DMC-3 (Disaster Monitoring Constellation-3)
In 2011, SSTL (Surrey Satellite Technology Ltd.) is building a new high-resolution satellite constellation, DMC-3 , a three spacecraft optical imaging constellation to be owned and operated by DMCii (DMC International Imaging Ltd.) of Guildford, Surrey, UK. DMCii is a subsidiary of SSTL, the world leader in small satellite technology. The objective of this innovative commercial DMCii service model is to lease capacity of these spacecraft - the service will provide daily access opportunities to anywhere in the world. 1)
The first data customer to this DMCii service model is the Chinese company 21AT (Twenty First Century Aerospace Technology Company Ltd.) of Beijing, China. The contract between 21AT and DMCii was signed in June 2011 in London during the Chinese premier's UK visit. 21AT will lease from DMCii 100% of the imaging capacity of the three high resolution satellites under a seven-year contract and SSTL will design the satellite constellation to meet the Earth Observation requirements of 21AT. 2) 3) 4) 5)
In return 21AT will receive timely, high resolution (0.75 m GSD Pan and 3 m MS data) satellite imagery with planning and tasking but without the complexity associated with procuring and operating a satellite constellation. A launch of the DMC-3 constellation is planned for 2014.
21AT is no stranger to SSTL, having built its business in selling imagery from its very successful small satellite mission Beijing-1 which was manufactured by SSTL and launched in 2005. Beijing-1 has been used extensively by 21AT to provide EO information services over the last 5 years, primarily to the Chinese government, for applications such as monitoring land and water resources, agriculture, urban development, desertification, earthquake and snow disasters, pollution, and environmental impact assessment.
By leasing capacity from the three new satellites in the DMC-3 constellation, 21AT will meet growing demand for high-resolution image data and value-added EO (Earth Observation) geo-information services from its established user community. It is anticipated that more satellites will be added to the constellation as additional capacity is required for DMCii's international customer base and other early adopters of this unique service model.
Table 1: General operational service concept of DMC-3
Table 2: Overview of DMC missions 6)
Figure 1: Artist's view of the SSTL-300 S1 bus for the DMC-3 mission (image credit: SSTL)
The DMC-3 constellation will be the first three spacecraft series of the SSTL-300 S1 bus - building on the heritage of the SSTL-300 platform and avionics of NigeriaSat-2. The SSTL 300 S1 bus is ideal for a wide range of commercial, civil and security applications, the agile SSTL 300 S1 Earth imaging system offers 1 m panchromatic and 4 m multispectral GSD imagery with a high speed downlink and a±45º fast slew off-pointing capability. 7)
Continuous technology innovation means that sub-meter imagery is possible on a minisatellite platform, along with quality and quantity of images to meet demanding rapid response applications.
Table 3: Overview of system parameters
A second generation X-band payload downlink chain has been developed for use on SSTL’s future missions, incorporating a HSDR (High Speed Data Recorder) with FMMU (Flash Mass Memory Unit), next generation XTx (X-band Transmitter) and higher gain APM (Antenna Pointing Mechanism).
The FMMU, shown in Figure 2 (left), utilizes a commercially available non-volatile flash memory to deliver total data storage of 128 GByte (i.e., 1 Tbit). Since the data read/write speed of flash memory is relatively low compared to that of DDR2 (Double Data Rate), and since the mass memory must be able to acquire data at extremely high data rates from the payload, the FMMU is being used as second-level storage, with the HSDR still used for data acquisition, processing and downlink. However, the FMMU will also be upgraded in due to course to allow it to function as an independent, high capacity storage solution for missions with a lower payload data acquisition rate (Ref. 6).
