NovaSAR-S is a joint technology demonstration initiative of SSTL (Surrey Satellite Technology Ltd.), UK, and EADS Astrium Ltd (Stevenage, UK). The overall objective is to make SAR (Synthetic Aperture Radar) observation missions more affordable to a customer base and to open up new application-oriented in the microwave region of the spectrum.
In the past decade, programs such as the DMC (Disaster Monitoring Constellation) have proven that a low cost, small satellite approach can provide solutions for medium resolution EO (Earth Observation) applications. However, these have all been optical missions. SAR missions have well known night time and all weather advantages over optical missions but have not yet made the same evolutionary step. To date, SAR satellites have mostly served large budget, institutional missions with a performance driven, rather than applications driven, design. 1) 2) 3) 4) 5) 6) 7)
A SAR payload design, which optimizes all imaging parameters (resolution, swath, sensitivity, duty cycle etc.), leads to a satellite design which is at odds with a low cost approach. An applications driven approach, focused on medium resolution applications, enables solutions that can make the evolutionary step to the low cost, small satellite arena. In the same way that low cost optical satellites have enabled numerous optical EO programs, a low cost approach can bring SAR solutions to a wider EO user community.
Rather than follow a traditional development with a flow down of requirements from an external customer, our approach has been to design a baseline mission which answers the question "what imaging performance can we achieve with a spacecraft that can be built and operated at low cost, and is compatible with low cost launches?" This approach leverages heavily SSTL's experience of low cost optical EO missions and Astrium's experience of SAR missions.
NovaSAR-S provides medium resolution (6-30 m) imagery ideal for applications in the following fields:
- flood monitoring
- agricultural crop assessment
- forest monitoring (temperate and rain forest)
- land use mapping
- disaster management
- maritime applications (e.g. ship detection and oil spill monitoring).
However, applications that can be served by a medium resolution (10-30 m) system are not limited to the list above.
In November 2011, the British government announced an investment of L 21 M to SSTL to assist in the development and launch of the first NovaSAR spacecraft, an innovative and highly competitive new spaceborne radar remote sensing program, in the international market. 8) 9) 10)
The industrial team developing the NovaSAR spacecraft is led by SSTL. NovaSAR-S combines heritage avionics (of the SSTL-300 bus) with a new structural design, to accommodate a payload, developed under a joint initiative between SSTL and Astrium. The design lifetime of the satellite is 7 years. A trade-off between imaging requirements, drag and propulsion subsystem mass/NRE/costs settled on an optimum orbital altitude of 580 km. 11) 12)
Figure 1: NovaSAR-S spacecraft with payload antenna (left), component accommodation (center), and solar panel (right), image credit: SSTL, Astrium
EPS (Electrical Power Subsystem):
Super capacitor experiment: The objective is to test a new energy storage technology in space - by providing a high-power energy source for relatively short operating times (to make for instance a radar operation possible on a small spacecraft. 13)
To provide a highly reliable system with a high probability of meeting the 7+ year lifetime requirement, SSTL's platforms make use of wide-ranging redundancy architectures. The EPS architecture is shown in Figure 2. There are two redundant CAN data buses and two of each of the key platform units, each connected to each data bus allowing cross strapping of units. Additionally some of the units provide internal redundancy e.g. triple redundant program memory and EDAC protection on the data memory.
SSTL is developing a prototype of a super-capacitor based power system, including a capacitor charge regulator (with maximum peak power tracking and control charging), a cell monitoring and management module (to monitor and protect each cell), and capacitor-bank (energy storage module). A COTS LIC (Lithium-Ion Capacitor) is employed in the capacitor-bank. The LIC uses activated carbon as a positive electrode that makes an electric double layer, and has lithium-ions pre-doped into a carbonaceous negative electrode.
Avionics: NovaSAR-S features the same avionics as those on the on the SSTL-300 bus of NigeriaSat-2.
