The SAR train

Abstract

The concept implements the coherent combination of N separate SAR flying along a same orbital arc as seen from ground. A "Signal Cleaning" mode of SAR train keeps unchanged the antenna area requirement of each individual SAR and brings a factor N advantage that applies on SNR and ambiguity protection. The main formation flying constraint is the width of the tube containing the satellite trajectories. The multiplication by N of the total antenna area is the other counterpart to these advantages. A "Antenna Dilution" mode of SAR train enables the distribution of an unchanged total antenna area into N smaller elementary antennas, together with the multiplication by N of the SAR Merit Factor (Swath over Resolution ratio). With respect to the first mode, the tube width constraint is increased and the space-time separation along the track has to be very accurate. Use of appropriate spread spectrum waveforms instead of conventional pulse waveforms removes the major part of the extra orbit constraints introduced by the "antenna dilution" class. A train of N SAR in visibility with a single transmit makes the concept more robust against lost of coherence and eases the metrology of the formation (DGPS). Moreover the global energy efficiency is increased by N since with only a single transmit SAR the same performance is achieved. However, the along track separation constraints for antenna dilution are made more stringent because restrained to the space domain, which reinforces the spread spectrum interest. As part of its applications, the concept can circumvent the matter of huge antenna size for SAR mission in very low frequency (P band) or at high altitude (surveillance).