The RA-2 individual echoes processing description and some scientific results

Abstract

The RA-2 in its nominal operation provides averaged waveforms at the rate of 18 Hz (one averaged waveform over 100 individual echoes, every 0.0557 seconds). It has also the capability to provide limited bursts of individual, unaveraged echo sample data in phase (I) and quadrature (Q), at the full PRF rate. In this concept the full-rate data are stored, for a short burst, into an internal buffer memory, in parallel to the normal averaging and other functions of the instrument. The buffered data are subsequently read out at a much lower rate and appended to the normal science data. These Individual Echoes (IE) are, therefore, not processed onboard in the same way that the nominal RA-2 waveforms are.Recent studies have demonstrated that through the full rate data it is possible to discover some behaviour than cannot be seen with the averaged data. Moreover, it is the first time in altimetry that we have echoes that contain the information of the phase. This is a great potential for new science studies. This paper describes the algorithm applied on-ground to the IE of the RA-2 Burst Waveforms to reproduce the same process done by the instrument on-board (except for the averaging). Once this algorithm is applied to the IE they will be in the same condition than the normal RA-2 telemetred average waveform, but at 100 time higher surface sampling and with the phase information. The final objective of this work is the use of the IE fully processed and instrument calibrated for calibration, validation and science exploitation purposes. We will present results of studies carried out using these IE. The blurring on the averaged waveforms depends on the total movement of the range window [1], which in turn depends on slope of the terrain, the orbit slope and ultimately, how well the on-board tracker tracks that particular waveform shape. In early studies using ERS data, the blurring on the averaged waveforms has been estimated by simulating the ERS range window movement during tracking. Using individual echoes there is no longer the need to do so, we can directly use these echoes. The IE will be averaged in the correct way and compared to the averaged waveform provided in the nominal RA-2 product. Changes on the retracked epoch and slope of the leading edge can be assessed for different type of waveforms over different surfaces. In particular we will present results of the analysis of the behaviour of IE over the "Salar D'Uyuni" in Bolivia, to better understand biases in retracking of specular echoes. The results can be used to improve the current understanding of retracked elevations over sea ice and help to improve tuning of current sea ice retracking schemes for the EnviSat RA-2 instrument. ESA has run a study on this topic to seed the use of individual echoes by scientists. This study is completed and reconstructed echoes will be made available for the first time to the scientific community. Final results from the technical and scientific application of individual echoes and S band data are described in [3].