10.0253/TUPRINTS-00009015
Schriftenreihe Fachrichtung Geodäsie der Technischen Universität Darmstadt
Model Improvement for SAR Altimetry
During the last decades Radar altimetry became an important tool to observe the sea surface with respect to sea level and sea state. Since the launch of CryoSat-2 a new technique called SAR altimetry improves the signal to noise ration of retrieved geophysical parameters and due to the smaller along track footprint a better performance in coastal zones is reached. In the last years several new processing modes were developed and tested to improve the understanding of this new technique such as reduced SAR (RDSAR), low resolution mode including range cell migration correction (LRMC) and fully focused SAR processing. The scope of this work is to improve the retrieval of geophysical parameters by developing a new stack and waveform model capable of including more general sea surface representations such as the geoid and introducing vertical wave particle motions. This new model will be included in different retracking schemes to investigate how the retrieved geophysical parameters change with respect to accuracy and precision.
It is shown that 20 Hz SAR processing, which is used in the official level two data products, results in an aliasing of the sea surface and that a surface sampling rate of 40 Hz will yield a lower pink noise floor and more precise retrieved parameters whereas the other two considered processing modes RDSAR and LRMC do not benefit from a higher sampling rate.
Another contribution of this work is the investigation of possible effects of sea surface slopes and curvatures on RDSAR, LRMC and unfocused SAR processing. It can be concluded that the impact of these quantities is small in RDSAR and SAR processing whereas in LRMC processing significant wave height biases up to three metre and sea level biases between minus twenty and plus twenty centimetres can occur if the surface slopes and curvatures are not considered correctly in the signal processing and parameter retrieval process. On the other hand LRMC is capable of estimating geophysical parameters consistent to the other two considered processing modes with a better precision as unfocused SAR.
In the last part it will be shown that the standard deviation of vertical wave particle velocities is an estimable parameter with a high correlation with respect to model and in-situ data. Additionally considering this geophysical parameter in the retracking can lead to consistent wave height estimations with RDSAR at high sea states which was an open issue as mean differences up to twenty centimetres occurred between these two processing modes. However, at small sea states the accuracy is not good due to differences in significant wave height up to one metre and sea level differences up to seven centimetres. This means that there is still a need for further improvements within zero to two metres significant wave heights which needs to be addressed in further studies.
Buchhaupt, Christopher
Christopher
Buchhaupt
CC-BY-NC-SA 4.0 International - Creative Commons, Attribution Non-commercial, Share-alike
2019
Thesis