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Ocean Surface Topography from Space
Development of Fully-Focused SAR Altimetry for Oceanographic Applications


Alejandro Egido - (University of Maryland, College Park)

This proposal is centered on developing critical SAR Altimetry aspects applicable to current and future altimetry missions such as, CryoSat-2, Sentinel-3 and Jason-CS/Sentinel-6. The ultimate goal of this project is to address key questions that still need to be answered to be able to optimize the use of this technology for oceanographic applications.

The work to be undertaken in this project leverages on and builds upon the proposing team’s expertise. Specifically, we have developed a novel data processing scheme to exploit both the intra-burst and inter-burst phase coherency of SAR Altimeters. Intra-burst phase coherency is widely known and routinely exploited by delay/Doppler altimeters, such as the CryoSat mission, and is planned for the Sentinel-3 and Jason-CS/Sentinel-6 missions as well. However, by accounting for the phase evolution of the scatterers in the scene, it is possible to correct the phase of the complex echoes along the aperture and focus for as long as the illumination time of a scatterer on the surface, thus reducing the achievable along-track resolution to the theoretical limit, equal to half the antenna length. We call this fully focused SAR (FF-SAR) altimetry. We have developed and demonstrated this technique with CryoSat-2 SAR mode data, but it is perfectly applicable to Sentinel-3 and Jason-CS/Sentinel-6.

The project will concentrate on three main aspects related to SAR Altimetry which are relevant for this particular NRA: Firstly, to determine an optimal coherent integration vs multi-looking strategy for SAR Altimeters; secondly, to develop an improved retracking methodology for partially specular surfaces; and thirdly, to develop a novel sea-sate bias model for SAR Altimetry. All these three key technical questions are of paramount importance to ensure the best possible estimation of geophysical parameters from SAR Altimeters and to guarantee seamless transition in the retrieval of sea surface height between the open-ocean and more complicated regions, such as the coastal areas and marginal ice zones. The project will conclude with the development of applications in the coastal zones, open ocean, and polar ocean. Finally, we will evaluate the performance of FF-SAR in comparison with conventional and delay/Doppler altimeters.

The work proposed here is therefore responsive to this NRA in the following solicited research topics: first and foremost, in providing expertise for the preparation of future altimetry missions, particularly Jason-CS/Sentinel-6; secondly, in providing a full characterization of sea-state bias for SAR altimeters; and finally, in providing support to complementary studies on coastal zones, and sea ice marginal zones.

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