68 Mbit/s.
LI (Lightning Imager):
The overall objective of the LI system is to deliver on a continuous basis information on total lightning over the full disk (high timeliness, data quality homogeneity in time and space), allowing to extend “locally developed” algorithms for NWC (National Weather Center) severe weather warning to be applied over wider areas like Europe or the full Earth Disk (Ref #.
• The LI measurements of total lightning [IC (Intra Cloud)+CG (Cloud Ground)] are complementing the global measurements of CG lightning as provided by ground based systems and will improve the quality of information which is essential for air traffic routing and safety.
• The information on IC+CG will allow to assess the impact of climate change on thunderstorm activity by monitoring and long-term analyzing lightning characteristics – in cooperation with the two NOAA GLMs on GOES-R and GOES-S - a major part of the globe is covered by a long term committed GEO lightning (IC+GC) observing system.
• Providing IC+CG information on a global scale will be a prerequisite for studying and monitoring the physical and chemical processes in the atmosphere regarding NOx, playing a key role in the ozone conversion process and acid rain generation.
• Error characterized IC+CG information can be assimilated to improve very short range forecasts of severe convective events or used to verify/validate other satellite data based NWC algorithms to forecast time and location of initiation of lightning.
The LI system is conceived for the detection of lightning in Earth's atmosphere from a geostationary orbit, where the FOV (Field of View) of Earth's full disk extents to an angle of 17.5º. The basic LI observation concept is to cover this FOV with four lenses and four detectors, each one covering a square FOV of 8.7º x 8.7º, corresponding to 12.3º diagonal FOV, in order to reduce the incidence angle on the interferential filter placed on the pupil of each subsystem. An important advantage in splitting the interference filter in four parts is related to the possibility to reduce its dimensions and to ease its manufacturing. 31) 32) 33)
TAS (Thales Alenia Space) as the MTG prime contractor is responsible for the procurement of the Lightning Imager instrument developed and manufactured by Selex Galileo of Campi Bisenzio, Italy. 34) 35) 36)
Table 6: Main requirements of the LI instrument
The instrument works in a staring mode, detecting lightning events within its FOV of its 4 cameras. The lightning detection is achieved implementing the following functions:
• Earth image acquisition for continuous monitoring of the lightning’s presence in the FOV
• Calculation of pixel by pixel adaptive background to cope with non-uniformities and low terms variations of the image (oceans, clouds, area in night conditions and areas with daylight conditions) and to reject at the same time noise effects and spurious events
• Removal of the background level from the overall pixel signal to obtain the net lightning illumination level
• Use of adaptive threshold; lower thresholds can be used in low noise dark areas of the scene, using higher thresholds only in highly illuminated areas (with corresponding higher shot noise)
• Pixels for which the difference between the pixel value and the estimated background signal exceeds the threshold are kept as DTs (Detected Transients)
• Collection of the DT video data and additional information for the ground processing with a dedicated processing electronics
• In flight processing of DTs to reduce the number of FT (False Transients) to a level compatible with the platform downlink data rate constraints (30 Mbit/s).
In addition LI is capable to acquire, process and transmit to the ground an Earth background image.
Instrument overview: LI is composed of one LOH (LI Optical Head) and one electronics unit, the LMI (LI Main Electronics). The LOH consists of four identical OCs (Optical Channels), each one including (Figure 14):
• a protective cover on the baffle aperture to prevent baffle and optics contamination during launch and prelaunch activities
• a baffle for stray light suppression and thermal load minimization
• a SRF (Solar Rejection Filter), to minimize both the background level and the thermal load inside the OC
• a NBF (Narrow Band Filter) to reduce the bandwidth in the range of the lightning spectral pulse (Figure 13)
• an optical system with F# 1.73, 110 mm entrance pupil diameter (determined by radiometry required to achieve the IADP performance) and 190 mm effective focal length [determined by the targeted GSD of 4.5 km at SSP (Sub Satellite Point) and the size of detector pixels]
• a CMOS detector with 1000 x 1170 pixels, 24 µm pitch, 1000 frame/s 37)
• a processing electronics device implementing the detection functions.
The LMI (LI Main Electronics) performs the overall payload functions, the interface to the platform, the configuration of the processing electronics, the data flow regulation, and finally compacts and packetizes the scientific data.
Figure 13: Optical emission from lightning (image credit: Selex Galileo)
Figure 14: Illustration of the LOH (LI Optical Head), image credit: Selex Galileo
System tradeoffs: The main tradeoffs for the selection of the LI configuration are:
• Single channel versus multi-channel architecture (this defines the FOV of the OC).
