Minimize UK-DMC-2

UK-DMC-2 (United Kingdom - Disaster Monitoring Constellition-2)

Overview    Spacecraft    Launch    Mission Status    Sensor Complement   References

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)

• Actually, SSTL considers already the Beijing-1 spacecraft (launched Oct. 27. 2005) of the DMC "first-generation missions" as its first second-generation DMC satellite - due to the fact that the satellite was using already an enhanced type platform (SSTL-150) of TopSat heritage. The X-band downlink capacity and the onboard data storage capability of Beijing-1 was considerably improved permitting observation scenes of 4100 km in along-track (the other first-generation DMC satellites were limited to 250 km scenes in along-track, and they were using S-band communications). In addition, the spacecraft was flown with a second imager, namely CMT (China Mapping Telescope) a high-resolution panchromatic pushbroom imager (4 m GSD on a swath of 24 km), next to the medium-resolution wide-swath imager SLIM6 of the DMC series.

• The imagery produced by the DMC satellites was initially used for international disaster monitoring and national campaigns by the satellite owners. Through DMC International Imaging Ltd. (DMCii), the DMC imagery was also exploited commercially in campaigns that needed large area coverage such as vegetation monitoring of the whole of Europe for the European Space Agency (ESA), and yearly campaigns of the Amazon rainforest, keeping an eye on illegal logging. These commercial campaigns generated significant revenue that flowed back to each of the satellite owners (Ref. 28).

Table 1: Some background of the DMC constellation

Timeframe

DMC constellation generation

Description

2003

DMC-1

First generation with 32 m GSD m/s & 600 km wide swath (4 m GSD pan on Beijing-1), in orbit since 2003

2009

DMC-2

Current generation providing 650 km wide-swath, 22 m GSD, in orbit since mid-2009 (Deimos-1, UK-DMC-2); 2.5 m GSD pan; NigeriaSat-2 and NigeriaSat-X were launched on August 17, 2011.

2014

DMC-3

Next generation VHR constellation of 3 satellites with ~1 m GSD pan and 4 m MS data on a 20 km swath

2015

DMC-4

Hyperspectral imaging

2016

DMC-5

SAR (Synthetic Aperture Radar) mission to provide all-weather, day-&-night coverage

Table 2: Sustainability - the DMC Road-Map 7)

 


 

UK-DMC-2

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)

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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).

Selected swath width (km)

Digitization (bit)

Maximum strip length (km)

650

8
10

1800
1450

325

8
10

3600
2900

185

8
10

6300
5100

Table 3: Maximum imaging strip length for selected swath widths and digitization (windowing scheme)

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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.

Wheel type

Peak momentum

Peak torque

Mass+electronics

Power constant speed

Power at peak torque

Heritage

MicroWheel-10S-E

0.42 Nms

10 mNm

1.1 kg

1.2 W

5.0 W

AlSat-1 (2), UK-DMC (2), BILSAT-1 (4), TopSat (4), NigeriaSat-1(2), Beijing-1 (4), CFESat (3)

MicroWheel-10SP-S

0.42 Nms

10 mNm

1.1 kg

1.2 W

5.0 W

RapidEye (20), Deimos-1 (3)

MicroWheel-10SP-M

0.42 Nms

10 mNm

0.96 kg

0.7 W

5.0 W

NigeriaSat-2(4), UK-DMC-2 (3), NX (4)

SmallWheel-200SP

12 Nms

200 mNm

5.4 kg

16 W @ 12 Nms

120 W

GIOVE-A (4), Kanopus (12), GIOVE-A2 (4), NigeriaSat-2 (4)

Table 4: Overview of MicroWheel parameters in various spacecraft of SSTL manufacture 13)

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Figure 3: Photo of the new SGR-07 receiver (image credit: SSTL) 14)

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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.

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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.

