A Parameterized ASCAT Measurement Spatial Response Function

The Advanced SCATterometer (ASCAT) measurement spatial response function (SRF) relates the weighted contribution of every location within the measurement footprint to the measured normalized radar cross section σ°. The SRF results from a combination of the antenna response and the onboard processing and is computed during ground processing by modeling in detail the measurement geometry, as this is required for an accurate σ° estimation. However, the computed SRF is not disseminated as part of the L1B data. For some applications of the L1B data, the SRF is additionally required. For these applications, an approximate description of the SRF is often sufficiently accurate, estimated from information contained in the L1B data, rather than from a full calculation based on the measurement geometry. This paper describes a parameterized model of the ASCAT SRF for each measurement. First, an SRF reference estimate that incorporates details on the ASCAT design and onboard measurement processing is created. A parameterized model is fit to the reference estimate. The parameterized SRF is computationally less demanding than the reference estimate and as such more useful for near-real-time processing. The two estimates are validated with the computed SRF used in ground processing and with the transponder data from calibration campaigns. Finally, to validate the SRF in a simple application, the land fraction (a measure of land contamination in near-coastal ocean measurements) is computed and compared to actual data for a sample region.