Dual Carrier Probing for Spaceborne SAR Imaging

Spaceborne imaging of the Earth’s surface by synthetic aperture radar (SAR) may be adversely affected by the ionosphere that causes distortions of the signals emitted and received by the radar antenna. In our previous publication on the subject [SIAM J. Imaging Sci., 2 (2009) pp. 140–182], we have analyzed those distortions for the inhomogeneous ionosphere described by the cold plasma model. Based on the analysis, we have concluded that the deterioration of SAR images was due to the mismatch between certain parameters of the actual received signal, which is slowed down by the temporal dispersion in the ionosphere, and the corresponding parameters of the matched filter, which is taken as if the propagation were unobstructed. Consequently, to improve the quality of the images, the filter must be corrected. However, to get the appropriate correction, one needs to know some key characteristics of the ionosphere precisely at the time and place the image is taken. To obtain those characteristics, we currently propose probing the terrain, and hence the ionosphere, on two distinct carrier frequencies. We also investigate the performance of the matched filters that were corrected this way and show that the final quality of the images, i.e., their resolution and sharpness evaluated using the SAR ambiguity theory, indeed improves.