Development and Implementation of the Hyperbolic Pretracker

Abstract

This report details the progress made on the design and implementation of a hyperbolic pretracker specifically for the Envisat RA-2 altimeter. First considerable care was taken to construct a waveform simulator whose characteristics matched those of the instrument — specifically, this meant some tuning to get leading edge slope and position of half-power point in exact agreement with the RA-2 altimeter. This simulator was used to generate
waveforms as the virtual instrument overflies a small rectangular patch of enhanced variability (a "bright target") possibly representing glassy seas. From a number of such (noise-free) simulations spanning the narrow altimeter swath it was possible to use mathematical inversion techniques to produce a set of weights for estimating intensities of such hyperbolae, and then remove these features from the 2-D waveform space. This is demonstrated in a number of worked examples (datasets provided for validation). A key aspect of the technique is that it can equally well be used to compensate for weak targets (rain cells or absence of ocean due to land); this is particularly effective in the transits of Pianosa. This technique is much better at coping with multiple discrete targets than an approach that treats each waveform in isolation thus neglecting the contextual information from neighbouring waveforms. The implementation with real RA-2 altimeter data is still problematic, because there are movements in the tracker window, which can still be important at the sub-binwidth scale, yet are hard to correct for. A further challenge is estimating the intensity of hyperbolic features when not all of the feature is present in the waveform anomalies e.g. because of
data over land generated using a different chirp bandwidth.

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