PROBA-V (Project for On-Board Autonomy - Vegetation)
PROBA-V (Project for On-Board Autonomy - Vegetation)
The PROBA-V (Vegetation) mission definition is an attempt, spearheaded by ESA and CNES, to accommodate an improved smaller version of the large VGT (Vegetation) optical instrument of SPOT-4 and SPOT-5 mission heritage on a small satellite bus, such as the one of PROBA-2.
As of 2008, small satellite technologies have reached a level of maturity and reliability to be used as a platform for an operational Earth observation mission. Furthermore, advancements in the techniques of detectors, optics fabrication and metrology are considered sufficiently mature to permit the design of a compact multispectral optical instrument. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16)
The C/D Phase started in July 2010. The system CDR (Critical Design Review) took place in the spring of 2011. The project is currently (summer 2012) in its Phase D, with a Final Acceptance Review planned for December 2012. ESA is responsible for the overall mission, the technological payloads and for the launcher selection.
Background: The VGT instruments (VGT1 & VGT2), each with a mass of ~140 kg and fairly large size, have provided the user community with almost daily global observations of continental surfaces at a resolution of 1.15 km on a swath of ~2200 km. The instruments VGT1 on SPOT-4 (launch March 24, 1998) and VGT2 on SPOT-5 (launch May 4, 2002) are quasi similar optical instruments operating in the VNIR (3 bands) and SWIR (1 band) range.
The Vegetation instruments were jointly developed and funded by France, Belgium, Italy, Sweden, and the EC (European Commission). The consortium of CNES, BelSPO (Federal Public Planning Service Science Policy), SNSB (Swedish National Space Board) and VITO (Flemish Institute for Technological Research) is providing the user segment services (data processing, archiving, distribution). Vegetation principally addresses key observations in the following application domains:
• General land use in relation to vegetation cover and its changes
• Vegetation behavior to strong meteorological events (severe droughts) and climate changes (long-term behavior of the vegetation cover)
• Disaster management (detection of fires and surface water bodies)
• Biophysical parameters for model input devoted to water budgets and primary productivity (agriculture, ecosystem vulnerability, etc.).
As of 2008, a Vegetation archive of 10 years of consistent global data sets has been established permitting researchers access on a long-term basis. The SPOT-5 operational lifetime is estimated to expire in 2012. Pleiades, the next French satellite for Earth Observation, is solely dedicated to high-resolution imaging (on a fairly narrow swath) and will not embark any instrument providing vegetation data.
Since the SPOT series spacecraft will not be continued and the SPOT-5 spacecraft will eventually fail — there is of course a great interest in the EO user community to the Vegetation observation in the context of a smaller mission, affordable to all concerned. 17)
PROBA-V will continue the production of Vegetation products exploiting advanced small satellite technology. However, this implies in particular a redesign of the Vegetation payload into a much smaller unit to be able to accommodate it onto the PROBA bus.
Overview of key requirements of the PROBA-V mission - and some improvements compared to SPOT/Vegetation:
- Data and service continuity: filling the gap between SPOT-VGT and the Sentinel-3 mission
- Spectral and radiometric performance identical to VGT
- GSD: 1 km mandatory, improved GSD is highly disirable: 300 m (VNIR bands), 600 m (SWIR band). Image quality and geometric accuracy, equal to or better than SPOT-VGT
- Provision of daily global coverage of the land masses in the latitudes 35º and 75º North and in the latitudes between 35° and 56° South, with a 90% daily coverage of equatorial zones - and 100% two-daily imaging, during day time, of the land masses in the latitudes between 35º North and 35º South..
Figure 1: Scenario of the PROBA-V gap-filler mission between SPOT-5 and Sentinel-3 (image credit: ESA) 18)
Figure 2: Illustration of the PROBA-V spacecraft in orbit (image credit: ESA)
An extensive feasibility study and trade-off work was undertaken to identify a solution that could meet not only the technical challenges, but that could also be developed and tested within a tight budget of a small satellite mission. 19) 20)
The PROBA-V project of ESA includes the Space Segment (platform contract award to QinetiQ Space NV of Kruibeke, Belgium - formerly Verhaert), the Mission Control Center (Redu, Belgium) and the User Segment (data processing facility) at VITO NV. VITO (Vlaamse instelling voor technologisch onderzoek - Flemish Institute for Technological Research) is located in northern Belgium. VITO's processing center of VGT1 and 2 data (SPOT-4 and SPOT-5) is operational since 1999. VITO is also the prime investigator and data service provider of PROBA-V for the user community including product quality control. 21)
• The Phase B of the project started in January 2009
• SRR (System Requirements Review) is in Q4 of 2009
• PDR (Preliminary Design Review) in Q2 of 2010
• HMA (Heterogeneous Mission Access) and QA4EO (Quality Assurance for Earth Observation) implementation for user data. Planned interoperability with GSCDA V2 (GMES Space Component Data Access Version 2).
Figure 3: PROBA-V project organization (image credit: ESA, Ref. 9)
An industrial team, led by QinetiQ Space NV (Belgium), is supported by several European subcontractors and suppliers, and is responsible for the development of the flight satellite platform, the vegetation payload and the Ground Segment.
The spacecraft bus (fully redundant) is of heritage from the PROBA-1 and PROBA-2 missions (structure, avionics, AOCS, OBS with minor modifications). The PROBA-V spacecraft has a total mass of 138 kg, and a volume of 80 cm x 80 cm x 100 cm. The three-axis stabilized platform is designed for a nominal mission lifetime of 2.5 years (Ref. 7). 22) 23) 24) 25)
The spacecraft resources management is built around ADPMS (Advanced Data and Power Management System), which is currently flying on PROBA-2. The data handling part of ADPMS is partitioned using compact PCI modules. A cold redundant mass memory module of 16 Gbit is foreseen for PROBA-V. The newly developed mass memory will use NAND flash technology.
The avionics architecture can be divided in several sections:
• The AOCS block, containing all AOCS equipment and the required additional electronics in order to adapt or convert interfaces and supply voltages.
• The ADPMS, featuring the two redundant data handling lanes and its power section. It is the center of all data handling, communications and power conditioning and distribution.
• The main payload (Vegetation Instrument).
• The technology demonstrators. Four technology demonstrator payloads have been incorporated in the design.
• The communication section, featuring full redundancy for all modules. It comprises the S-band TT&C subsystem and the X-band data downlink subsystem.
• The power distribution and conditioning part of ADPMS supplies an unregulated bus, with each equipment having its internal DC/DC converter. The power conditioning system is designed around a Li-ion battery and a dump resistor ((to dissipate excess current).
The data handling part of ADPMS is built up from several modules, each based on the compact PCI standard. The ADPMS design is fully redundant. The PROBA-V configuration of ADPMS comprises:
• MPM (Main Processor Module), based on a LEON2 processor (ASIC), providing the processing power, memory and physical interfaces to control all other peripheral boards
• TTM (Telecommand and Telemetry Module), providing the bidirectional interface between the spacecraft and the ground stations
• SIM (Spacecraft Interface Module), providing the bidirectional communication interfaces between the ADPMS and the other spacecraft units
• DAM (Data Acquisition Module), providing the data acquisition of analog, digital and temperature signals
• MMM (Mass Memory Module), providing a data storage capability of 16 GByte EDAC protected and based on NAND flash technology
• REM (Reconfiguration and Emergency Module (REM), providing hardware functions to allow an easy reconfiguration and recovery of ADPMS directly from ground in case of problems.
The Power Management System of ADPMS supplies a battery-regulated bus, designed around a Li-ion battery. The power management system is built-up from the following modules:
• PCM (Power Conditioning Module), managing and regulating the incoming and outgoing power
• PDM (Power Distribution Module), managing the power distribution and over current protections.
The power management system can manage a total power of 300 W. Each power output line can handle a current of 2 A.
Figure 5: PROBA-V spacecraft accommodation, outer platform views on left, inner platform views on right (image credit: QinetiQ Space)
AOCS (Attitude and Orbit Control Subsystem) provides three-axis attitude control including high accuracy pointing and maneuvering in different spacecraft attitude modes. The AOCS SW is an extension of the one of PROBA-2, including the following algorithms required by the on-board autonomous mission and payload management: 26) 27)
- Prediction of land/sea transitions using a land sea mask to reduce the amount of data generated
- Optimization of attitude in Sun Bathing mode to enhance incoming power while avoiding star tracker blinding
- Momentum dumping without zero wheel speed crossings during imaging
- Estimations of remaining spacecraft magnetic dipole to reduce pointing error
- Autonomous avoidance of star tracker Earth/Sun blinding
- Inertial mode with fixed scanning rate for moon calibration.
The AOCS hardware selection for PROBA-V consists of a high accuracy double star tracker head, a set of reaction wheels, magnetotorquers, magnetometers and a GPS receiver.
The main AOCS modes are: Safe, Geodetic, Sun Bathing and Inertial mode.
- The satellite Safe mode is used to detumble the spacecraft after separation from the launcher and it will be used to recover from spacecraft anomalies.
- The Geodetic mode is used during nominal observation of the Earth's vegetation. In this mode the payload is pointed towards the geodetic normal to the Earth's surface. An extra steering compensation, yaw-steering, is added in this mode, to minimize the image distortion caused by the rotation of the Earth. This yaw-steering maneuver ensures that the spectral imagers are oriented such that the lines of pixels are perpendicular to the ground-trace at each moment. In this mode the star trackers and the GPS receiver are used as sensors and the reaction wheels as actuators.
- On each orbit, the spacecraft enters the Sun Bathing mode from -56º latitude until entry of eclipse. This is to enhance the incoming power.
- The Inertial mode coupled with an inertial scanning of the Moon at a fixed rate is used for monthly radiometric full moon instrument calibration purposes. The pointing towards the moon takes 2.5 min, 9 min for scanning the moon and 2.5 min to return to nominal observation mode. It is sufficient to have the moon in the FOV of the SI (Spectral Imager) for a number of pixels.
NGC Aerospace of Canada was responsible for the design, implementation and validation of the autonomous GNC (Guidance, Navigation and Control) algorithms implemented as part of the PROBA-1 and PROBA-2 AOCS software and has the same responsibilities for the PROBA-V mission.
