UK-DMC-2 (United Kingdom - Disaster Monitoring Constellation-2)
In the timeframe 2005/6, SSTL (Surrey Satellite Technology Ltd.) started planning for second generation missions (with improved imaging capabilities) to be added to the current first generation DMC (Disaster Monitoring Constellation), coordinated by DMCii (DMC International Imaging Ltd.). The basic enhancement of the second generation missions, also referred to as DMC-NG (Next Generation), is to provide wide-swath multispectral imagery at higher resolutions (660 km swath with 22 m pixel size at nadir). The DMC satellites are built on a platform developed by SSTL under the BNSC MOSAIC (Micro Satellite Applications in Collaboration) program. 1) 2) 3) 4) 5) 6)
The DMC-2G (DMC -Second Generation) satellites not only provide data continuity, but also a greatly enhanced imaging capability to cover large areas of territory at enhanced resolution. The objective is to offer the timely imagery (broadcast service) to a global community of customers with the capability of a direct downlink to a customer ground station. The combination of multiple spacecraft in constellation provides for daily coverage at a resolution that enables effective monitoring of the rapidly changing environment.
The following DMC second generation missions are in orbit:
• Deimos-1, launch July 29, 2009
• UK-DMC-2, launch July 29, 2009
• NigeriaSat-2 (launch August 17, 2011, the mission is described under a separate entry on the eoPortal)
• NigeriaSat-X (launch August 17, 2011, the mission is described under a separate entry on the eoPortal)
Table 1: Some background of the DMC constellation
Table 2: Sustainability - the DMC Road-Map 7)
In May 2007, Surrey Satellite Technology Ltd (SSTL) embarked on an internal development to add an SSTL-owned satellite to the second generation of the Disaster Monitoring Constellation (DMC-2G). The objective of the DMCii / SSTL mission is to provide continuous continent-level imaging with direct downlink of data to customer's ground stations. In June 2008, the UK-DMC-2 spacecraft passed its TRR (Test Readiness Review) which assesses the status of the fully integrated spacecraft. 8) 9) 10) 11)
Figure 1: Illustration of the UK-DMC-2 spacecraft in launch configuration (image credit: SSTL)
UK-DMC-2 operations and performance:
As part of the Disaster Monitoring Constellation, UK-DMC-2 will be used to maintain rapid revisit times needed to monitor fast changing phenomena such as fires, floods and crops. Furthermore it also covers large areas quickly including the Amazon Basin, Australia, and Europe, providing the capability for multi-season coverage of major agriculture. To enhance the capability of imaging very large areas as quickly as possible and being able to offer custom data delivery contracts to customers, a number of operational modes are implemented (Ref 3).
• Store-and-forward imaging and downlink: This is the standard imaging and downlink operational mode employed by the current DMC spacecraft. One or more images are recorded on the data recorders after which they are put in a lower power storage mode. - On approach of a ground station access, the recorders are put back into full-power mode and all or part of the data is transmitted using the SSTL Saratoga hole-filling downlink protocol. With the full 650 km swath of the imager, strips of up to 1800 km can be imaged, as can be seen in Figure 2, an image from the south of Italy to Denmark can be recorded in one go.
A swath may also be windowed to acquire the imaging targets more efficiently (only one window at a time can be used).
Table 3: Maximum imaging strip length for selected swath widths and digitization (windowing scheme)
Figure 2: Typical store-and-forward imaging scheme (image credit: SSTL)
In addition to store-and-forward operations it is possible to perform a so-called near real time mode. This mode allows imagery acquired within a 2000 km radius of a ground station to be downlinked within the same pass. In this way imagery can be delivered directly to customers with only a few minutes delay (not taking into account ground processing).
• Broadcast downlink: To allow receive-only ground stations access to UK-DMC-2 imagery, an asynchronous "broadcast" downlink is implemented. It provides a substitute service for customers currently receiving Landsat data. This downlink mode doesn't use the standard SSTL hole-filling algorithm and therefore requires a very low BER to ensure error-free reception of imagery.
• Multiple ground station support: Another advancement compared to the original DMC spacecraft is the possibility of making use of multiple ground stations. UK-DMC-2 has the capability of taking images every orbit and is therefore not power limited but ground station access time and/or storage limited. The total data throughput is greatly increased when making use of more than one ground station.
• Throughput performance: Based on spacecraft performance and provided there are enough ground stations available, UK-DMC-2 is able to image and downlink 11 million km2 every day, which is more than the total area of the USA.