• NBF (Narrow Band Filter) position within the optical path: close to entrance pupil for a parallel incidence beam working concept versus close to the focal plane for a convergent incidence beam working concept. 38)
• Optical system sizing: optimization of the entrance pupil diameter with respect to lightning ADP (Average Detection Probability), mass and volume constraints and definition of the GSD (Ground Sampling Distance) with respect to ADP and the electronics processing load constraints and data rate bottlenecks.
• Optical system design: single primary optics with four relay systems; catadioptric approach; dioptric approach.
Figure 15: Alternate view of the LI instrument (image credit: EUMETSAT)
Single channel vs. multichannel architecture:
The separation of the LI global FOV into multiple OCs, taking into account the NBF positioned in the entrance pupil, allows mitigating the development risk of the critical items (NBF, detector and proximity electronics) and the optimization of the NBF performances. Figure 16 shows the FOV layout for the single and double OC solutions fulfilling the coverage requirement.
The large FOV (8.7°) of the single OC concept cannot be sustained by the NBF due to the blue spectral shift induced by the high angle of incidence which is not compatible with the bandwidth requirement. To limit such effects, a Galilean telescope can be placed in front of the NBF (Figure 17) reducing the angle of incidence to 5.5°, but enlarging at the same time the filter diameter (~1.6 X). This makes the achievement of the coating uniformity requirements more challenging. In addition, this solution imposes the development of a very large detector array (about 5 Mpixel) and of a huge processing demand on the electronics (5 Gpixel/s to be processed in real-time) both considered unfeasible.
In case the global coverage is achieved by means of two or four OCs, the detector size is reduced and the use of the Galilean telescope, requiring more mass and envelope than available, is no more required.
A larger number of OCs (Optical Channels) improves the NBF performance and the feasibility of detector and relevant processing electronics.
Figure 16: LI coverage for the single (left) and double OC concepts (image credit: Selex Galileo)
Figure 17: Single optical channel layout with the Galilean telescope (image credit: Selex Galileo)
Table 7: Comparison of OC (Optical Channel) main parameters for single and multichannel configurations vs LI concept
The NBF parallel beam concept (as defined in Table 7) has been compared with an alternative one, for which the filter is positioned close to focal plane (convergent beam optical configuration). In this case to limit the spectral shift of the NBF bandwidth, the optical system must be telecentric and with an F# of 4.75, corresponding to a convergent beam of 6° maximum. The resulting pixel pitch of the detector is 65 µm producing a very large detector size and thus an increase of the effective focal length (~2.7 X) when compared with the NBF parallel beam concept.
Optical system design:
The results of the previous evaluations and tradeoffs limit the study to optical configurations with four detectors and an NBF in a parallel beam arrangement.
The selected optical layout (after a lengthy tradeoff analysis) is composed of four optical channels, each one with an independent single stage lens, detector and baffle. Figure 18 shows the optical layout of the baseline solution, whose main optical characteristics are reported in Table .
Figure 18: Selected optical layout of LI (image credit: Selex Galileo)
Legend to Figure 18: The first two parallel plates () are the SRF (Solar Rejection Filter) and the NBF (Narrow Band Filter), both in green. The imager is composed of 6 lenses, all made of radiation resistant glass. All the lenses have a spherical profile except one. The lens diameters are larger sized to insert some “light traps” and to limit the straylight caused from the lens borders and the internal objective walls. The SRF, the first from left, is placed tilted to mitigate the ghosts images due to multiple reflections between filters.
Table 8: Summary of the LI optical system parameters
Figure 19: LI coverage using the 4 Optical Channel concept (image credit: Selex Galileo)
IRS (Infrared Sounder):
The IRS (also referred to as GeoSounder) objectives are to provide “break through” measurements on the time evolution of horizontal and vertical water vapor structures ins the atmosphere – an unprecedented source of information available for the operational services in NWCs (National Weather Centers) and regional/global NWP (Numerical Weather Prediction). The IRS has no heritage instrument in previous Meteosat missions.
• The IRS (30 minute repeat cycle over Europe) will fill large spatial and temporal voids in the 12 hour time standard radiosonde observations and will allow time and space interpolation of moisture/temperature observations taken from the polar orbit (Ref #.
• The IRS derived information on low tropospheric moisture and its changes in time is expected to lead to a better depiction of the hydrological cycle in models, potentially providing better precipitation forecasts.