Figure 6: Functional breakdown of the AOCS software (NGC Aerospace, ESA, QinetiQ Space)
Beyond the technology demonstration through the PROBA program, it is also noted that the AOCS software technology developed in the course of this program is now the baseline of the AOCS of a major operational mission of the GMES (Global Monitoring for Environment and Security) program: Sentinel-3. NGC Aerospace Ltd (NGC) of Sherbrooke, (Québec), Canada was responsible for the design, implementation and validation of the autonomous GNC (Guidance, Navigation and Control) algorithms implemented as part of the AOCS software of PROBA-1 and PROBA-2. NGC has the same responsibilities for the PROBA-V mission (Ref. 26).
Operational modes of AOCS:
The PROBA-V operational modes are illustrated in Figure 7 (critical modes in blue) and briefly described in Table 2. Transitions between any of the modes are possible and can be commanded by the ground station or, autonomously, by the on-board mission manager. The Magnetic mode is represented in a dotted rectangle in Figure 7 because it was not part of the launch configuration. The addition of the Magnetic mode was decided and implemented after the launch. It was decided to include it as an added safety feature for PROBA-V. The Magnetic mode replaces the Bdot mode as the spacecraft Safe mode. However, the Bdot mode remains available upon ground request or as an autonomous fallback mode in case of an anomaly with the Magnetic mode. 28)
The requirements for the PROBA-V AOCS software were first defined by QinetiQ Space. Then, NGC (NGC Aerospace Ltd.) completed the software design and the related validation tests. Finally, NGC and QinetiQ Space conducted, in close collaboration, the AOCS Software Acceptance Tests (SAT). In addition to this on-ground validation, prior to the addition of the Magnetic mode, the AOCS software was validated in flight. For this purpose, the various AOCS software functions and operational modes were activated and validated one at a time in an incremental manner in the weeks following the launch. This validation successfully demonstrated that the AOCS software meets the stringent PROBA-V requirements.
With the addition of the Magnetic mode, the validation process had to be followed again not only for the Magnetic mode, but to ensure non-regression of the already fully validated operational modes. The validation approach followed for the addition of the Magnetic mode is described in the next section after an overview of the Magnetic mode algorithm and of its implementation.
Magnetic mode design: The reader is referred to Ref. 28) , since this is a too lengthy discussion. However, from the in-flight results, it can be stated that the Magnetic mode meets its requirements both in terms of functionality and robustness. Indeed, it has been demonstrated that the spacecraft attitude is controlled as required and that maneuvers can be performed.
Since its successful in-flight validation, the Magnetic mode is the baseline Safe mode for PROBA-V. With its successful operation on-board PROBA-V, the Magnetic mode provides an alternative to the traditional B-dot algorithm for safe modes of missions where it might be desired to maintain three-axis pointing in order to point one of the spacecraft axis (e.g. antenna) towards the Earth.
The PROBA-V design was successful in meeting the challenges as can be seen by the pointing performance achieved in flight (Table 3).
Table 3: PROBA-V geodetic mode pointing requirements and achieved performance (Ref. 28)
EPS (Electric Power Subsystem): The PVA (Photo-Voltaic Array) uses GaAs triple junction cells with an of efficiency of 28%. To obtain the operating voltage of 31.5 V, 18 cells are included in each string in series with a blocking diode. The PVA consists of a total of 25 solar strings taken into account the loss of one string on the most contributing PVA panel. The average solar string power under EOL conditions (summer solstice and T = 40°C) yields 12.8 W. The maximal incoming power at EOL during an orbit is 144 W. The energy budget for PROBA-V is derived for a bus power consumption of 140 W assuming a worst case day in the summer and while not taken into account the effect of albedo. A worst case power budget analysis indicated a maximum capacity discharge of 1.66 Ah. Use of a Li-ion battery. The battery cells provide a capacity of 1.5 Ah per string. The PROBA-V battery is sized to 12 Ah taking into account capacity fading and loss of a string.
RF communications (PROBA-V): S-band for TT&C transmissions and low-gain antennas with omni-directional up- and downlink capability. The uplink symbol rate will be fixed at 64 ks/s, while the downlink can be set to a high rate (< 2 Ms/s) for nominal imaging or to a low rate at 329 ks/s for off-nominal conditions. The CCSDS protocol is used for the TT&C transmissions.
X-band downlink of the payload data is in X-band at a data rate of 35 Mbit/s. The onboard mass memory is 88 Gbit. The Redu station (Belgium) is being used for TT&C communication services. The X-band uses two cold redundant high-rate X-band transmitters (developed by Syrlinks, France) and two nadir pointing isoflux antennas, both RHCP.
The S-band transceivers will be connected to RS422 outputs (cross strapped) of ADPMS while the X-band transmitters (8090 MHz) will be connected to the LVDS outputs not cross-strapped. The X-band link budget results in a link margin of 6 dB which will allow a reduction of the RF output power. Therefore the X-band transmitter will be designed (customer furnished item) to support various output power settings such that after commissioning, a lower output power might be selected.
Data compression: The massive amount of data produced by the instrument is beyond the capabilities of the bandwidth available on board of a small satellite. Data are reduced by using a lossless data compression algorithm implemented in a specific electronics. The data compression ratio obtained using standard CCSDS compression algorithms (CCSDS 133.0 B-1) is shown in Table 4.
The CCSDS image data compression standard turned out to meet all the requirements in terms of image quality and reachable compression ratio, accordingly reaching the required target data rate. This compression algorithm has been implemented in specific electronics (FPGA) on the satellite. Among many other notable firsts, PROBA-V has therefore become the first European mission to fly the CCSDS image data compression standard.
The selection of an S-band transceiver and the development of an innovative and generic X-band transmitter for small satellites has been initiated in a collaborative program between CNES and ESA and is funded under GSTP-5 (General Support Technology Program-5). The X-band transmitter is a high-performance device optimized for the needs and constraints of small platforms for which small volume, low mass, low power consumption, and low cost cost are important parameters. Moreover, some key features such as modulation (filtered Offset-QSK), coding scheme (convolutional 7 ½), data and clock interfaces (LVDS packet wire serial interface) have been selected in compliance with CCSDS recommendations, but also to ease the interoperability with most of the existing on-board computers and ground station demodulators. 29)
Following CNES studies under ESA contract, two low-cost X-band transmitters compatible with data rates up to 100 Mbit/s were designed and manufactured by Syrlinks of Bruz, France. One transmitter uses a GaAs RF power amplifier and one X-band transmitter uses a new GaN (Gallium Nitride) RF power amplifier, part of ESA's developed GREAT2 (GaN Reliability Enhancement and Technology Transfer Initiative). 30) 31)
The development of the new X-band transmitter is based almost exclusively on COTS components to achieve at the same time high performances and low recurrent cost. The transmitter also features an innovative functionality with an on-board programmable RF output power from 1-10 W which allows to match finely with the chosen bit rate, and to reduce as much as possible the margins of the link budget and therefore the consumption power. PROBA-V is the first mission to use this newly developed transmitter. The transmitter has a mass of 1 kg, a size of 160 mm x 115 mm x 46 mm, an in-orbit life time of 5 years, and a radiation hardness of 10 krad. Data rates from 10-100 Mbit/s are available.
Table 5: Typical specification of the X-band transmitter
Figure 8: Overview of the X-band transmitter architecture (Syrlinks, CNES)
Incoming telemetry data is coded in a programmable device (CPLD) at the clock rate. I and Q baseband signals are filtered (7th order Butterworth filter) and then applied to the RF I/Q modulator. This modulator works in S-band. The S-to-X converter translates the modulated signal from S-band to X-band. X-Band signal is then amplified. The microcontroller manages the serial link with the OBC (On-Board Computer) of the platform. It decodes the commands and executes the actions. The microcontroller reads the different internal indicators (temperature, currents, ...) and controls the functions such as the RF synthesizer or the power supplies. The power supply is a galvanic isolated Dc/Dc converter followed by non-isolated Dc/Dc converters and linear regulators. Internal supplies are protected against over-current consumption, in case of latch-up.
The GaAs and GaN transmitter measured performances are similar. Table 6 gives the main typical results.
Table 6: GaAs and GaN transmitter performances overview (Ref. 30)
Figure 9: Photo of the X-band transmitter (image credit: Syrlinks, CNES, ESA)
• June 25, 2015: Europe's first item of high-performance gallium nitride technology to fly in space has completed its second year of operations. Hosted on ESA's Earth-observing PROBA-V minisatellite in 2013 as a test prototype, the transmitter is today used routinely to return mission imagery to the ground. "The X-band transmitter in question incorporates an experimental gallium nitride (GaN) amplifier," explains Andrew Barnes, overseeing ESA's work in GaN. "It is still working seamlessly today after two years in orbit, showing no drift in performance" (Ref. 31).
- The GaN-based transmitter is used to downlink data to PROBA-V's Kiruna ground station – in the Swedish Arctic – once per orbit for a week at a time, alternating with a second transmitter using a conventional gallium arsenide amplifier.
- With its data coming down at a standard rate of 42.22 Mbit/s during each roughly 12 minute pass, the m3-sized PROBA-V builds up a complete picture of all Earth's vegetation cover every two days.
- Access to the GaN-based transmitter also increases the operational flexibility of the satellite – in principle its data rate can be boosted to 100 Mbit/s , while its programmable radio frequency output power can also be increased as needed, while operating at a lower voltage than its conventional equivalent.
- Gallium nitride has been described as the most promising semiconductor since silicon, capable of operating at much higher voltages and temperatures than comparable materials. As an additional advantage, GaN also possesses inherent resistance against the radiation encountered in space.
- "In terms of communications for space, GaN offers a five- to ten-fold increase in communications power, while requiring no additional cooling systems," adds Andrew.
- "Its promise is such that back in 2008 ESA launched the ‘GaN Reliability Enhancement and Technology Transfer Initiative' (GREAT2), bringing together leading universities, research institutes and industry to develop space-compatible production processes for making GaN microwave power transistors and integrated circuits.
