PROBA-1 (Project for On-Board Autonomy - 1)
PROBA is minisatellite technology demonstration mission in ESA's General Study Program with the objective to address issues of on-board operational autonomy of a generic platform. The following functional capabilities and/or techniques are to be demonstrated: 1) 2) 3) 4) 5)
• Command management coordination of on-board resources and house-keeping functions
• Scheduling, preparation, and execution of instrument observations (coordination of sensor settings, pointing angles, etc.)
• Source data handling functions (data collection, processing, storage, distribution)
• Source data communications management
• Performance evaluation and estimation of drifts, trends
• Failure detection and failure handling procedures
Further (secondary) objectives of PROBA are space environment investigation and Earth observation. The scientific interest relates to the use of the imaging spectrometer CHRIS. 6)
Figure 1: Artist's impression of the PROBA satellite (image credit: Verhaert Space, ESA)
PROBA was designed/developed by a consortium led by Verhaert Design and Development N. V. of Kruibeke, Belgium and sponsored by OSTC (Federal Office for Scientific, Technical, and Cultural Affairs) of Belgium.
The S/C structure resembles a box (60 cm x 60 cm x 80 cm) of conventional honeycomb design with body-mounted solar panels (GaAs) on five sides. The satellite is three-axis stabilized. The ACNS (Attitude Control and Navigation Subsystem) performs autonomous on-board navigation. Attitude measurements are provided by an autonomous star tracker [referred to as ASC (Advanced Stellar Compass), a GPS-based attitude sensor (SRG-20), gyroscopes (one resonating gyro, built by Sagem of France, flies for the first time) and four three-axis magnetometers; attitude control is provided with four miniaturized reaction wheels and four magnetorquers. The absolute pointing accuracy is 150 arcsec; the relative pointing stability is 10 arcsec over a 10 s period. All ACNS sensors and actuators are controlled by the ACNS software package (developed at the Université de Sherbrouk) running on the central ERC-32 RISC processor. There is no propulsion capability for orbit correction.
S/C power = 90 W (peak) provided by body-mounted GaAs solar cells with integrated diode, a 36 Li-ion cell battery of 9 Ah capacity is used for energy storage (PROBA was the first ESA mission to use a Li-ion battery). A centrally switched 28 V regulated bus distributes power to all sensors and subsystems. PROBA has a Memory Management Unit (MMU) with a 1.2 Gbit capacity for data storage.
S/C total mass = 94 kg including the payload mass. The S/C design life is 2 years. The S/C provides nadir and inertial pointing capabilities; along-track body pointing is up to 38º, while cross-track pointing is up to 30º (the slew rate is up to 1º/s). Yaw steering of up to 24º is only used for calibration purposes. A further feature of the platform maneuvers is the ability to implement Forward Motion Compensation (FMC) during imaging to enable the CHRIS instrument to enhance signal to noise and increase the number of spectral bands that can be read out. The FMC factor is 5.
PROBA provides full on-board flight dynamics and orbital navigation computation, as well as automated onboard functions handling nominal spacecraft operations and control and resources management. Payload resources management, payload operations scheduling and execution, target fly-by prediction and computation, and the control of imager pointing and scanning from high-level requests from users (target latitude, longitude and altitude) are hence fully automated. 7) 8) 9) 10)
Figure 2: Block diagram of the PROBA spacecraft (image credit: ESA)
The avionics is composed by:
• A high-performance redundant central computer (DHS) responsible for spacecraft telecommand and telemetry, all spacecraft computing tasks and interfaces to every unit of the spacecraft
• PPU (Payload Processor Unit) with a solid-state recorder and a DSP for payload processing and data storage
• A redundant set of S-band receivers and transmitters.
The DHS (Data Handling System) was designed to integrate in a single redundant unit all the core functions of the spacecraft avionics (Figure 3) and to provide sufficiently high-performance computing to support not only the traditional attitude control and data handling tasks but also spacecraft autonomy (i.e. the processing normally performed on-ground has been migrated on-board in the case of PROBA). A RISC processor, the ERC 32, has been used. The ERC 32 is a radiation tolerant (> 80 Krad) SPARC V7 processor providing 10 MIPS and 2 MFLOPS with a floating-point unit. A memory controller includes all the peripheral functions needed by the processor, such as the address decoders, the bus arbiter, the EDAC, 2 UARTS, 3 timers and a watchdog. The chip set is manufactured with the MHS 0.8 µm CMOS/EPI radiation tolerant technology.
Figure 3: Block diagram of the DHS (image credit: ESA)
Table 1: Overview of PROBA platform parameters
Figure 4: Illustration of the Li-ion battery (image credit: ABSL Space Products)
Figure 5: PROBA exploded view (image credit: ESA)
Launch: The PROBA spacecraft was launched on Oct. 22, 2001 on the PSLV-C3 launcher of ISRO (secondary payload to TES (Technology Experiment Satellite) of ISRO and BIRD of DLR) from SHAR (ISRO Sriharikota Range, the ISRO launch site on the East Coast of India).
RF communications: CCSDS-compatible uplink (2-4 kbit/s, PSK/PM modulation) and downlink (up to 1 Mbit/s, transmitter power of 2 W, BPSK modulation) packetized communications in S-band. Mission operations are located at the ESA Redu (Belgium) ground station. - The main elements of the ground segment are a fully steerable S-band antenna of 2.4 m diameter, the baseband equipment, a control system based on the ESA SCOS II system used also during the ground test and integration phase, a planning system and a data server. 11)
Orbit: Sun-synchronous elliptical polar orbit, perigee= 542 km, apogee=657 km (mean altitude of 615 km), inclination = 97.9º, period = 96.97 min, repeat cycle = 7 days (approximately), equator crossing time at 10:30 AM on descending node. The pointing capability of PROBA permits a CHRIS imagery repeat cycle of approximately 2 days at mid latitudes.
