On the Impact of Mispointing Error and Hamming Filtering on Altimeter Waveform Retracking and Skewness Retrieval

Abstract

Ground-based processing of radar altimeter signals has improved the accuracy of geophysical information retrieved from the shape of the waveforms over the ocean. Information about the nonlinear nature of ocean waves (wave skewness) can now be extracted (Gómez-Enri et al. 2006), but first estimates of wave skewness returned a large percentage of negative values, which have no physical meaning. Here, we investigate two possible causes of these negative skewness values using simulations of ocean waveforms for the Envisat RA-2 altimeter: the Hamming filter applied in the altimeter on-board processing and discrepancies between the real mispointing of the altimeter and the value assumed by the retracking algorithm. The Hamming filter strongly affects the retrieval of the parameters at low wave conditions, but it is not the responsible for the negative skewness. It is the second effect that plays a key role in the value of the skewness retrieved.

Keywords:

• Radar altimetry
• retracking
• attitude
• signal processing
• wave-skewness

Acknowledgements

This work has been done under the auspices of the European Space Agency in the frame of the Postdoctoral Research Fellowship Program and was partially funded by the Spanish authorities under their I+D research program (project code: CGL2004-01473/CLI). Pierre Féménias and Jérôme Benveniste, from ESRIN-ESA, supplied the SGDR records used in this work under a Category-1 Project lead by Graham Quartly (NOCS). Paolo Cipollini (NOCS) helped us understand the signal processing. We would also like to thank the EOHelp Team at ESRIN-ESA for their help in the distribution of the SGDR data.