- With GREAT2 , ESA has come in at an early stage of industrialization to ensure that the resulting products meet the demanding requirements of space use, such as resistance to shock and temperature extremes, as well as continuous operations for years at a time."
- The GREAT2 partners include UMS (United Monolithic Semiconductors) based in Germany and France, responsible for the industrial foundry used for manufacturing GaN products.
Figure 10: GaN amplifier supply chain for PROBA-V (image credit: ESA)
- Since then, while the transmitter has been proving its worth in space, the first industrial prototypes have successfully completed their testing for reliability and robustness.
- "As a result of GREAT2 we were able to place the UMS GaN manufacturing process onto the European Preferred Parts List of the European Space Components Coordination – a list of recommended parts for space missions – in 2012," adds Andrew. "This was two years earlier than originally planned.
Figure 11: GaN on SiC wafers (image credit: ESA)
Figure 12: Photo of a PROBA-V integration test at QinetiQ Space (image credit: ESA) 32)
Table 7: PROBA-V key features
Launch: The PROBA-V spacecraft (primary payload) was launched on May 7, 2013 (02:06:31 UTC). The launch vehicle was Vega (with Arianespace as launch provider); the launch site was the Guiana Space Center, Kourou. This marks the first VERTA (Vega Research, Technology and Accompaniment) flight of VEGA (designated as VERTA-1). The mission is designated Flight VV02 in Arianespace's numbering system. 34) 35) 36)
The secondary payloads on this flight were:
• VNREDSat-1A (Vietnam Natural Resources, Environment and Disaster-monitoring Satellite) of STI-VAST (Space Technology Institute-Vietnam Academy of Science and Technology). The microsatellite VNREDSat-1A (115.3 kg) has been built by EADS Astrium, Toulouse, France. 37) 38)
• ESTCube-1 (Estonian Student Satellite-1), a 1U CubeSat (1.3 kg) technology demonstration mission of the University of Tartu.
PROBA-V will ride in the upper position of the VESPA (Vega Secondary Payload Adapter), while VNREDSat-1A and ESTCube-1 will sit in the lower position in the structure. The upper stage of the Vega vehicle is a liquid propulsion module with multiple re-ignition capability. The secondary payloads will be deployed last after re-ignition of the Vega upper stage.
Orbit of PROBA-V: Sun-synchronous orbit, altitude = 820 km, inclination = 98.8º, LTDN (Local Time on Descending Node) = 10:30 hours (with a drift limited between 10:30 and 11:30 AM during the mission lifetime). Note: In contrast to the SPOT-4 and SPOT-5 missions, PROBA-V will not have the capability to maintain its orbit.
Orbit of VNREDSat-1 and ESTCube-1: Sun-synchronous orbit, altitude =704 km, inclination = 98.7º. VNREDSat-1A was released 1 hour 57 minutes into flight. ESTCube-1 was ejected from its dispenser three minutes later. A last burn will now place the spent upper stage on a trajectory that ensures a safe reentry that complies with new debris mitigation regulations.
Figure 14: Artist's view of the PROBA-V minisatellite in orbit (image credit: ESA)
• May 04, 2018: ESA's Proba-V minisatellite has imaged all of the Antarctic after users asked for a survey of the icy southern continent. Proba-V collected the data between November 2017 and February 2018. 39)
- Imagery is available at 1 km, 300 m and 100 m resolution. For information on how to access it, click here. VITO Remote Sensing in Belgium processes and then distributes Proba-V data to users worldwide.
- On 7 May, 2018, PROBA-V will be 5 years on orbit. A PROBA-V symposium will be held in Ostend, Belgium (29-31 May, 2018) to bring researchers together to discuss the work being performed using the minisatellite.
Figure 15: Antarctic survey image of PROBA-V, collected between Nov. 2017 and Feb. 2018 (image credit: ESA/Belspo – produced by VITO)
• February 14, 2018: Figure 16 is a false-color image of Pyeongchang county and surrounding territory in South Korea – currently hosting the 2018 Winter Olympics. This is a view of the northern part of the country, with vegetation in red and built-up areas seen in grey, including capital city Seoul, astride the banks of the Hangang River, seen left. Pyeongchang county is located towards the east coast. Mountainous regions are seen dusted with snow. 40)
- The Winter Olympics run from 8 to 25 February. By adding four new disciplines, this international event is the first Winter Games to extend over 100 medals, spread across 15 sports.
- PROBA-V's main camera's continent-spanning 2250 km swath width collects light in the blue, red, near-infrared and mid-infrared wavebands at 300 m resolution and down to 100 m resolution in its central field of view.
- VITO Remote Sensing in Belgium processes and then distributes PROBA-V data to users worldwide. An online image gallery highlights some of the mission's most striking images so far, including views of storms, fires and deforestation.
- Located just north of the border with Kazakhstan, Lake Chany is a large but shallow freshwater lake surrounded by wetlands, salt marshes and birch and aspen forests, making it an important stop for birds migrating southwards from colder Siberia.
- VITO Remote Sensing in Belgium processes and then distributes PROBA-V data to users worldwide. An online image gallery highlights some of the mission's most striking images so far, including views of storms, fires and deforestation.
- The mountains – an extension of Europe's Alpine system – stretch some 2400 km through Morocco, seen here, into Algeria and Tunisia. The Atlas mountains are actually a set of five ranges dividing the northern Mediterranean climate from the arid Sahara to the south. A second, darker, range, the Anti-Atlas mountains, are seen to the south, with the the Draa River valley cutting through them – seen as a reddish line. The Draa, Morocco's longest river, flows south from the city of Ouarzazate city into the Sahara.
- The Berber-speaking Ouarzazate is a popular location for filmmakers, with productions such as Lawrence of Arabia (1962), The Mummy (1999) and Game of Thrones (2011–present) having been shot here.
- VITO Remote Sensing in Belgium processes and then distributes PROBA-V data to users worldwide. An online image gallery highlights some of the mission's most striking images so far, including views of storms, fires and deforestation.
• September 20, 2017: PROBA-V captures Bolivia's Salar de Uyuni, the world's largest salt plain – its 10,500 km2 make it larger than some countries. Located in the highlands of southwestern Bolivia at an altitude of 3650 m, Salar de Uyuni is also extremely flat, varying less than 1 m across its expanse (Figure 19). It is so flat that it is often used to calibrate laser and radar altimeters on satellites. 43)
- The salt plains were formed 42 000–30 000 years ago as a result of transformations between several prehistoric lakes. The crusty top layer, several meters thick in places, lies on a brine rich in lithium (containing 50–70% of the world's reserves), potassium and magnesium.
Figure 19: The false-color PROBA-V image was acquired on 5 April 2017. On the western side of the Salar de Uyuni, some wavy patterns are visible, while blue shades on the northern and eastern edges indicate flooded areas. The small rectangular patches to the south of the salt flat indicate a large lithium mining area (image credit: ESA/Belspo – produced by VITO)
• July 27, 2017: ESA's Proba-V minisatellite reveals the seasonal changes in Africa's sub-Saharan Sahel, with the rainy season allowing vegetation to blossom between February (Figure 20 top) and September (bottom). 44)
- The semi-arid Sahel stretches more than 5000 km across Africa, from the Atlantic Ocean (Senegal, Mauritania) to the Red Sea (Sudan). The few months of the rainy season in the Sahel are much needed in these hot and sunny parts of Africa, and are critical for the food security and livelihood of their inhabitants.
- The name Sahel can be translated from Arabic as coast or shore, considered as the ever-shifting landward ‘coastline' of the arid Sahara Desert.
• June 21, 2017: ESA's PROBA-V minisatellite has captured the forest fire raging in central Portugal, revealing blackened scars and columns of smoke as well as pinpointing active fire hotspots. More than a thousand firefighters are tackling the forest fire in the Pedrógão Grande region, north east of Lisbon, which has been aflame since Saturday. Some 64 people have been reported dead and more than 130 injured. 45)
Figure 21: A forest fire in Portugal's Pedrógão Grande region, showing burnt scars, smoke plumes and hotspots (seen in red). This 100 m-resolution image was acquired on Tuesday 20 June 2017 by ESA's PROBA-V satellite (image credit: ESA/BELSPO produced by VITO)
- PROBA-V has a 2250 km-wide field of view with an overall 300 m resolution, narrowing to 100 m at the center. The satellite contributes to Europe's world-monitoring Copernicus program, which makes imagery and data freely available to authorities. The V stands for Vegetation – a lighter but fully functional redesign of the camera previously flown on France's full-sized Spot-4 and Spot-5 satellites.
- Launched on 7 May 2013, PROBA-V continues the supply of this much-needed information for applications such as assessing climate impact, managing water resources and monitoring crops.
- PROBA-V's wide view and polar orbit means it revisits every spot on Earth's land every two days, building up a new global composite for researchers every 10 days.
Figure 22: Before the blaze: Portugal's Pedrógão Grande region. This 100 m-resolution image was acquired on 19 May 2017 by PROBA-V, showing the region before the forest fire that started on 17 June (image credit: ESA/BELSPO produced by VITO)
- The dammed Zêzere river is seen in the center of the main image, with burnt scars and fires burning to its north. The village of Nodeirinho – home of many of the casualties – is situated amid the scars.
- The forest fire is believed to have been started by a lightning strike during an intense heatwave. Aircraft have been used to drop water over the Pedrógão Grande region.