• January 2014: PROBA-1 is exploited as a 'Third Part Mission' by ESA's D/EOP (Directorate of Earth Observation Programs). The spacecraft is fully functional and multi spectral images are delivered to the CHRIS/PROBA registered scientists. 12)
• June 2012: To date PROBA-1 remains the most agile and stable satellite platform in its range. The company QinetiQ Space Belgium served as ESA’s prime contractor for the development of the mission (previously known as Verhaert Design and Development). For PROBA-1 — and the other PROBA missions that have followed — the approach that has been taken by QinetiQ Space and ESA was to develop small satellites at affordable costs where reliability is an important parameter. 13)
ESA acquires, processes and distributes PROBA-1 data as part of its ‘Third Party Mission’ data portfolio (together with international partner missions such as NASA’s Landsat satellites, the Japanese ALOS satellite, the French SPOT satellites or the UK–built DMC (Disaster Monitoring Constellation) missions). 14)
For the majority of Earth-observing missions, image acquisition is simply a matter of opening a viewing aperture, but PROBA-1 is very different. A technology demonstration satellite turned Earth observation mission, the satellite’s platform and payload effectively work as one. Using only a star tracker for guidance, the spacecraft can roll the satellite off to 25º off-nadir in the cross-track direction and 55º in its along-track. This tracking ability allows PROBA-1 to compensate for the effective satellite groundtrack speed. This ‘forward motion compensation’ boosts its overall integration time per image, giving CHRIS an imaging performance and signal-to-noise equivalent to that of an instrument with an aperture area five times larger. Cross-track tilts also increase the frequency with which the satellite is able to revisit areas of interest to less than a week.
PROBA-1 can acquire different views of the same target at up to five different viewing angles: at ±55º and 36º , as well as the standard nadir view. It is this capacity in particular that has proved invaluable to many scientists investigating the BRDF (Bi-Reflectance Distribution Function) of vegetation and other land cover features – meaning how the light they reflect changes with shifts in illumination or view angle.
A worldwide community of users have based their research over the past years largely on data from CHRIS (Compact High Resolution Imaging Spectrometer) on PROBA-1. The following list represents a few examples of application projects and achievements with the CHRIS instrument:
- CHRIS’s multi-angle observations of Canadian forests and associated land cover reduced classification errors by more than half compared to traditional multispectral data.
- Teams in Switzerland, Germany and Australia meanwhile have been using CHRIS observations to classify the leaf angle and therefore maturity of maize, wheat and cotton crops respectively (the angle of the leaves increases as the structure of the growing crops change, the stems and heads coming to dominate).
- A Beijing project has done the same for seasonal observations of wheat, cotton and apple orchards in the Hengshui area of China’s Hebei Province.
- Other CHRIS users within the land cover field concentrate on spectral data alone –multi-angular views not being relevant. A project team at the University of Kaiserslautern in Germany has employed CHRIS imagery to measure the coverage and estimate the biomass of lichens and biological soil crusts in the Namibian Desert. Such crusts play an important ecological role, anchoring soil in place, so their loss over time can flag up erosion patterns.
- CHRIS data has also found favor in combination with other, lower spatial and spectral resolution imagers, such as NASA’s MODIS (Moderate-resolution Imaging Spectroradiometer) or the MERIS (Medium Resolution Imaging Spectrometer) on ESA’s Envisat satellite. Here it can give extra detail (higher spectral resolution) of a local section of a larger satellite image acquired at lower spatial resolution.
CHRIS over water and imaging the air:
- A community of users are using CHRIS data to sample water quality, its multispectral imagery serving to derive standardized parameters such as chlorophyll content, total suspended soils and dissolved organic matter within inland water bodies such as the Rosarito Reservoir and the Aracena Dam in Spain – research of growing significance due to the European Commission’s Water Framework and Drinking Water Directives.
- PROBA-1 acquired a notable image of the 2010 Gulf of Mexico oil spill and has also been used to study spills within Venezuela’s Lake Maracaibo, one of the most ancient, oil-rich (and hence polluted) water bodies in the world. In Germany, CHRIS data has been harnessed to assess the ecological condition of abandoned open-cast mining areas, identifying impacts on vegetation, sediments and water – iron oxide tailings can not only turn water brown but also acidic.
- Similar techniques can be used within coastal waters, with one survey of chlorophyll and suspended particles performed off the coast of Ostend (also spelled Ostende) in Belgium. But CHRIS is also being used to survey the bathymetry of coastal waters as well as their contents: a team at the University of New South Wales in Australia has developed an innovative algorithm to estimate the shallowness of coastal waters by identifying the shifting frequency of waves as they approach the coast or underlying reefs, employing the fact that the wave’s spatial frequency increases as the water gets shallower. This technique can be used in highly sedimented water without needing to rely on the optical visibility through the water as typically used in bathymetry.
- CHRIS is also used for atmospheric aerosol monitoring; its higher resolution offering enhanced insight into highly polluted regions. A team at the University Polytechnic of Hong Kong adapted an algorithm, originally developed for MODIS, to retrieve AOT (Aerosol Optical Thickness) over Hong Kong, with results checked against by a Sun photometer and lidar measurements, as well as air quality data gathered by ground stations. They estimated an error of around 6%, compared to up to 20% from comparable MODIS data products – the entire sampling area of 11 km x 11 km being only slightly larger than a single MODIS AOT pixel (Ref. 13).
Figure 6: The largest Third Party Missions of ESA are: PROBA-1, ALOS and Landsat (image credit: ESA) 15)
• In May 2012, PROBA-1 received a software update to fix its star tracker. After more than a decade in orbit, ESA’s PROBA-1 was showing its age – even hibernating last winter. But a software fix to its star tracker, radiation-impaired after surpassing its design lifetime fivefold, has returned the veteran Earth-observing microsatellite to full operation. The new software from DTU (Technical University of Denmark ) allows PROBA-1 to distinguish between genuine star constellations to measure its pointing direction versus clusters of radiation-induced ‘hotspots’. As a result, the mission is back in business. 16)
• The funding from Third Party Missions and the current Earth Observation Envelope Programs has ensured the mission operations until the end of 2012 (Ref. 19).
