VENµS (Vegetation and Environment monitoring on a New MicroSatellite)
VENµS is the first cooperative Earth observation program of Israel (ISA) and France (CNES). The minisatellite mission is being developed jointly by ISA (Israel Space Agency) and CNES, under a memorandum of understanding between the two space agencies, signed in April 2005. In this setup, ISA and CNES are sharing responsibilities for the VENµS program. ISA is responsible for the spacecraft bus, satellite integration, engineering data, and the satellite control center including mission operations. CNES is responsible for the science mission center, including the science data processing center and programming center. CNES is also providing the superspectral camera and is in charge of the launcher interface. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)
The main program participants are: ISA, CNES, CESBIO (Centre d'Etudes Spatiales de la BIOsphère, Toulouse, France), ELOp (Elbit Systems Ltd. Electro-Optics of Rehovot, Israel), BGU/BIRD/RSL (Ben Gurion University of the Negev/Jacob Blaustein Institute for Desert Research/Remote Sensing Laboratory), IAI/MBT Space Division (Israel Aerospace Industries Ltd.), and Rafael.
Scientific mission (VM1):
Global models of biosphere processes and dynamics have been developed in the past few years and are used for carbon cycle studies. Today, they are mainly based on ground measurements provided by networks of experimental stations. On the other hand, the water cycle is driving important research activities on the interface between land and atmosphere for improving atmospheric models. A major limitation of these approaches is the lack of consistent ground network and measurements and the imperfection of models. The development of Earth observation data assimilation techniques and aggregation or desegregation procedures is leading to more promising integrated approaches.
The main scientific objectives of the Earth observation missions for terrestrial environment monitoring are:
• to monitor and analyze surface functioning under the influence of environmental factors (climate, topography, soils etc.) as well as human activities
• to study processes and their interactions with natural and human factors
• to develop and validate natural and cultivated ecosystem functioning models and to improve and validate global carbon cycle models
• to develop remote sensing data assimilation techniques within vegetation and surface flux models
• to define theoretical and practical methods for scale transfer, i.e. up and downscaling
• to interpret data collected by those low spatial resolution sensors
• to develop, validate and implement integrated modelling platforms.
Technological mission (VM2, VM3):
There are two main objectives for the Technological mission: Hall Effect Thruster verification and validation in space. The verification is achieved by operating the thrusters in the space environment. Its performances will be tested and qualified. Validation will be demonstrated for some mission enhancement operations.
The Technological mission is split into 3 parts:
• orbit maintenance and small experiments combined with the Scientific mission
• transfer to a lower altitude orbit from 720 km SSO to 410 km SSO (VM2)
• low altitude orbit keeping, compensating drag, while continuing to image as a secondary mission (VM3).
Figure 1: Overview of VENµS program responsibilities (image credit: CNES)
VENµS is a research demonstrator mission for the GMES (Global Monitoring for Environment and Security) program, a joint initiative of ESA (European Space Agency) and the EC (European Commission). Dedicated to monitoring vegetation, it will lay the foundations of an operational GMES observatory designed to keep track of the environment and manage natural resources. The basic requirements call for: 17) 18) 19)
• Land use mapping. In this respect, VENµS is opening new horizons. Every 2 days, the satellite will cover 50 sites representative of the world's main inland and coastal ecosystems in 12 spectral bands in the visible and near-infrared.
• By mapping vegetation at high resolution using a superspectral camera, the satellite will make it possible to derive enhanced information from this type of data. More significantly, it will also help to automate and improve thematic mapping procedures.
• Other VENµS applications include assessment of carbon flux and monitoring and management of crop production and water resources. The mission will also be used to characterize water color for applications in continental hydrology and coastal oceanography.
The nominal science mission at an altitude of 720 km is planned to last for 2.5 years. This is being followed by a 1 year technology mission at an altitude of 410 km. In addition, the mission is used for a technology demonstration/validation of a new instrument: IHET (Israeli Hall Effect Thruster).
