Detection of Objects Below Uneven Surfaces With a UAV-Based GPSAR

Airborne ground-penetrating synthetic aperture radar (GPSAR) using unmanned aerial vehicles (UAVs) offers advantages for many applications compared to established methods. In particular, these systems are useful for nontraversable terrain, where they could be used for the detection and localization of buried objects and structures. At distances of a few meters to the ground, refraction effects at the surface need to be considered in processing even for shallowly buried objects. Often the ground is assumed to be horizontal, which is not the case in many situations, such as uneven meadows or acres. This work investigates the influence of uneven ground on GPSAR images. It is shown that neglecting tilt in backprojection (BP) for shallowly buried targets leads to an apparent target shift. A model is derived to predict the shift for circular measurement trajectories and to estimate whether a loss of quality is to be expected. The model is verified using simulations, which are verified by measurement. It is concluded that tilt can be neglected in processing for shallowly buried targets. This is confirmed in a realistic measurement scenario.