NovaSAR-S — A low cost S-band SAR mission on a Minisatellite
NovaSAR-S is a joint technology demonstration initiative of SSTL (Surrey Satellite Technology Ltd.), UK, and Airbus DS (former EADS Astrium Ltd, Stevenage, UK), funded by the UK Government via the UKSA (UK Space Agency). 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) 8) 9)
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, oil spill monitoring, maritime safety, and security of defence applications). 10)
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 £ 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. 11) 12) 13)
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. 14) 15)
Figure 1: NovaSAR-S spacecraft with payload antenna (left), component accommodation (center), and solar panel (right), image credit: SSTL, Airbus DS
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. 16)
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 <500 kg, a xenon propulsion system, and a design life of 7 years.
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. 15).
Figure 3: Photo of the NovaSAR platform during assembly and integration in November of 2014 (image credit: SSTL) 17)
Figure 4: Artist's rendition of the NovaSAR radar platform in orbit (image credit: SSTL)
• September 26, 2017: SSTL signed an agreement in Adelaide today at the IAC (International Astronautical Congress) to provide Australia's CSIRO (Commonwealth Scientific and Industrial Research Organisation) a 10% share of the tasking and data acquisition capabilities from NovaSAR-S, a first-in-class small radar satellite due for launch later this year. 18)
- NovaSAR-S is a technology demonstration mission designed to complement much larger, complex radar satellites with a smaller, lighter and more cost effective platform that delivers Earth observation Synthetic Aperture Radar imagery day and night, and through cloud cover. Managing the energy use on board the small SAR platform has been made possible by using a new, highly efficient S-band solid-state amplifier technology and flying an innovative S-band SAR payload developed by Airbus UK in Portsmouth .
- Speaking at the IAC in Adelaide, Luis Gomes, SSTL's Commercial Director said "We are delighted to be here in Adelaide for the IAC, and to see the Australian government's "Big Country, Big Sky, Big Ideas" slogan underpinned by today's announcement of our partnership with CSIRO on the NovaSAR-S mission. We anticipate that CSIRO will greatly enhance Australia's sovereign Earth observation capability with the addition of SAR data, particularly for this continent which has a tropical climate and a large coastline territory."
- Gomes also confirmed that further data shares on the mission are available, commenting "With the launch of the satellite due later this year, we have a number of discussions open with potential partners on the mission but we also welcome new partners looking to take a data and tasking share in NovaSAR-S."
- The agreement gives CSIRO tasking priorities and the ability to access the raw data directly from the satellite, and a license to use and share the data with other Australian companies and organizations over an initial 7 year period.
• The NovaSAR-S payload was delivered by Airbus DS UK to SSTL for platform integration in November 2015.
• The flight build commenced in January 2014 and was completed on schedule and on budget in July 2015.
• The Engineering Qualification program was completed in Q1 2014.
Launch: A launch of NovaSAR-S as a secondary payload is expected in late 2017 on a PSLV vehicle of ISRO.
Orbit: Sun-synchronous orbit, nominal altitude of 580 km.
Sensor complement: (S-SAR, AIS)
S-SAR (S-band Synthetic Aperture Radar):
The innovative S-SAR payload is being developed collaboratively by Airbus Defence & Space Ltd (formerly EADS Astrium Ltd.) of Portsmouth, UK, and SSTL, the payload activities are led by Airbus DS, UK. 19) 20) 21) 22) 23)
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: 24)
• 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.
Detailed trade-off analyses were performed to ascertain the performance of an S-band Phased Array Antenna of this architecture, with many phase-center configurations considered. The result is an innovative Front-End Architecture (low phase center count) with 18 x 100 W phase centers, arranged in a 3-column by 6-column configuration, all accommodated in a 3 m x 1 m panel.
Figure 5: Low phase center count antenna (image credit: Airbus DS)
The S-band antenna is a microstrip 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 TWTA (Traveling Wave Tube Amplifier) reflectors 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 (Gallium Mitride) technology 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 6).
This arrangement enables ScanSAR modes with up to around five subswaths with acceptable grating lobe performance. In fact, the use of the longer wavelength at S-band improves the grating lobe performance, enabling more elevation steering than would be possible with just six elevation phase centers at shorter wavelengths. With each phase center delivering around 100 Watts of peak RF power, GaN devices were readily available, and measures to avoid multipaction remained practicable. The total DC power demand was also kept within achievable limits for the small satellite platform with this arrangement.
Bandwidth: With an ITU regulation bandwidth limit of 200 MHz at S-band, the six-element subarrays are sufficiently short for this bandwidth to be readily achieved. However, in terms of power aperture product, the need for credible sensitivity performance limited the bandwidth used on NovaSAR-S to around 100 MHz, even though the payload can implement up to 200 MHz.
Antenna size considerations: Clearly a 3m2 antenna will be restricted in terms of access capability due to early range ambiguity degradation with increasing incidence angle. Indeed, an S-band SAR solution to access out to 50° incidence (single-polar) would typically need around 14m2 of antenna area. However with careful tuning of the processed Doppler bandwidth and the pulse repetition frequency, an antenna area of just 3m2 can deliver access out to 31° with acceptable ambiguity performance, achieving an access width of around 125 km for 20-31° incidence. Additional access width can be achieved (for compatible applications) by venturing below the traditional 20° incidence angle lower limit. Extending down to 16° incidence, for example, widens the access to 165 km.
It is important to note that the design of the front-end for NovaSAR-S provides a fully scalable solution, so that a larger antenna can easily be realized simply by building more of the same phase center units and configuring them with the required number of rows and columns. The performance and access will then improve accordingly.
