# On-Board Wind Scatterometry
## 3 Wind Scatterometry Processing Chain and On-Board Processing Modifications
### On-Board Wind Inversion  
When the NRCS of slices are obtained, the WVC they belong to is determined before wind field inversion. WVCs are divided according to their predicted satellite routine. A 25-km sized grid is generally applied in wind products [34] and was adopted for this research. An LUT records WVC information that is then stored in an on-board storage device. This LUT is generated with predicted satellite and slice positions located on the Earth's surface. The inputs for searching WVCs with the NRCS of slices by using an LUT are satellite latitude, observing azimuthal angle, and track number. Output is the number of WVCs. When measurements are obtained on-board, the differences between the actual and the predicted observing geometry are generally less than a WVC lateral length. The differences are calculated; if they are not zero, the location of the slice is compared with the center of the WVC found from the LUT. If it is located outside the WVC, an adjacent WVC from the LUT is checked for this slice. Searching continues until the correct corresponding WVC number is obtained. Land and ocean marks of NRCS are also obtained during WVC locating.  
Besides slices, particularly for rotating fan-beam scatterometers, view divisions are generally applied before wind inversion. A view includes slices with similar observing geometries, and it can be defined as slices in a WVC that are obtained in the same rotation of the antenna. The NRCS, geometry, and Kp of a view are obtained by averaging those values of slices within it. For fan-beam scatterometers, the NRCS of views within a WVC, instead of slices, are used as inputs for wind inversion.  
The wind fields of a WVC can be inverted from GMFs in the MLE method. After MLE, the ambiguity removal step is required for the selection of wind directions as output results. In traditional wind inversion, circular medium filtering and two-dimensional variational ambiguity removal (2-DVAR) are optional methods [19; 42]. In this procedure, background information from NWP is used. For on-board processing, the renewal of NWP information can be achieved in a future up-link routine [29; 30]; for the simplicity of algorithms, in this research, circular medium filtering without NWP was used in wind direction inversion. The accuracy of this processing chain is evaluated in the following experiment section. Figure 4 illustrates the wind inversion steps developed in this research. To guarantee wind product generation speed, two steps were modified. First was in the MLE procedure; instead of the estimation of all solutions, the searching for maximum likelihood solution was divided into searching in coarse and finer steps. In general wind inversion, the searching step size is 0.2 m/s and 5\({}^{\circ}\) for wind speed and direction, respectively. The step for coarse searching in on-board wind inversion is 2 m/s and 30\({}^{\circ}\). Traditional step sizes are used for finer step searching. Second is that the GMF LUT used in on-board processing was interpolated into finer resolution. The resolution for incidence angle was 0.1\({}^{\circ}\), the wind direction was 0.5\({}^{\circ}\), and the wind speed was 0.1 m/s.