Figure 23: Regional view of fire: A forest fire in Portugal's Pedrógão Grande region, showing burnt scars, smoke plumes and hotspots (seen in red). This 330 m-resolution image was acquired on 18 June 2017 by ESA's PROBA-V satellite (image credit: ESA/BELSPO produced by VITO)
• May 19, 2017: The Korean the mid-sized city of Mokpo (~250,000 inhabitants) is a historic naval base and gateway to the country's Honam Plain. Mokpo is visible in this false-color image as a blue–grey area on the estuary of the Yeonsang River (Figure 24). The port city is surrounded by more than 1,400 islands, which provide fishing grounds while safeguarding Mokpo from the effects of large typhoons and tsunamis. An extensive region of high sediment concentrations is also visible, extending into the Yellow Sea in a bow shape. 46)
Figure 24: South Korea's island-studded Mokpo port region was captured by ESA's Earth-observing PROBA-V minisatellite on October 6, 2016 at a nadir resolution of 100 m on a swath of 2250 km (image credit: ESA/Belspo – produced by VITO)
• March 29, 2017: Snow-dusted Norwegian fjords imaged by ESA's Earth-observing PROBA-V minisatellite. Norway's coastline is one of the world's longest – with a total length recently calculated at 80 000–100 000 km – owing to its famous fjords, narrow inlets bordered by steep cliffs created by glacial erosion during previous Ice Ages. 47)
- After these glaciers melted and Earth's crust rebounded, seawater flooded the valleys, leading to some fjords becoming very deep: the Sognefjord fjord (visible to the upper left) has a depth of 1300 m. From bottom to top the Bokna and Hardanger fjords are also seen. The white region in the middle is the Hardangervidda National Park, an extensive plateau at around 1200 m altitude, inhabited by wild reindeer.
Figure 25: Norwegian fjords imaged by PROBA-V, the image was acquired on 14 February 2017 (image credit: ESA/BELSPO – produced by VITO)
• The PROBA-V minisatellite of ESA with the vegetation instrument on board is fully operational in February 2017. — The VITO processing center in Belgium provided the image of the week (Figure 26) with the Flinders Ranges in Australia. 48)
- The high plateau of the Gammon Ranges on the eastern side and the alternating hills and ridges, often with a gentle slope on one side and steep slope on the other (cuesta landforms), make for a dramatic and beautiful landscape.
- The region has a semi-arid climate with hot dry summers and cool winters. It's a place rich in Aboriginal history and home to a vast array of wildlife such as kangaroos, parrots, emus and snakes. The flora is well adapted to this environment, with species such as cypress-pine, black oak and mallee, a low-growing, bush-like eucalyptus that is common in Australia.
Figure 26: PROBA-V false-color image of Flinders Range, Australia, showing the northern part of the rugged, weathered peaks and rocky gorges of the Flinders Ranges, the largest mountain range in the South Australian Outback (image credit: ESA-BELSPO 2017, produced by VITO)
• December 15, 2016: In honor of the UN "World Mountain Day" on 11 December, the image of Figure 27 depicts the snow-capped Himalayas, with Nepal to the south (with vegetation shown in red) and the bleaker Tibetan Plateau to the north. 49)
- Mount Everest, the tallest mountain of the world at 8848 m, is shown in white along with a few of its 8000 m-plus neighbors, including Kangchenjunga (8586 m), the third tallest mountain of the world, to the east of Everest. The Himalayas, which can be translated from Sanskrit as ‘abode of snow', are the source of many major Asian rivers.
Figure 27: PROBA-V – ESA's smallest Earth-observing mission – overflies Mount Everest, the highest mountain in the world, its peak seen left of center in this false-color image. This 100 m-resolution image was acquired by PROBA-V on 27 October 2016 (image credit: ESA/BELSPO – produced by VITO)
• October 19, 2016: ESA's PROBA-V minisatellite gives a false-color view of circular fields fed by underground water resource in the mist of the desert. This 100 m resolution image Figure 28) shows the Wadi As Sirhan basin of Saudi Arabia, with agricultural fields fed water by circular-pivot irrigation systems, amid the yellowish desert sands and surrounding low hills and rocks. 50)
- The VGT-P (Vegetation Instrument - PROBA) camera has a swath width of 2250 km, collecting light in the blue, red, near-infrared and mid-infrared wavebands at 300 m resolution and down to 100 m resolution in its central field of view.
• August 2016: The compact SATRAM (Space Application of Timepix-based Radiation Monitor) is operating nominally in LEO orbit since 2013 on board the PROBA-V satellite and provides high-resolution wide-range radiation monitoring of the satellite environment. Equipped with the pixel detector Timepix, the technology demonstration payload determines the composition (particle types) and spectral characterization (stopping power) of the mixed radiation field with quantum imaging sensitivity, charged particle tracking, energy loss and directionality capability. The space radiation field is continuously sampled over the entire planet every few days. Results are given in the form of spatial- and time-correlated maps of dose rate and particle flux. 51)
- Preliminary exploitation of data from the SATRAM/Timepix payload serves for detailed radiation effects studies but also for physics research and space weather studies.
• August 26, 2016: ESA released a PROBA-V image of the Great Salt Lake in Utah, USA (Figure 29). The VGT-P instrumentation of the minisatellite provides a swath width of 2250 km in the blue, red, near-infrared and mid-infrared wavebands at 300 m resolution and down to 100 m resolution in its central field of view. 52)
• April 6, 2016: ESA's PROBA-V minisatellite gazes down at Earth's largest volcano – Mauna Loa, or ‘long mountain' which covers half of the island of Hawaii (Figure 30). Mauna Loa remains active, having last erupted in 1984. To the north of its distinctive blackened ridges is the still-higher Mauna Kea volcano – an extinct volcano whose crests are home to some of the world's leading astronomical observatories. 53)
- To its east is the very active Kilauea volcano, which has been erupting for more than three decades, the Hawaii Island being formed of five volcanoes altogether. The island's forest reserves are shown in green.
- Mauna Loa stands 4169 m above sea level, and extends another 5 km beneath the sea. Its 75,000 km3 volume depresses the adjacent sea bed another 6 km or so. This PROBA-V image was acquired on 19 February, 2016 with a resolution of 100 m. - VITO Remote Sensing in Belgium processes and then distributes PROBA-V data to users worldwide.
• February 8, 2016: Monitoring Earth's surface every day, ESA's PROBA-V minisatellite has had a ringside seat as the second largest lake in Bolivia, Lake Poopó, gradually dried up. Lake Poopó has now been declared fully evaporated. Occupying a depression in the Altiplano mountains, the saline Lake Poopó has in the past spanned an area of 3000 km2 — greater than France's Réunion Island. 54)
- But the lake's shallow nature, with an average depth of just 3 m, coupled with its arid highland surroundings, means that it is very sensitive to fluctuations in climate. Its official evaporation was declared last December. This is not the first time Lake Poopó has evaporated – the last time was in 1994 – but the fear is that any refilling might take many years, if it occurs at all.
- In the meantime local fishermen are left without livelihoods and the lake ecosystem is extremely vulnerable – Lake Poopó being recognized as a conservation wetland through the international Ramsar Convention.
- The evaporation has been variously linked to diversions of the lake's water sources for mining and agriculture, a persistent drought linked to El Niño warming in the Pacific Ocean and climate change.
Figure 31: PROBA-V tracks Lake Poopó evaporation in Bolivia. The three 100-m resolution PROBA-V images shown here were acquired on 27 April 2014, 20 July 2015 and 22 January 2016 respectively (image credit: ESA, BELSPO, VITO) 55)
• January 25, 2016: The PROBA-V minisatellite collected imagery in early 2016 of a large smoke plume from a bushfire south of Perth in Western Australia. Bushfires are frequent events during the long, dry Australian summer. In this fire, several hundred houses and an area exceeding 700 km2 have burned. Bushfires in the area are responsible for two deaths so far in 2016. Particularly hard hit was the town of Yarloop, which lost factories, a fire station, part of a local school and the heritage-listed Yarloop Timber Mill Workshops, which had been the most-intact example of a historical railway workshop in Australia. 56)
Figure 32: This 300 m resolution false-color image from ESA's PROBA-V satellite was acquired on Jan. 7, 2016, showing blue-gray smoke over Geographe Bay south of Perth in Western Australia (image credit: ESA/BELSPO, produced by VITO)
• January 2016: The PROBA-V mission has a nominal lifetime of 2.5 years; in 2015 the mission life was extended to 5 years. 57)
- Platform availability 99.9%; No safe mode in the last 12 months, all platform subsystems are nominal; Thermal situation very stable
- LTDN well within 10:30 – 11:30 hours; LTDN reaches 10:30 around Sept 2017; Stays well above 10:00 after 5 years.
- X-band data downlink DRS status: reliable!
1) Contract with SSC for the acquisition of PROBA-V X-band data at 3 stations: Kiruna (Sweden), North Pole (USA - Alaska) and Inuvik (Canada), allowing the PROBA-V reception of 10-11 passes per day
2) Acquisition performances are very good (in December over 315 passes acquired, only 4 had small data gaps = 99%); however rare delays in the data transfer from Inuvik
3) The Inuvik station will be upgraded with a fiber connection to the Canadian backbone during 2016, in order to improve the data transfer performances.
- PROBA-V Archive + Dissemination Status: Current PROBA-V Archive: 465 TB, with Disaster recovery: 930 TB; Total size of downloaded products: 252 TB.
• Dec. 15, 2015: ESA is pleased to announce that accessing PROBA-V data is easier than ever (and always free): 58)
- All level 1C (NRT), all 1 km products (NRT), and all 333 m and 100 m products (older than 1 month) can be downloaded immediately after registration to the PROBA-V portal.
- All 333 m and 100 m in NRT (Near Real Time) are also offered to ESA PIs by ESA (free of charge). Access is granted after ESA project proposal acceptance (proposal submission), see the PROBA-V Information Area on Earth
• October 21, 2015: Figure 33 presents a view of the glacier atop Africa's highest peak, as observed by ESA's PROBA-V minisatellite. The dormant volcano known as Mount Kilimanjaro is Africa's highest mountain, at 5895 m above sea level. It is also the tallest free-standing mountain in the world, rising about 4900 m above its surrounding plain. 59)
- Located close to the equator at 3ºS in Tanzania, only its summit is covered with snow and ice. The ascent towards the top is a journey through most of the world's climate zones, from the tropical to the Arctic. On the way the landscape shifts from tropical rain forest to moorland, alpine heather to desert and finally snow and ice. - The mountain is part of the Kilimanjaro National Park and is a major climbing destination. The mountain has been the subject of many scientific studies because of its shrinking glaciers.
