INVESTIGATION ON CIRCULAR MAPPING BY FMCW-SAR ON SMALL AIRPLANES
The evaluation of local damages after natural disasters by using remote sensing demands for flexible platforms as well as sensor systems, which guarantee both weather- and daylight-independence. Due to the fact of small energy consumption, small size and light weight millimeter-wave FMCW radar sensors on small airplanes are very promising for this task. Especially in urban environments the side looking SAR geometry causes shadowing and foreshortening effects, which lead to gaps in the reconstructed scene and misinterpretation. Multiple views from different aspect angles can reduce the shadowing effects but especially in unknown areas the best line of sight cannot be investigated in advance. This is achievable by perfoming circular flight trajectories. However, as millimeter-wave FMCW radar sensors typically have very narrow beams and small airplanes are sensitive to air turbulences, a beam stabilization method is necessary to provide full illumination of the target scene. In this paper the calculations for system specific optimal flight parameters are demonstrated and validated by simulations. The impact of air turbulences, causing angular deviations to the sensor and thus misplacement of the main beam lobe on the ground are explored by experimental data performed with our SUMATRA system. The effects of a potential mechanical beam stabilization are visualized and requirements for such a system are formulated. Our experiments show that for typical flight conditions a stabilized platform is well suitable to stabilize a narrow radar beam in order to keep a target scene constantly illuminated over a full circular trajectory. Typically these stabilized platforms can handle angular corrections in all three geometries (pitch, roll, squint) of up to 12 °−15 ° by a speed of 15 ° per second. Therefore a more cost intensive full gimbal system which is known to be used in optical applications and which can handle a full 360 ° tracking is not neccessarily needed.