• November 2011; During the mission life of PROBA-1, the SREM (Standard Radiation Environment Monitor) device has been monitoring the high-energy charged particles, whether radiating from the Sun, the cosmos or trapped inside radiation belts entwined in Earth’s magnetic field. The monitor’s main purpose is to identify radiation hazards to its host mission but it is also building a detailed picture of the space radiation environment. 17)
PROBA-1’s data (Figure 7) demonstrate how Earth’s radiation belts change with time and location, In particular, these data show the effects of magnetospheric storms and solar particle events that inject more radiation into the near-Earth environment. At high polar latitudes it crosses geomagnetic field lines that are linked to higher altitudes, allowing it to monitor the state of the ‘outer’ radiation belt.
Figure 7 provides a complete summary of the measurements from some of the PROBA-1 energy channels. It represents the evolution of a cross-section through the radiation belts during the mission. The upper panel shows the dynamic outer radiation belt where radiation levels can rise and fall as a consequence of 'solar storms' and vertical streaks appear where solar particle events occur. The vertical axis represents approximately where the field line crosses the equator in units of Earth-radii, and so is also closely related to geomagnetic latitude. - A field line with L = 6.6 Re crosses the equator up at about the geostationary orbit and crosses the PROBA-1 orbit at a geomagnetic latitude of around 65º.
The lower panel of Figure 7 shows the inner radiation belt encountered by PROBA-1 in the SAA (South Atlantic Anomaly). This is much more stable and the regular short term variations are due to orbital effects. The long term trend is due to the slow effect of the solar cycle on the Earth's atmosphere - radiation belt particles are lost in collisions with atmospheric neutrals and as solar activity declines the atmosphere shrinks, allowing radiation levels to rise. During much of 2009 and 2010 the SREM was kept switched off to control PROBA-1's internal temperature, but smart operations have allowed data acquisition to resume as before (Ref. 17).
Figure 7: Illustration of the SREM measurements of the PROBA-1 mission (image credit: ESA)
• On October 22, 2011, the ESA PROBA-1 microsatellite was celebrating 10 years of on-orbit life. During its life, the CHRIS (Compact High Resolution Imaging Spectrometer) instrument has acquired nearly 20,000 environmental science images, used by a total of 446 research groups in 60 countries. 18) 19) 20)
- There are currently a total of 42 262 CHRIS data sets available for scientific users in the ESRIN data archive, each set comprising between one and five images of the same target. The archive also contains more than 13 000 HRC images.
- The only noticeable ageing effect experienced by the satellite comes from a combination of cumulative radiation damage and higher seasonal temperatures (counter-intuitively for us in the Northern hemisphere, Earth comes closest to the Sun in January, leading to an effective 10% increase in solar flux). After 10 years in orbit (and a design life of only two years) PROBA-1’s star tracker shows sensitivity to the resulting change in temperature.
- Time is gradually running out for PROBA-1 in another way, nothing to do with its design but resulting from stern laws of orbital mechanics. Like most Earth observation missions, PROBA-1 was placed into a sun-synchronous orbit, meaning it keeps pace with the Sun so that it crosses the equator northward at a fixed local time, helping to keep lighting conditions constant for comparing images.
At launch, this local time was 10:30 hours, the orbit chosen so it would advance up to 10:46 within the first three years and then gradually drift back earlier. This gave the ops team very good local times for about eight years. However, it’s becoming more of a problem as the local time moved past 9:00 hours, and there will come a point it simply gets too dark.
- The funding from Third Party Missions and the current Earth Observation Envelope Program has ensured the mission operations until the end of 2012. It would be great to have CHRIS around even longer until national hyperspectral instruments such as the Italian PRISMA or the German EnMAP missions become available, currently foreseen for 2013 and 2014, respectively (Ref. 19).
• In the spring of 2011, PROBA-1 is in its tenth year in orbit as an Earth Observation third-party mission. It is operating normally, acquiring about 450 images per month, at five angles (–55, –36, 0, +36, +55 degrees) and up to 62 spectral channels (400–1050 nm). It covers several types of observations: land and vegetation, inland water, coastal areas, atmosphere and snow and ice. The spacecraft and instruments are healthy, allowing a continuation of the mission for a few more years, despite the degradation of the local time of the descending node. 21)
• In 2010, the CHRIS instrument of PROBA-1 continuous to provide some exceedingly good images for the scientific applications. However, the platform drift with respect to the equator crossing time may well limit its utility before any significant instrument degradation. In Oct. 2009, PROBA completed its 9th year in orbit (Ref. 24).
• In the summer 2009, ESA conducted the Sentinel-3 Experiment (SEN3EXP) field campaign at different test sites in Southern Europe. The main purpose of this activity was proving feedback on key issues for the definition of Sentinel-3 OLCI and SLSTR instruments (GMES) and the development of data processing algorithms. Ground-based, airborne and spaceborne data were acquired from multi- and hyperspectral instruments over four study sites: Barrax in Spain, San Rossore in Italy, Boussole ocean buoy in the Ligurian Sea and the Acqua Alta Oceanographic Tower in the Adriatic Sea. The collected spaceborne data sets included observations of Envisat (MERIS, AASTR) and of PROBA (CHRIS). 22) 23)
• PROBA is operating nominally in 2009. Over the last eight years the mission has provided the opportunity for users to test a number of applications covering both land, aerosol and marine scenes and a large body of experience now exists in the use of satellite derived multi-angular hyperspectral data of the CHRIS instrument (see Figure 17). 24)
• PROBA is in its 8th year of operation, design life of 2 years, (so far no backup systems have been used) and continues to serve the science community with daily images. PROBA data are supporting 98 Earth-observation research projects in 26 countries. The relatively low operational cost of the mission (provided by autonomy and simplicity of operation) permit the extension of the project (several additional years of the mission are expected. 25) 26) 27)
- After a period of platform and payload commissioning in late 2001 and early 2002 a science program was established to formalize the prioritization of acquisitions.