• Large area mapping: In the past years, demand for large area DMC imagery has grown steadily and UK-DMC-2 will be used to carry out these campaigns. The UK-DMC-2 increased resolution and improved dynamic range will assist in providing the detail needed to accurately map areas of rapid deforestation in the Amazon region and other parts of the world.
Common design of the Deimos-1 and UK-DMC-2 spacecraft
The next generation spacecraft are designed, building upon the extensive experience built up through the first generation DMC system. Both Deimos-1 and UK-DMC-2 spacecraft are developed to carry out a commercially-focused operational imaging mission, supporting rapid-response, large-area mapping for a range of applications.
Enhancements in these new-generation DMC spacecraft include a modified payload design allowing a transition to a lower ground sampling distance whilst maintaining the 600 km swath. Improved power and data handling subsystems allow a greater imaging capacity so that swath lengths of up to 1000 km can be captured. The higher imaging capacity is augmented by an X-band downlink operating at 40 Mbit/s.
Each satellite incorporates many enhancements over the existing DMC-1G series, effectively starting a new series with a platform, referred to as SSTL-100 (first used in 2002), based on current technologies. The spacecraft structure employs Aluminum-alloy and Aluminum honeycomb panels, and has been designed to be compatible with a wide range of launchers. The internal structure includes a stack of "micro-tray" modules traditionally used by SSTL in all its microsatellites. However, a number of "nano-trays" of SNAP nanosatellite heritage are also included. The stack of trays carries an optical platform, and between the stack and the panels, the battery, wheels and propulsion system are carried. Each spacecraft employs a fully passive thermal control system.
Each spacecraft features a mass envelope equivalent to the current DMC series (~88 kg for Deimos-1, and 96 kg for UK-DMC-2). The size is (X, Y, Z): 630 mm x 660 mm x 640 mm). The design life is 5 years.
The AODCS (Attitude and Orbit Determination and Control Subsystem) provides 3-axis momentum biased attitude control. An in-house developed GPS receiver, SGR-07 (Space GPS Receiver-07), is used to determine the orbit and to provide accurate timing information. The SGR-07 is a lower power and stripped down version (reduced size) of the SGR-10, with 12 C/A channels, and a single antenna (Figure 3). The mass is 450 g unit, plus 55 g for the patch antenna, and it takes approximately 1.6 W power at 28 V. The UK-DMC-2 flight represents the first installation of the SGR-07 receiver. 12)
Dual-axis sun sensors on the four sides of the spacecraft and dual redundant vector magnetometers provide attitude knowledge by measuring the sun-angle and Earth's magnetic field. - Based on the sensor knowledge, three magnetorquers which generate a torque by interacting with the Earth's magnetic field, and three reaction wheels (MicroWheel) are used to control the attitude and attitude rates and maintain attitude pointing during various mission stages (attitude knowledge ) to < ±0.25º. A gravity gradient boom is employed to provide a high degree of platform stability, constraining the body-rotation rates to < ±2.5 mdeg/s.
Table 4: Overview of MicroWheel parameters in various spacecraft of SSTL manufacture 13)
Figure 4: Photo of the MicroWheel-10SP-S (left) and MicroWheel-10SP-M (right), image credit: SSTL
EPS (Electric Propulsion Subsystem): Deimos-1: Four body mounted GaAs solar panels and a 15 Ah Li-ion battery. A raw 28 V bus is distributed, alongside a regulated 5 V bus. Lines are electronically switched and over-current protected with electronic switches. The system delivers over 30 W orbit-average to the platform and payloads, with 12 W for the platform, ~ 16-20 W are available for payload operations.
UK-DMC-2: Three body-mounted heritage solar arrays are used containing GaAs/Ge single-junction solar cells providing 65 W at AM0 and 25ºC. The fourth body-mounted solar array has been replaced by a deployable solar array containing Emcore triple-junction InGaP/InGaAs/Ge solar cells which provides 65 W at AM0 and 28ºC. Because this panel is deployed by an angle of 110º, it increases the total OAP (Orbit Average Power) by 60 %.
A Li-ion battery (15 Ah capacity, of ABSL Space Products) is used to be able to continue operations through the Earth's eclipse phase and to provide extra power during heavy operations. - The rest of the power subsystem consists of a BCR (Battery Charge Regulator) module and a PCM/PDM (Power Conditioning and Power Distribution Module) to regulate the power coming from the solar panels and battery and to distribute the power to all subsystems. A raw 28 V bus is distributed, alongside a regulated 5 V bus.
