Strip-map Phase Gradient Autofocus

This paper describes a generalisation of the Phase Gradient Autofocus (PGA) algorithm that allows strip-map operation. A standard autofocus technique, PGA, uses prominent points within the target scene to estimate the point spread function of the system. PGA was developed for tomographic mode spotlight synthetic aperture radar (SAR) but has limited applicability for side-scan synthetic aperture operation. We show how it can be generalised to work with stripmap geometries and relate our new method to the previous PGA extension to strip-map systems. Strip-map PGA (SPGA) works by employing the (image domain) 2D phase correction technique. Using the scale transform of the imaging system, the 2D phase errors are related to aperture position errors. Robust phase error estimates are obtained by utilising range redundancies. We also present a modified Phase Curvature Autofocus (PCA) algorithm applicable to strip-map operation that also uses the scale transform. Preliminary results of the two algorithms are presented showing a similar performance, and a discussion of the limitations of the algorithms is given.