Inflight vertical antenna patterns for X-SAR from Amazon rain-forest observations

The full pattern for a large antenna is usually not well known because of the difficulty in measuring it on a range. For a spaceborne SAR, the antenna may also experience different gravitational stresses and temperature gradients that might deform its structure, so it is very likely that its pattern will be different from that on the ground. In these cases, one can extract the relative vertical antenna pattern from the SAR images of target areas that are uniform on average. In this note, images taken from Amazon rain forest region are analyzed to yield an estimate of the in-flight relative-gain vertical antenna pattern for the X-SAR. The method is the same as that Moore and Hemmat used with SIR-B and used for scatterometer antennas on Seasat. The authors give the vertical antenna pattern over an angular range of 8° for X-SAR. The two-way 6-dB elevation beamwidth of the X-SAR antenna is found to be 5.9°, which is fairly close to the 5.46° preflight beamwidth given by DLP, Deutsche Forschungsanstalt fur Luft-und Raumfahrt, but somewhat larger