Maximizing the Intrinsic Precision of Radar Altimetric Measurements

This letter derives the best obtainable sea-surface height measurement precision for the general case of a partially coherent altimeter. Analogous to the Walsh upper bound on pulse repetition frequency for a conventional (noncoherent) altimeter, there is a lower bound on burst period for an advanced synthetic aperture radar (SAR)-mode altimeter that operates coherently on groups of radar returns. The optimal pulse repetition frequency falls between the Walsh upper bound and the Nyquist lower bound. Interleaved (open burst) operation is required. Postprocessing along-track resolution-open to the designer´s choice-is the principal performance-determining parameter. At the unfocused limit, the maximum range (height) precision for a Ku-band altimeter is on the order of 5 mm, which is relatively constant with altitude variations. Along-track resolution smaller than the unfocused limit implies better measurement precision but requires a more complex processing algorithm. An optimal design approach is suggested in which the altimeter operates in SAR mode and conventional mode simultaneously, thus capturing the most favorable measurement capabilities of either paradigm.