Figure 2: Photo of the FFMU (left) and +18 / +15 dBi APMs (right), image credit: SSTL
As with many of SSTL’s products, the next generation XTx has been developed from a heritage design in order to maximize confidence in its suitability for the space environment. This transmitter variant is initially designed to deliver a maximum data rate of 400 Mbit/s, using 8PSK modulation and 2/3 TCM (Trellis Coded Modulation) FEC (Forward Error Correction) coding. The use of this higher order modulation scheme allows the signal bandwidth to be reduced, whilst the 2/3 TCM coding adds a lower coding overhead (albeit with reduced FEC coding gain). This means that it is possible to support a data rate approximately four times higher than the previous variant with a bandwidth less than double; this is advantageous as it makes better usage of the limited X-band spectrum and simplifies the HPA (High Power Amplifier) design.
Two major changes have been made to this transmitter to enable the higher data rates. Firstly, the modulator module has been upgraded to use a Xilinx Virtex-5 SX FPGA; initial radiation characterization performed on this FPGA has been favorable and its high performance means that it can be used to support higher data rates and higher order modulation schemes such as 16PSK. Secondly, the HPA has been upgraded to support the higher bandwidth required and to deliver a total RF output power of 12 W. The next generation XTx has been designed with an eye to the future, potentially allowing the development of higher data rate solution. In the short term, it is planned to develop VHDL [VHSIC (Very High Speed Integrated Circuit) Hardware Description Language] to operate the transmitter with more efficient FEC coding scheme of 5/6 TCM, allowing a data rate of 500 Mbit/s to be supported while keeping the symbol rate unchanged at 200 Msample/s.
Also, a new variant of the APM has been developed, using a +18 dBi carbon fiber horn antenna. The advantage gained from carbon fiber over aluminum (as used on the +15 dBi variant) is that the motor and bearing does not need to be modified to accommodate a higher mass antenna. In turn, this maximizes the flight heritage that can be inherited from the NigeriaSat-2 mission. It was found that the composite horn was electromagnetically opaque; hence, it did not need the addition of a metallic coating for use as an antenna.
SSTL’s next generation payload downlink chain, comprising 128 GByte FMMU, HSDR, 400 Mbit/s XTx and +18 dBi APM, will fly for the first time on SSTL’s TechDemoSat-1 mission, currently expected to launch during the first quarter of 2013.
Figure 3: Artist's rendition of the DMC-3 constellation in orbit (image credit: SSTL)
Figure 4: Photo of the DMC-3 SQM (Structural Qualification Model), image credit: SSTL
Figure 5: CAD drawing of the SSTL-300S1 platform (image credit: SSTL)
Launch: A common launch of the DMC-3 constellation (3 spacecraft) is planned for Q4 2014/Q1 2015 on a PSLV-XL vehicle of ISRO from SDSC (Satish Dhawan Space Center) SHAR on the south-east coast of India. 8)
Orbit: Sun-synchronous orbit, altitude = 630 km, inclination = 97.8º, LTAN (Local Time on Ascending Node) = 10:30 hours. The three spacecraft will be evenly spaced in the same orbital plane with daily revisit times which is crucial for change detection, disaster monitoring and response planning, and essential for acquiring cloud-free imagery.
Sensor complement: SSTL300-S1 Imager
The imager used in the S1 spacecraft is a modified Newtonian telescope which gives the best optical image quality within the constraints of the platform structure and launch accommodation. The focal plane makes use of a TDI (Time Delayed Integration) sensor which gives a great SNR (Signal-to-Noise Ratio) for low albedo targets over extended periods of time, independent of seasonal variations.
The 1m panchromatic and 4m multispectral imagery from the DMC-3 satellites will provide 21AT's customers with greatly enhanced data continuity from in 2005 launched Beijing-1 and by leasing capacity from three satellites, 21AT will have the power of the new constellation at its disposal - providing both increased resolution with rapid imaging and revisit. 21AT will use the image data mostly for high resolution mapping of vast areas of the Chinese landmass by imaging strips with a length of up to 4000 km.
