Abstract
Although the Brown mathematical model is the standard model for waveform retracking over open oceans, due to heterogeneous surface reflections within altimeter footprints, coastal waveforms usually deviate from open ocean waveform shapes and thus cannot be directly interpreted by the Brown model. Generally, the two primary sources of heterogeneous surface reflections are land surfaces and bright targets such as calm surface water. The former reduces echo power, while the latter often produces particularly strong echoes. In previous studies, sub-waveform retrackers, which use waveform samples collected from around leading edges in order to avoid trailing edge noise, have been recommended for coastal waveform retracking. In the present study, the peaky-type noise caused by fixed-point bright targets is explicitly detected and masked using the parabolic signature in the sequential along-track waveforms (or, azimuth-range echograms). Moreover, the power deficit of waveform trailing edges caused by weak land reflections is compensated for by estimating the ratio of sea surface area within each annular footprint in order to produce pseudo-homogeneous reflected waveforms suitable for the Brown model. Using this method, altimeter waveforms measured over the Tsushima Islands in Japan by the Ocean Surface Topography Mission (OSTM)/Jason-2 satellite are retracked. Our results show that both the correlation coefficient and root mean square difference between the derived sea surface height anomalies and tide gauge records retain similar values at the open ocean (0.9 and 20 cm) level, even in areas approaching 3 km from coastlines, which is considerably improved from the 10 km correlation coefficient limit of the conventional MLE4 retracker and the 7 km sub-waveform ALES retracker limit. These values, however, depend on the topography of the study areas because the approach distance limit increases (decreases) in areas with complicated (straight) coastlines.
Highlights
Radar altimeters transmit modulated chirp pulses towards the sea at nadir, and record the echoes reflected from the sea surface in an altimeter footprint [1]
The derived along-track sea surface height anomalies (SSHAs) are in noise land caused by bright targets through their parabolic signatures within an echogram is introduced validated by tide gauge measurements and compared with sensor geophysical data record (SGDR)
Geophysical parameters can be correctly estimated over open ocean areas
Summary
Radar altimeters transmit modulated chirp pulses towards the sea at nadir, and record the echoes reflected from the sea surface in an altimeter footprint [1]. Trailing edge post-processing using along-track waveforms because waveform noise at a given location can be expected noise can be explicitly determined based on its spatial relationship in the echogram. Trailing edge noise can be explicitly retrackers limit the analysis of waveform samples around the leading edge to avoid trailing edge determined based on its spatial relationship in the echogram. Sub-waveform retrackers limit the analysis of noise This is equivalent to limiting the altimeter footprint size near the nadir points where waveform samples around the conditions leading edge to avoid trailingeven edge noise.the. This approach assist in obtaining homogeneous seafootprints surface conditions, which is necessary which bright peaks were removed by comparing them with waveforms in the adjacent open water, to adopt the Brown model, by keeping the number of samples within the footprints constant.
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