Exploiting Formation Flying for Earth Science: P-band Distributed Synthetic Aperture Radar

This paper deals with a spaceborne P-band mission based on an innovative distributed architecture. This mission was considered in the Phase A study for an Earth Observation Mission based on Satellite Formation funded by the Italian Space Agency (ASI). The study has been conducted under the coordination and leadership of Thales Alenia Space Italia (TAS-I), and CO.RI.S.T.A. has been in charge of assessing the scientific applications and defining the payload architecture. Low frequencies and P-band are widely considered of high interest from a scientific point of view, in particular for biosphere and bioclimatology studies, glaciology and geophysics. However, in spite of its scientific value, a spaceborne P-band radar poses significant technological challenges, which are mainly connected to the necessity to use huge antennas (order 100 m2) because of requirements on power and ambiguities. For example, full polarization (necessary to correct Faraday rotation) leads to double the PRF for given azimuth ambiguities. As a consequence, swath has to be reduced. A distributed SAR allows to overcome these constraints by exploiting the enhanced sampling capability of the system. Basically, it is made up by a number of cooperating antennas which are able to receive the radar signal sent by one of them and reflected by the Earth surface, so that global PRF depends on effective PRF and the number of satellites. The system is based on accurate positioning and synchronization among all the satellites which fly in formation. Moreover, each antenna is relatively small. While each antenna by itself is relatively useless, the combined processing of all the received signals leads to high observation performance. The paper gives an overview of the distributed system options and trade-offs, focusing on the payload related aspects. In particular, the next section is devoted to the scientific applications and the consequent performance requirements. Then, formation mission architecture is briefly presented, and details about the distributed SAR performance are provided. Different sensors architecture are traded-off against each other, in order to find a good compromise between number of satellites, antennas dimensions, global system performance. Finally, preliminary considerations on the possibility to combine biomass and ice sounding in a formation flying mission are presented