Figure 6: Radiator units arranged in three columns of six phase centers (image credit: Airbus DS)
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.
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.
Maritime Mode: An additional, experimental, mode was also defined, known as Maritime Mode, which implements a novel low PRF mode for wide-area ship detection in the open ocean. This is expected to enable effective ship detection via deliberately azimuth ambiguous ScanSAR imaging over a swath more than 400 km wide in the 32-56° incidence angle range. Analysis of S-band airborne imagery of ships using the Airborne SAR Demonstrator developed at Airbus DS in Portsmouth has enabled tuning of the baseline mode design to maximize detection probability and swath width, while maintaining an acceptable false alarm rate of less than 1 image pixel in 107.
Polarimetric capability: Since the antenna is dual polarized (H and V), the instrument is capable of transmitting H or V pulses. To keep cost low, a single receive chain is implemented, but this can receive either H or V in any given echo window, enabling incoherent polarimetric imaging to be acquired. Sensitivity is compatible with primarily using HH and/or VV imaging, since signal to noise ratios will be limited in cross-polar operation for typical rural distributed scenes. However cross-polar operation is available and may also be useful in some circumstances, for example for detecting ships at low incidence angles where unwanted sea clutter will be heavily attenuated in cross polar imagery while the target cross section remains high.
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.
Table 3: Baseline operating modes with a single satellite and with a constellation of 3 satellites
Ship detection is one of the main SAR applications since the ships are usually built from large metal sheet and therefore detectable in SAR imagery; commonly a ship detection algorithm consists of three stages: pre-processing (land masking), detection (also known as prescreening) and discrimination. The flow-chart of the ship detection scheme is shown in Figure 7, where all the steps are highlighted and elements of novelty are introduced in each stage Ref. 25).
In the pre-processing, a land mask is obtained considering the different statistics between the sea and the land's backscattered field; the detection stage isolates the bright points over the sea background employing a CFAR (Constant False Alarm) method; while the ships are retrieved, in the discrimination step, by evaluating the scattering contributions of the possible targets detected in the previous stage.
Figure 7: Ship detection flow-chart. The rounded rectangles represents input and output data, while the rectangles are all the processes needed to perform the algorithm. The scheme is divided into three sections: pre-processing (masking), detection (CFAR) and discrimination (clustering, ambiguities removal and scattering evaluation), image credit: SSC, SSTL
Calibration: The architecture of the front-end subsystem is shown in Figure 8. Calibration paths have been included to enable characterization of the phase center distortions for replica generation, antenna beam pattern maintenance, and system diagnostics. The calibration scheme is based on the scheme developed for ASAR on ENVISAT, with a P1 path that includes the transmit electronics but bypasses the receive electronics, a P2 path that includes the receive electronics but bypasses the transmit electronics, and a P3 path that bypasses both the transmit and the receive electronics. The P1 and P2 paths each have an H and a V variant.
Figure 8: NovaSAR-S front-end architecture (image credit: Airbus DS)
Backend (central electronics): A separate, but directly relevant, development at Airbus DS in Portsmouth is the NIA (New Instrument Architecture) generic space radar central electronics. This equipment exploits the power and flexibility of the Xilinx Virtex 5 (XQR5V) FPGA (Field Programmable Gate Array). The XQR5V is the first high performance RAM based FPGA to integrate effective single event effect mitigation into its core architecture. This has created the opportunity to develop a truly generic backend solution that can easily be applied to a very wide range of space radar missions with minimal non-recurring cost. In addition, the power of the Virtex 5 enables functionality that would normally be implemented within a dedicated CPU module within the central electronics to be moved into the FPGA, making the CPU module potentially unnecessary. Missions which are able to omit the CPU module from the central electronics not only save the mass, power, cost and size of the CPU module itself, but also have a fully functional equipment that requires no flight software, which is a major cost saving factor. NovaSAR-S is an example of such a mission, and is fully functional without the need for a CPU module or any flight software. The NIA central electronics is compact, lightweight (10.9 kg on NovaSAR-S) and consumes just 46 W, the size of the instrument is 211 mm (H) x 265 mm (W) x 272 mm (D). NIA's first mission is NovaSAR-S. 26) 27) 28)
Figure 9: The NIA payload physical implementation for NovaSAR-S (image credit: Airbus DS)
Figure 10: Nominal architecture of NIA (image credit: Airbus DS)
S-SAR payload qualification campaigns:
An extensive development program 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. 29)
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 provided by COM DEV.
When the AIS instrument is being used to collect data simultaneously with SAR imagery of the same area, this will provide additional information on the identification of detected ships and highlight non-AIS transmitting vessels located in restricted areas. AIS is required to be carried for any ship over 300 tons, or for any passenger carrying ship, and the transmitted messages include unique identification, position, course, and speed data.
Note: The instrument will be described when information is available.
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18) "SSTL Announces NovaSAR-S Data Deal with Australia's CSIRO," SSTL Press Release, 26 Sept. 2017, URL: http://www.sstl.co.uk/Press/SSTL-
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27) Martin Cohen, Andrew Larkins, Phil Wason, "NIA SAR Central Electronics Product," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016
28) José Márquez-Martínez, Martin Cohen, Sam Doody, Pedro Lau-Semedo, Andrew Larkins, "Next Generation Low Cost Payloads: NovaSAR-S and beyond," Proceedings of IGARSS 2017 (IEEE International Geoscience and Remote Sensing Symposium), Fort Worth, Texas, USA, July 23–28, 2017
29) 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 (firstname.lastname@example.org).