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Figure 6: Block diagram of the HSDR (image credit: SSTL)

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Figure 7: Block diagram of the UK-DMC-2 spacecraft (image credit: SSTL)

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Figure 8: The SSTL-100 compact modular platform in its various assembly stages (image credit: SSTL)

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Figure 9: Integration of UK-DMC-2 and Deimos-1 spacecraft at the launch facility in Baikonur (image credit: SSTL, Ref. 25)

 

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:

• Feb. 2016: The UK-DMC-2 satellite is operational in 2016, in its 7th year on orbit.

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Figure 10: A series of UK-DMC-2 images were collected over Poopó Lake in Bolivia showing the decline in Water levels in 2013 (left) to 2016 (right), image credit: DMCii, Airbus DS 15)

Legend to Figure 10: 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. 16)

• 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. 17)

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Figure 11: 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 12) 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. 18)

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.

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Figure 12: 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: 19)

- 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. 19).

• 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: 20) 21)

- 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.

USDA 2011 crop land collection requirements

• 20 m-60 m resolution, 3-band imagery

• Wide swath (>185 km)

• Twice a month coverage of the lower 48: May 1, 2011 – October 31, 2011

• 30% or less cloud cover of the defined crop areas

• Orthorectified (within one pixel GSD @ CE90)

• Delivery within 72 hours of acquisition

Data collection

• Two 15 day collection campaigns per month (12 collection campaigns for the project)

• Deimos-1 was used for primary collection

- UK-DMC2 used to fill gap and recollect cloudy areas

- Collection distribution was 80% Deimos-1, 20% UK-DMC2

Collection results

• A total of 1,578 scenes were collected

• 1,462 scenes were delivered to the USDA (On average 122 scenes were delivered for each 15 day campaign)

• Astrium delivered more then 186 million km2 of data to the USDA (150 million km2 of cloud-free data)

• On average, 15.6 million km2 of imagery was delivered every 15 days ( the area of CONUS is 8.1million km2)

• On average, 91% cloud-free data was delivered per campaign ( the requirement was 70% cloud-free)

• On average, 95% of the AOI was delivered cloud-free each month

• The average delivery time was 33 hours from acquisition (Deimos-1 Data – 29 hours, UK-DMC-2 Data – 41 hours)

• Only 4 images out of 1462 exceeded 72 hours

• All data meet the requirement (within one pixel GSD @ CE90) and w.r.t. to the Landsat reference dataset

Table 5: Overview of the USDA 2011 crop land collection campaigns of Deimos-1 and UK-DMC-2 (Ref. 20)

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Figure 13: 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%. 22)

• 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. 27). 23)

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. 27).

• 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. 24) 25) 26)

• 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.

 

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Figure 14: The first image from UK-DMC2, showing the states of Texas and Oklahoma, USA (image credit: DMCii, Ref. 25)

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Figure 15: UK-DMC-2 mission overview (left) and in-orbit artist impression (right), image credit: SSTL/DMCii 27)

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).

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Figure 16: 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.

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Figure 17: 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.

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Figure 18: 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.

Sensor (detector)

Eastman Kodak KLI14403 linear CCD sensor. 14,400 array with 5.0 x 5.0 µm/ pixel

Spectral bands (3)

0.52-0.62 µm (green), 0.63-0.69 µm (red), and 0.76-0.9 µm (NIR)

Lens

Custom design, 155.9 mm focal length lens, f/5

IFOV

32.07 µm (or 0.00184º or 6.62 arcsec)

GSD

22 m

FOV per channel

26.6º, TFOV = 52º (due to two banks of imagers)

Swath per channel

330 km

Total imaging swath

~ 660 km

Standard scenes

80 km x 80 km

Data quantization

8 bit or 10 bit digitization (selectable)

SNR

> 100 (rms across field)

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). 28) 29)

• 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.
The first spacecraft based on the SSTL-100 platform, AlSat-1, was launched in 2002. Designed and built together with the Algerian Centre National des Techniques Spatiales (CNTS), it marked the start of the DMC (Disaster Monitoring Constellation) and was the first SSTL spacecraft to carry SSTL's newly developed SLIM6 Line Scan Imager payload. In August 2010 the mission was completed after 7 years and 9 months of operations, exceeding its 5 year design lifetime by 50%.