Table 4: Key performance parameters of the DMC-3 S1Imager
The SSTL300-S1 imager is the next evolution from the 2.5 m resolution imager flown on NigeriaSat-2. In both imagers, on-orbit focusing is performed by adjusting the position of a focus lens linearly along the lens optical axis. 9)
The S1 Imager has more stringent requirements for the focus lens’ alignment, linear position, and stability, as well as a more challenging environment and very different location on the imager. This required the development and qualification of a new focus mechanism design integrated into one of the optical subassemblies on the imager, the RLA ( Relay Lens Assembly), a location depicted in Figure 6.
The focusing lens is the final lens on the RLA along the optical path. This configuration was traded off against other locations, including at the focal plane; however, the logistical challenges and criticality of the thermal solution proved more onerous than the mechanical environment at the RLA. As a result, the focus mechanism is fully integrated into the RLA.
Figure 6: DMC-3 Imager focus mechanism EM (left),PFM model with RLA assembly (center), and RLA mass dummy on SQM Imager at top right (image credit: SSTL)
The fundamental purpose of the focus mechanism is to position the focusing lens along the optical axis. In order to ensure that the Earth image is focused with the focal plane’s required depth of focus, the lens must be positioned within 5 µm. Due to various factors affecting the focal length of the imager, it must be able to position the lens along a 10 mm length of travel. The required life and total travel are small, driven mostly by ground cycles.
In doing so, the lens must not be perturbed in any other degree of freedom, either during focusing or due to test, launch, micro-vibration, or thermoelastic loads. The parallel shift of the lens optical axis (referred to as decentration) must be within 5 µm of the nominal position. The tilt of the lens optical axis must be within 15 arcsec of the nominal orientation – also expressed as 10 µm TIR (Total Indicated Run-out) on the lens surface.
The lens must also be able to be installed after the mechanism has already been qualified; the sequence of assembly has to consider the large size of the lens (148 mm diameter).
Table 5: Requirements of the DMC-3 S1 Imager
The design load used for the mechanism is 60 g quasi-static in the optical axis, and 30 g in the transverse axes. Also, the mechanism must provide independent feedback on focusing movement, not require launch locks, and hold the focus position while unpowered.
EM (Engineering Model) concept:
The mechanism concept settled upon for an engineering model utilizes a stepper motor-driven ball screw to drive the lens carriage along three linear ball bushing shafts. The design allowed two different configurations to be tested: a single screw configuration which drives the carriage at one edge of the lens, and a multi-screw configuration which drives the carriage at three points roughly equally spaced around the lens. The motion of the three screws is synchronized by a three pinion and ring gear set. The EM mechanism is built largely to a flight standard, but using COTS tribological components and eliminating costly custom modifications allowed the EM to be assembled at a low cost.
Figure 7: RLA with optics model (image credit: SSTL)
Actuator: As a result of the requirement to position the lens with a high resolution and to do so over a comparatively large distance, concepts from other focus and linear positioning mechanisms quickly fell by the wayside. In order to avoid windup, low friction and high stiffness along the line of action are required. For this reason a 440C stainless steel ball screw with a 2 mm lead pitch was selected. The accompanying ball nut has an integral thread that easily clamps on to the lens carriage. In keeping with SSTL’s ethos, both the ball screw and its supporting spindle bearings are COTS components, with the ball screw undergoing post-machining and lubrication under the direction of SSTL.
Structure: The focus lens has a diameter of 148 mm, and is truncated at two sides with a width of 110 mm. It is first installed into a titanium lens cell, and then subsequently the cell is bonded into place in the titanium carriage. Instead of the typical installation of the linear bushings into the carriage, the focus mechanism has the bushing shafts installed in the carriage. This is for two reasons.
Firstly, the linear bushings are designed to have two bushings along each shaft to react moment loads. Installing two bushings on the carriage would have a smaller distance between the two bushings, but would again unnecessarily extend the length of the structure, with the corresponding increase in mass.
