Abstract
Microsatellites are already playing a major role in operational optical-imaging remote sensing applications, viz. the Disaster Monitoring Constellation. However, such satellites are currently limited to providing useful images over cloud free regions during daylight. Whilst the use of microsatellite constellations does increase the probability of achieving useful images within short timescales over most regions, there are still areas, such as the tropics (that have significant cloud cover), or extreme latitudes (which can have low light levels), where the all weather, day/night imaging capability of Synthetic Aperture Radar (SAR) would be beneficial. Current SAR satellites typically have complex, large volume, large mass, and high power consumption payloads, that are expensive and typically take a few years to build. Usually they operate singly with relatively long repeat cycles. Recent missions have shown that SAR payloads can be flown on smaller satellites, however, these are not “low cost” and there are still significant size/cost barriers to being able to set up affordable SAR satellite constellations. This research analyses the theoretical aspects of designing a low cost SAR satellite based on an existing low-cost SSTL microsatellite platform (the SSTL-150). SAR payloads have numerous practical applications, each with its own set of requirements, so to provide a focus the applications of forestry and flood monitoring are selected to be the principal applications considered. The study concludes that a suitable low-cost SAR payload to meet these applications would operate in L-band, have dual polarisation, and have medium (30 m) resolution and medium (30 km x 30 km) swath. Such a payload can be accommodated on the SSTL-150 bus with a simple deployable parabolic mesh antenna. To further reduce the cost of the SAR payload, a low cost Frequency Modulated Continuous Wave (FMCW) SAR was selected, inspired by the use of such SARs on Unmanned Aerial Vehicles (UAVs). However, the research showed that the transmitter and receiver elements of the payload would require much greater isolation in the case of a satellite compared to a UAV, due to the much greater path loss involved with operation at space altitudes. This can only be achieved by placing the transmitter and receiver on separate platforms necessitating a bi-static configuration. This bi-static configuration would have the satellites separated along-track by a several km distance, but, given the altitude of the satellites (600-800km) the payload would still operate pseudo-monostatically as far as imaging/signal processing is concerned. The separate elements of the payload would require careful synchronisation, however, this research shows that such synchronisation can be achieved relatively easily using current technology. This leads to a novel low cost microsatellite SAR design, with potential to be extended to multi-static or interferometric SAR configurations.