The minisatellite is funded by ISA. It uses an IMPS (Improved Multi Purpose Satellite) platform, designed and developed by IAI/MBT (Israel Aerospace Industries) Ltd. and Rafael (Advanced Defense Systems Ltd.). Rafael is a former sub-division of the Israeli Defense ministry and is considered a governmental firm.
Figure 2: Illustration of the deployed VENµS minisatellite (image credit: CNES, IAI)
The spacecraft is 3-axis stabilized using a modified IMPS (Improved Multi Purpose Satellite) platform of IAI/MBT (the same minisatellite bus is being used on the TecSAR and OPSat missions of ISA) - and is referred to as OptSat-2000. The spacecraft bus is a cylinder, 1.6 m in height x 1.2 m in diameter. The spacecraft features a tilting capability of ±30º in cross-track or along-track direction - thus, providing a FOR (Field of Regard) of 720 km. The estimated launch mass is 260 kg, the power is 800 W, the planned lifetime is 4.5 years.
The IMPS modifications concern mainly the base plate - to accommodate the two IHET engines and the tanks - and the solar panels - to provide the amount of power needed for the IHET. As the S/C contains two different propulsion systems, it has one hydrazine tank of 7 kg to feed four 1 N (+4 redundant 1 N) thrusters, and one xenon tank of 16 kg for one (+1 redundant) IHET.
The power management system size can drive the high energy demand of the IHET system (operating range of up to 600 W).
The AOCS features star trackers, a GPS receiver and reaction wheels, providing good pointing accuracy performance during imaging periods.
Figure 3: Block diagram of the VENµS spacecraft (image credit: IAI, CNES)
Table 1: Overview of satellite parameters
Figure 4: The VENµS spacecraft in stowed configuration at launch (image credit: CNES, IAI)
RF communications: The TT&C communication is in S-band. The science data are downlinked in X-band (data rate of 2 x 155 Mbit/s). An onboard data storage capacity of 30 GByte is provided.
Arianespace and the Italian company CGS S.p.A. signed a launch contract for OptSat-3000 and for the VENµS satellite with IAI in Feb. 2014. 21)
Orbit: Sun-synchronous orbit: altitude = 720 km, inclination = 98.28º, LTDN (Local Time on Descending Node) = 10:30 hours. The repeat cycle is 2 days with 29 orbits per cycle. The instrument swath allows 27 km x 27 km individual images at nadir.
Figure 5: Artist's rendition of the deployed VENµS spacecraft (image credit: IAI/MBT, CNES)
The operational mission time is divided into three distinct phases.
1) VM1 (VENµS Mission 1): VM1 will start after the IOT (In Orbit Test) phase; it will mainly be devoted to the science mission objectives and is planned to last for 2.5 years. During VM1, the spacecraft will acquire images from the nominal orbit at 720 km altitude.
2) VM2 (VENµS Mission 2): VM2 is the orbit transfer phase in which the spacecraft will descend to a new lower operational orbit. The phase will last for up to six months during which no imaging is taking place. The technological mission is being performed during this period in which IHET will be operated in each orbit until the spacecraft reaches the new VM3 orbit at a nominal altitude of 410 km.
3) VM3 (VENµS Mission 3): In the VM3 phase the technological mission and the science mission will be operated in an interleaving mode - requiring the spacecraft to alternate constantly between imaging and IHET firings. The IHET is expected to correct the orbit after every three imaging orbits due to the increased drag rate. - The VM3 mission phase is planned for 1 year after which the spacecraft will be disposed.
At the altitude of 410 km, VENµS will keep on imaging in a circular sun-synchronous orbit while maintaining the 2-day revisit ground track. Since the VENµS satellite was not designed for a very low altitude, its cross-section area is quite large and the atmospheric drag at this altitude plays a significant role. The thrusters will have to be used intensively, to keep the imaging requirements accuracy. This task is hardly achievable by a comparable traditional chemical propulsion system. In this phase, the images resolution increases and its swath decreases with respect to the scientific mission phase.
