The SAR train concept:: An along-track formation of SAR satellites for diluting the antenna area over N smaller satellites, while increasing performance by N

This concept implements the coherent combination of N separate SAR flying along the same orbital arc as seen from ground. A particular case is when the N SAR are in visibility with a single one that transmits. A "signal cleaning" class of trains keeps the antenna area requirement of each individual SAR unchanged 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. An "antenna dilution" class of train enables the distribution of an unchanged total antenna area into N smaller elementary antennas, together with multiplication by N of the SAR merit factor (swath over resolution ratio). With respect to the first class, 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 makes the concept more robust against loss 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 of being 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). (c) 2005 Elsevier Ltd. All rights reserved.