DHS (Data Handling Subsystem): Two redundant Intel 386 based OBCs (On-board Computers) are used for onboard data handling. The OBCs provide communications between subsystems, monitor the temperature and current consumptions, maintain log files and execute the imaging schedule. The data handling subsystem uses a CAN (Controller Aerea Network) bus for onboard data exchange.
Propulsion subsystem: A propulsion subsystem is needed to be able to operate as part of the constellation. The heritage DMC Butane cold gas monopropellant subsystem is used to correct for possible launcher injection errors, phasing maneuvers with other spacecraft and a potential EOL (End-of-Life) maneuver to reduce the time it takes before re-entry into the atmosphere. At BOL, the orbit parameters are selected to optimize the orbit altitude reduction and LTAN (Local Time of the Ascending Node) drift without use of the propulsion subsystem, but small corrections over life are supported.
The propulsion subsystem delivers ~ 20 m/s ΔV, and comprises 2 cylindrical tanks holding 2.3 kg of propellant, and a single low-thrust thruster augmented by a small resistojet to boost its efficiency. The system is housed on the spacecraft base-plate, alongside the propulsion-controller electronics.
Figure 5: Photo of the propulsion subsystem (image credit: SSTL)
RF communications: The S-band is used for TT&C transmissions while the X-band downlink is used for the transmission of payload imagery. The S-band uplink uses dual redundant 9.6 kbit/s CPFSK receivers, each having 2 patch antennas; they are providing an omni-directional antenna pattern to maintain the RF link at all times. The TT&C downlink is provided by redundant 38.4 kbit/s BPSK (Bi-Phase Shift Keying) modulated S-band transmitters, each with 2 monopole antennas to provide an omni-directional antenna pattern.
There are two redundant X-band transmitters, each is using the DQPSK (Differential Quadrature Phase Shift Keying) modulation technique and Viterbi convolutional encoding with r=1/2 and k=7 for optimized spectral efficiency. Both transmitters are connected to a Saab isoflux antenna on the Earth Facing Facet. The data rate is selectable on orbit between 20 and 80 Mbit/s.
The payload downlink is designed to maintain a positive link margin when transmitting data to the UK-DMC-2 ground station with a BER (Bit Error Rate) of < 10-6. For downlinks to receive-only ground stations, a BER of <10-9 is required and for smaller ground stations a data rate of 20 Mbit/s may be selected to maintain a positive link margin.
An onboard data storage capacity of 8 GByte is provided using two 2 GByte SSDRs (Solid-State Data Recorders). For UK-DMC-2, a newly developed 8 Gbyte HSDR (High-Speed Data Recorder) has been added for increased storage and a faster downlink data rate. For full swath imaging, both SSDRs are used, each connected to one of the imager banks. The HSDR is able to receive imager data from both banks in parallel. Where the SSDRs can downlink their data at 20 Mbit/s, the HSDR can support the high-speed 80 Mbit/s downlink as well.
Figure 6: Block diagram of the HSDR (image credit: SSTL)
Figure 7: Block diagram of the UK-DMC-2 spacecraft (image credit: SSTL)
Figure 8: The SSTL-100 compact modular platform in its various assembly stages (image credit: SSTL)
Figure 9: Integration of UK-DMC-2 and Deimos-1 spacecraft at the launch facility in Baikonur (image credit: SSTL, Ref. 28)
Launch: The Deimos-1 and UK-DMC-2 spacecraft were launched on July 29, 2009. The launch vehicle is Dnepr-1, the launch site is Baikonur, Kazakhstan. The launch provider is ISC Kosmotras (a joint venture of Russia, Ukraine and Kazakhstan).
The shared payload manifest of the launch was:
• DubaiSat-1 an imaging minisatellite of the UEA (United Arab Emirates) Institution for Advanced Science and Technology (EIAST) is the main payload on this flight (mass of ~ 190 kg)
• Deimos-1, UK-DMC-2
• AprizeSat-3, AprizeSat-4, LEO communication experiments of Aprize Satellite Inc., Fairfax, VA, USA (each with a mass of 12 kg)
• NanoSat-1B of INTA, Spain (23.5 kg).
Orbit of main payload: Sun-synchronous near-circular orbit, altitude = 686 km, inclination = 98.13º, the orbital period is about 97.7 minutes, the LTAN (Local Time on Ascending Node) is at 10:30 hours.
Deimos-1 and UK-DMC-2 will separate from the launch vehicle marginally earlier than the main payload.
