BISSAT: A Bistatic Radar for Earth Observation

This paper describes scientific rationale and technical approach for a spaceborne bistatic synthetic aperture radar (SAR) mission (BISSAT). The studied configuration is based on a small satellite, equipped with a receiving-only microwave system that catches the echoes of an already existing, main orbiting SAR, without requiring its design modification or additional operating complexities. This experiment has been proposed to the Italian Space Agency as candidate for the third Italian Small Scientific and Technological Mission and preliminary founded for a competitive phase A study. BISSAT experiment offer unique features: (a) to conduct original scientific experiments and to exploit novel applications; (b) to perform a technology demonstration of an original space formation; (c) to keep cost within a small mission budget. Bistatic scattering has proven to be of fundamental importance to many branches of earth sciences and several investigations have been conducted, but considering one or both earth-based antennas and targets of limited dimensions because bistatic observation requires accurate time synchronization and antenna pointing. As an example, bistatic echoes allows innovative characterization of vegetated and urban surfaces for biomass evaluation, or sea surface waves and current to be mapped. Proposed bistatic experiment will allow original scientific activities to be carried out for the first time from space, such as: (i) evaluation of bistatic radar cross section of natural and man-made targets, by means of multi-angle bistatic SAR observations; (ii) acquisition of terrain elevation and slope by means of range and bistatic scattering measurements; (iii) acquisition of velocity measurements thanks to the simultaneous measurement of two Doppler frequencies; (iv) stereogrammetric applications thanks to the large antenna separation involved; (v) improvement of image classification and pattern recognition procedures; (vi) high-resolution measurements of components of sea wave spectra; (vii) signal processing of bistatic data; (viii) across-track and along-track interferometry. Technical feasibility of the mission has been demonstrated, in particular taking advantage from Italian COSMO/SkyMed X-band SAR2000 experience, which has been selected as main mission. Development of bistatic receiver chain can be carried out with wide reuse of components and instruments already qualified within COSMO project. Furthermore, BISSAT will add value and exploit further scientific applications of COSMO mission. Ground segment and bistatic SAR processing require novel, although limited, implementations, again founded on well-assessed expertise. Regarding the platform, BISSAT mission will allow novel experience and potentialities to be fulfilled, such as: propulsion for formation flying and orbit maintenance, and fine attitude and pointing control. It is worth noting that formation flying is an internationally recognized important scenario.