A phenomenon, referred to as differential synthetic aperture ladar (DSAL) image inclination, is reported. The phenomenon states that in a stably operating DSAL system, the high-resolution image generated by DSAL technique is possibly inclined in the range direction. The range-inclined DSAL image does not change its resolution but makes the DSAL image differ from the real target structure, resulting in a kind of geometric deformation. Theoretical analysis shows that this inclination phenomenon originates from the geometric setting of the two receiving subapertures in the DSAL hardware and is independent of the target distance. By properly setting the subapertures, this phenomenon could be minimized. Detailed DSAL experimental results verified the theoretical interpretation. Methods to correct the inclined DSAL image are suggested.
The performance of differential synthetic aperture ladar (DSAL) processing versus phase-gradient autofocus (PGA) technique in SAL image formation is demonstrated experimentally using phase history data (PHD) with large random phase errors and various target returns generated from a laboratory DSAL setup operating at 1550-nm wavelength. Results show that at strong return levels, both DSAL and PGA are very robust in focusing high-resolution images from the PHD; whereas, as target returns become weak and PHD is dominated by noise, the iterative PGA performs less effectively than the DSAL does. It is concluded that, on the whole, the DSAL technique is possibly more robust than PGA in both effectiveness and efficiencies.