- The PROBA-1/CHRIS mission was initiated as part of a one-year technology demonstration program; however, the success of the program and the considerable interest in the utilization of the CHRIS hyperspectral data has led to the formation of an “operational” program to fully exploit the available data opportunities.
- Since the end of 2003, PROBA has been supporting the `International Charter on Space and Major Disasters' and collecting images of disaster areas on request, such as volcano eruptions, floods and forest fires.
- Having completed its demonstration objectives, PROBA-1 became an Earth Observation third-party mission in order to continue exploiting the excellent performance of its main scientific payload.
• On Oct. 22, 2006, the PROBA spacecraft reached the milestone of 5 years of successful operations in orbit. PROBA is powered by an ABSL Li-ion battery which is the longest serving in the industry.
Figure 8: Portion of the Maldive Islands as seen by CHRIS on PROBA, observed on July 05, 2005 (image credit: ESA)
Legend to Figure 8: The Republic of Maledives in the Indian Ocean, formed by a double chain of twenty-six atolls, is the flattest country in the world, with altitudes no greater than 2.4 m above sea level (the average ground level is 1.5 m above sea level). The country sustained serious damage during the Indian Ocean Tsunami on December 26, 2004, where around a hundred people were killed or reported missing. - The atolls of the Maldives encompass a territory spread over roughly 90,000 km2, making it one of the world's most dispersed countries in geographic terms. Its population of 313,920 (2010) inhabits 200 of its 1,192 islands.
• 2004: As a result of the success of the technology and the continuing excellent performance of the satellite and its payloads, PROBA-1 became an ESA Earth observation ‘Third Party Mission’ in 2004 (Ref.26) . In addition, the success of the PROBA-1 mission resulted in a planning and preparation phase for a follow-up mission, namely PROBA-2 (as a secondary payload to SMOS of ESA) with a scientific payload dedicated to sun observations and monitoring of space weather. The PROBA-2 spacecraft was launched successfully on Nov. 2, 2009.
During its nominal design lifetime of 2 years, PROBA-1 has in particular demonstrated that: 28)
- A technology demonstration mission can also support a user oriented mission, Earth observation in the case of PROBA-1
- A microsatellite, using advanced platform and payload technologies, can support demanding and new scientific missions, using for example agility
- Embedded autonomy allows low cost and highly reactive missions and in particular the process by which the imaging of targets could be accomplished by specifying only its geographic coordinates, was demonstrated to be effective and simple for the ground operators and the users. The main goal of leaving PROBA-1 to operate itself with minimum ground control was long ago achieved.
- Advanced development methods (such as code generation) are sufficiently mature and are cost efficient
- An attitude control system based only on an autonomous star tracker is sufficient to support the pointing and stability requirements of an Earth observation mission, as well as the execution of fast and accurate attitude maneuvers requirement by the “point and stare” and the “BRDF” requirements.
Figure 9: CHRIS image of Uluru, also known as Ayers Rock, in Australia as taken in April 2004 (image credit: ESA)
Sensor complement: (CHRIS, SREM, DEBIE)
CHRIS (Compact High Resolution Imaging Spectrometer):
CHRIS is an AO hyperspectral instrument funded by the British National Space Agency (BNSC) and Sira Electro-Optics Ltd of Chislehurst, Kent, UK, and developed/built by Sira Technology Ltd (formerly Sira Electro-Optics Ltd). Objective: collection of BRDF (Bidirectional Reflectance Distribution Function) data for a better understanding of spectral reflectances. CHRIS is the prime instrument of the PROBA mission. Note: In April 2006, the Space Group of Sira Technology Ltd. was acquired by SSTL.
The technology objective is to explore the capabilities of imaging spectrometers on agile small satellite platforms. CHRIS provides 19 spectral bands (fully programmable) in the VNIR range (400 - 1050 nm) at a GSD (Ground Sampling Distance) of 17 m. Each nominal image forms a square of 13 km x 13 km on the ground (at perigee). The observation of the square target area consists in 5 consecutive pushbroom scans by the single-line array detectors, each scan is executed at different view angles to the target within a 55º cone centered at the target zenith. Furthermore, the pushbroom velocity at the target must be reduced by a factor of 5 compared to nominal nadir-pointing velocity in order to increase optimal exposure time. - CHRIS can be reconfigured to provide 63 spectral bands at a spatial resolution of about 34 m. The CHRIS design is capable to provide up to 150 channels over the spectral range of 400-1050 nm. The initial bandset selection for the science mission were defined prior to launch: 29) 30) 31) 32) 33) 34) 35) 36)
• Bands for full swath, high resolution land/aerosols (18 bands, plus 1 band for calibration)
• Water bands, full swath, high resolution (18 bands, plus 1 band for calibration)
• Full swath, 34 m resolution bands, all sites (62 bands, plus 1 band for calibration)
• Half swath, high resolution bands over land (37 bands, plus 1 band for calibration)
• Full swath, chlorophyll band set, high resolution (18 bands, plus 1 band for calibration)
Other band sets are utilized for calibration purposes. There is a very broad range of application areas, including vegetation mapping, agricultural crop forecasting, forestry, water quality, air quality and pollution monitoring.
The CHRIS instrument design comprises a catadioptric telescope, an imaging spectrometer, and an area detector array at the focal plane of the spectrometer. The focal length of the telescope is 746 mm, the aperture diameter is 120 mm (f/6). All refracting elements in the design are made of fused quartz. The telescope is axially symmetrical and has only spherical surfaces.
The spectrometer uses “prisms” with curved surfaces integrated into a modified Offner relay. The dispersion of the spectrometer varies from about 1.25 nm to 11 nm across the spectral range, with the highest dispersion at 400 nm and the lowest at the high end (1050 nm) of the spectral range. A pixel registration of better than 5% is provided in the spectral and spatial directions, with resolution limited essentially by the detector pixel size.