The NovaSAR-S spacecraft has a mass of ~400 kg, a xenon propulsion system, and a design life of 7 years.
Launch: A launch of NovaSAR is planned for 2015. The flight readiness review is scheduled for Q1 2015.
Orbit: Sun-synchronous orbit, nominal altitude of 580 km,
RF communications: The TT&C data are transmitted in S-band (2025-2110 MHz, 2200-2290 MHz); the payload data are downlinked in X-band (8.025-8.4 GHz) at a data rate of 500 Mbit/s. An onboard data storage capacity of 544 GByte is provided (Ref. 12).
Sensor complement: (S-SAR, AIS)
S-SAR (S-band Synthetic Aperture Radar):
The innovative S-SAR payload is being developed collaboratively by EADS Astrium of Portsmouth, UK, and SSTL, the payload activities are led by Astrium UK. 14)
Background: The S-SAR payload is a derivative from Astrium's airborne radar demonstrator technologies with the benefit of significant heritage and risk reduction from the airborne demonstrator development: 15)
· architecture and implementation of back end radar electronics
· development and demonstration of imaging modes
· provision of data sets to support proving of SAR applications
· key enabler for low cost approach to space-based radar instruments.
Demonstrator developed by Astrium over the last decade:
· designed and built under UK government contract and Astrium R&D
· system/instrument exercised on extensive flight trials campaigns
· has been an important tool as a radar test bed to provide support to SAR research and development
· SAR image processor developed to evaluate acquired imagery.
Demonstrator now evolved to a multi-frequency high performance capability offering potential to support both research and operational users.
The S-band antenna is a micro-strip patch phased array of ~ 3 m x 1 m in size. The antenna size drives the satellite size which has been deliberately constrained to meet low cost launch vehicle requirements.
The payload can transmit and receive on both horizontal and vertical polarizations. The baseline payload configuration can be operated to produce imagery in polarimetric mixes that include single polar (HH, VV), dual polar (any 2 from HH, VV, HV or VH), tri-polar (any 3 from HH, VV, HV or VH), or quad polar (HH, VV, HV & VH) .
Table 1: S-SAR payload parameters
The payload front-end RF electronics are mounted on the reverse side of the antenna panel making the payload front-end self-contained. The NovaSAR platform has been designed to support payloads that operate in different frequency bands.
Traditional space qualified TWTAs offer good efficiency but are relatively expensive. GaAs SSPAs (Solid-State Power Amplifiers) can be implemented using COTS technology but do not offer such good efficiency. Recently, the terrestrial use of GaN in SSPAs has become mature at frequencies up to S-band and offers efficiencies > 40%. Therefore, our payload has been designed around S-band GaN SSPAs.
By using GaN SSPAs and designing for an imaging duty cycle of ? 2 minutes/orbit (~2%) the orbit average power consumption of the payload is in the region of 100 W. This greatly helps to reduce the overall spacecraft mass and volume, and hence costs associated with manufacturing, test, transportation, launch etc. The 2 minute imaging period has been selected as a baseline to enable an 800 km long strip of imaging each orbit that is well suited to regional observations, comparable with the size of many nation states. In fact, depending on operational mode and downlinking opportunities, larger payload duty cycles can also be supported by the NovaSAR-S satellite providing the overall 2% duty is respected across a number of orbits.
The payload can transmit and receive on both horizontal and vertical polarizations. The baseline payload configuration can be operated to produce imagery in polarimetric mixes that include single polar (HH, VV), and then by trading swath width and/or resolution for more diverse polarimetric capability, dual polar (any 2 from HH, VV, HV or VH) or tripolar (any 3 from HH, VV, HV or VH).
