The delay/Doppler radar altimeter

The key innovation in the delay/Doppler radar altimeter is delay compensation, analogous to range curvature correction in a burst-mode synthetic aperture radar (SAR). Following delay compensation, height estimates are sorted by Doppler frequency, and integrated in parallel. More equivalent looks are accumulated than in a conventional altimeter. The relatively small along-track footprint size is a constant of the system, typically on the order of 250 m for a Ku-band altimeter. The flat-surface response is an impulse rather than the more familiar step function produced by conventional satellite radar altimeters. The radar equation for the delay/Doppler radar altimeter has an h^-5/2(CT)^1/2 dependence on height h and compressed pulse length τ, which is more efficient than the corresponding h³CT factor for a pulse-limited altimeter. The radiometric response obtained by the new approach would be 10 dB stronger than that of the TOPEX/Poseidon altimeter, for example, if the same hardware were used in the delay/Doppler altimeter mode. This new technique leads to a smaller instrument that requires less power, yet performs better than a conventional radar altimeter. The concept represents a new generation of altimeter for Earth observation, with particular suitability for coastal ocean regions and polar ice sheets as well as open oceans