An area-array CCD detector at the focal plane provides pushbroom imagery. The detector is a thinned, back-illuminated, frame-transfer CCD (1152 rows and 780 columns) - the rows are assigned to separate wavelengths, while the CCD columns are used to separate resolved points in the image. The detector array operates in a frame transfer mode, with image and masked storage zones. The spectrometer fills < 200 of the CCD rows, part of the nominally-unexposed area is used to calibrate for stray light and CCD smear effects.
Table 2: Some specification parameters of the CHRIS hyperspectral instrument
The absolute pointing accuracy of 150 arcsec and the relative pointing stability of 10 arcsec over 10 s, as well as the spacecraft agility (along- and across-track) and slew rate requirements (up to 1º/s), enable multiple images acquisition (typically 5) of the same target during a single orbital overpass.
Figure 10: Schematic of instrument optical design (image credit: Sira)
The CHRIS commands are transferred from the platform telemetry system to CHRIS via a TEMIC DSP (Data Signal Processor), a radiation tolerant and space-qualified TSC 21020 device (TSC 21020 is commercially available since the end of 1998). The resulting images are stored in MMU (Memory Management Unit). Both the DSP and MMU are part of PPU (Payload Processing Unit). 37) 38)
The PPU provides also several additional interfaces for other on-board experiments, interfaces to solid state gyroscopes (SSG), an interface to an extra star tracker (PASS), and an interface to a house-keeping bus.
Figure 11: Block diagram of the PPU and attached payloads (image credit: ESA)
Figure 12: Functional block diagram of CHRIS (image credit: Sira)
Figure 13: Illustration of the CHRIS instrument (image credit: Sira)
The platform/instrument can be commanded to perform the following functions:
• Target location - requiring roll maneuvers to point cross-track
• Viewing directions for each target in one orbit - requiring pitch maneuvers to point along-track
• Spectral bands and spectral sampling interval in each band
- Programmed line integration and dumping on chip for spectral band selection
- Pixel integration on chip for spatial resolution control
- Correlated double sampling (noise reduction circuit)
- .Dynamic gain switch for optimum usage of the ADC resolution
• Spatial sampling interval
Atmospheric science objectives of CHRIS focus on aerosols, which as well as being important for weather and climate, are also a consideration for accurate atmospheric correction of satellite data. Operational plans call for a total of 30 test sites: 15 for aerosol/atmosphere studies, 10 for land surface studies and 5 for coastal studies. Aerosol studies include a number of different continental, marine, urban and desert test sites. Land surface sites include temperate agricultural areas, boreal forests and semi-arid areas.
The combination of the PROBA platform and the CHRIS instrument provides unique potential for Earth imaging. It allows hyperspectral image data to be obtained at up to five different sensor view angles during a single orbital overpass through along-track pointing and, cloud-cover permitting, up to 15 looks at the same target within a period of a few days from multiple orbital overpasses. These data can be used to derive information on the biophysical and biochemical properties of the land surface, atmosphere and coastal and inland waters through, for example, the numerical or analytical inversion of BRDF models. The latter describe the physical mechanisms by which solar radiation is scattered anisotropically at the Earth surface.
Table 3: Overview of CHRIS operating modes
Figure 14: Illustration of the FPA (Focal Plane Array) of CHRIS (image credit: Sira)
Figure 15: CHRIS image of Mumbai (formerly Bombay), India, taken on Nov. 14, 2004 (image credit: Sira and ESA)
Figure 16: CHRIS image of the Three Gorges Dam in China (image credit: SSTL)
SREM (Standard Radiation Environment Monitor):
SREM is an ESA/ESTEC instrument of REM heritage flown on STRV-1b, PROBA, the MIR space station (Oerlikon-Contraves of Switzerland is the prime contractor of the instrument), Integral (launch Oct. 17, 2002), Rosetta, and Planck. The objective of SREM is to measure energetic electrons (0.3-6 MeV), protons (8-300 MeV), and heavy ions, as well as the total accumulated dose, encountered during the mission. Thus, SREM provides data on the space weather conditions in the vicinity of the spacecraft.
Table 4: SREM instrument performance/design parameters
Figure 18: Illustration of the SREM instrument (image credit: ESA, Paul Scherrer Institut)
DEBIE (DEBris In-orbit Evaluator):
DEBIE was built by Metorex International and Patria Finavitec, both of Oy, Finland. The objective is to monitor sub-millimeter sized particles which impact the detector surfaces - measurement of mass (>10-14 g), impact speed and penetration power. The instrument consists of a central processing unit and up to 4 separate sensor units (each 10 cm x 10 cm) which can be placed on different spacecraft surfaces (two are on the ram side and two on the deep space face). DEBIE uses a combination of impact ionization, momentum and foil penetration detection. The instrument is designed as a standard detector to be flown on different spacecraft and missions with little or no modifications (planned to fly on ISS). DEBIE on PROBA uses two impact detectors located on the panel; one detector is looking in the flight direction, the other detector is looking in the cross-track direction. The DEBIE data is used in risk assessment models and for the design of protective shielding. 39)
Figure 19: Illustration of DEBIE (image credit: ESA)
Demonstration of Autonomy Technologies:
The attitude control and avionics subsystems accommodate the core technologies for S/C autonomy. The particular subsystems are:
• SGR-20 (Space GPS Receiver 20), built by SSTL, UK (heritage of UoSat-12 and TMSat). Objective: demonstration of autonomous operations for orbit and attitude determination (alternatively to ASC). The instrument consists of a 24 channel GPS L1 and C/A receiver, and four antennas for position and medium-accuracy attitude determination. The SRG-20 instrument is designed to be tolerant to radiation effects.
