Producing High-Quality Digital Elevation Models from Ultra-Wideband Drone-Borne SAR Interferometry

Across-track synthetic aperture radar (SAR) interferometry (InSAR) is a remote sensing technique that exploits the phase difference between two complex SAR images to generate digital elevation models (DEMs), which are essential for monitoring the Earth's surface. DEMs have been obtained on a large scale from space- and airborne InSAR systems, which have, however, stringent bandwidth regulations, constrained revisit times, and costly deployment and operation. A complementary approach is addressed in this work and consists of obtaining DEMs from SAR images acquired by drone-borne radars. It enables the surveillance of local areas with unprecedented resolution and height accuracy by exploiting wider range bandwidths and larger baselines with respect to the flight height. However, due to low flight altitudes and stability issues, DEM generation presents significant challenges, especially in the case of SAR images acquired from antennas located on distinct platforms or from the same platform at different times. We propose a SAR processing scheme based on the omega-k algorithm that allows fast processing of the drone-based raw SAR data and an InSAR algorithm that takes into consideration the wide bandwidth of the system and exploits it to support phase unwrapping using radargrammetry. The system capabilities are demonstrated through experimental acquisitions over flat and hilly areas, which resulted in DEMs with a height accuracy of a decimeter at a posting of 25 cm x 25 cm. The results obtained show that drone-based InSAR is a promising technology for very frequent and cost-effective monitoring of local areas.