Analysis of ASCAT-A transponder calibration and wind processing

The EUMETSAT transponder calibration effectively results in new ASCAT-A instrument gain values at each Wind Vector Cell (WVC) across the swath and for each of the fore, mid and aft beams. In this report we describe and evaluate level 1b (L1b) corrections to the operational L1b ASCAT backscatter data version 902 relative to 901. Version 902 is based on all three transponder calibration campaign carried out by EUMETSAT so far. Version 902 is now the reference radiometric calibration for Metop-A ASCAT and part of the re-processing of the ASCAT-A mission. Based on the EUMETSAT OSI SAF cone visualisation tools at KNMI and the NWP Ocean Calibration (NOC) procedure, calibration of the ASCAT-A scatterometer is checked. Indeed, after reverse corrections the ASCAT wind product based on L1b version 902 shows very similar characteristics to the ASCAT scatterometer wind product based on L1b version 901 and therefore meets the wind product requirements.
The operational OSI SAF Metop-A ASCAT level 2 wind product streams run at KNMI, using the validated ASCAT level 1b stream at full resolution and 25 km sampling as input, may be maintained without any significant effects on product quality. The new L1b σ0 stream will be corrected using the new (v902) linear scaling factors in the transformed z domain, which correspond to addition factors in the logarithmic domain (dB). These changes correspond to slightly resetting the ASCAT instrument gain per beam and per Wind Vector Cell (WVC) in order to maintain backscatter data consistency and wind product quality across the EUMETSAT L1b processing versions 901 and 902.