Abstract
Waveforms of radar altimeters are often corrupted due to heterogeneous sea surface roughness within footprints, such as slicks. In past studies, subwaveform retrackers such as the adaptive leading edge subwaveform retracker (ALES) which use only a section of the waveform have been proposed. However, it is difficult to choose a reasonable estimation window from an individual waveform. In the present study, a post-processed subwaveform retracker is proposed which identifies the waveforms of surrounding along-track points. The size of the estimation window is variable and is determined to keep the sea surface roughness within the corresponding footprint homogeneous. The method was applied to seven years of 20 Hz Jason-2 altimeter data over the slick-rich Sulawesi Sea of Indonesia and compared with ALES and sensor geophysical data record (SGDR) products. The standard deviation of the sea surface dynamic heights was around 0.13 m, even without spatial smoothing or some geophysical corrections. This is only 75% and 25% of the ALES and SGDR results, respectively. Moreover, all retrievals of the range, SWH, and sigma0 include less outliers than the other products. These results indicate that the variable estimation windows determined in the present study can adapt well to the variation of sea surface roughness.
Highlights
Radar altimeters transmit pulses towards the sea at a nadir and receive the echoes reflected from the sea surface within an altimeter footprint
The present study proposes a subwaveform retracker which considers the possible effects of various corruption sources through automatic adjustment of the length of the estimation window
A new post-processed subwaveform retracking method is proposed in the present study, which uses variable footprint sizes with homogeneous sea surface roughness in order to hold proper fitting conditions of the Brown model
Summary
Radar altimeters transmit pulses towards the sea at a nadir and receive the echoes reflected from the sea surface within an altimeter footprint. Several subwaveform retrackers have been proposed in recent years [7,8] These retrackers use only a portion of the waveform trailing edge close to the leading edge, which is equivalent to reducing the size of the altimeter footprint. Significant echoes from a point target, such as a calm sea surface in a semi-closed bay, result in a parabolic signature in the sequential along-track waveforms (hereafter, referred to as echograms). By masking these parabolic shapes, the SSH information in the mid-latitudes (near Taiwan and the Tsushima Islands, Japan) were successfully retracked. Different from the ALES retracker, the estimation window is no longer determined by a single waveform but depends on the footprint size with homogeneous sea surface roughness, using spatial restriction conditions in echograms
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