Figure 33: This 100 m resolution false-color image from PROBA-V's main Vegetation camera, acquired on 14 June 2015, shows Kilimanjaro enveloped by clouds to the south and north. The gradual decrease of vegetation with altitude can be seen by the colors changing from green to brown and finally light blue, representing the summit's glacier (image credit: ESA, BelSPO, VITO)
• August 2015: PROBA-V provides continuation of SPOT-Vegetation (SPOT-VGT) products in its 4 bands: Blue, Red, NIR and SWIR. Aside from the global daily 1 km time series available from SPOT-VGT since 1998, PROBA-V brings new assets with a global daily 300 m time series for the same set of bands. In addition, 100 m products are available from March 16 2014, delivering a global coverage every 5 days. 60)
- The VGT-P (Vegetation-PROBA) instrument has several specific properties influencing its products : 1) VNIR and SWIR detectors mounted on different locations; 2) VNIR and SWIR with different GSD; 3) mechanically staggered SWIR detector composed by three overlapping detectors with an overlap area.
- Like SPOT-VGT, PROBA-V's spectral bands allow to discriminate between different types of land cover and land use, in particular plant species and crops, often measured by derived products such as the NDVI (Normalized Difference Vegetation Index). Vital uses of these data include day-by-day tracking of vegetation growth, early warnings to authorities of crop failures, monitoring of inland water resources and tracing trends of soil erosion and deforestation.
Table 8: Available products for PROBA-V
- Operational status: After more than a year of calibration monitoring, PROBA-V is able to show impressive results, showing a stable radiometric and geometric performance, and a consistency with missions such as SPOT-VGT, MERIS and Landsat-8. All performance requirements for the 1 km and 1/3 km products have been successfully achieved (Tables 9 and 10).
• On May 7, 2015, PROBA-V was two years on orbit, operating nominally. - As ESA's PROBA-V works quietly on its main task of monitoring vegetation growth across Earth, the minisatellite is also picking up something from a little higher: signals from thousands of aircraft.
- During its operational life, the PROBA-V minisatellite has picked up aircraft positions, using an experimental ADS-B (Automatic Dependent Surveillance – Broadcast) receiver (Figure 34). The ADS-B receiver was built by DLR. These signals are regularly broadcast from aircraft, giving flight information such as speed, position and altitude. All aircraft entering European airspace are envisaged to carry ADS-B in the coming years. 61)
- The ADS-B signals include GPS data on the aircraft's position, speed and altitude, and the signals are designed to be detected by ground stations and nearby airplanes. The 1500 x 750 km footprint of the single satellite is relatively small and a fully world-wide operational system would require a constellation of satellites. Several commercial operators have indicated interest in establishing such a system.
Legend to Figure 34: There are roughly 20 000 aircraft worldwide from which the DLR (German Aerospace Center) and the SES Techcom team has captured more than 25 million positions. The team has identified more than 22,000 unique callsigns, identifying more than 15,000 aircraft by their unique ICAO (International Civil Aviation Organization) addresses (one aircraft can share a callsign with others, depending on the flight route).
• May 6, 2015: The Drakensberg mountain range – Afrikaans for ‘Dragon mountains' – is located in southern Africa and extends from northeast to southwest for about 1125 km. This 100 m resolution image (Figure 35) centers on uKhahlamba–Drakensberg Park, located at Lesotho's northeastern border with the South African province of Kwazulu-Natal, covering roughly 240,000 hectares. Lesotho's Malibamats'o River is visible to the upper left. 62)
- Rich in its diversity of habitats, the park is home to various populations of endemic birds, mammals and reptiles. It is also home to the largest collection of rock paintings in Africa, south of the Sahara. The paintings reflect the way of life of the San people, who lived in the area for more than four millennia. In 2000, UNESCO named the park as a World Heritage Site.
• March 6, 2015: A high-speed camera for monitoring vegetation from space and combating famine in Africa is being adapted to spot changes in human skin cells, invisible to the naked eye, to help diagnose skin diseases like cancer. 63)
- In fact, the extraordinary digital infrared sensor from ESA's PROBA-V vegetation-scanning satellite is being adapted for several non-space applications. Mounted on a standard medical scanner, the space sensor can help doctors to look deeper into human tissues for detecting skin diseases earlier. It also has a bright future in industry: it has already been shown to improve solar cell production as well as spotting defective items on production lines.
- Leading-edge space technology: The PROBA-V camera has such a unique wide field of view that it allows the small satellite to build a fresh picture of our entire planet's flora every two days. Developed for ESA by the Belgian company Xenics, the camera sees light we cannot by looking in the shortwave infrared range.
- PROBA-V's ability to ‘see the unseeable' as Earth revolves beneath made the commercialization of the camera a natural step. With support from ESA and the Belgian Space Technology Transfer Program, the Xenics team created ‘Machine Vision', integrating cameras on inspection systems to replace humans in looking for imperfections. The high-speed resolution of our ‘line-scan' cameras makes them ideal for detecting hidden defects on fast-moving production lines, such as bottle manufacturing or sorting different types of plastics for recycling – all of which look similar to the human eye.
Figure 36: An OCT (Optical Coherence Tomography) scan for skin diseases, in particular to detect melanomas (image credit: Xenics)
• On March 4, 2015, ESA released Figure 37 in which the PROBA-V minisatellite captured the rare sight of standing water in the arid south Australian outback. Lake Frome, one of the whitest salt lakes in the southern hemisphere is visible to the right. Unusually, this 12 February image shows it filled with brackish water from the surrounding creeks in the area, which are typically dry. - Covering most of this 100 m spatial resolution image are the ranges and gorges of Vulkathunha-Gammon Ranges National Park, haven to many rare and endangered plants and animals. 64)
• Fall 2014: Study for PROBA-V Successor Mission: PROBA-V has been successfully launched on May 7,2013 and is providing a global monitoring in the continuity of the SPOT-VEGETATION mission. The progress in terms of ground resolution between Spot VGT and PROBA-V is a factor 3 (1 km to 300 m ground resolution product). The User Community requirements for the next generation of global monitoring call for a 100 m ground resolution product. This means an additional factor 3 improvement, but in a short time frame (5 years). After the success of the PROBA-V mission, the Belgian Science Policy (BELSPO) initiated a PROBA-V Successor feasibility study. This study was undertaken by VITO and CSL to identify potential tracks to achieve a follow-on mission which is expected to be relevant for the User Community. The mission analyses for each of these tracks was evaluated. Today the PROBA-V mission lifetime is expected to expire by mid of 2018. 65)
Since the interest for global land monitoring is expected to continue in the future, this study proposes mission requirements and a shortlist of optimal mission scenarios for a follow-on mission in this short time frame. The goal of such a new PROBA-V mission is clear: it should ensure the data continuity of global vegetation monitoring, while taking the opportunity to further improve the data quality. Data continuity is essential for understanding long term trends of land use that may affect the global equilibrium of the planet (in the context of scarcity for land or food, natural disasters, climate change). As for added value, a fine example is the improvement of spatial resolution when comparing PROBA-V with the spatial resolution in SPOT-VEGETATION products. An improvement in spatial resolution towards a full 100 m product is considered by the user community as the main target for a PROBA-V follow-on mission.
Figure 38: Comparison of coverage frequency and spatial resolution (image credit: VITO, CSL)
• Dec. 4, 2014: ESA released Figure 39 as 'image of the week': PROBA-V catches the Sangeang Api volcano on the island of Sangeang in Indonesia as a thick column of ash and sulphur dioxide pumps into the atmosphere. The dense ash grounded flights across much of the archipelago nation. 66)
• The PROBA-V image (Figure 40) of western Brazil was released on October 16, 2014. This 300 m resolution image reveals the human impact on the world's largest tropical rainforest. The brownish colors indicate deforested areas – note the distinctive ‘fishbone' pattern as main roads are cut through an area, followed by secondary roads for further clearing. 67)
- INPE, the Brazilian Institute for Space Research, uses satellites to monitor Brazil's rainforests. Its results show the annual rate of deforestation has fallen from some 3900 km2 in 2004 to 900 km2 in 2013, although substantial amounts of precious forest are still disappearing every day.
- PROBA-V's images are processed and distributed to hundreds of scientific users by VITO, Belgium's Flemish Institute for Technological Research, extending the coverage of previous generations of the Vegetation camera flown on the SPOT-4 and SPOT-5 satellites.
• The PROBA-V mission is operating nominally in August 2014. — For the annual Small Satellite Conference of AIAA/USU, Logan, UT, the PROBA-V project published a paper with the title: "PROBA-V: The example of onboard and onground autonomy," virtually the entire paper is presented in the last chapter of this file. It demonstrates vividly many autonomy concepts and aspects implemented successively for the PROBA series. The content might be of interest and value to the entire EO community.
• June 6, 2014: The SPOT Vegetation mission, flown on SPOT-4 and SPOT-5 for 16 years, marked 16 years of service in May, and has now passed the torch to its European counterpart. As of June 1, 2014, PROBA-V is the official successor mission — mapping land cover and vegetation growth across the entire planet every two days. The data can also be used for day-by-day tracking of extreme weather, alerting authorities to crop failures, monitoring inland water resources and tracing the steady spread of deserts and deforestation. 68)
- PROBA-V has demonstrated the benefit of the utilization of small satellites for an operational mission, as well as for the in-orbit demonstration of new technologies and technological payloads that can benefit from an early flight opportunity and quick implementation from decision to flight (Ref. 25). All embarked technological payloads are working properly and continuously on-board thanks to the allocation of the system resources that became available along with the project implementation. The flexibility of the PROBA platform and the integration process of the satellite has allowed to manifest even late flight opportunities for new technological payloads.- Due to the autonomous character of the flight and the ground segments, virtually all operational activities don't require human intervention, while keeping the availability of the system very high.
• Status of ADS-B in late May 2014: General aspects of the results. 69)
The ADS-B receiver on board the satellite was the first experiment of its kind, receiving 1090ES ADS-B squitter signals transmitted from aircraft. Therefore the experimenter could not build on experiences or any evaluation results. The assessment of the achieved results should take into account the constraints under which this experiment was realized, as there were limitations in cost and time but in particular available resources on the satellite in terms of available power and geometry.
