Research On CryoSat-2 Sea Ice Thickness Retrieval Method

Sea ice thickness is one of the most sensitive environmental factors to indicate climate change.It is also a key factor to control regional heat balance and to understand the climate change process.The use of satellite altimeter provides opportunities for large-scale sea ice thickness retrieval.In present,CryoSat-2(CS-2)carries the most advanced synthetic aperture radar altimeter,its spatial coverage and measurement accuracy are both superior to other traditional satellite altimeters.There are currently two problems in the sea ice thickness retrieval method using satellite altimeter.On the one hand,it is necessary to use the external ice type data to realize ice classification,which will bring uncertainty to the calculation of sea ice thickness.On the other hand,the common waveform retracking algorithms are less robust and inefficient,and thus an improved waveform retracking algorithm is urgently needed.Therefore,a new CS-2 sea ice thickness retrieval method is developed,including the sea ice classification algorithm which is based on echo waveform and the waveform retracking algorithm which is based on waveform fitting.In the aspect of sea ice classification algorithm,a sea ice classification algorithm which is based on CS-2 waveform is proposed in this paper.6 classifiers(convolutional neural network,Bayesian,K nearest-neighbor,support vector machine,random forest,and back propagation neural network)and 63 feture combinations which are consisted of 6 waveform features(leading-edge width,trailing-edge width,pulse peakiness,stack standard deviation,maximum of the returned power waveform and radar backscatter coefficient)are used.The results show that the best classification combination is the random forest with leading-edge width,the trailing-edge width,radar backscatter coefficient,maximum of the returned power waveform and pulse peakiness,and the overall classification accuracy is about 91.45%,which outperforms the other state-of-art algorithms by 9%.In terms of waveform retracking algorithm,a new waveform retracking algorithm which is based on Bezier curve fitting is proposed in this paper.A composite Bezier curve is used to fit the the CS-2 waveforms,retracking points are fixed on positions at which the rise reaches 70%of the Bezier curve peak in case of lead,and 50%in case of sea ice.The results show that retracking algorithm which is based on Bezier curve fitting can accurately correct tracking errors and then realize sea ice freeboard cauclation,the retrieval accuracy is higher than the commonly used Threshold-First-Maximum-Retracker-Algorithm and the European Space Agency CS-2 In-depth Level-2 algorithm.Based on the above-mentioned algorithms,the sea ice freeboard can be obtained.Two kinds of sea ice thickness calculation formula are deduced when sea ice freeboard is positive and not positive respectively,and then completing the Arctic sea ice thickness retrieval.The results show that the sea ice thickness retrieved by this method is much more accurate than other two commonly used methods'.In the future,we can apply this method in other satellite altimeter(HaiYang-2,Sentinel-3 and so on)to obtain long-time sea ice thickness observation data,and then study the variation of sea ice thickness and explore its relationship with the climate change.