Waveform Retracking for Improving Level Estimations From TOPEX/Poseidon, Jason-1, and Jason-2 Altimetry Observations Over African Lakes

Estimating accurate water heights from complex radar return waveforms, as observed by satellite altimeters over inland water bodies, requires the application of postprocessing algorithms known as retrackers. This study introduces a new retracking algorithm, the Institute for Theoretical Geodesy Retracker (ITGR), which is applied to TOPEX/Poseidon (T/P), Jason-1, and Jason-2 waveforms over African lakes. The ITGR algorithm first analyzes the pattern of returned waveforms to identify retrackable waveforms. To provide range corrections, it then applies a maximum-likelihood estimator to a flexible waveform model, which is constructed based on the number of identified peaks and their position in the waveform. ITGR also adopts an adjustable peak model to deal with both symmetric and asymmetric shaped peaks. As a result, the introduced method exhibits some skill in mitigating the impact of peaks in the waveform pattern. We validated our retracked lake level heights (LLHs) over Lake Volta by comparing them to daily tide gauge and LLHs derived from various retracking algorithms, as well as to LLHs from the Global Reservoir and Lake Monitoring database. Our results over Lake Volta indicate that ITGR LLHs can be obtained with a standard deviation of 33 cm from T/P, 10 cm from Jason-1, and 8 cm from the Jason-2 altimeter; this represents an improved performance when compared to existing retrackers. Investigations over a land-contaminated region of Lake Victoria confirm the superior performance of subwaveform retrackers such as ITGR. Over Lake Naivasha, where mostly specular peak waveform patterns were observed, the performance of these retrackers was found similar to that of conventional threshold retrackers.