Estimating the impact of the ionosphere on space-based SAR autofocus using GPS signals

Low-frequency space-based synthetic aperture radar (SAR) can suffer from the degrading effects of a scintillating ionosphere, which modulates both the phase and amplitude of the radar signal. In this paper, we use the received signal from GNSS receivers to simulate the effect of the ionosphere on a point target. The process for transforming the one-way GNSS signal, which contains a variety of signal biases and geometric factors, is described. Data recorded on Ascension Island during scintillation events is then used to determine the peak to sidelobe ratio (PSLR) that would be obtained from a SAR. A phase correction is applied to one GNSS receiver using another located along a magnetic east-west baseline. It is shown that this improves the SAR point spread function (PSF) and the variation with baseline distance is illustrated. It is concluded that, up to a baseline distance of at least 2305 m, the phase correction improves the focus.