- The result is that these satellites will have a small eccentricity in their orbits
- Apogee will be at 680 km
- Deimos-1 perigee will be ~ 642 km, UK-DMC-2 perigee will be ~ 632 km (both spacecraft will have 10:30 LTAN). Both spacecraft have also sufficient ΔV to circularize into a sun synchronous orbit suitable for long term operations and mission requirements (ΔV carried is ~ 20 m/s).
Status of UK-DMC-2:
• February 27, 2019: Airbus Defence and Space is now offering new capabilities for wide area monitoring through the DMC Constellation, bringing increased resolution (from 12 to 24m) and extra spectral bands, including Red-Edge, Blue, Coastal Blue and Panchromatic. These new assets will boost key capabilities for agriculture, environmental monitoring and disaster management applications. 15)
- Complementing existing DMC Constellation satellites, this expansion of imaging capability is possible through exclusive commercial imagery agreements for the Alsat-1B satellite mission with the Algerian Space Agency (ASAL), and for the KazSTSAT satellite mission with Ghalam LLP. This is a significant step for all parties, which ensures partners leverage Airbus' expertise in end-to-end mission operation services, while improving Airbus satellites' fleet capacities.
- "Our team in Guildford, UK, has been managing imagery operations and processing since 2004 with the DMC Constellation and this opportunity will allow our customers to benefit from continuity, consistency of pricing, a wider offer and our long term experience. We are also committed to expand the UK contribution to the International Charter for Space and Major Disaster through the addition of these new missions," said Phil Brownnett, Head of UK Programs at Airbus Defence and Space, Intelligence.
- The DMC Constellation has been integral to the success of major monitoring programs over the USA and Europe for agriculture, as well as helping to combat deforestation in the Amazon Basin, and will continue to complement imagery from the Sentinel-2 and Landsat-8 missions. Additionally, future seamless integration into existing Airbus offers such as OneAtlas, Verde and Starling will ensure more effective global monitoring. Specifically, the Red-Edge band is crucial to calculate biophysical parameters such as chlorophyll content, enabling insight into leaf nitrogen content.
- Along with the DMC Constellation, Airbus Defence and Space offers exclusive commercial access to the largest fleet of high-resolution Earth observation satellites: the Pléiades twins, the SPOT-6 and SPOT-7 satellites and the weather-independent radar satellites TerraSAR-X, TanDEM-X and PAZ. The constellation is also reinforced by satellite partners such as TripleSat and KazEOSat.
- Surrey Satellite Technology Ltd., a subsidiary of Airbus, has jointly developed and manufactured the Alsat-1B satellite with ASAL, and the KazSTSAT satellite with Ghalam LLP.
Figure 10: Mississippi River, Louisiana - DMC constellation, AlSat-1B image (image credit: ASAL, Distribution: Airbus DS, 2019)
• January 31, 2018: According to Kimberley Wilson, Operational Marketing Manager Intelligence Communications, Intelligence & Securityof Airbus DS, the UK-DMC-2 saatellite is still in operation, and continues to deliver data daily. UK-DMC-2 operates as a commercial service.
- Greenland's National Park is the largest and most northerly in the world. Established in 1974 and expanded to its present size in 1988, it protects 972,001 km2 of the interior and northeastern coast of Greenland and is bigger than all but twenty-nine countries in the world. It was the first national park to be created in the Kingdom of Denmark and remains Greenland's only national park. -The park mostly comprises of inland ice and composite fjord landscape. This Arctic wilderness remains largely inaccessible and is home to nine species of land mammal, including the largest predator, the polar bear, and giant walruses.
Figure 11: Northeast Greenland National Park (image credit: Wikipedia) 17)
Figure 12: This UK-DMC-2 image of a portion the Northeast Greenland National Park was acquired in 2017 (image credit: Airbus DS)
• Feb. 2016: The UK-DMC-2 satellite is operational in 2016, in its 7th year on orbit.
