Optimal Formation Flying for Single-Pass Multi-Baseline Across-Track Synthetic Aperture Radar Interferometry

Single-pass across-track Synthetic Aperture Radar (SAR) interferometry is a well-established technique for the generation of high-quality digital elevation models (DEMs) that strongly benefits from formation flying. In this context, the TanDEM-X mission of the German Aerospace Center (DLR) has generated the most accurate global DEM by employing a formation of two satellites flying with a time-variant baseline. The achievable accuracy of the DEM, however, is intrinsically limited by phase unwrapping, which poses a limit to the maximum adoptable across-track baseline. Moreover, the time-variant baseline represents a trade-off between height accuracy and robustness and phase unwrapping errors and propellant consumption. To tackle these challenges, this study proposes an optimized single-pass multi-baseline approach, based on a nested helix configuration that overcomes such limitations. We determine the optimal orbital parameters to produce a high-quality DEM with uniform height of ambiguity for all latitudes, while also ensuring passive safety for the relative trajectories. Formations of three or four satellites allow at least two desired baselines to be achieved at all latitudes, the smallest of which aids phase unwrapping. The resulting deviations in the height of ambiguity are about 7–10% and 2–3.5% for the three- and four-satellite formations, respectively. The proposed approach therefore paves the way to multi-baseline interferometric SAR missions for the generation of DEMs with considerably improved quality.