The front surface of the payload frontend consists of the SAR antenna panel with a total of 18 pairs of sub-arrays, each pair forming an individually controllable phase center. The payload frontend RF electronics are mounted on the reverse side of the antenna panel, making the payload frontend self-contained. Six beam control units apply transmit and receive phase adjustments to the eighteen phase centers. Each phase center consists of a transmit unit (100 W RF o/p), a power conditioning unit, a receive unit and a radiator unit. The radiator units are arranged in three columns of six sub-array pairs, each having 24 patches (Figure 3). The cold redundant payload backends are an S-band variant of the Astrium NIA (New Instrument Architechture) equipment (Figure 4).
Figure 3: Radiator units arranged in three columns of six (image credit: Astrium)
The NovaSAR-S platform has been designed to support payloads that operate in different frequency bands. Efficient SSPA technologies in other bands are being investigated so that alternate payload frontends can be implemented in the future.
The applications focused approach to the NovaSAR-S system design has resulted in a platform where both solar panels and the payload antenna can follow fixed, body mounted forms. Hence, the satellite is without deployable appendages, a unique achievement for a SAR satellite.
Figure 4: Illustration of the NIA payload backend (image credit: Astrium)
Baseline imaging modes: An orbit altitude of 580 km has been used to derive the baseline imaging modes given in Table 2. Mode 1 is expected to be the mode most commonly used for the target applications identified. Mode 2 is an unconventional, ultra-wide swath mode intended for ship detection. Modes 1, 2 and 4 are ScanSAR modes. Mode 3 is a Stripmap mode which trades swath width for an improved resolution. However, this reduced swath can be selected from a 150 km access range. Mode 4 is similar to mode 1, but trades resolution to get a wider swath. Modes 1, 3 and 4 have options to increase the available incidence angles and reduce revisit times.
Table 2: Baseline single polar imaging modes
The payload is highly flexible and is capable of delivering a wider range of imaging modes than those baseline modes presented in Table 2 which have been designed for maximum coverage. In-orbit tuning of beam shape and PRF (Pulse Repetition Frequency) can be used to define new modes.
S-SAR payload qualification campaigns:
An extensive development programme has been in progress for 3 years. This involves four stages of development for the NovaSAR-S payload equipment:
1) Prototype hardware ground based inverse SAR test which imaged the ISS (International Space Station).
2) Airborne demonstrator using prototype hardware for providing sample imagery. 16)
3) In-orbit validation of payload technology by flying one phase center as a radar altimeter on the TechDemoSat-1 (Technology Demonstration Satellite-1) / TDS-1 mission of SSTL (a launch of TechDemoSat-1 is scheduled for late 2013).
4) Qualification of the payload via an EQM test campaign.
AIS (Automatic Identification System) payload:
AIS is a secondary payload on the NovaSAR-S mission. The instrument will be descibed when information is available.
1) Philip Whittaker, Martin Cohen, David Hall, Luis Gomes, "An Affordable Small Satellite SAR Mission," 8th IAA (International Academy of Astronautics) Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 4-8, 2011; URL of the presentation, IAA-B8-0203, URL: http://media.dlr.de:8080/erez4/erez?cmd=get&src=os/IAA/archiv8/Presentations/IAA-B8-0203.pdf
2) NovaSAR - setting a new benchmark in affordability and performance for spaceborne Synthetic Aperture Radar (SAR) systems," SSTL, URL:.http://www.sstl.co.uk/Downloads/Datasheets/1767-SSTL-SAR-Datasheet
