Performance Analysis of Autofocus Algorithms for Compensating Ionospheric Dispersion Effect on Spaceborne Low-Frequency SAR Focusing

Spaceborne low-frequency synthetic aperture radar with high resolution will be affected by ionospheric dispersion, which introduces a quadratic range phase error, leading to range defocusing. The dependence of the performance of contrast optimization (CO) autofocus and multilook registration (MLR) autofocus for dispersion compensation on the signal-to-clutter ratio (SCR) is analyzed. For SCR >20 dB, two autofocus algorithms have similar performances, and CO is a little better. For SCR < 20 dB, CO, and MLR performance will be affected by initial quadratic phase error (QPE) and clutter size, respectively. When the initial QPE is less than 530 degrees, and clutter size is at least 100 resolution cells, CO performance will be better than MLR. Taking 45 degrees QPE as the threshold for accurate focusing, in the case of the single point object, the threshold SCR for CO to be effective is approximately between 12.5 and 18 dB, corresponding to initial QPE between 0 degrees and 800 degrees. For MLR, the threshold SCR is approximately between 17.5 and 18.5 dB, corresponding to clutter size between 100 and 500 resolution cells. For the case of multiple point objects, the threshold SCR values of both algorithms will decrease. Simulations based on scenes derived from PALSAR2 data demonstrate the effectiveness of the CO and MLR autofocus algorithm.