The challenge at 410 km is to comply with as many scientific mission requirements as possible and to accomplish the combined mission, of achieving the scientific goals with the proper technology use.
Table 2: Summary of mission phase parameters
Figure 6: Timeline of the VENµS mission (image credit: IAI/MBT, Rafael Ltd, CNES)
Sensor complement: (VSSC, IHET)
VSSC (VENµS Superspectral Camera):
The prime objective of the VENµS science mission is to provide digital imagery to be able to study the monitoring, analysis, and modeling of land surface parameter behavior. The goal is also to demonstrate the relevance of superspectral, high spatial resolution observations with frequent revisit capabilities in the context of the European Copernicus/GMES (Global Monitoring for Environment and Security) program.
CNES is responsible for the provision of the camera and the image ground segment. The camera, being built by ELOp under CNES contract, is of MSRS (Multi-Spectral high Resolution Sensor) heritage which was developed by ELOp and OHB System of Bremen, Germany. The camera provides 12 simultaneous overlapping Earth images (12 parallel detector arrays in the focal plane) with high spatial and high spectral resolution. With 12 narrow spectral bands in the VNIR (Visible Near Infrared) spectral region and 5.3 m ground resolution, the VENµS camera introduces a new level of superspectral high spatial resolution Earth imaging for a wide range of commercial and scientific applications.
• Swath: 27.56 km
• Spatial resolution: 5.3 m
• Number of spectral bands: 12 (VNIR)
• Spacecraft body tilting capability: ±30º in cross-track and along-track
• Data quantization: 10 bit
• Lmin (minimum radiance) defined for the mission.
Table 3: Spectral characteristics of the VSSC instrument
VSSC is a pushbroom-type imager consisting of the following major elements:
• A catadioptric objective
• A focal plane with 4 detector units, each with 3 separate CCD-TDI arrays
• Operating electronics and interface to the satellite transmitter
• Thermal control
• A sun shield and thermal shield.
The objective is a Ritchey-Chrétien type with auxiliary lenses for field correction and telecentricity. There is no refocus mechanism but properties are maintained thanks to the careful choice of materials and processes and a close thermal control loop which maintains constant temperature of the camera. The VSSC focal length is 1.75 m, with an objective aperture diameter of 0.25 m.
Figure 7: Illustration of the VSSC instrument (image credit: ELOp, CNES)
Figure 8: Camera configuration (image credit: ELOp, CNES)
Figure 9: The focal plane configuration of the VSSC (image credit: ELOp, CNES)
The optics subsystem employs a Ritchie-Cretien reflective type objective fitted with auxiliary lenses for the correction of aberrations. Narrow-band interference filters are used to ensure the required wavelength characteristics. The primary and secondary lenses are made of Zerodur; they are lightweighted to reduce the camera mass. The optics subsystem is maintained at a constant temperature of 20 ±3ºC to ensure proper focusing of the system.
Figure 10: Photo of a lightweighted mirror (image credit: ELOp, CNES)
Table 4: Some parameters of the VSSC instrument
Focal plane assembly (FPA): Four detector units, each with 3 separate CCD-TDI (Time Delay Integration) arrays, are combined in the focal plane assembly. The detectors selected are triple-junction units. Each array has 32 TDI rows with 5200 pixels per row. The number of TDI stages may be selected individually for each array. The TDI options are: 1, 2, 4, 8, 16, or 32 rows.
The physical layout of the focal plane is shown in Figure 9. Two of the detectors receive the image directly from the objective while in the other two, the image is reflected by a folding mirror. This enables the distance between the outermost bands in the focal plane to be minimized.
Figure 11: Illustration of a three-array TDI detector unit (image credit: ElOp Ltd.)
Figure 12: Schematic layout of the TriT detector (image credit: ELOp, CNES)
The telescope structure consists of a lightweighted bezel of titanium to which the primary mirror is fixed, and a composite-material tube which keeps the secondary mirror at the correct distance. The focal plane uses invar material to provide a stable thermal environment for the detector units. In front of each detector unit is a field stop, on which the absorption filters are mounted.
