Incidence Angle Optimization for Formation-Flying Across-Track SAR Interferometry

Single-pass across-track synthetic aperture radar (SAR) interferometry using formation flying is a well-established technique for the generation of high-quality digital elevation models (DEMs), as it avoids temporal decorrelation and allows for long baselines, thus leading to high height accuracy. A successful implementation, employed in the TanDEM-X mission, foresees the use of a Helix formation, which entails a reduced control effort to be maintained in the presence of external perturbations. However, an intrinsic limitation of the Helix concept lies in the inherently time-varying baseline and height of ambiguity (HoA), which results in nonhomogeneous DEM performance. This letter proposes an analytical method to minimize the HoA variability at the global level by optimizing the range of incidence angles used at each latitude. The approach is primarily aimed at systems, such as TanDEM-X, for which the area to be imaged becomes significantly smaller than the ground access range as the latitude increases. A two-step approach is presented: an initial approximate solution is derived in closed form and subsequently refined. The method is validated against numerical results and compared with simpler strategies involving fixed incidence angles, showing reduced variation of the HoA for different formation parameters. The procedure presented in this work could enable a better operation of current interferometric SAR systems and foster an improved design of future ones.