• 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.

UKDMC2_Auto2

Figure 19: Overview of the first and second generation DMC (Disaster Monitoring Constellation), image credit: SSTL

Legend to Figure 19: 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.

UKDMC2_Auto1

Figure 20: 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 21 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.

UKDMC2_Auto0

Figure 21: 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.

 


1) P. Stephens, D. Hodgson, S. Mackin, G. Crowley, "DMC: New sensors & capabilities enhance the coordinated constellation EO service," Proceedings of the 58th IAC (International Astronautical Congress), International Space Expo, Hyderabad, India, Sept. 24-28, 2007, IAC-07-B1.6.02

2) A. M. Baker, P. Davies, L. Boland, "Moving Towards Commercial Earth Observation Services with Small Satellite Constellations," Proceedings of the 6th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 23 - 26, 2007

3) Zeger de Groot, James Penson, Adam Baker, Paul Stephens, "Getting the bigger picture: more bytes for your buck," Proceedings of the 22nd Annual AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 11-14, 2008, SSC08-III-4

4) A. Fernandez García, A. da Silva Curiel, P. Davies, J. Penson, Z. de Groot, D. Hodgson, P. Stephens, "Deimos-1 - The Next Generation of Commercial DMC Spacecraft," Proceedings of the IAA Symposium on Small Satellite Systems and Services (4S), Rhodes, Greece, May 26-30, 2008

5) A. M. Baker, P. Davies, L. Boland, "Moving Towards Commercial Earth Observation Services with Small Satellite Constellations," Proceedings of the 6th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 23 - 26, 2007

6) Paul Stephens, "The Disaster Monitoring Constellation, New Sensors & Data Policy," BARSC (British Association of Remote Sensing Companies) Workshop 2008, London, June 12, 2008

7) Martin Sweeting, "Microsatellites moving from research to operational missions," UN/Austria/ESA Symposium on Small Satellite Programs for Sustainable Development: Payloads for Small Satellite Programs, Sept. 21-24, 2010, Graz, Austria

8)) "DMCii To Launch New Higher-Resolution Satellite Imaging Service," Spacemart March 21, 2007, URL: http://www.spacemart.com/reports/DMCii_To_Launch_New_Higher_Resolution-Satellite_Imaging_Service_999.html

9) Information provided by Audrey Nice of SSTL

10) "Enhanced EO satellite UK-DMC-2 passes Test Readiness Review," June 19, 2008, URL: http://www.ballard.co.uk/press_releases/files/1187/PR%202020%20UKDMC-2%20Test%20Readiness%20Review.pdf

11) Alex da Silva Curiel, Zeger de Groot, James Penson, Adam M Baker, Paul Stephens, "UK-DMC2, stimulating small satellite based Earth Observation Services," Proceedings of the 59th IAC (International Astronautical Congress), Glasgow, Scotland, UK, Sept. 29 to Oct. 3, 2008, IAC-08-B4.4A.12

12) Martin Unwin, R. de Vos van Steenwijk, S. Duncan, "GPS / GNSS Receivers and Applications for Small Satellites in Low and High Earth Orbit," Proceedings of the 32nd AAS Guidance and Control Conference, Breckenridge, CO, USA, Jan. 31.- Feb. 4, 2009, AAS 09-0707

13) "Small Satellite Reaction Wheel - Microwheel 10SP-M," URL: http://www.sst-us.com/getdoc/bf3b6476-5559-429f-8426-11120cc2db6f

14) "SGR-07 Space GPS Receiver," URL: http://www.sst-us.com/getdoc/e594a1ef-823d-4449-87ae-f0b636863acb

15) https://www.flickr.com/photos/dmcii/24713038192/in/dateposted-public/

16) "The Rapid Decline of Lake Poopó, Bolivia," DMCii, Feb. 5, 2016, URL: http://www.dmcii.com/?p=10821