• ASC (Advanced Stellar Compass), developed and built by DTU (Technical University of Denmark). ASC is already flown on Ørsted, a Danish geomagnetic research microsatellite mission that was launched Feb. 23, 1999 (see also ASC description under Ørsted and CHAMP). ASC is an autonomous star tracker, the main attitude sensor of PROBA, with the objective to provide full-sky coverage and to achieve a high pointing accuracy (precision of a few arcseconds) for Earth observation. ASC is capable to reconstruct autonomously the S/C inertial attitude starting from the condition “lost in space.” The camera of the star imager is a 752 x 588 pixel CCD device. ASC consists of a camera head unit (CHU) connected to a DPU (Data Processing Unit), i.e. a microcomputer fitted to a frame-grabber. The CHU acquires star images within its FOV, while the DPU provides the processing power to perform image analysis, pattern recognition, data reduction, and communication.
Figure 20: Illustration of the ASC with 2 CHU and a DPU (image credit: ESA)
• PASS (Payload Autonomous Star Sensor), developed and built in collaboration by Sira Electro-Optics Ltd and MMS (UK) for a demonstration flight on PROBA. 40) The instrument is an autonomous star tracker with a FOV of 19.3º x 14.4º, capable of immediate recovery from a “lost-in-space” condition. The PASS elements are:
• An optical head (50 mm focal length, f/2) with a CCD detector array (770 x 576) and an ADC unit (12 bit quantization)
• A DSP-based processor (radiation-tolerant STAR DSP board), which reads relevant pixels from the head buffer, calculates centroids of star events, and hosts the star identification software
• Star identification software, which identifies stars in each camera frame and estimates the pointing direction from this data
PASS provides pointing accuracies of 3 arcseconds in pitch and yaw and 30 arcseconds in roll at update rates of 5 Hz (up to 20 Hz with reduced accuracy). The instrument has a mass of about 2.4 kg, power consumption of 9 W, and standard RS 422 / IEEE 1355 interfaces.
Note: There are several names for the instrument: a) WASS (Wide Angle Star Sensor), the original name used for the first customer, DERA; b) PASS (Payload Autonomous Star Sensor) is the name used in the PROBA technology demonstration; and c) AST20 (Autonomous Star Tracker 20) is the name chosen for commercial applications.
Figure 21: PASS (left) and the PPU (Payload Processing Unit), image credit: EADS Astrium Ltd.
• VMC (Visual Monitoring Camera) is a stand-alone wide-angle digital camera system, based on CMOS APS imaging technology, and designed and built by DSS/OIP and IMEC of Belgium. The camera is modular, offering a choice of various support options with regard to interfaces, detectors, color or grey-scale, and exchangeable optics. VMC accepts image capture and exposure control commands, performs image acquisition, and is capable of storing one image in an on-board buffer to facilitate readout at a lower data rate. In addition, VMC accepts commands from, and transmits imagery to, standard serial digital interfaces such as TTC-B-01 synchronous and RS-422-like asynchronous.
Figure 22: View of the wide-angle VMC (image credit: ESA)
VMC employs IMEC's IRIS-1 (Integrated Radiation-tolerant Imaging System), a single-chip integrating CMOS imager. IRIS-1 was developed for general purpose imaging applications in space, including visual telemetry, robot vision, planetary imaging, and land/rover imaging. The following functions are integrated on one chip: a focal plane detector array, double sampling readout structures, ADC (Analog Digital Converter), and control/interfaces. VMC flown on PROBA implies a technology demonstration.
Table 5: VMC instrument parameters
• ERC-32 high-performance RISC processor, funded by ESA, is provided by TEMIC Semiconductors (the DHS uses two of these processors). ERC-32 is a space version of a standard commercial processor, a high-performance radiation tolerant (>80 krad) SPARC V7 processor, providing 10 MIPS and 2 MFLOPS with a floating point unit. A memory controller includes all peripheral functions. The ERC32 computer is able to support a number of on-board processing functions including science data processing (image compression). This capability enhances the S/C autonomy with respect to data distribution through the selective use of the downlink and the on-board mass memory.
In addition, a TCS 21020 DSP (Digital Signal Processor) is employed (>100 krad, 15 MIPS, 45 MFLOPS). This processor provides the processing power for the imaging payloads.
• HRC (High Resolution Camera), an ESA/ESTEC instrument built by DSS/OIP of Belgium. HRC is of VTS (Visual Telemetry System) heritage, test-flown on TEAMSAT and XMM (X-Ray Multi-Mirror Mission) of ESA. The objective is to demonstrate a high-resolution imager, primarily intended for technology, educational and general public information purposes. HRC is a miniaturized imager providing gray scale images of size 4 km x 4 km with a ground resolution of 8 m (from an altitude of 600 km). The field of view is 0.358º. The telescope is of Cassegrain type with an aperture size of 115 mm and a focal length of 2296 mm. The CCD detector array uses 3D packaging technology. It contains 1026 x 1026 pixels of 14 µm size. Images are digitized to 10 bits before transmission to the spacecraft. HRC has a total mass of 2.1 kg.
Figure 23: Illustration of HRC (image credit: ESA)
Mission operations concept:
PROBA features four operational modes each characterized by the hardware units and SW modules used. 41)
• In “safe mode” all ACNS HW units are switched off and the ACNS SW is not computing. The spacecraft is then free tumbling. The PROBA S/C is designed as such that, even in this case, the body-mounted solar cells provide enough energy to guarantee S/C survival.
• The “magnetic mode” uses a limited set of sensors and actuators and relies mainly on the magnetometer and the magnetotorquers. The mode is used to detumble the S/C after the separation from the launcher. Due to its robust behavior, this mode is also used in case of onboard problems.
• The “celestial mode” uses the star tracker and the reaction wheels and allows pointing to celestial targets, such as for emergency sun pointing and sensor calibration.
