A Solution to Conflicting User Requirements Based on Simultaneous Single-/Dual- and Quad-Pol SAR Imaging

Synthetic aperture radar (SAR) concepts based on digital beamforming (DBF) in elevation and multiple receive beams, e.g., staggered SAR, allow high-resolution imaging of wide swaths and are therefore well suited for the systematic observation of dynamic processes on the Earth’s surface. While these concepts are compatible with quad-polarimetric (quad-pol) operation, due to severe ambiguity constraints, the achievable swath in quad-pol mode is narrower than in single- (or dual-) pol mode. Therefore, either single- (or dual-) pol data over a wider swath or quad-pol data over a narrower swath can be acquired. This paper discusses a novel SAR acquisition mode, based on pulse-to-pulse alternation of the antenna pattern on transmit, that delivers at the same time single- (or dual-) pol data over a wider swath and quad-pol data over a narrower swath at the expense of an acceptable degradation of the ambiguity performance. The proposed mode for simultaneous single-/dual- and quad-pol SAR imaging bears great potential for the design and operation of future SAR missions, where it is well suited to resolve conflicts among different user groups with different acquisition needs. For illustration, one may consider the contradicting requirements for the deformation mode and the 3 D structure mode in Tandem-L. On the one hand, the deformation mode asks for wide-swath acquisitions from every ascending and descending orbit to provide 3-D vector deformation measurements with unprecedented accuracy by minimizing the errors from temporal decorrelation as well as tropospheric and ionospheric disturbances. On the other hand, the 3-D structure mode needs less frequent acquisitions, but asks instead for full polarization to resolve and discriminate different scattering mechanisms in vegetated areas and to improve biomass estimates. As the regions to be acquired in the deformation and 3 D structure mode are characterized by a substantial degree of mutual overlap, the current observation concept had to find a compromise among the different user requests, accepting a somewhat reduced quality of the higher level products. Such a performance loss can be avoided with the novel proposed SAR mode with only a minor increase of the SAR system complexity. It should be evident that the novel technique for simultaneous single-/dual- and quad-pol SAR imaging introduced in this paper also bears great potential for the design and operation of several other future SAR systems and missions like the next generation of Sentinel-1, where it represents an effective solution to acquire fully polarimetric SAR images without disturbing the request for frequent wide-swath monitoring in regular time intervals. This paper can therefore be regarded as an important step towards the development of a new generation of hybrid SAR systems, which have the capability to acquire SAR data in multiple imaging modes and resolutions within one and the same data take, thereby maximizing the information that can be gained for different Earth observation applications under the constraint of a limited power and downlink budget