Airborne radar altimeter return waveform computations

Radar altimeter experiments have shown the need for expanding the downward-looking, single-beam system into a multiple beam radar altimeter employing off-nadir altimetry so that information from a wider swath can be obtained from a single overflight. Problems associated with off-nadir altimetry include the effects of pointing angle errors on the return information and difficulty in performing accurate range tracking. In order to understand these problems, investigation of the sensitivity of the average return waveforms to pointing errors is necessary. These waveforms are computed using a convolutional model, including the effects of asymmetric antenna patterns, which is representative of NASA’s Multimode Airborne Radar Altimeter. The necessary convolutions are most efficiently performed by a method that uses the fast Fourier transform. The modeled waveforms are then used to devise a method that provides an estimate the pointing angle.