RADARSAT SAR antenna RF design

The synthetic aperture radar (SAR) antenna on-board the Canadian remote sensing satellite, RADARSAT, is a large deployable planar array (1.5m × 15m) [1], [2]. It provides a set of selectable pseudo-trapezoidal elevation beam shapes (see figure 1–1) which afford flexible coverage within a 500 km wide accessibility region, which runs parallel to, and to the side of, the orbital plane of the spacecraft. In order to achieve the desired SAR image quality, the antenna is required to meet stringent gain requirements, while maintaining sufficiently low sidelobe levels, as well as sharp beam fall-off slopes, consistent with low SAR ambiguity ratios. The azimuth plane radiation pattern is a pencil beam with tightly controlled beamwidth to preserve the SAR along-track resolution. The required cross-track resolution determines the SAR antenna frequency bandwidth specification of ±10 MHz at 5.3 GHz, and up to ±16 MHz for finer resolution beams. Pulse distortion within the frequency band is controlled through phase and amplitude pulseerror specifications, and RF coherence over each SAR integration period, which is of the order of 1 second, is imposed by aperture synthesis error requirements on the antenna short-term stability. The design of the SAR antenna under these constraints constituted a challenge. Section 2 describes briefly the RF design and the major trade-offs behind the different design choices. Section 3 gives a brief overview of the design and analysis methods used to design the SAR antenna and predict its performance. Section 4 compares the performance predictions with measured data so as to validate the analysis and software tools.