Figure 13: Longitudinal section of the VSSC imager (image credit: ELOP)
The objective is covered with a cylindrical shield which extends also in front of the telescope thereby acting as a sunshield. The inner part of the telescope, referred to as TCSA (Thermal Control Shield Assembly), provides thermal control to the optical subsystem.
Instrument calibration: Without the availability of an on-board calibration system, periodic imaging of known targets (instrumented sites) will be used together with occasional imaging of the moon at a specific phase. Also, extensive pre-launch calibration of the instrument will be carried out (Ref 25).
The VSSC data will be acquired over existing or planned experimental sites with size ranging from 27 km x 27 km to 27 km x 54 km or more. All data for a given site will be acquired with the same observation angle in order to minimize directional effects. The baseline product for these selected sites is time composite images of geometrically registered surface reflectances at 10 m resolution. Strong efforts are devoted to provide high quality data, both in term of radiometry (e.g. SNR around 100), geometry (e.g. multitemporal registration better than 3 m), and atmospheric corrections.
Particular attention is paid to stray light since it has a serious impact on the radiometric performances. The sources of stray light are diverse, either scattering due to mirror roughness and contamination or ghost images due to reflection on the optics surfaces. The camera baffle design was done in order to minimize the stray light and keep the particle contamination level on optical surfaces at a maximum of 300 ppm.
IHET (Israeli Hall Effect Thruster):
IHET was developed by Rafael Ltd., Israel for the technological mission. The IHET will be used for autonomous orbit maintenance to enable continuation of the scientific mission. The newly developed instrument is to be space qualified during a separate technology demonstration mission at the end of the science mission. The objective of IHET is to provide the capabilities of general orbit maintenance as well as to demonstrate a LEO-LEO orbit transfer (Ref 3). IHET is also codenamed HET-300. 31) 32) 33) 34) 35)
The electric thruster will first be used with the satellite on its orbit at an altitude of 720 km (science mission of 2.5 years). Following a descent phase down to an altitude of 410 km, the plasma motor will then be used to compensate for drag and to keep the satellite at that altitude for one year.
The orbit change and orbit maintenance shall be performed by operating the EPS (Electric Propulsion System) autonomously in a cyclic mode according to power availability at each orbit. In total, each thruster will be activated for about 2500 cycles and will accumulate more than 1000 operating hours to accomplish the above mentioned missions.
Thruster operating principles:
- Xe gas directed to distribution channel (Anode)
- Electrons emitted from cathode, collide with Xe atoms, and ionizing them
- Applied magnetic field, spiral electrons in the thruster channel
- Electric field accelerate ions out of the channel
- Ions neutralization at exit by electrons from the cathode.
Figure 14: Schematic illustration of the ion thruster operating principle (image credit: Rafael)
The PBA (Propulsion Base Assembly) is designed, manufactured and tested as a standalone module by Rafael. The PBA includes two propulsion systems: the HPS (Hydrazine Propulsion System) and the EPS, as shown in Figure 15. The two Hall thrusters of the EPS are positioned on opposite edges of the plate where their thrust vectors are aimed toward the satellite COG (Center of Gravity).
Figure 16 shows a schematic flow diagram of the EPS. Xenon at a pressure of 86 bar is stored in the propellant tank and locked by the high pressure LVM (Latch Valve Module). Pressure is then reduced by the pressure regulator assembly to 1.9 bar absolute. Xenon flow is distributed to the cathodes the anodes line. Cathodes lines include a common getter for the two cathodes and at the end of each line there is CVM (Cathode Valve Module) which shut / open the flow to the selected cathode. The CVM includes flow restrictors that control the flow rate to the cathode. The two anode lines include a single DXFC (Digital Xenon Flow Controller) that delivers controlled mass flow rate according to mission requirements.