• The IRS will provide information on vertically resolved atmospheric motion vectors with improved height assignment, which in particular is beneficial for the tropical areas having only a weak coupling between the dynamic and thermodynamic atmospheric fields.
• The IRS will provide information to identify pre-convective situations supporting NWC applications to forecast convective initiation.
• IRS will support forecasting pollution and monitoring of atmospheric minor constituents through its capability to provide estimates of diurnal variations of tropospheric contributions of atmospheric trace gases such as O3 and CO.
Table 9: IRS atmospheric sounder requirements (Ref. 7)
The IRS operates at millimeter wavelengths which are capable to penetrate clouds and rain. Its design relies on a principle called interferometry, with separate signals from multiple antennas correlated together to produce an image of otherwise impossible sharpness. To further reduce the number of antennas needed, the device rotates at 1º/s, filling in further detail.
Geostationary infrared sounding missions offer good temporal coverage, however due to the large distance to the observed targets on Earth, the effect of diffraction is increased compared to sounding from LEO (Low Earth Orbit). Due to the wavelength dependence of diffraction, the spectral channels do not sample the same volume of air, as in general assumed by the retrieval algorithms for LEO infrared sounder data. This additional error introduced in the retrieval by diffraction limited instruments is in general referred to as ‘pseudo noise.’ 39) 40)
Contrary to infrared sounders in LEO, the main objective of the IRS (in GEO) is not absolute temperature/humidity sounding. Rather, the prime objective of IRS is to support the dynamics via tracking of vertical water vapor structures. The user of IRS data is mostly interested in the information on vertical structures (temperature, humidity and wind at high horizontal, vertical and temporal resolution), which comes from the fidelity of the spectral information. This fidelity would be destroyed by excessive and spectrally uncorrelated random noise. In case of a spectrally high correlated noise (spectral bias) the information on vertical structure is not equally destroyed, i.e. it will have a lesser impact.
The IRS data will contribute both through assimilation into convective-scale, regional and global NWP (Numerical Weather Prediction) models and through nowcasting products. The data will be particularly important for observing the advection and convergence of low-level moisture associated with some types of severe weather in Europe. The main goal is to document the added value of water vapor observations derived from a hyperspectral infrared sounding instrument on a geostationary satellite for regional forecasting.
Preliminary IRS instrument parameters are (Ref. 29) :
• Volume: 1.44 m x 1.30 m x 1.25 m
• Mass: ~438 kg
• Power: ~858 W
• Data rate: ~167 Mbit/s
The state-of-the-art IRS instrument is being developed by Kayser Threde of Munich, an OHB company. The objective of IRS is to probe the atmosphere and provide information on the horizontal, vertical and temporal resolution of water vapor and temperature distributions. 41)
The IRS instrument is designed to detect, with a high radiometric accuracy, the signals emitted from gases in the atmosphere. A demanding spectral resolution for velocity determination is used to determine the wind profiles at various heights above ground together with a high spatial and temporal resolution.
The instrument will be able to scan the full earth circle within 1 hour with a spatial on ground resolution of 4 km x 4 km from geostationary orbit. This resolution can be achieved by means of a high resolution telescope operating in the Infrared spectral range with a scan mirror assembly allowing a step and stare of the line of sight. Radiometric, spectral and geometric requirements are met both in nominal and restricted operations conditions.
Figure 20: Illustration of the IRS instrument (preliminary design of Kayser Threde)
The IRS instrument is a sounding Fourier transform spectrometer and comprises the following main subsystems:
• The scan assembly allows performing a complete scan of the earth disk in a fully programmable scan pattern including stares to cold space for fast and repeated recalibration of the full optical path.
• The unique design of the entrance cavity allows the sun to enter the instrument through an entrance baffle together with an internal baffle thermally decoupled from sensitive elements like mirrors and optical bench.
• The front-telescope reduces the entrance pupil, located at the scan mirror to the back-telescope, by a factor of 4, while an in-built imager allows, via a small band beam splitter, a direct and parallel detection of the investigated ground area in the spectral band between visible and infrared.
• The Michelson interferometer generates the interferograms at each position of the scan assembly by highly accurate motion of a corner cube. At each observation position 2 x 25,600 interferograms are recorded simultaneously in two spectral bands, by two detector arrays. The detectors are integrated and co-aligned in order to achieve a high inter-band co-registration.
• The back-telescope is an in-size reduced afocal front-telescope to adapt the interferometer to the cold optics. Efficient straylight reduction is achieved by means of an intermediate field stop located in the front telescope before the interferometer. The cooling of the detectors and optics behind the back telescope, down to 55 K, is realized by means of two active cryocoolers for redundancy.