Secondly, it is of paramount importance is that thermoelastic distortions are minimized across the focus mechanism structure. For this reason, the main housing, end plate and carriage are all titanium and largely axisymmetric; the other structural elements driving the position of the carriage, the bushing shafts and the bushings themselves, are 440C stainless and only a mismatch of 3.4 ppm/°C. Since the only CTE mismatch (the length of the shafts) is compensated by the linear bushings; if the shaft was mounted to the end plate and housing, it would impart a load on the structure at the operating temperature of -2.5°C which is thus avoided.
The structure is designed so that the bearing bores (for the ball screw spindle bearings) and the bushing bores can be match machined, thereby ensuring their concentricity to within a few microns. The fit of the housing and end plate is retained upon disassembly and assembly with six tapered dowels fitted before match machining takes place.
Figure 8: PFM (Proto-Flight Model) focus mechanism components (image credit: SSTL)
Sensor: A further requirement on the focus mechanism is that when a focusing movement is commanded, independent feedback can confirm that the movement has taken place. This had to be measured linearly; an input or output encoder on the motor/gearbox would not suffice. To that end a vacuum-rated non-contacting linear encoder was selected and implemented in a location on the mechanism expected to see <1 krad TID. Selecting a vacuum-rated EEE part provided it is effectively shielded is in keeping with SSTL’s COTS approach.
Figure 9: Photo of Sir Marting Sweeting with the DMC-3 imager barrel (image credit: SSTL)
1) Phil Davies, Andrew Cawthorne, Paul Carter, Liam Sills, Alex da Silva Curiel, “The DMC-3 1 m Small Satellite Constellation,” 8th IAA (International Academy of Astronautics) Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 4-8, 2011, IAA-B8-0401, URL: http://media.dlr.de:8080/erez4/erez?cmd=get&src=os/IAA/archiv8/Presentations/IAA-B8-0401.pdf
2) “David Cameron andWen Jiabao witness DMCii landmark satellite imaging deal with Chinese company,” DMCii Press Release, June 29, 2011, URL: http://www.srpnet.co.uk/.../DavidCameronandWenJiabaowitnessDMCii
3) “DMCii signs landmark satellite imaging deal with Chinese company,” Space Daily, June 30, 2011, URL: http://www.spacedaily.com/.../DMCii_signs_landmark_satellite_imaging_deal_with_Chinese_company
5) Stefanie Kohl, Zeger de Groot, Andrew Cawthorne, Luis Gomes, Martin Sweeting, “DMC3 Constellation: Sub-meter resolution imagery at lowest costs,” Proceedings of the 64th International Astronautical Congress (IAC 2013), Beijing, China, Sept. 23-27, 2013, paper: IAC-13-B1.2.1
6) Mark Brenchley, Peter Garner, Andrew Cawthorne, Katarzyna Wisniewska, Philip Davies, “Bridging the Abyss - Agile data downlink solutions for the Disaster Monitoring Constellation,” Proceedings of the 4S (Small Satellites Systems and Services) Symposium, Portoroz, Slovenia, June 4-8, 2012
7) J. Paul Stephens, “New Sensors: Update on developments in the DMC Constellation,” 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/Stephens_JACIE_DMCii_presentationApr2012.pdf
8) “Antrix signs agreements for launching satellites from UK and Singapore on-board ISRO’s workhorse Polar Satellite Launch Vehicle (PSLV) ,” ISRO, Feb. 06,2014, URL: http://www.isro.org/scripts/news-6-2-14.aspx
9) Ryan Kraliz, “Multi-Screw Focusing Mechanism for DMC3 Sub-1 m Imager,” Proceedings of the 15th ESMATS (European Space Mechanisms and Tribology Symposium) 2013, Noordwijk, The Netherlands, Sept. 25-27, 2013, ESA, SP-718, URL: http://www.esmats.eu/esmatspapers/pastpapers/pdfs/2013/kraliz.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.
The Disaster Monitoring Constellation series: