Impact of 2-D Antenna Pattern on Radiometric Calibration of Azimuthal Multi-angle Observation Spaceborne SAR

Antenna pattern is essential for the radiometric calibration of synthetic aperture radar (SAR). For the conventional SAR with low-squint narrow beams, the azimuth antenna pattern and the elevation antenna pattern play separate roles in scattering measurements. In recent years, a novel spaceborne SAR imaging mode — azimuthal multi-angle observation — has been proposed based on the capability of beam steering. However, in the case of squint and high resolution, the antenna pattern becomes two-dimensional (2-D) inseparable along with the beam steering. Therefore, a spatially varying radiometric error is induced, which makes the calibration more complicated. This paper analyzed the impact of 2-D antenna pattern on radiometric calibration. First, the antenna pattern angles varying with the observation angles are analyzed. Then, an equivalent antenna pattern model for squint steering beam seen by an on-ground point scatterer is proposed. Finally, the relative radiometric error induced by the 2-D antenna pattern is assessed based on the integral method. The results show that the impact is related to the squint angle and azimuth resolution. The larger the squint angle, the higher the resolution, and the higher the impact of the 2-D antenna pattern on radiometric calibration. In addition, the beam steering needs to be controlled not only in azimuth but also in elevation. Otherwise, the radiometric error can increase several times.