# On-Board Wind Scatterometry
## 2 The On-Board Processing Environment and System  
The space-borne scatterometer is an implementation of real aperture radar, with capability of obtaining better accuracy and precision of the received power measurement [32]. These scatterometers transmit and receive the backscattered signals from the ocean surface. Received signals are detected and processed as digitalized power values, together with noise measurements and auxiliary data describing observing geometry and polarization; they are then transferred to ground stations [33; 34]. Wind products are obtained with computers distributed in institutes or agencies before being provided to application end-users. The proposed on-board environment integrates traditional observing and ground processing chains for wind products. Figure 1 illustrates both routines.  
Processing modules and hardware implementations are also shown in Figure 1. The functions required for the on-board processing system can be realized from the control system, data processing, high-speed input-output (I/O), data storing, and swapping modules. The control module receives and interprets orders to specific tasks and sends them to other modules. The I/O module provides  
Figure 1: Comparison of traditional and the proposed on-board processing routines.  
data interchange channels capable of meeting the speed of data observing and product generating. The swapping module manages data transmission flows between modules and processors within a module. These modules form the on-board processing environment. Digital signal processor (DSP), a field-programmable gate array (FPGA), and an embedded graphics processing unit (GPU) can be used to implement these modules. On an FPGA mother-board, together with two sets of Double-Data-Rate of version 3 (DDR3) Random Access Memory (RAM), FPGA chips realize I/O and control procedures. Data compression before transmission can also be done. Since the wind scatterometry requires multi-inputs and iterations in wind retrieval procedure, embedded GPUs or DSP are suitable choices to meet its requirements. In this research, an embedded Jetson TX2 GPU [35] was adopted for its better calculating ability and flexibility in applications. Together with FPGA chips that swap data with the GPU and the I/O module, the processing module was complemented, as illustrated in the middle part of Figure 2. Based on these settings, a specific on-board processor (SOBP) was designed and developed. The hardware scale of this SOBP is shown in Table 1. Its parameters indicate that it is suitable for satellite applications [36]. Results obtained on-board are downlinked in communication channels provided by communication satellites and broadband low Earth orbit (LEO) satellites [37]. For the validation of the on-board-scatterometer wind product generation algorithms proposed in this research, the simulation of the on-board environment was necessary. Figure 2 illustrates a simulated environment where the SOBP has the same set of parameters as described above. Differences with the actual on-board environment are for data input and product output only. Measurements with simultaneous auxiliary attributes of observations are provided by a personal computer (PC), while the result products are output to another instead of being downlinked for RT applications.