Improved sea level determination in the Arctic regions through development of tolerant altimetry retracking

Abstract

This PhD project involves the development of a suitable retracking strategy for processing ofCryosat-2 SAR (Synthetic Aperture Radar) altimetry waveforms in the Arctic Ocean. The Cryosat-2SAR altimetry waveforms are processed for precise and accurate SSH determination. Precise and accurate knowledge of SSH has various applications like gravity field determination, climate prediction, weather forecasting and studies of ocean currents and circulations.Cryosat-2 SAR altimetry waveforms in the Arctic can have a variety of shapes because of the superposition of the echoes from the water and the sea ice. Consequently, the waveforms are not well fitted with the existing physical retrackers and provide erroneous results with existing empirical retrackers like threshold retracker and OCOG (Offset Centre of Gravity) retracker.The research performed in this project is primarily divided in three segments. The first segment deals with the development of an improved and customized empirical retracker for the sea icecovered regions in the Arctic. The improved retrackers which are termed as primary peak empiricalretrackers work on just the primary peak of the waveform rather than the complete waveform. It is demonstrated through performance evaluation of the retracked SSHA, that the primary peak empirical retrackers demonstrate a more precise SSHA as compared to empirical retrackers like threshold retracker and OCOG retracker. Retracker performance evaluation is done for the primary peak threshold retracker, primary peak COG retracker, threshold retracker, OCOG retracker and the ESA retracker. For retracker performance evaluation, the standard deviation (STD) of the 1 HzSSHA is computed and a lower value of this STD indicates a more precisely retracked SSHA.The second segment deals with the customized application of physical retracking to Cryosat-2 SAR altimetry waveforms available in the Arctic Ocean. The SAMOSA3-C has been applied in the Arctic and is a combination of two modes – SAMOSA3-O mode and SAMOSA3-L Mode. The two modes deal with SSHA determination from the ocean type waveforms and the lead type waveforms. It is demonstrated that the SAMOSA3-C retracker exhibits a more precise SSHA as compared tothe primary peak retrackers and the ESA retracker. The accuracy evaluation of the retrackers showsthat the SAMOSA3-O retracker has the best accuracy when compared to tide gauge data in the Arctic. Combination of the physical and empirical retrackers is attempted in order to get the advantages of both the retrackers. The third segment deals with the combination of the physical (SAMOSA3-C)retracker and primary peak COG (empirical) retracker. It has the advantage of high precision fromthe SAMOSA3-C retracker. It also has the advantage of primary peak COG retracker with capability of estimating SSH in the sea ice areas where irregular type waveforms are present, which are neither lead type nor ocean type. Prior to combining the physical and empirical retracking, bias is removed and the primary peak COG retrackers SSHA values are corrected with the estimated bias. The removal of bias between the physical and empirical retrackers is a complex procedure. Various bias removal methods were tried and the best approach has been presented in this thesis.The combined physical empirical retracker results in a better precision than the primary peakretrackers and has a larger dataset of the estimated SSHA as compared to the SAMOSA3-Cretracker with inclusion of the SSHA in sea ice areas.