Legend to Figure 13: The Bolivian Government has recently placed the Oruro Department under a state of natural disaster following the rapid decline of Lake Poopó. The second largest water body in Bolivia after Lake Titicaca, Poopó is a saline lake sitting at an altitude of 3,700 m, high on the Andean altipano. Occupying 2,000 km2 in the 1990s, the lake has been an important resource, with nearly 50,000 inhabitants relying on it for their livelihoods. Yet the lake has been deteriorating over the past decade, with 2015 a critical year. In December, the local government's office estimated, that Lake Poopó was down to just 2% of its former water level. However it has since been declared evaporated. 19)
• Nov. 6, 2014: DMCii is providing free UK-DMC-2 imagery within the International Charter to assist in the management of the Ebola crisis in West Africa. The recent Charter activation will allow WHO (World Health Organization) to acquire satellite imagery of Sierra Leone and Guinea – areas of West Africa that have seen a high number of Ebola cases. The epidemic has so far claimed over four thousand lives in West Africa and people are still falling victim to the spreading disease. It is the worst outbreak since the discovery of the disease in 1976. 20)
Figure 14: This image is located over the KanKan prefecture in Guinea. The grey city to the left of the image is Kankan itself. The image was acquired by UK-DMC-2 on October 17, 2014 (image credit: UKSA, DMCii)
- The satellite images provided by the Charter will assist in the response to the epidemic by providing international teams with maps that will allow them to better characterize where and how to deploy overseas medical staff and their support bases. The UK-built and operated DMC-2 is one of the satellites that has acquired images of West Africa, an excellent example of a UK satellite being used in support of our international humanitarian efforts.
• The UK-DMC-2 spacecraft and its payload are operating nominally in 2014.
• Feb. 12, 2014: Satellite images (Figure 15) taken by the UK's disaster monitoring satellite, UK-DMC2, are assisting UK agencies in their response to the severe flooding caused by storms in early Feb. 2014. The images, which show serious flooding on the Somerset Levels, were taken when the UK activated the International Charter ‘Space and Major Disasters' – an international effort to task Earth Observation (EO) satellites to provide free satellite data during natural emergencies. 21)
Satellite technology can make a tremendous difference in the immediate aftermath of an emergency, providing invaluable and immediate satellite images and data during times of crisis. In the image of the Somerset Levels, acquired on February 8, 2014, one can clearly see where the River Parrett has burst its banks and flooded the surrounding areas.
Figure 15: UK-DMC-2 satellite images are assisting UK agencies in their response to the severe flooding caused by the storms of early Feb. 2014 (image credit: DMCii, UKSA)
• The UK-DMC-2 spacecraft and its payload are operating nominally in 2013. - During the 2012 crop growing season, UK-DMC-2 and Deimos-1 participated again in the USDA Crop Collection Program.
• The UK-DMC-2 spacecraft and its payload are operating nominally in the summer of 2012. The spacecraft provides now an improved service: 22)
- Increased downlink rate of 80 Mbit/s
- Acquisition and processing of a single 8 Gbit file of imagery
- "Direct Downlink" service for UK-DMC-2 customers providing: a) Near real-time imaging over the ground station region; b) Store & Forward image downlink; c) Customers across Europe, Asia and The Americas.
Data Flow Concept:
- UK-DMC2 downlinks data to a designated groundstation
- Raw data is sent back to the DMCii central archive
- Data is processed and cataloged
- Products are generated and delivered to customers from DMCii
- Data downlinked at a designated groundstation can also be processed and the products delivered to their customers using their own systems.
Global EO Network:
- The use of additional groundstations has improved the throughput of UK-DMC-2
- The limitation of satellite access and data downlink has been removed
- The bottle neck is now on UK-DMC-2 spacecraft which shows the true capability of the mission (Ref. 22).
• The UK-DMC-2 spacecraft and its payload are operating nominally in 2011.
• During the 2011 US crop growing season, the USDA (United States Department of Agriculture) received imagery of Deimos-1 and UK-DMC-2 (complete coverage of the lower 48 states) to support its USDA users. The project team consisted of: 23) 24)
- Astrium GEO-Information Services (USDA awarded the contract to Astrium Geo-Information Services as prime contractor for the 2011 monitoring campaigns)
- Elecnor Deimos Imaging – subcontractor and DMCii as subcontractor to Elecnor Deimos Imaging.
Table 5: Overview of the USDA 2011 crop land collection campaigns of Deimos-1 and UK-DMC-2 (Ref. 23)
Figure 16: Example of USA multitemporal coverage (12 coverages in the period May-October 2011), image credit: Astrium GEO Information Services
• In August 2010, DEIMOS Imaging, through the DMC consortium, participated with Deimos-1 data in the 2010 CEOS Land Cal Event Campain at Tuz Golu salt lake, along with seven other major space institutions (CMA, CNES, ESA, JAXA, KARI, NASA and USGS) and 11 other participants who comprised the ground crew. - As a result of these activities, absolute calibration coefficients have been accurately computed for all bands (error <5%). Moreover, the cross-calibration errors with the other considered sensors was characterised to be lower than 2%. In the case of Landsat, this error was found to be very stable, taking values between 1% and 2%. 25)
• The UK-DMC-2 spacecraft and its payload are operating nominally in 2010, being part of the Disaster Monitoring Constellation and providing a new level of imagery output with 22 m multispectral imagery to customers worldwide. The MTF and noise characteristics of the in-orbit imagery show a significant step forward compared to the original DMC spacecraft (Ref. 30). 26)
The in-orbit calibration campaign has shown a very good cross-calibration with Landsat-7. By making use of the technological advancements of SSTL subsystems over the years the data throughput of the constellation has been significantly increased and new operational modes such as the broadcast mode have made it possible for customers with an existing ground station to receive DMC data directly and in Near Real Time (Ref. 30).