17) "Satellites assist in management of Ebola crisis," UKSA, Nov. 6, 2014, URL: https://www.gov.uk/government/news/satellites-assist-in-management-of-ebola-crisis

18) "Surveying storm damage from space: UK satellite provides images of Somerset floods," UKSA, Feb. 12, 2014, URL: http://www.bis.gov.uk/ukspaceagency/news-and-events/2014/Feb/somerset-floods

19) Gary Crowley, "DMC Missions and Ground Segment," 3rd GSCB (Ground Segment Coordination Body) Workshop, 2012, ESA/ESRIN, Frascati, Italy, June 6-7, 2012, URL: http://earth.esa.int/gscb/papers/2012/19-DMC_missions_and_ground_segment.pdf

20) Drew Hopwood, "Using Deimos-1 & UK-DMC2 to Monitor US Crop Conditions During the 2011 Crop Season," Proceedings of the 11th Annual JACIE (Joint Agency Commercial Imagery Evaluation ) Workshop, Fairfax, VA, USA, April 17-19, 2012, URL: http://calval.cr.usgs.gov/wordpress/wp-content/uploads/Hopwood_JACIE-2012-2011_Crop_Collection.pdf

21) Fabrizio Pirondini, Mónica Díez, Alfredo Romo, Cristina Moclan, Jorge Gil, "Data Quality of Deimos-1 & UK-DMC2 Imagery for the Monitoring of 2011 US Crop Season," Proceedings of the 11th Annual JACIE (Joint Agency Commercial Imagery Evaluation ) Workshop, Fairfax, VA, USA, April 17-19, 2012, URL: http://calval.cr.usgs.gov/wordpress/wp-content/uploads/JACIE-2012-Deimos-1-FPirondini.pdf

22) Francisco J. Lozano, Alfredo Romo, Cristina Moclan, Jorge Gil, Fabrizio Pirondini, "The Deimos-1 Mission: Absolute and relative calibration activities and radiometric optimization," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Munich, Germany, July 22-27, 2012

23) Information provided by Paul Stephens of DMCii, Surrey, UK

24) "UK-DMC2 satellite delivers first images just one week after launch," Press Release Aug. 11, 2009, URL: http://www.technologynewsroom.com/press_releases/files/1436/PR%202090%20UK-DMC2%20satellite%20delivers%20first%20images%20just%20one% 20week%20after%20launch%20pres.pdf

25) http://www.sstl.co.uk/News_and_Events/Latest_News/?story=1436

26) A New Era for the Constellation, New Satellites Improve Specifications and Expand the Scope of Service of the DMC," DMCii Newsletter, Vol. 3, Issue 2, Sept. 2009

27) Zeger de Groot, Stephen Mackin, Phil Davies, "In Orbit Results of the Next Generation Disaster Monitoring Constellation Satellite UK-DMC-2," Proceedings of the Symposium on Small Satellite Systems and Services (4S), Funchal, Madeira, Portugal, May 31-June 4, 2010

28) James Penson, Zeger de Groot, L. Sills, R. Taylor, Paul Stephens, Martin Sweeting, "In Orbit Results of the Next Generation Disaster Monitoring Constellation Satellite UK-DMC-2, Proceedings of the 61st IAC (International Astronautical Congress), Prague, Czech Republic, Sept. 27-Oct. 1, 2010, IAC-10.B1.5.2

29) Mike A. Cutter, Luis Gomes, Alex da Silva Curiel, Philip E. Davies, Zeger de Groot, Liam R. Sills, Andrew Cawthrorne, Martin N. Sweeting, "A New Generation of Disaster Monitoring Constellation Imagers," Proceedings of IAC 2011 (62nd International Astronautical Congress), Cape Town, South Africa, Oct. 3-7, 2011, paper: IAC-11.B1.3.2
 


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 (herb.kramer@gmx.net).

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