Satellite radar altimetry waveform retracking over the Caspian Sea

Post-processing of complex altimetry-derived echo shapes is necessary over the Caspian Sea, in which the altimetry-inferred level fluctuations, derived from on-board ‘retrackers’, represent errors within a wide boundary close to the coastlines, i.e. up to ~10–30 km. Four different techniques were implemented to retrack 10 Hz radar waveforms of the TOPEX/Poseidon (T/P) mission and 20 Hz waveforms of its follow-on mission Jason-1. These include three previously introduced algorithms of the Off Center of Gravity, the standard NASA β-5, and Threshold Retracking, after some improvements. We also introduced a new method called ‘Extrema Retracking (ExtR)’, which works based on edge detection and extracting extremum points of the returned waveforms. Before implementing each technique, a two-step preprocessing was applied to exclude unwanted waveforms. Then, an unsupervised classification approach was applied to classify the remaining waveforms into multiple groups, according to their shape and geometrical properties. Finally, the waveforms of each group were post-processed using an appropriate retracking algorithm, and subsequently, the derived corrected ranges were converted to sea surface height (SSH) values. Our post-processed results were evaluated (1) by comparing the time series of SSHs with in situ tide gauge measurements as well as (2) by comparing the along-track retracked SSHs to their corresponding geoid profiles, computed from the EGM2008 model. For each evaluation, in situ measurements or geoid profiles were subtracted from the retracked SSH values to compute residuals. Thereafter, the mean and root mean squares of the residuals were calculated and interpreted, respectively, as a measure of bias and noise level of the processed SSHs. Our results indicated a reduction in noise and bias after the application of retrackers. Specifically, the bias and noise of the ExtR-derived SSH time series were decreased at locations close to the Anzali, Noshahr and Neka tide-gauge stations (along the southern coast). For example, the correlation between altimetry observations and tide gauge measurements close to the Anzali Port over the T/P era was increased from 0.31 (using original altimetry data) to 0.83 (using ExtR), which is equivalent to a noise reduction of 0.58 to 0.39 m. Comparisons of the along-track SSHs with geoid profiles indicated a significant mitigation of land contamination over the coastal areas.