Accommodation of space-variant effects in spaceborne SAR image formation

The spaceborne SAR geometry is space-variant due to the curved orbit and Earth´s rotation. A first effect is the azimuth variance, which in current missions is accommodated by processing the data in azimuth blocks larger than the synthetic aperture and updating the processing parameters of the kernel. However, such option is not valid when the variation occurs within the observation time, as it happens in high-resolution modes. A second effect occurs due to the topography dependence of the geometry, i.e., targets at the same zero-Doppler slant-range distance but different altitudes have slightly different range curvatures. This topography dependence is neglected in current SAR missions. However, it has been recently shown that in very high-resolution spaceborne SAR image formation (e.g., resolutions a factor 10 of the wavelength or smaller) the azimuth- and topography-dependence become critical. This paper analyses first these two effects and then proposes a processing scheme to efficiently handle the geometry. The suggested approach is validated with simulations and TerraSAR-X staring spotlight data.