The active thruster to be fired is selected by the TSU (Thruster Selecting Unit) and the active AVM (Anode Valve Module), which open / shut the flow path to the relevant thruster. The AVM and the CVM constitute the TVM (Thruster Valve Module). In case of a power failure, the TVM's solenoid valves shall shut off and will prevent Xenon loss to space. The PPU (Power Processing Unit) that incorporates the TSU and the SCU (Sequence Control Unit) supplies electric power to operate the thrusters and electricity to the active valves. The SCU controls thruster power level by commanding the DXFC setting i.e. Xenon flow rate in a close loop with the discharge current.
Figure 16: Flow diagram of the EPS (image credit: Rafael)
Figure 17: Photo of the IHET instrument, the HET-300 model (image credit: Rafael)
Table 5: Key parameters of the IHET-300 W instrument
Two ground facility locations will be used to process and analyze the data. SMIGS (Scientific Mission Image Ground Segment) is the processing facility for the imaging data, located at CNES in France. The objective is to command the spacecraft imager for the imaging targets and to process and distribute the products to the scientific community. The TMC (Technological Mission Center) takes care of the technological mission operations in space. It will also process the data and analyze the performance of the IHET. TMC is located at Rafael in Israel. 37)
CNES is responsible for the provision of the camera and the image ground segment. ISA is in charge of the platform (provided by IAI - Israel Aerospace Industry and RAFAEL - Advanced Defense Systems), the IHET system, the satellite control center and the operation of the satellite.
The ground segment consists of:
• Command and control center (IAI/MBT, Israel)
• X-band receiving station (Kiruna, Sweden), in charge of receiving the imaging data from the satellite.
• A Data Processing Ground Segment in charge of acquiring, processing, archiving and distributing the VENµS scientific and associated data generated in-orbit with:
- Scientific Mission Image Ground Segment (SMIGS), located at CNES which processes the data from raw telemetry to level 1, 2 and 3 products, and performs the image quality check
- VENµS Products Distribution Server (VPDS), hosted by POSTEL for the interface with the scientific community for VENµS products.
• Technological Mission Center (Rafael, Israel), in charge of planning the Technological Mission, analyzing and archiving the VENµS Technological Payload (IHET) data.
Figure 18: Overview of the VENµS mission elements (image credit: Rafael)
The scientific mission program activities are split between the French SMIGS and the GCS (Ground Segment Center) located at IAI/MBT in Israel. 38)
• GCS is responsible for the overall satellite command and control. It includes a SCC (Satellite Control Center) and a TT&C for S-band communications. In addition, there is an X-band station for optional direct downloads of images.
• The TMC (Technological Mission Center) at Rafael (Israel) processes the IHET telemetry. TMC has also the task of to program the satellite during IHET operations.
• VRS (VENµS Receiving Station), an X-band station in Kiruna, receives the image raw data and some auxiliary data. This auxiliary data includes AOCS, GPS, and star tracker data, but also some IHET auxiliary data.
• SMIGS at CNES is responsible for the programming of the scientific mission and the processing of the imaging products.
Figure 19: Overview of ground segment elements (image credit: CNES)
VENµS image products (Ref. 15):
The ground processing center provides 3 product levels:
• Level 1 will provide geolocated top of atmosphere reflectances with a sub-pixel multi-date registration.
• Level 2 provides surface reflectances ("Top of Canopy"). Its aim is to suppress as much as possible the atmospheric contribution to the signal. During this process, a high quality cloud and cloud shadow mask is generated.
• The Level 3 product is a 10 day composite of level 2 products. The aim is to provide the users with cloud-free, easy to use images of surface reflectances, every 10 days.
The ground sampling distance of level 2 and 3 products will be 10 m, whereas the level 1 product is delivered at 5 m resolution.
The level 2 and 3 algorithms for cloud detection and atmospheric correction intensively use the VENµS unique characteristics: frequent observations at high resolution with constant viewing angles. Thanks to these features, the evolutions between two successive images are very likely to be related to changes in the atmosphere rather than in the surface. These characteristics are used to detect clouds and aerosols before atmospheric correction. In addition, Clouds altitude detection benefits from the stereoscopic viewing feature of VENµS.
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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 (firstname.lastname@example.org).