Presenting Author: Ms. Julia Figa-Saldana

The Advanced Scatterometer (ASCAT) on METOP-A is real aperture, vertical polarisation, C band radar. It has been designed with the primary objective of providing near surface winds over the global oceans in all weather conditions. Winds are retrieved using a model from the primary measurement of the radar, the surface normalized radar cross section (sigma0), which is provided over two swaths of 550 km at both sides of the satellite ground track, at approximately 50 and 34 km resolution, on a 25 km and 12.5 km swath grid spacing, respectively. Three finely calibrated ground transponders located in Turkey provide a means of sending to the ASCAT point target signals with a precisely known cross section. As the position of the transponders is also very precisely known, this provides a fine sampling of the antenna gain patterns of all six beams. These data are used to fit an antenna gain model to achieve the absolute calibration of the radar. This paper describes the calibration process and the accuracy levels achieved. ASCAT is a successor instrument to the Active Microwave Instruments on ERS-1 and 2 and shares the same beam geometry, polarisation and almost the same radar frequency. Research using data from ERS-1 and 2 has resulted in a number of accurate backscatter models for ocean, sea ice and rainforest regions of the Earth. We describe how these models are used with ASCAT data to confirm the calibration levels and to monitor the instrument stability in between transponder calibration campaigns.

2025 International OVWST Meeting May 5th - 8th Darmstadt, Germany

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