The reception of 1090 Extended Squitter ADS-B messages on board of the PROBA-V satellite is mainly affected by the following issues, which may lead to a loss of ADS-B information:
- RF signal loss due to the low signal level resulting from the distance between the receiving satellite at an altitude of approximately 820 km and the transmitting aircraft at an altitude of 0 to 12 km.
- RF signal loss due to the shapes of the satellite antenna vertical radiation pattern and the aircraft antenna vertical radiation pattern.
- Corruption of messages by garbling, when several messages arriving at the ADS-B antenna onboard of the satellite at the same time overlap and thus cannot be decoded by the ADS-B receiver.
- Speed of the satellite of about 27000 km/h, which leads to a limited time of observation for each detected aircraft of about 3 minutes maximum (Ref. 69).
A separate file of ADS-B was added to the eoPortal in June 2014, providing more information of ADS-B (Automatic Dependent Surveillance-Broadcast) over Satellite.
• On May 31, 2014, the SPOT-Vegetation program will hand over the torch to the PROBA-V mission. 70)
Figure 42: Central Asia's receding Aral Sea, acquired by ESA's PROBA-V minisatellite on May 13, 2013 at a resolution of 300 m (image credit: ESA, VITO) 71)
Legend to Figure 42: The Aral Sea is a striking example of the kind of changes the satellite series (SPOT-4, -5 and PROBA-V) with the Vegetation instrument has been tracking. Once the world's fourth-largest inland water body, it has lost around 90% of its water volume since 1960 because of Soviet-era irrigation schemes. The image depicts the white salt terrain left behind by the southern Aral Sea receding, now called the Aral Karakum Desert. The greenery to the south is cultivated land irrigated by the Amu Darya river.
Table 11: Some background on the SPOT Vegetation program (Ref. 70)
• On May 7, 2014, the PROBA-V minisatellite was 1 year on orbit. For this occasion, ESA and VITO provided some recent imagery of the PROBA-V spacecraft (Figures 43, 44 and 45). — PROBA-V is set to make a giant leap for a minisatellite: formally taking over the task of continuously charting our planet's global vegetation. 72)
• February 2014: In its first two months of work, the vegetation-monitoring PROBA-V minisatellite has yielded a valuable harvest for around a hundred scientific teams around the globe: more than 5000 images, 65 daily global maps and six 10-day global syntheses, plus a quick peek at the Olympics. ESA released the PROBA-V image of Figure 46 (333 m resolution) on February 17, 2014. Cloud covers much of the Black Sea itself. The city of Sochi is a sea port on the coast of the Black Sea (right next to the border of Georgia), while the snow events are taking place in the resort settlement of Krasnaya Polyana in the Caucasus Mountains. 73)
• January 2014: After the commissioning phase, the PROBA-V mission operations has been transferred within ESA to D/EOP (Directorate of Earth Observation Programs) as an ESA Earthwatch mission. The Vegetation images are online from the VITO/Copernicus user segment. 74)
• December 03, 2013: Less than seven months after launch, Earth-watcher PROBA-V is ready to provide global vegetation data for operational and scientific use. The crucial commissioning phase is now complete and the satellite has been declared ready for operations. 75)
It is anticipated, that with the data from PROBA-V, the user community, ranging from operational Copernicus services to scientific users, will be able to answer questions related to the state of global vegetation and its dynamic changes in a seasonal context. Furthermore, PROBA-V will extend the valuable time series that was started by the SPOT-4/5 Vegetation instruments 15 years ago.
Figure 47: PROBA-V image acquired on Oct. 26, 2013, showing Sicily, Italy with the twin volcanic plumes – one ash, one gas – from Mt. Etna (image credit: ESA, BELSPO)
• On July 10, 2013, the first global VGT map of PROBA-V was provided, demonstrating that the minisatellite is on track to continue a 15-year legacy of global vegetation monitoring from space. 76)
Figure 48: First uncalibrated global mosaic of vegetation from PROBA-V, June 2013 (image credit: ESA)
• July 02, 2013: The satellite is currently in its commissioning phase, which includes a careful cross-calibration of the Vegetation imager with its predecessor on France's Spot-5 satellite, to ensure data compatibility. 77)
Legend to Figure 49: The area pictured is about 500 km across, with large reservoir lakes along the Euphrates River visible on the left, and another along the Tigris River on the right. Reservoirs and rivers secure water supply in the very dry region. They also form the basis for what is visible in different shades of green along the river water's edges and irrigated agricultural plots to provide food and income to the population.
• June 13, 2013: An A320 Airbus overflying Scotland was the first aircraft 'seen' from space by a new receiver, the ADS-B (Automatic Dependant Surveillance-Broadcast) device of DLR. This verifies that tracking aircraft from space is possible. On May 23, 2013, the experiment was switched on for the first time, recording over 12,000 ADS-B messages within two hours, at an altitude of 820 km. The project team detected over 100 aircraft during the first pass over the British Isles, East Asia and Australia when the receiver was switched on. 78) 79)
ADS-B signals are broadcast by aircraft every second; they include aircraft position and velocity information. ADS-B equipment is being introduced on aircraft as a supplementary data source to the ground-based radar currently used to monitor air traffic. The problem with radar is that its coverage is restricted. Once out of the range of terrestrial radar stations, the continuous air traffic surveillance stops.
Figure 50: Tracking aircraft with the ADS-B receiver on board PROBA-V over Great Britain and the Atlantic Ocean (image credit: DLR)
• May 17, 2013: The PROBA-V spacecraft is in good health following its launch on May 7, 2013. The Vegetation imager has been switched on and the first image has been captured over western France. 80)
- PROBA-V reached its orbit of 820 km altitude just 55 minutes after launch. The first LEOP (Launch and Early Operations Phase) milestone was to check the first signs of life from the satellite as it flew over the ESA ground station in Kourou 40 minutes after separation. Then a full telemetry session confirmed the stabilization of the satellite's attitude. The onboard computer used its magnetorquers to control the satellite's attitude and compensate for the spin imparted by the separation.
- Since then, the project has been checking the various subsystems one by one, confirming that they have made it through the stress of launch in working order. These initial checks are now being followed by a diligent commissioning of every single detail of the overall system platform, instrument and technology demonstration payloads, which will take the next few months.
Legend to Figure 51: The image was generated using the three VNIR bands, blue, red and near-infrared (NIR) superposed, the green being replaced by the NIR. It has not yet been radiometrically or geometrically corrected. Less than a cubic meter in volume, the miniaturized ESA satellite is tasked to map land cover and vegetation growth across the entire planet every two days.
Sensor complement: (VGT-P, Technology Payloads)
The major challenge in designing the payload is to make it compatible with the resources available on a small satellite like PROBA and at the same time accommodate the large swath. The selected solution is to divide the FOV (Field Of View) into three smaller parts and to use compact reflective optics elements using TMA ((Three Mirror Anastigmat) telescopes for each part. Each TMA is equipped with large VNIR and SWIR FPAs (Focal Plane Assemblies) to cover the large swath.
In addition to VGT-P, as a secondary mission objective, PROBA-V will fly four technology demonstration payloads.
VGT-P (Vegetation Instrument - PROBA):
The PROBA-Vegetation payload is a multispectral pushbroom spectrometer with 4 spectral bands and with a very large swath of 2285 km to guarantee daily coverage above 35 latitude. The payload consists of 3 identical SI (Spectral Imagers), each with a very compact TMA telescope. Each TMA, having a FOV of 34º, contains 4 spectral bands: 3 bands in the visible range and one band in the SWIR spectral range. The swath TFOV is 103º. 81) 82) 83) 84)
VGT-P is restricted to imaging land and dedicated calibration zones. On-board the spacecraft, there is for each spectral imager a land sea mask that is provided by the PI (Principal Investigator). The land sea mask removes the pixels that contain only sea and it dictates when each SI should be in imaging mode.
OIP (Optronic Instruments & Products, Belgium) is the industrial prime contractor for the payload and is responsible for the design and development of the PROBA-V instrument and AMOS (Belgium) is responsible for the manufacturing and alignment of the telescope. The major payload challenge lies in the fact that the wide-swath imaging instrument has to fit into a small satellite with limited resources. The TMAs and the SWIR FPA have to be developed for the VGT-P since no COTS products are available. 85) 86) 87)
Table 12: Overview of VGT-P parameters
Each SI (Spectral Imager) contains one telescope, a beam splitter to separate the VNIR from the SWIR spectral bands, spectral bandpass filters to select the spectral bands, and the VNIR and SWIR focal plane arrays. The spectral bands will be realized by spectral bandpass filters centered on 460, 658, 834 and 1610 nm, with bandwidths of respectively 42, 82, 121 and 80 nm. The filters will be applied on the detector windows.
The optical axis of the central telescope will point to nadir and the two outer telescopes will point 34º from nadir. Together the three TMAs will cover a complete FOV of 102º. The optical system is telecentric, and the aperture is located at the position of the second (spherical) mirror.
The optical bench is passively cooled through a radiator and heaters compatible with the geo-location performances. The mass of the complete optical bench is about 20 kg, the total instrument mass is 35 kg.
Figure 52 shows the payload mounted on the PROBA-V platform. Given the reduced size of the platform, a H-shape structure, the only practical location of the payload is on the anti-velocity panel. This accommodation, with respect to a solution with the payload in the middle of the structure, has the advantage of a very simple assembly and clean mechanical interface. The drawback is a larger temperature gradient due to the close vicinity of the payload to the solar panel.
Legend to Figure 53:
- ROE (Read Out Electronics)
- PSU (Power Supply Unit)
- DHU (Data Handling Unit)
- PEU (Peripheral Electronics Unit)
- MLI (Multi-Layered Insulation)
TMA telescope development: VGT-P makes use of a set of three such telescopes, identical to each other. The purpose of the related ESA GSTP (General Support Technology Program) development is to demonstrate the feasibility of one item of the set with respect to its required optical quality, and to secure the instrument development. The entire telescope (structure and mirrors included) is an athermal design made of the same aluminum material. The mirrors quality is achieved by SPDT and the alignment rely on the very precise matching of the mirrors with the mounting structure.