• In December 2009, DMCii introduced 22 m imagery to its geospatial image library, complementing its extensive 32 m imagery.
Since the launch of UK-DMC-2 and Deimos-1, both UK-DMC2 and Deimos-1 have been commissioned and carefully calibrated to match the existing DMC constellation satellites.
• Within just one week after launch, all the primary avionics systems have been commissioned and the first commercial grade 22 m image acquired. On Aug. 11, 2009, UK-DMC-2 acquired the first full resolution commercial grade image. 27) 28) 29)
• The SSTL mission control in Guildford, UK established communication with UK-DMC-2 during its first pass, less than 2 hours after the satellite's launch from Baikonur. In-orbit tests commenced immediately as each of the operating systems were activated, including the deployment of UK-DMC-2's additional solar panel.
Figure 17: The first image from UK-DMC2, showing the states of Texas and Oklahoma, USA (image credit: DMCii, Ref. 28)
Figure 18: UK-DMC-2 mission overview (left) and in-orbit artist impression (right), image credit: SSTL/DMCii 30)
LCMS (Low-cost low mass Star Tracker): The UK-DMC-2 satellite carries a star tracker experiment. The aim of the experiment are to evaluate the LCMS CMOS Image Sensor and to provide a platform for in-orbit testing of new star tracker algorithms. FillFactory NV (a company acquired by Cypress Semiconductor in 2004) developed the LCMS device. LCMS features a CMOS APS (Active Pixel Sensor) resulting in a more power efficient and mass-reduced star tracker (the CCD drive electronics can be dropped in this design).
Figure 19: Photos of the LCMS CMOS image sensor (left) and optics (right), image credit: SSTL/DMCii
Since launch, the star tracker has been switched on in orbit on a number of occasions and the series of star images have been captured and downloaded. In the coming months of 2010, the star camera will be characterized and will be used to test new star tracker algorithms.
Common sensor complement of UK-DMC-2 and Deimos-1: (SLIM6)
SLIM6 (Surrey Linear Imager Multispectral 6 channels - but 3 spectral bands):
The upgraded SLIM6 consists of two bore‐sighted instrument banks. The SLIM6 design provides for a nadir-viewing, three-band multispectral scanning camera capable of providing mid-resolution image information of the Earth's surface.
Three spectral bands are provided in the ranges: 0.52-0.62 µm (green), 0.63-0.69 µm (red), and 0.76-0.9 µm (NIR). The SLIM6 bands come closely to those of Landsat-7 bands 2, 3, and 4. SLIM6 employs the pushbroom imaging technology using two cameras per band (mounted in a double-barrel cross-track configuration - or in two banks) thus providing a dual (slightly overlapping) swath with a combined swath width of > 600 km with a spatial resolution 22 m GSD (Ground Sampling Distance) at nadir.
Figure 20: Schematic illustration of the SLIM6 configuration (image credit, SSTL)
The imager comprises six lens and sensor pairs, configured in two banks as part of an overall optical bench assembly. The banks are mounted angled away from nadir by approximately 13º, to double the swath width, but with a small overlap of approximately 5% to aid image stitching. The FOV (Field of View) from each bank is 26.6º.
Figure 21: Alternate view of the SLIM6 banks of channels (image credit: SSTL)
The instrument design has been improved since the first generation, in order to support data quality, radiometry, and calibration. The lens system has been modified to improve the GSD from 32m to 22m, and by employing a larger dimension linear array, the same swath width can be maintained. The SNR (Signal-To-Noise) ratio goal has also increased.
On-orbit calibration techniques have evolved given experience with the first generation system, the radiometric accuracy prior and following launch, and the relative radiometric stability are specified. Finally, a new alignment and focusing mechanism is included to support rapid setup of the instrument.
