Scatterometer data interpretation: measurement space and inversion.

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The geophysical interpretation of the radar measurements from the ERS-1 scatterometer, called sigma o, is considered. An important tool in the interpretation of the data is the visualization of the triplets of radar backscatter in measurement space. For a given position (or node) across the swath it is shown that the measured triplets of sigma o are distributed around a well-defined "conical" surface and hence that the signal largely depends on just two geophysical parameters, which can be taken to be wind speed and direction. In general, the scatter of triplets is comparable to the instrumental measurement noise of 0.2 dB, which corresponds to an uncertainty in vector wind of only 0.5 m s to power -1. In extreme meteorological conditions, a small number of anomalous triplets is found, but these can be identified by their distance from the conical surface and flagged or rejected by the authors' quality control procedure. The prelaunch transfer function developed by the European Space Agency (ESA), denoted CMOD2, is shown to give a poor repreesentation of the conical surface, with typical errors an order of magnitude larger than instrumental noise. Its sensitivity to both wind speed and direction needs revision in order to fit the backscatter characteristics, as quantified in this paper. A fourth-order ...

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