POST-EPS WIND SCATTEROMETER CONCEPT TRADE-OFFS AND WIND RETRIEVAL PERFORMANCE ASSESSMENT

The Post-EPS (or EPS-Second Generation) mission will be deployed in the 2019 – 2021 time frame in order to ensure continuity of the EUMETSAT Polar System (EPS) observation missions, currently realised with the MetOp satellite series, to support operational meteorology, in particular for numerical weather prediction (NWP), climate monitoring and to develop new environmental services. The Scatterometer (SCA) is one of the high-priority payload instruments to provide vector surface wind observations over the ocean, which constitutes an important input to the NWP as well as provides valuable information for tracking of extreme weather events. The Post-EPS SCA shall offer observations with higher spatial resolution than those provided by ASCAT on board MetOp. Furthermore, addition of HH polarization is considered as an option. Phase 0 industrial studies, addressing the complete system design, have taken place from 2008 to 2009. Two study teams, constituted respectively by Astrium SAS and Thales Alenia Space Italy, have performed comprehensive analyses of the system requirements, trade-offs of various concepts and preliminary design of the selected concepts, which included both the single and dual satellite configurations. Three distinct SCA concepts were considered for trade-offs: (1) Fixed fanbeam concept with 6 fixed antennas; (2) Rotating fan-beam concept with a single rotating antenna; (3) Rotating pencil-beam concept. The first two concepts were further elaborated during Phase 0,
and the fixed fan-beam concept was selected as baseline after a final trade-off. For supporting the above instrument concept elaboration by the industrial study teams during Phase 0, KNMI has developed retrieval algorithms tailored to the those concepts, derived from the ASCAT operational algorithms, and specific metrics to characterize the associated retrieval performance. The metrics used for the present performance assessment were: (a) Wind vector RMS error; (b) Ambiguity susceptibility, and; (c) Wind biases. The end-to-end performance evaluation makes use of an ensemble of wind-fields as input having the mean climatology distribution, generates the output wind-fields which account for the measurement system imperfections and geophysical noise, and compute the performance metrics for comparisons. This paper describes the three SCA concepts as analysed in Phase 0 studies by the industrial study teams and summarises the technical trade-offs carried out. The performance metrics are described and applied to two of the concepts in order to compare their respective merits. It is shown that both concepts are able to meet the observation requirements of Post-EPS.