3) Philip Whittaker, Martin Cohen, David Hall, Rachel Bird, Luis Gomes, "NovaSAR - A Novel, Low Cost, Medium Resolution Spaceborne SAR System Development," Proceedings of the 3rd Workshop on Advanced RF Sensors and Remote Sensing Instruments (ARSI), Noordwijk, The Netherlands, Sept. 13-15, 2011, URL: http://www.congrex.nl/11c11/ARSI%20papers/WHITTAKER_ARSI_Paper.pdf
4) Philip Whittaker, Rachel Bird, Martin Cohen, David Hall, Luis Gomes, Alex da Silva Curiel, Philip Davies, "A Low Cost SAR Solution for Disaster Management and Environmental Monitoring Applications," Proceedings of IAC 2011 (62nd International Astronautical Congress), Cape Town, South Africa, Oct. 3-7, 2011, paper: IAC-11-B4.4.10
5) Philip Davies, Phil Whittaker, Rachel Bird, Luis Gomes, Ben Stern, Martin Sweeting, Martin Cohen, David Hall, "NovaSAR - Bringing Radar Capability to the Disaster Monitoring Constellation," Proceedings of the 4S (Small Satellites Systems and Services) Symposium, Portoroz, Slovenia, June 4-8, 2012
6) Philip Davies, Alex da Silva Curiel, Rachel Bird, Philip Whittaker, Martin Cohen, David Hall, Luis Gomes, "Changing the Radar paradigm - Technology and cost trades in establishing the NovaSAR constellation," 6th International Conference 'Remote Sensing- the Synergy of High Technology,' Moscow, Russia, April 25-27, 2012," URL: http://www.sovzondconference.ru/upload/medialibrary/d77/d77b716f1e4a26f89bd6f786c68e0d0b.pdf
7) Philip Davies, Phil Whittaker, Rachel Bird, Luis Gomes, Ben Stern, Martin Sweeting, Martin Cohen, David Hall, "NovaSAR - Bringing Radar Capability to the Disaster Monitoring Constellation," Proceedings of the 26th Annual AIAA/USU Conference on Small Satellites, Logan, Utah, USA, August 13-16, 2012, paper: SSC-I-7
9) "Government and industry to invest in world-leading UK satellite constellation," Nov. 29, 2011, URL: http://www.bis.gov.uk/ukspaceagency/news-and-events/2011/Nov/government-and-industry-to-invest-in-world
10) "UK and Japan commit to greater collaboration on space," UKSA, April 18, 2012, URL: http://www.bis.gov.uk/ukspaceagency/news-and-events/2012/Apr/uk-and-japan-commit-to-greater-collaboration-on-space
11) NovaSAR-S the small satellite approach to Synthetic Aperture Radar, SSTL brochure, URL: http://www.sstl.co.uk/Downloads/SSTL-Brochure-pdfs/1904-SSTL-NovaSAR-Brochure
12) Phil Davies, Andrew Cawthorne, Phil Whittaker, Sir Martin Sweeting, Martin Cohen, "Development and Test the First NovaSAR-S Mission," Proceedings of the 9th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 8-12, 2013, paper: IAA-B9-1303
13) Tatsuo Shimizu, Craig Underwood, "Commercial Lithium-Ion Capacitor for Spacecraft Energy Storage: Performance Characterization and Safety Evaluation," Proceedings of the 9th European Space Power Conference, Saint Raphael, France, June 6-10, 2011, ESA SP-690
14) "NovaSAR-S Mission and Imaging Overview," Annual Conference of the Remote Sensing and Photogrammetry Society 2012, University of Greenwich, London, UK, Sept. 13-14, 2012, URL: http://www.eotechcluster.org.uk/eotechcluster/documents/novasar-overview-for-rspsoc-sep12.pdf
15) Geoff Burbidge, David Hall, "Astrium S-band airborne demonstrator opportunity," Earth Observation Technology Cluster, UK, Sept. 13, 2012, URL: http://www.eotechcluster.org.uk/eotechcluster/documents/astrium-s-band-airborne-demonstrator-opportunity-final-v2.pdf
16) Antonio Natale, Raffaella Guida, Rachel Bird, Philip Whittaker, Martin Cohen, David Hall, "Demonstration and Analysis of the Applications of S-Band SAR," APSAR (The Asia-Pacific Conference on Synthetic Aperture Radar), Seoul, Korea, Sept. 26-30, 2011, URL: http://epubs.surrey.ac.uk/726887/2/PID1983553.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.