Non-iterative spotlight SAR autofocusing using a modified phase-gradient approach

The phase-gradient autofocus (PGA) technique is robust over a wide range of imagery and phase error functions but the convergence usually requires 4-6 iterations. The authors propose to speed up the estimation convergence by selectively increasing the pool of quality synchronisation sources and not to be limited by the range pixels of the SAR map. This is highly probable since each range bin contains more than 1 prominent scatterers across the integration aperture. It is also highly probable that the least brightest selected scatterer in a range gate may turn out to be of higher energy as compared to the maximum brightest scatterer of another gate. With appropriate target filtering to finally select the quality scatterers out of the large pool and higher-order phase error measurement tool, the new algorithm achieves near convergence quality without iteration. The authors named this solution the Quality PGA algorithm (QPGA)