A New Retracking Algorithm for Retrieving Sea Ice Freeboard from CryoSat-2 Radar Altimeter Data during Winter–Spring Transition

Xiaoyi Shen,Changqing Ke,Meijie Liu,Markku Similä,Xi Zhang,Manman Wang,Wolfgang Dierking

doi:10.3390/rs11101194

Abstract

A new method called Bézier curve fitting (BCF) for approximating CryoSat-2 (CS-2) SAR-mode waveform is developed to optimize the retrieval of surface elevation of both sea ice and leads for the period of late winter/early spring. We found that the best results are achieved when the retracking points are fixed on positions at which the rise of the fitted Bézier curve reaches 70% of its peak in case of leads, and 50% in case of sea ice. In order to evaluate the proposed retracking algorithm, we compare it to other empirically-based methods currently reported in the literature, namely the threshold first-maximum retracker algorithm (TFMRA) and the European Space Agency (ESA) CS-2 in-depth Level-2 algorithm (L2I). The results of the retracking procedure for the different algorithms are validated using data of the Operation Ice Bridge (OIB) airborne mission. For two OIB campaign periods in March 2015 and April 2016, the mean absolute differences between freeboard values retrieved from CS-2 and OIB data were 9.22 and 7.79 cm when using the BCF method, 10.41 cm and 8.16 cm for TFMRA, and 10.01 cm and 8.42 cm for L2I. This suggests that the sea ice freeboard data can be obtained with a higher accuracy when using the proposed BCF method instead of the TFMRA or the CS-2 L2I algorithm.

Highlights

Sea ice thickness is an important environmental parameter, which is related to the ice mass balance and which controls the local and regional heat budget
We propose a new retracking correction based on Bézier curve fitting applied to the CS-2 echo waveform
In the study we presented here, a new method is introduced for the retracking correction of the Cryosat-2 radar waveform, which we denote ‘Bézier curve fitting (BCF)’ (Bézier Curve Fitting)

Summary

Introduction

Sea ice thickness is an important environmental parameter, which is related to the ice mass balance and which controls the local and regional heat budget. CryoSat-2 (CS-2), which was launched in April 2010, is equipped with a Ku-band Synthetic Aperture Interferometric Radar Altimeter (SIRAL) and offers much higher spatial resolution (~360 m along track) [17] and larger global coverage (up to 88◦N) than earlier space-borne altimetry missions. It is one of the most advanced satellite altimeters for sea ice observation, providing more detailed information about spatial and temporal ice thickness variations than hitherto available

Results

Discussion

Conclusion