• The “terrestrial mode” uses the same HW as the celestial mode plus the GPS receiver. This mode consists of the following submodes:
- The nominal nadir-pointing attitude
- The fixed-target mode where the cameras are pointed to an Earth-fixed target
- The imaging mode for the CHRIS spectrometer, where pitch and roll profiles are generated to provide the five back-and-forth scans of the target
- The high-power-generation mode where an inertially-fixed rotation of the spacecraft is superimposed on the nominal nadir-pointing attitude to increase solar electrical input power.
PROBA software provides considerable flexibility in the allocation of on-board resources and in scheduling of operations when compared to the relatively rigid concepts used in conventional missions. In particular, the following operations functions are being planned for implementation on PROBA:
1) On-board housekeeping: PROBA monitors autonomously all routine housekeeping and resource management tasks. This includes also the decision-making process in the case of (foreseeable only!) anomalies, i.e. failure detection, failure identification and first-level recovery actions. A summary of the information available on-board is downlinked to the control centre at regular intervals.
2) On-board data management: PROBA takes care autonomously of all management tasks related to the on-board data handling, storage, and downlinks. The S/C uses a 1 Gbit mass memory for data recording and a tuneable 2 Kbit/s to 1 Mbit/s downlink. As a worst case, two passes of about 10 min every 12 hours are available for downlink of telemetry data (as well as for command uplinks).
3) On-board resources usage: PROBA takes care autonomously of all management tasks related to the on-board power usage. Any excess power and energy (above the basic spacecraft control requirements during daylight and eclipse phases) is allocated to the instruments and to the spacecraft subsystems supporting the specific operations of the instruments, for instance to the wheels for attitude maneuvers. The allocation is performed on a dynamic basis, resolving task constraints and priorities. Constraints include for each activity the power and data storage area needed, the pointing requested, etc.
4) Instrument commanding: All preparatory, commanding, and data processing activities related to the instrument operations are performed on-board (after an appropriate initialization period). This includes the planning, scheduling, resource management, navigation, and instrument pointing as well as the downlinks of the processed data. In the case of CHRIS, the main EO instrument, a specific attitude (pointing and rates) of the S/C is required to perform its Earth observation function. The calculations of the relevant slew characteristics are based on a request file (containing the coordinates of the target area and the observation duration) which is uplinked from ground.
5) Science data distribution: The collected science data are normally downlinked to the (nominal) ground station from where they may be routed automatically or on request to a user's site using Internet links. Furthermore, it is planned to demonstrate an automatic direct data distribution capability to different user ground antennae upon their requests without human involvement and with minimum possible delay. The optimal downlink times may be uplinked in the request file or calculated on-board.
The in-flight performance analysis of PROBA is in line with the payload mission requirements. Besides, the PROBA mission successfully demonstrated innovative AOCS technology, such as:
• Attitude estimation with delayed measurement
• Onboard generation of attitude guidance profiles
• Optimal large-angle attitude control
• High-accuracy attitude control during imaging
• Failure detection and identification
• MATRIX case tool: testing, code size and CPU load
The ground segment of PROBA is based on a portable ground station, located at Redu, Belgium, and additional support from small stations to validate the concept of distributed users. Users around the world can ask the satellite to take pictures by sending a request through internet. A web server at the ground station handles the request and uplinks it to the satellite, which schedules autonomously and takes the imagery. The picture data are stored in the on-board mass memory unit, they are downlinked during a ground station pass, after which it becomes available to the user.
1) D. Bernaerts, F. Teston, J. Bermyn, “PROBA (Project for On-Board Autonomy),” Proceedings of the 6th ISU Symposium on Smaller Satellites: Bigger Business?, Strasbourg, France, May 21-23, 2001
2) F. Teston, R. Creasey, J. Bermyn, D. Bernaerts, K. Mellab, “PROBA: ESA's Autonomy and Technology Demonstration Mission,” Proceedings of the 13th AIAA/USU Conference on Small Satellites, Logan UT, Sept. 23-26,1999, SSC99-V-8, http://www.estec.esa.nl/wawww/ES/PROBA.html
3) M. A. Cutter, “A Small Satellite Hyperspectral Mission,” Proceedings of the 4S Symposium: Small Satellites, Systems and Services, Sept. 20-24, 2004, La Rochelle, France
4) F. Teston, “PROBA-1,” CHRIS/PROBA Workshop, April 28, 2004, URL: http://earth.esa.int/workshops/chris_proba_04/pres/2_FTeston.pdf
6) F. Teston, R. Creasey, J. van der Ha, “PROBA: ESA's Autonomy and Technology Demonstration Mission,” IAA-97-1.3.05, 48th International Astronautical Congress, Oct. 6-10, 1997, Turin, Italy
7) J. de Lafontaine, J. Buijs, P. Vuilleumier, P. van den Braembussche, “Development of the PROBA Attitude Control and Navigation Software,” 4th ESA International Conference on Spacecraft Guidance, Navigation and Control Systems, Oct. 18-21, 1999, pp. 427-442, ESA/ESTEC, Noordwijk, The Netherlands, (ESA SP-425, Feb. 2000)