Taking into account the mission constraints and objectives, including the innovative features of the instrument, a full-aluminum design was selected. This choice allows taking benefit from the recent developments in ultra-precision milling and turning techniques, as well as in optical aluminum production. Furthermore, this leads to a homothetic telescope behavior. The optical performance requirement of the telescope with regard to MTF (Modulation Transfer Function) is given in Table 13.
SPDT (Single Point Diamond Turning): Diamond turning is a process of mechanical machining of precision elements using Computer Numerical Control (CNC) lathes equipped with natural or synthetic diamond-tipped cutting elements. The SPDT process is widely used to manufacture high-quality aspheric optical elements from crystals, metals, acrylic, and other materials. Optical elements produced by the means of diamond turning are used in optical assemblies in telescopes, scientific research instruments and numerous other systems and devices. Diamond turning is specifically useful when cutting materials that feature aspheric shapes such as TMA surfaces.
Figure 54: Optical design concept of the TMA (ray tracing diagram), image credit: OIP
Baffle design (Ref. 83): The aim of the baffle design is to block the out-of-field light which could enter the instrument and reach the detector, directly or through one or several reflections on the mirrors. This 1st order analysis didn't consider vanes on the baffles and diffusion on M1 of out-of-field light.
The preliminary baffle layout is presented in Figure 55. It comprises 7 baffles: 1 at the entrance aperture of the instrument and 6 placed inside the instrument. An aperture stop is also placed at the level of the secondary mirror.
The baffle #1 is placed at the entrance of the instrument. Its role is to limit the out-of-field light that could directly reach the mirrors. The combination of the baffles #1 and #2 stops the direct view of the M3 mirror through the instrument entrance. The length of the upper side of the entrance baffle is defined to stop the light which could directly reach the M3 mirror and that could not be stopped by the lower side of the entrance baffle and by baffle #2. Some out-of-field light can also reach the M2 and M3 mirrors after reflecting on M1. This cannot be totally avoided but the length of the lower side of baffle #1 has been chosen in such a way that this straylight is stopped by the baffle #3 after reflecting on M3. The baffle #3 is placed below the M2 mirror and stops the direct view of the M1 mirror by the VNIR detector. The baffle #4 is a critical location where reflection or diffusion on the M2 structure can occur and bring stray light to the VNIR detector which is very close. Vanes will be placed at this location. The baffles #5 and #6 are placed near the focal planes to isolate the detectors from each other. The baffle #7 avoids a direct view to the SWIR detector from the M1 or M3 mirrors.
FPA (Focal Plane Assembly) design: The FPA is a very precise optical sub-module that comprises of a very accurately machined interconnecting structure in titanium, the VNIR and SWIR detectors, the detector windows, and a folding mirror used to optimize the packaging of the detectors, and thus minimizing the sub-module's volume claim.
Within this assembly, the VNIR and SWIR detectors are precisely mounted relative to the FPA interconnecting structure. The FPA sub-module is mounted and aligned onto the TMA interconnecting structure and its position is justified via the VNIR detector - the SWIR detector inherits its position.
VNIR detector: The VNIR detector is a AT71547 (ex TH31547) quadrilinear detector from E2V. It consists of 4 photodetector lines, each line containing 6000 photodiodes HAD type (Hole Accumulation Diode) with 13 µm pitch. Each line includes its own anti-blooming system. Only 3 lines (blue, red and NIR) out of the four are used as separate array at a different wavelength. The spectral separation is done by the use of a spectral windows from Barr mounted on the detector transparent window (Figure 56). To reduce stray light falling on the arrays, the front and back surfaces of the detector window are coated with an anti-reflective coating, and the front surface also has a black mask applied.
Figure 57: VNIR detector layout (image credit: OIP, ESA)
Figure 58: Illustration of the optical assembly of VGT-P and two star trackers on the optical bench (image credit: OIP)
SWIR detector: The SWIR detector was specifically designed for PROBA-V by the company XenICs considering the large FOV. Due to the required imaging length of over 2700 pixels, sensor, it was decided to make the device in three sections, each consisting of a ROIC (Read-Out Integrated Circuit) chip and a PDA (Photodiode Array) chip with 1024 pixels on 25 µm pitch. The ROIC chip has been custom designed for the mission. The assembly is realized in a custom-designed kovar package of ~103 mm length with 72 pins shown in Figure 59. The large number of pins is required to operate and read-out the three ROICs independently for improved redundancy in case of failures during the mission. Due to the tight pitch, two rows of wire bonds on a 50 µm pitch are used to connect the ROIC chips to the PDA fan out (Ref. 81). 88)
Table 14: Vegetation instrument parameter comparison on SPOT series and on PROBA-V spacecraft (Ref. 17)
Thermal design of the VGT-P instrument: 89)
One of the major drawbacks of using multiple optical systems in parallel while imaging, is the effect of pointing inaccuracies due to thermo-elastic and mechanical deformations. It is obvious that such pointing errors can easily destroy the quality of the images. For the VGT-P, the stringent geo-location requirements demand the instrument to be thermally stabilized as much as possible to reduce any thermo-elastic disturbances.
Since the PROBA platform is fairly limited in the delivery of power, VGT-P needs to be very efficient in its power use. As a direct consequence, there is no possibility to have an active thermal control system to stabilize the instrument. The thermal design of the instrument must therefore be very carefully assessed and engineered.
• Thermal isolation: Firstly, as the surrounding satellite panels are heavily fluctuating in temperature during the orbit, it is of the utmost importance to shield the instrument thermally from these platform variations. To reduce the radiative heat loads from the environment, the instrument is completely wrapped in a 12 layer MLI. To reduce the conductive heat loads from the mounting plane, the instrument is mounted by means of titanium quasi isostatic mounting feet. These quasi isostatic mounting feet also play a major role in the transfer of the thermo-mechanical deformations from the underlying platform to the optical bench as they strongly reduce these deformations. Therefore, these titanium flexures as they are called not only serve as a thermal isolation, but also acts as a thermo-elastic isolator.
Power reduction: A natural step to reduce the thermo-elastic effects on the instrument is to reduce as much as possible the heat load on the optomechanics. Therefore, all non critical and heavy heat dissipating detector read-out electronics are separated from the optics. The FPAs of the telescope only contain the detector and electronic components which drive the radiometric performances of the instrument. These FPA electronics are connected through a flex rigid to the ROE (Read-Out Electronics) which is thermally and structurally disconnected from the optomechanics. All major heat dissipating components are located in there.
Obviously, also the central electronics (DHU and PSU) are separated from the optomechanical imaging system. By doing this, the total power dissipation on the optical bench is only 9W, which is less than ¼ of the total power dissipation of the complete VTG-P instrument.
• Heat dissipation: To dissipate this heat load, a radiator is needed. Several concepts were proposed and analyzed. The most efficient radiators point towards deep space which would enable us to cool down the complete instrument to very cold temperatures. This had a drawback that additional heaters would have been needed to stabilize the thermal regime of the instrument to normal working temperature. Moreover, as the instrument is always pointing downwards towards Earth, the radiator would have been located on the side of the instrument which naturally induces an asymmetry in the optomechanics. Such asymmetry is not desired in an imaging sensor with stringent pointing requirements. Moreover, heat pipes would have been mandatory to extract as efficient as possible all heat of the detectors towards the radiator which unnecessarily complicated the complete design.
From a thermo-elastic point of view, it was highly desirable to respect the symmetry of the instrument as much as possible and to symmetrically extract the heat from the FPA's on the optical bench. Thus, it was chosen to locate the radiator in front of the instrument and point it towards the earth surface. As the earth is thermally quite stable at a fairly modest temperature and as the payload is always pointed nadir, this is the perfect heat drain for the instrument. The implementation of this concept reduces the complexity dramatically: the radiator, covered with aluminized Teflon, is connected through two thermal straps towards the front of the instrument without the need to install heat pipes.
• Stability: Stability is the key aspect of thermo-elastic performance. Of course, without the possibility of an active thermal control system, stability is quite difficult to achieve in a thermal environment which is constantly varying over the orbit.
To tackle this problem, the first stage was to avoid the randomness in the heat loads on the instrument and to have constant thermal regime along the orbit. As the payload is encircling the Earth with its radiator pointing at nadir, the heat load on the radiator is subjected to a varying regime from sunlit to eclipse and back. From the point of view of efficient power use, the imaging circuits on the instrument are switched off by the satellite if no imaging is needed (over the oceans, over the poles, during eclipse). This would induce different thermal regimes from one orbit to the other, which is not acceptable from pointing point of view. But leaving all electronics switched on during non operation is a no go considering the lack of power. As a compromise, during sunlit conditions and when the imaging electronics is powered off, a heater located on the detector with a heat load equal to the heat load of the detector and FPA is powered. In this way, the heat loads on the optical system remain constant during sunlit. During eclipse, all is switched off. - As a consequence, a constant thermal regime on the optics is established: during 1/3 of the orbit (eclipse) the radiator faces only IR and the instrument is switched off. During the 2/3 of the orbit, the radiator sees IR and albedo and the instrument is switched on.
• Gradients: The final challenge in the thermal design is to avoid thermal gradients in the instrument as gradients are hard to control and can severely affect the thermo-elastic performance. As already described, the heat extraction has respected the symmetry of the instrument. An unavoidable asymmetry is the location of the Star Trackers as they have their own limitations. The heat load from the FPA and the detectors on the telescopes is normally entering the instrument through the TMAs to the top skin of the optical bench. However, this would heavily distort and bend the optical bench as the top skin will expand more than the bottom. To reduce this effect, thermal straps are designed to extract most of the heat (4/5) from the detectors and the FPA towards the optical bench, the rest is still entering the TMA structure. To reduce the thermal bending, the heat straps are mounted on the side of the optical bench to avoid the bending of the bench.
Electronics design: The functional conceptual electronic design consists of three major building blocks:
4) The VNIR and SWIR ROE (Read-Out Electronics): The VNIR and SWIR ROE are the interface between the respective detector and the DHU (Data Handling Unit). The main functions of the ROE modules are to control the detectors, perform the A/D conversion, generate accurate bias to the detectors, and transmit the raw digital image data to the DHU. Additional functions are generating local biases and performing housekeeping measurements.
