Improved Range Ambiguity Performance in Quad-Pol SAR

Conventional quadrature-polarimetric (quad-pol) synthetic aperture radar (SAR) systems operating from space are severely constrained by their limited range of useful incident angles and their reduced swath widths particularly at larger incidence. These limitations are due primarily to relatively severe range ambiguities in the cross-polarized measurement channels. The conventional approach for quad-pol SAR systems uses linear polarizations on both transmit and receive. Range ambiguities can be markedly reduced by adopting hybrid-polarimetric architecture. In this approach, the radar transmits circularly polarized waves but receives on orthogonal linear polarizations. The sense of the circular polarization-left or right-is reversed on alternate transmissions. Hybrid-polarimetric quad-pol architecture leads to hardware that is more readily calibrated because neither receive channel is cross polarized with respect to the transmitted polarization; hence, their mean signal levels are the same. The data provided by a hybrid-polarimetric quad-pol SAR may be transformed into the conventional linearly polarized scattering matrix, thus preserving compatibility with the rich heritage of analysis tools developed for such radars.