GLOBAL ANALYSIS OF THE ENVISAT RA-2 BURST ECHOES
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The unique capability of the RA-2 to return not only the 18 Hz averaged waveform data but also short segments of the full resolution 1800 Hz 'burst' echoes gives a valuable opportunity to study both the detailed response of different land and ocean surfaces, as well as the operation of the instrument. The first stage in utilising these data is to duplicate the onboard processing, applying the Fourier transforms and other engineering algorithms to transform the raw data into a useable product; this must then be co-located with the SGDR dataset to derive a useable product. This paper presents a global analysis of the behaviour and information content of these data, analysed and interpreted using a rule-based expert system, and illustrates some of the applications of this valuable and unique dataset in both geophysical applications and analysis of instrument behaviour. 1.INTRODUCTION The RA-2 has the ability to transmit to ground a small proportion of echoes at full 1800Hz resolution (one 1Hz packet every 180 seconds) [1]. This gives a dataset with extremely high along-track resolution. In order to use these data, the onboard processing must be duplicated and the reconstructed echoes co-located with SGDR data to obtain orbit and corrections data. In this paper we present sample results from a global analysis using the RAIES processor developed by SciSys [2,3]. The intent is to demonstrate the high information content of these echoes, which will shortly be made available by ESA to the user community [4]. 2. BURST ECHO ANALYSIS To illustrate the quality and utility of these data, examples are taken from one pass over Africa (Fig. 1) from cycle 33, passing from South to North across the continent. Here, the circles show the position of many burst echoes over Africa during this cycle; the plotted pass contains 7 bursts. For each burst, the top plot shown the 18 Hz waveform sequence, with each waveform plotted vertically and successive echoes stacked horizontally from left to right. The middle plot shows the equivalent 18Hz waveform data from the marked location, expanded out for comparison with the detailed burst, which is plotted in the lower part of the figures. The burst echoes have been retracked at full resolution using the EAPRS expert system [5] and the height plots are shown for each burst in the following sections. Figure 1 Burst echo locations from one cycle over Africa including bursts shown in the paper _____________________________________________________ Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)