Synthetic aperture radar autofocus and calibration

Summary

Introduction Radar drone. Since the first post I have added a Gopro camera and a second GPS for redundancy. Earlier this year I made a polarimetric synthetic aperture radar (SAR) mounted on a drone. I have since worked a lot on the software side to improve the image quality and now the same hardware can generate much better quality images. The main contribution of this article is description of a new SAR autofocus algorithm that combines some of the existing SAR autofocus algorithms to make an algorithm that is well-suited for drone mounted SAR. Also antenna pattern normalization and polarimetric calibration suitable for drone mounted SAR with non-linear track is described. Radar signal SAR image formation geometry. While the drone flies along a track, the radar measures the distances and phases of the targets in the antenna beam. Neither azimuth (\(\varphi\)) nor elevation (\(\theta\)) angle of a target can be determined from a single measurement, and all targets at the same range will overlap in the measurement. However, by recording many radar measurements from multiple locations, a radar image can be formed. Magnitude of a single recorded range compressed radar signal. Depending on the radar parameters and radiated waveform, the signal processing to obtain a range-compressed signal from the raw recorded data might vary. After range compression, however, the image formation process is essentially the same. Range resolution is the ability to separate two closely spaced targets. Ideally it's calculated as \(\Delta r = c/2B\), where \(c\) is speed of light, and \(B\) is the bandwidth of the radiated waveform. For example, with a 150 MHz bandwidth the range resolution is 1 m. For a linear flight track this is also the image resolution in range direction. The phase of the received signal depends on the distance to the target. Because the signal travels to the target and back, a distance change equal to half the wavelength causes a full wavelength (360 degrees) shift i...