Retracking Altimeter Waveforms Near the Coasts

There has been considerable interest in the past few years in addressing some of the long-term technical difficulties associated with retrieving valid measurements from satellite altimeters in coastal areas, where high levels of human activities are putting increasing demand for information about sea level, wind and wave conditions. Developments of altimeter waveform retracking techniques, together with the now-established practice of giving users access to altimeter waveform data, has led to rapid progress in our understanding of the challenges posed by waveform shapes in the vicinity of land. In this chapter, we present observational evidence of the huge diversity and complexity of waveforms seen by contemporary altimeters in coastal areas. We proceed with a review of waveform retracking methods, examining first empirical methods, then so-called physically-based methods, including discussion of some of their implementation intricacies. We proceed with providing examples of the application of waveform retracking methods to coastal altimeter waveforms in coastal regions around the world. Finally, we explore some of the new ideas on how it may be possible to exploit prior knowledge, for example about the statistics or the along-track evolution of ocean properties in the coastal domain, to improve the estimation of geophysical parameters. Innovative schemes, such as iterative retracking or simultaneous batch retracking, are discussed as new ways to yield unbiased parameter estimation for land-contaminated waveforms much closer to the land/water interface than is currently possible.