The concept implements the coherent combination of the N SAR flying along a same arc as seen from ground. A particular case is with N SAR in visibility and a single transmit. A "Signal Cleaning" SAR Train keeps unchanged the minimum antenna area of each individual SAR and brings a factor N advantage on both SNR and ambiguity protection. The main formation constraint is the width of the tube containing the satellite trajectories on a earth frame. An "Antenna distributing" SAR Train enables the distribution of an unchanged total minimum antenna area into N smaller elementary antennas, together with the multiplication by N of the merit factor (swath over azimuth resolution ratio). Application for N> 2 requires very tight along track SAR spacing accuracy. Use of appropriate spread spectrum waveforms instead of conventional pulse waveforms removes the major part of the extra orbit constraints induced by "Antenna distributing" and enables it for any value of N. A Train of N SAR in visibility eases the metrology of the formation (DGPS) and the global energy efficiency is increased by N since with only a single transmit SAR the same performance is achieved. Moreover, the orbit constraints are relaxed by 2. As part of its applications, the concept can circumvent the matter of huge antenna size for SAR mission in P band or at very high altitude, all the more as in theses cases the tube width constraints are reduced (function of λ λ λ λ and altitude). I INTRODUCTIONIt is well known that antenna area and power are the major design constraints of a spaceborne SAR. Both limit the Swath over Resolution ratio that is the key performance criteria and that we call here merit factor.There is a particular SAR sizing called critical sizing [1] which maximises the achievable merit factor at a given antenna area. Merit factor at critical sizing is about 10000 for a maximum incidence in the range 50 ° to 60°. There are modes and design [2] for circumventing theses constraints although they cannot practically push the merit barrier by more than about a factor 3. We describe here (see [3] and [4] for more extensive developments) the concept of collaboration of N SAR with two cumulative purposes: -to dramatically push the practical limitation for power or antenna area by distributing them on several bodies -to remove the SAR merit factor barrier and to multiply by N the merit factor under a given total antenna area. The N SAR fly on the same orbital arc in an earth frame, which means that the SAR are generally on different orbit planes because of the earth rotation between the satellite fly-over. This time delay is supposed to be under the limits which cause the temporal decorrelation of the backscatter, the SAR are not necessarily in visibility although visibility enable multistatic configurations where a single one transmits and all receive. II A TRAIN OF N MONOSTATIC SAR A N element Antenna array model and basic advantagesWe use the representation of a synthetic antenna output as the addition along the orbital arc of S identical signal ...

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