Table 6: Parameters of the SLIM6 (SLIM6) instrument
Each SLIM6 imager channel has a solid-state detector at the focal plane. The spectral filters for the bands are located in front of each channel lens. To protect the filter a fused silica radiation protection window is set in front of the filter.
The detector output is digitized to 12 bits and processed either to 10 bits or 8 bits radiometric resolution. The pushbroom system is capable of providing continuous imagery in flight path direction. The source data are stored in an onboard solid-state memory of 2 x 2 GByte capacity. SLIM6 features also a windowing capability. This function was introduced to avoid a saturation of the storage units and add more flexibility during satellite operations.
The operations strategy targets maximum imaging time in the sunlit phase of the orbit. Operating modes with imaging and downlinking data during each orbit, and imaging orbits followed by downlinking orbits have been developed.
The SLIM6 instrument operates in store-and-forward mode, so that images can be taken out of range of any control stations. Payload data remain stored on-board until the spacecraft is commanded to return the data once in contact with one of the network ground stations. The payload therefore comprises two banks of channels, two solid-state data recorders, and two high-speed downlinks. A balance has been struck between cost and performance. Payload operations are fundamentally resource limited by the on-board power available at the end-of-life, and by on-board storage. The system design constrains the spacecraft at end-of-life, and under worst-case power availability, on every orbit to:
1) Take images filling half its data recorders
2) Return all these data to a single station.
In practice, the system is well balanced between these constraints, but there are cases where any of these becomes the limiting factor. Data storage becomes the limiting factor when the spacecraft does not transit stations on several successive orbits, and power is the limiting factor when the spacecraft transits several stations in a single orbit.
Technical advancements: comparing some DMC-2G and DMC-1G performances
The following list enumerates some of the improvements that have been implemented over the past seven years with successive launches of the SSTL-100 platform of the DMC (Disaster Management Constellation). 31) 32)
• The SSTL-100 platform has been a very successful platform in the SSTL family of spacecraft. With seven platforms launched, the heritage of the platform is extensive and can be built quickly and affordably.
• A further 3 spacecraft based on the SSTL-100 platform, UK-DMC, NigeriaSat-1 and BILSAT, were launched into the DMC in 2003. These spacecraft worked together with AlSat-1 to provide global disaster monitoring.
Figure 22: Overview of the first and second generation DMC (Disaster Monitoring Constellation), image credit: SSTL
Legend to Figure 22: Of the 1st generation constellation, the spacecraft NigeriaSat-1, UK-DMC and Beijing-1 are operational in the fall of 2011. The BILSAT whose operations were terminated in August 2006 after nearly 3 years of service provision. The AlSat-1 operations were terminated in the summer of 2010. - Of the 2nd generation constellation, Deimos-1 and UK-DMC-2 are operational in 2011 while NigeriaSat-2 and NigeriaSat-X (both spacecraft were launched on August 17, 2011) are still in the commissioning phase as of October 2011.
• In 2009, a further 2 SSTL-100 spacecraft, UK-DMC-2 and Deimos-1, were launched. The spacecraft Deimos-1 and UK-DMC-2 are being partially funded by projected data sales and are operated by the commercial companies Deimos Imaging SL and DMCii Ltd., respectively. Both spacecraft carry an improved SLIM6 Line Scan imager payload with a 22 m GSD (Ground Sampling Distance) and an order of magnitude more imagery throughput compared to the first generation DMC.
• In late October 2010, a further 2 SSTL-100 spacecraft will be launched. NigeriaSat-X will be another addition to the DMC carrying the same payload as Deimos-1 and UKDMC-2. This spacecraft has been built together with Nigerian engineers.
• The MTF (Modulation Transfer Function) characteristics and SNR (Signal to Noise Ratio) have been improved compared to the first generation 32 m DMC imagers. The MTF performance of UK-DMC-1 and UKDMC-2 is shown in Figure 2 as they were measured before launch.
Figure 23: MTF comparison of UK-DMC-1, all channels (top), and UK-DMC-2 green channel (bottom), image credit: SSTL
The MTF and spatial resolution improvement can be clearly seen when raw imagery is compared. Figure 24 shows an image of Texas Fort Worth airport taken by UK-DMC-1 and UK-DMC-2 where the striking difference in MTF and spatial resolution can be seen. Small urban roads are clearly visible on the UK-DMC-2 image, while blurred on the UK-DMC-1 image.