5) A centralized DHU (Data Handling Unit): The DHU is the central unit, acting as interface between the satellite and the VNIR and SWIR ROE. It is responsible for all the on-board data handling, image processing and temporal data storage, as well as for collecting the housekeeping data of the different sub-units. The main function are control and synchronization of the ORE, perform compression of the incoming image data as per CCSDS standard, packetizing of the image data before sending to the satellite OBC, collection and transmission of the available housekeeping data and processing of command data sent from the OBC over the serial communication link.
PROBA-V has three VNIR and three SWIR line sensors which compose three separated channels each. This leads to a total amount of 18 channels with 9 channels for VNIR each with a resolution of 5200 pixel and 9 SWIR channels each with a resolution of 1024 pixels. The 18 Channels have to be compressed independently with a variable compression ratio by the DHU. Furthermore, DHU supports bad pixel removal on basis of a bad pixel map which can be uploaded from. The compression is performed on an advanced overlapping tilling scheme. The DHU performs the following functionality: 90)
- Control & Monitoring of 3 VNIR and 3 SWIR
- Generation of camera syncs to the 3 VNIR and 3 SWIR with programmable time and delay
- Interface to VNIR/SWIR and separation of the incoming data streams into a total of 18 independent channel data streams
- Sorting of incoming data from a disordered arrangement to an adjacent pixel to pixel and adjacent line to line based arrangement supported
- Storage of data in SDRAM (required because of 2D wavelet based compression)
- Online lossless and lossy compression with selectable ratio and bypass capacity (independent selectable for each channel)
- Generation of source packets from the processed data with subsequent transmission via a PacketWire interface to S/C
- Interface for control & monitoring from S/C (TMTC Interface)
- RTC (Real Time Clock)
- Acquisition of HK data and provision of HK packet to the S/C.
The complete DHU functionality is implemented within two Microsemi RTAX 2000S FPGAs from ACTEL.
Image data handling approach: The advanced overlapping tiling scheme of the universal compression core is especially a challenge for DHU image data handling system. The number of lines according to the tile height (128) has to be collected prior to compression into a temporary SDRAM buffer to form one image. Since all spectral channels are read out simultaneously but have to be processed independently, the DHU has to format an overall of 162 (15 VNIR and 3 SWIR tiles per channel) overlapping tiles of the 18 channels and furthermore perform the bad pixel removal in real time. In parallel, due to the continuous data acquisition of the sensors, the subsequent received lines have to be stored in a different area of the temporary SDRAM buffer to provide a seamless data stream to the compression. With the constraints of spaceborne DHUs, a robust and deterministic architectural approach according to system constraints e.g. resource utilization, performance boundaries and requirements has to be provided.
The DHU has to perform the following processing steps on the acquired images:
- Storage of image line data from the instruments until a complete image is received
- Tile formatting and transfer to the compression core; in parallel bad pixel removal
- Storage of the compressed data from the compression core and additional packet formatting
- Transfer of compressed tiles over the PacketWire interface to the S/C.
All these processing steps have to be performed in parallel. The DHU comprises one centralized mass memory. To provide subchannel independent operations, the memory is sub-divided into VCs (Virtual Channels). Each VC is related to one of the SWIR and VNIR subchannel. Furthermore, to support the parallel processing of incoming data and image processing, a double buffer implementation is mandatory. Therefore, 18 x 2 VCs for image data exist in the architecture. Each VC allocates memory for 1 complete image (128 lines) in the mass memory and additional 18 VCs are provided for the bad pixel maps. On these VCs, constant data length transmissions are performed. Furthermore, the incoming compressed variable data length from the compression core is also stored in VC tile buffers. Two tile buffers are provided in the architecture in which data transmissions of variable length can be performed. Overall the DHU image data handling systems provides 56 VCs.
Figure 60: Photo of the PROBA-V FM DHU box (image credit: VGT consortium)
6) The PSU (Power Supply Unit): The PSU contains all the necessary DC/DC converters (i.e. LPLCs from TAS ETCA) to perform power conditioning to the different sub-units. It contains also the powering selection circuitry (by means of MOSFET) to individually switch on and off the different Spectral Imagers and heaters. The different power lines towards the spectral imager ROEs are also protected by current limiters within the PSU.
Figure 61: Architectural layout of the electronics design (image credit: OIP, ESA)
Since the electronics design is to be Single Point Failure Free, the above described concept is built in the following redundancy configuration:
• the DHU actually consists of 2 sub-units, one primary and one (cold) redundant
• the PSU has a more complex arrangement:
- fully redundant DC/DC converters with their associated TM/TCs
- fully redundant power distribution to DHU and survival heaters
- cross-strapped power distribution to ROEs, via a MOSFET diode –Oring (in line with the 3 ROEs partial redundancy principle).
• The VNIR and SWIR ROE cannot be built in redundant configuration as the detectors are directly mounted on these PCBs. However, if a failure should occur in one of the ROE units such that the instrument fails, the malfunctioning unit can be turned off and the instrument can continue functioning, but with reduced functionality.
Figure 62: Photo of the VGT-P instrument during the integration phase, the star trackers are on the left of the bench (image credit: VGT consortium, Ref. 84)
VGT-P testing and simulations:
A test version of PROBA-V's wide-viewing multispectral imager, referred to as CHIB (Compact Hyperspectral Imager Breadboard), has been subjected to a combination of hard vacuum and temperature extremes in ESTEC's Mechanical Systems Laboratory, simulating conditions it will face in space (Ref. 82). 91) 92)
CHIB consists of a wide (34º) FOV Three Mirror Anastigmat (TMA) telescope telecentric in the image space and equipped with a 2-dimensional focal plane array with a linear variable optical filter (LVF).
The CHIB optical design is shown in Figure 63. SPDT (Single Point Diamond Turning) enables the fabrication of complex aspheric shapes, which gives a freedom to design and manufacture a highly compact (90 mm x 110 mm x 140 mm) TMA.
A prototype TMA telescope was developed in 2009 through ESA's GSTP (General Support Technology Program).The idea was to combine the PROBA-V TMA with the wide 2-dimensional CMOS array (10000 x 1200), developed under the ESA-PRODEX program for the HALE UAV instrument MEDUSA, and a linear variable filter to create a compact wide swath hyperspectral imager. Based on a first analysis the following top level specifications have been derived for such type of instrument.
The mirrors and structure holding the mirrors are both manufactured in aluminum providing an athermal design. The telescope is telecentric: the variation of the angle of incidence of the chief rays on the focal plane is lower than 1º over the entire field of view, ensuring a negligible shift of the peak transmission over the interference filter. The LVF is positioned close to the focal plane, just in front of the detector.
To fully exploit the very wide FOV of the telescope, the instrument is equipped with the large detectors developed by Cypress for Medusa. The detector is a 1200 x 10000 pixel CMOS image sensor with a pixel size of 5.5 µm x 5.5 µm. The main characteristics of the sensor are reported in Table 15.
The LVF (Linear Variable Filter) is a fused silica plate coated plate with an interference filter with increasing thickness in along-track direction. The peak of the transmission curve varies with the thickness of the deposition. This implies that all detector pixels in a cross-track row receive information in the same spectral channel. The detector pixels in the along-track rows receive information in different spectral channels, and the closer pixels to each other, the less difference between the corresponding spectral channels. Thus, in principle, the total number of spectral channels is equal to the number of pixels in an along-track detector row covered by LVF. The filter used in the breadboard has operating spectral range 450 nm – 900 nm, FWHM spectral resolution of less than 15 nm and a gradient of the peak transmission wavelength of 60 nm/mm.
VGT-P calibration: Radiometric and geometric instrument performance measurements will be done both on ground and in-flight. The on-ground calibration of the PROBA-V instrument will be performed at CSL (Liège, Belgium) prior to integration on the platform at Verhaert Space. A complete calibration report describing the radiometric and geometric performance characteristics before launch will be compiled. Radiometric and spectral performance characteristics that will be verified on ground are: signal-to-noise, dark currents, linearity, stray light, pixel non-uniformity, polarization sensitivity, spectral response and spectral misregistration. Geometric performance characteristics include MTF, bore sight, spatial misregistration. 93)
The assessment of the PROBA-V performance, the analysis of the image quality and the calibration after launch will be performed by the PROBA-V IQC (Image Quality Center) located at VITO, Belgium. VITO is the processing center for the SPOT-4 and SPOT-5 Vegetation data and is operational since 1999. The Image Quality Center will ensure the highest possible image quality, both radiometrically and geometrically. Given the constraints on power consumption and the small size and weight of the platform, only vicarious calibration techniques will be used to monitor sensor performance over time; no on-board calibration facility is available.
Still, a complete calibration plan to assess the radiometric and geometric performances in-flight is being outlined. The objective of the calibration plan is to achieve a complete PROBA-V calibration at the end of the commissioning phase with :
• A full in-flight radiometric characterization and calibration including :
- Dark current determination
- Calibration of the absolute calibration coefficients of the three cameras
- Equalization among detectors or multi-angular calibration: to correct for sensitivity variation over the PROBA-V wide field-of-view
- Characterization of response non-linearity
- Radiometric image quality performance analysis: Noise, MTF, SNR
• A full in-flight geometric characterization and calibration including :
- Geometric sensor model calibration: Post-launch check and calibration of all parameters of the geometric sensor model for each sensor including 94)
- Continuous absolute geometric accuracy check
- Image geometric quality performance indicators such as absolute location accuracy, multi-temporal coregistration accuracy, multi-spectral co-registration accuracy.
Vicarious radiometric calibration: 95)
• Calibration over Rayleigh scattering (oceans)
• Calibration over deep convective clouds (oceans)
• Calibration over sun glint (oceans)
• Calibration over stable deserts
• Calibration over Antarctica (equalization)
• Calibration over oceans during night (dark signal)
• Stability check using the moon
• Calibration validation over Tüz Gölu (meaning slat lake). Tüz Golü is the second biggest lake in Turkey, located in the Central Anatolia Region.
• Calibration validation under flights with the APEX airborne sensor at high altitude.