New designs or modes for flexible space borne SAR

In a first step we recall the key constraints in spaceborne SAR and propose a new sizing typology for optimum trading around. The swath over azimuth resolution ratio, which is here called merit factor, has a strong impact on antenna area and power. A critical sizing is defined as giving the maximum merit factor at given antenna area. Then we introduce (or recall) different modes or designs for circumventing these constraints and we evaluate them with respect to the critical sizing reference. The DATS mode (doubling aperture by timing saturation) significantly reduces the power sensitivity to merit factor. The SETS mode (swath extension by timing saturation) doubles the swath at unchanged merit factor and antenna length and without the drawbacks of Scansar mode. Still at unchanged antenna area but at a price of some extra complexity in terms of antenna control (beam agility) or architecture (multi-antenna array), other modes improve the merit factor by up to 3.2. Echo tracking and steering are two possible modes for improving the merit on swath domain while DPCA (displacement phase centre) does it on resolution. To go further in merit factor and in reconfigurability needs radically different architectures with DBF (digital beam forming) possibly coupled with DPCA. We identify the limitations of all these modes/designs and particularly the matters of the timing and the nadir echo rejection. Rejection by beam notching is the only solution with standard SAR geometry while the vertical antenna geometry provides a natural nadir echo blocking and helps to reconcile antenna simplicity and reconfigurability. Finally we recall the ROSE concept (Radar Observation on Systematic and Economical way) that implements DBF (+ optionally DPCA) with vertical antenna and combines huge merit factor and design simplicity.