Figure 24: Comparison of raw imagery of Texas Fort Worth airport taken by UK-DMC-1 (top) and UK-DMC-2 (bottom)
• The original SLIM6 Line Scan Imager, first operational in space in 2002, consisted of a COTS (Commercial Off The Shelf) optics design and a COTS detector. This imager produced 32 m GSD imagery in three wavebands (red, green, NIR) and at a very wide swath (>600 km). - In 2009 the DMC-2G imager came into service. This imager consisted of a custom optics design and an upgraded detector and electronics. This upgrade improved the GSD to 22 m, doubling the data density of the imagery and improved the MTF significantly, while still maintaining the wide swath of the original imager.
• On-board data storage: In 2002 the first DMC generation used SSDRs (Solid State Data Recorders) capable of storing up to 1.5 GByte of imagery and downloading this data in S-band at a maximum rate of 8 Mbit/s. By 2009, the second generation spacecraft UK-DMC-2 and Deimos-1 carried a HSDR (High Speed Data Recorder) capable of storing up to 8 GByte worth of data, paired with X-band downlinks providing data rates of up to 80 Mbit/s.
In the last couple of years, minor changes to the design have expanded the capacity of one HSDR to 16 GByte. These data recorders can be accommodated in the SSTL-100 either in a set of 2 or 3, thus providing up to 48 GByte of storage. Real-time hardware JPEG-LS compression can be implemented inside the HSDR FPGA, providing a lossless compression ratio of 2.8:1 or lossy compression ratio of 4:1 or higher.
• X-band downlink developments: The first DMC spacecraft in 2002 used an S-band downlink system capable of downlinking at 8 Mbit/s. The UK-DMC-2 and Deimos-1 missions in 2009 made use of X-band technology that was developed under the Beijing-1 mission in 2004. The data rate of these transmitters is up to 80 Mbit/s, increasing the amount of data that can be downlinked to ground by a factor of 10.
Further development in X-band technology for the NigeriaSat-2 project makes use of dual polarization and APM (Antenna Pointing Mechanisms). On the SSTL-100 it is going to be challenging from a mechanical point of view to include these APMs.
• Generated power developments: The first DMC spacecraft made use of four body-mounted panels, each with 288 single junction GaAs/Ge solar cells with a cell efficiency of 19.2% at 28ºC. The sun-synchronous orbit with 10:30 hours LTAN causes the 4th panel to permanently point away from the sun in the operational life of the spacecraft. This configuration generated 26.5 W OPA (Orbit Average Power) under worst case temperature, beta angle and solar flux and at end of life.
- For the Deimos-1 mission, 10 cells per panel were added, increasing the generated power by more than 20% to 32 W.
- The UK-DMC-2 mission makes use of the technology developed during the CFESat program by adding a deployable panel with triple junction InGaP/InGaAs/Ge cells with a cell efficiency of 27.5%. This addition increases the OAP by 56% to 50 W.
In the near future solar cells with efficiencies of 30%-33% will be available commercially boosting the generated power even further into the range of 87 W to 96 W.
• AOCS (Attitude and Orbit Control Subsystem) developments: The current AOCS system of the SSTL-100 makes use of sun sensors, magnetometers and reaction wheels to control its attitude. This system is adequate for the applications of the DMC. Due to its wide swath, geolocation is relatively simple and the 20-30 m GSD has no strict requirements on pointing control. However, the continuing increase of the resolution of the imager payload (in the past and the future) and the requirement to be able to off-point the spacecraft when a high resolution imager is accommodated on the platform will require an update to the AOCS system.
- One of the new developments that has been included as a flight experiment on UK-DMC-2 are the FSS (Fine Sun Sensors). These sun sensors have minimal impact to the spacecraft and can be included inside the current sun sensor assembly. The sun sensors are designed to have a pointing knowledge of 0.1º, which would improve the pointing of the spacecraft by an order of magnitude. As the moments of inertia of the SSTL-100 platform are relatively small, off-pointing maneuvers are possible by using the current reaction wheels and when sufficient settling time is allowed the fine sun sensors should give enough pointing knowledge to adequately geolocate the image taken by the imager payload.
A further development that has been considered is the introduction of a star camera on the SSTL-100 platform. In the past, experimental star cameras have been included on the platform, but these sensors were not operated in-the-loop and had limited baffles. - In the future a star camera could be added to the AOCS system of the SSTL-100, which would improve the pointing knowledge especially when performing off-pointing maneuvers, but the mechanical impacts to the platform need to be assessed.
The list of this chapter shows clearly, that over the past 7 years, the capability of the SSTL-100 platform and its payload has improved dramatically, and will keep improving dramatically in the coming years. This was achieved thanks to the gradual improvement of the subsystems.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).