Scan Mechanism development for FCI and IRS instruments (requirements):
The scanning of the Earth along the E/W and the S/N directions shall be ensured by the “Scan Mechanisms” located ahead the instruments, corresponding to challenging pointing requirements. Key requirements are: scan angular velocity stability (accurate slope and low jitter), angular pointing restitution accuracy (angular pointing knowledge) and overall pointing position accuracy with respect to the commanded position. All these performances are in the rad – or even sub µrad - range over a wide angular stroke, corresponding to challenging capabilities. 42)
The driving technical requirements derived from instrument specification are:
• Scanning accuracy (stability): "The E/W scanning accuracy shall be such that 68,3% (1σ) of the PTV errors are lower than 1.6 µrad (target 0.58 µrad) over 203 ms."
• Pointing Restitution: "For each scanned line, the E/W scanning angle knowledge / restitution accuracy shall be better than:
- Bias < 0.5 µrad. Error average over the scan range
- Jitter < 0.5 µrad (1σ).
This is the error between the actual pointing angle and the associated measurement / restitution. This accuracy can include proper processing to be specified and/or applied by the Scan Mechanism responsible based on the knowledge of the Scan Mechanism components performances (encoder characterization, filtering, calibration & mapping as necessary)"
• Overall position accuracy (“achieved” versus commanded): "the maximum absolute angular error within the whole scan range shall be lower or equal to 100 µrad".
UVN (Ultra-Violet Near-infrared) Sounder:
The UVN sounder on MTG-S is the GEO component of the joint Copernicus Sentinel-4 (GEO) and Sentinel-5 (LEO) concept for climate protocol monitoring and air quality applications expected to deliver data products on ozone, nitrogen dioxide, sulphur dioxide, formaldehyde, aerosol optical depth, and aerosol scattering height.
A separate file, Copernicus: Sentinel-4 (or simply Sentinel-4), is provided on the eoPortal which describes only the UVN Sounder, a hosted payload on the MTG-S spacecraft series.
GEOSAR (Geostationary Search & Rescue) service:
GEOSAR is part of the COSPAS-SARSAT international system; the objective is to provide distress alert and location information to appropriate rescue authorities for maritime, aviation and land users in distress.
DCS (Data Collection System):
The DCS mission involves, as a continuity of the MSG mission, the collection and transmission of observations and data from the ground segment consisting of surface, buoy, ship, balloon or airborne DCP (Data Collection Platforms).
The ground segment contains the main ground elements necessary to support the mission. They are logically decomposed in facilities as follows:
• GSTF (Ground Station Facilities)
• MOF (Mission Operations Facility)
• IDPF (Instrument Data Processing Facility)
• MPF (Multi-Programme Facilities)
• Infrastructure Facilities and Supporting Facilities
• L2PF (Level 2 Processing Facility), as part of the Application Ground Processing System
• SAF (Satellite Application Facilities).
All system operations will be conducted from the EUMETSAT HQs in Darmstadt, except for the LEOP (Launch and Early Orbit Phase) which will be performed by a LEOP service provider, to be selected. Following the spacecraft hand-over from LEOP, EUMETSAT will then execute the commissioning of the system before entering the routine operations phase. The MTG in-orbit constellation will grow progressively until it will reach its FOC (Full Operational Capability), which consists of 3 operational in orbit satellites (two MTG- I and one MTG-S).
The operational (prime) MTG-I satellite, nominally at 0º longitude, will be set to fulfill the requirements of the FDHSI (Full Disc High Spectral resolution Imagery) mission, with an FCI operating in Full Disk Scanning mode with 10 min repeat cycle. It will also be the nominal operational spacecraft for the other payloads it carries, the LI, the DCP and GEOSAR payloads. The second in-orbit operational MTG-I satellite, expected to be at 9.5º east longitude, will be performing the HRFI (High spatial Resolution Fast Imagery) mission needs, with an FCI operating in Rapid Scanning mode over ¼ of Full Disk with 2.5 min repeat cycle. It will also constitute the in-orbit hot back up for the prime MTG-I satellite (noting that initially an MSG satellite will be the in-orbit hot backup for MTG-I1. The second in-orbit MTG-I satellite may also operate the LI, DCP and GEOSAR payloads in case the corresponding payload becomes not available or for cross-calibration or to improve the quality of the combined ground processing (Ref. 29).
Figure 21: Overview of the MTG ground segment (image credit: EUMETSAT) 43)
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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.