8) F. Teston, M. Barnsley, J. Settle, P. Vuilleumier, S. Santandrea, “PROBA: An ESA Technology Demonstration Mission with Earth Imaging Payload. First Year of In-Orbit Results,” 4th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 7-11, 2003
9) Announcement of Opportunity: Instruments for PROBA-2,” ESA, June 3, 2002, URL: : http://esamultimedia.esa.int/docs/industry/Proba%20call%20for%20ideas%20June%202002.pdf
11) E. Tilmans, “The PROBA Ground Infrastructure and Mission Operations,” STEC 2004 (Space Technology Education Conference), April 14-16, 2004, Lausanne,Switzerland
12) Information was provided by Frédéric Teston, Head of Systems & Cost Engineering Division, ESA
13) Bianca Hoersch, Frédéric Teston, “Autonomy in Action: Ten Years in PROBA-1,” Proceedings of the 4S (Small Satellites Systems and Services) Symposium, Portoroz, Slovenia, June 4-8, 2012
14) “ESA Third Party Missions,” URL: https://earth.esa.int/web/guest/missions/3rd-party-missions/overview
15) Henri Laur, “ESA >EO missions,” 3rd GSCB (Ground Segment Coordination Body) Workshop, 2012, ESA/ESRIN, Frascati, Italy, June 6-7, 2012, URL: http://earth.esa.int/gscb/papers/2012/12-ESA_missions.pdf
16) “‘Eye operation’ puts Proba-1 back in business,” ESA, May 14, 2012, URL: http://www.esa.int/export/SPECIALS/Proba_web_site/SEMDBPNW91H_0.html
17) “PROBA-1 charting Earth’s radiation belts for a decade,” ESA, Nov. 7, 2011, URL: http://www.esa.int/esaMI/Proba_web_site/SEMX52TWLUG_0.html
19) Sean Blair, “Autonomy in Action, Ten years of PROBA-1,” ESA Bulletin, No 148, Nov. 2011, pp. 23-31, URL: http://earth.esa.int/pub/ESA_DOC/Autonomy-in-action.pdf
20) “First Belgian satellite celebrates its 10th anniversary,” QinetiQ Space, URL: http://www.space.qinetiq.com/news/pressreleases/Pages/First-Belgian-satellite-celebrates-its-10th-anniversary.aspx
21) “PROBA status,” ESA Bulletin, No 145, February 2011, p. 86
23) Alessandro Barducci, Donatella Guzzi, Cinzia Lastri, Vanni Nardino, Ivan Pippi, Federico Magnani, Maurizio Pieri, Fabio Maselli, “CHRIS/PROBA-1 Acquisition on San Rossore Test Site for the ESA SEN3EXP Campaign,” Proceedings of the Hyperspectral Workshop 2010, Frascati, Italy, M;arch 17-19, 2010, ESA SP-683
24) Mike Cutter, Frédéric Teston, “A Small Satellite Mission Demonstrating Multi-Angular Hyperspectral Applications Over a Period of Eight Years,” Proceedings of the 60th IAC (International Astronautical Congress), Daejeon, Korea, Oct. 12-16, 2009, IAC-09.B4.2.8
25) M. A. Cutter, “CHRIS Hyperspectral Mission -Five Years since Launch,” Proceedings of the Asian Space Conference 2007, Nanyang Technological University (NTU), Singapore, March 21-23, 2007
26) Frederic Teston, Pierrik Vuilleumier, David Hardy, Etienne Tilmans, Kristof Gantois & the PROBA-1 Project Team, “PROBA Proves the Technology,” ESA Bulletin, No. 129, Feb. 2007, pp. 48-53, URL: http://www.esa.int/esapub/bulletin/bulletin129/bul129f_teston.pdf
27) In-orbit demonstration - Testing new technology in Earth orbit,” ESA, June 22, 2009, URL: http://www.esa.int/SPECIALS/Technology/SEMCPNBDNRF_0.html
28) F. Teston, D. Bernaerts, K. Gantois, “PROBA, an ESA technology demonstration mission, results after 3 years in orbit,” Proceedings of the 4S Symposium: Small Satellites, Systems and Services,” Sept. 20-24, 2004, La Rochelle, France, ESA SP-571
29) M. A. Cutter, D. R. Lobb, T. L. Williams, R. E. Renton, “Integration & Testing of the Compact High-Resolution Imaging Spectrometer (CHRIS),” Proceedings of SPIE, Vol. 3753, Denver, CO, July 19-21, 1999, pp. 180-191
30) M. A. Cutter, D. R. Lobb, R. A. Cockshott, “Compact High Resolution Imaging Spectrometer (CHRIS),” IAA 2nd International Symposium on Small Satellites for Earth Observation, Berlin, April 12-16, 1999, pp. 205-208
32) “Exploitation of CHRIS data from the PROBA Mission,” Experimenters Handbook, Issue 4, Oct. 18, 1999
33) M. Cutter, D. Lobb, “Design of the Compact High-Resolution Imaging Spectrometer (CHRIS), and Future Developments,” Proceedings of the 5th International Conference on Space Optics, March 30-April 2, 2004, Toulouse, France, ESA SP-554, URL: http://earth.esa.int/pub/ESA_DOC/PROBA/Sira5thICSO2004CHRISpaperSP-554.PDF
34) M. A. Cutter, “Review of a Small Satellite Hyperspectral Mission,” Proceedings of the 19th AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 8-11, 2005, SSC05-IV-02
35) M. A. Cutter, “The PROBA-1 / CHRIS Hyperspectral Mission - Five Years Since Launch,” Proceedings of the 4S Symposium: `Small Satellite Systems and Services,' Chia Laguna Sardinia, Italy, Sept. 25-29, 2006, ESA SP-618
36) M. Cutter, “Compact High Resolution Imaging Spectrometer (CHRIS),” URL: http://www.research.plymouth.ac.uk/geomatics/research/chris/SIRA-4_July_2000.pdf
37) TEMIC was a daughter of Daimler-Benz until 1997 when it was acquired by Vishay and in 1998 sold to Amtel (with plants in Heilbronn, Germany and Nantes, France)
39) “PROBA-1 payloads,” ESA, URL: http://www.esa.int/esaMI/Proba_web_site/ESARBKTHN6D_0.html
40) R. Cockshott, D. Purll, N. Fillery, V. Lewis, “The UK Wide Angle Star Sensor (WASS),” Presented at the poster session of the 4th ESA International Conference on Spacecraft Guidance, Navigation and Control Systems, Oct. 18-21, 1999, Noordwijk.
41) P. van den Braembussche, J. de Lafontaine, J. Buijs, P. Vuilleumier, “PROBA Attitude Control and Guidance In-Orbit Performance,” 5th International ESA Conference on Guidance Navigation and Control Systems, Frascati, Italy, Oct. 22-25, 2002, ESA SP-516
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 PROBA series: