# From GPS Receiver to GNSS Reflectometry Payload Development for the Triton Satellite Mission
## 3 GNSS Reflectometry (GNSS-R) Mission Payload
### GNSS-R Payload Validation
In this experiment, the Hs observations of two buoys (Chimei (22\({}^{\circ}\)57\({}^{\prime}\)09\({}^{\prime\prime}\) \({}^{\prime\prime}\) N 119\({}^{\circ}\)31\({}^{\prime}\)06\({}^{\prime\prime}\) E) and Pratas (21\({}^{\circ}\)04\({}^{\prime}\)39\({}^{\prime\prime}\) N 118\({}^{\circ}\)50\({}^{\prime}\)50\({}^{\prime\prime}\) E)) were used. These buoys belong to the Center Weather Bureau (CWB), Taiwan. Moreover, a Drifter Technology Co., Ltd. buoy [26] was anchored at (22\({}^{\circ}\)55\({}^{\prime}\)29\({}^{\prime\prime}\) N 119\({}^{\circ}\)52\({}^{\prime}\)10\({}^{\prime\prime}\) E) to obtain more Hs observations. The diamond, triangle, and square in Figure 14 indicate the buoy positions of the Chimei, Pratas, and Drifter Technology's buoy, respectively. The comparison between Hs and maxDDM is shown in Figure 15 and the symbol indicators of the three buoys are the same as those in Figure 14. The maxDDM information comes from the DDM where the specular point (indicated with the asterisk symbol in Figure 14) is closest to the buoy position, and the red line is the linear regression result. The correlation coefficient between Hs and  
Figure 11: A unmanned aerial vehicle (UAV) flight test with GNSS-R EM. maxDDM is close to -0.99. According to the result, it is a well negative relation between Hs and maxDDM. This means that the maxDDM decreases as Hs increases, and a higher Hs indicates higher ocean surface roughness. When a signal is reflected by a higher roughness ocean surface, more signal power is scattered and the signal power received is lowered. Hence, it is shown that the quality of the data of the GNSS-R EM is satisfactory. Figure 12: (**a**) One snapshot of the four DDMs (**b**) Four corresponding specular point positions (red circle points) at the UAV flight path. Moreover, since the scattered/reflected GNSS signals for space application are extremely weak, a link budget analysis is performed and compared with real collected DDM  
Figure 14: The actual flight path of AIDC reflectometry experiment conducted on 27 July 2020. Figure 13: An Aerospace Industrial Development Corporation (AIDC) flight test with GNSS-R EM. Figure 15: Comparison between the Hs of a buoy and maxDDM (Chimei, Pratas, and Drifter Technology’s buoy are indicated with the diamond, triangle, and square). SNR (defined in Figure 16) to validate the design parameters, including antenna gain and coherent/non-coherent processing parameters. The theoretical model DDM could be represented by the following GPS bi-static radar equation:  
Figure 11.0  
where the detailed parameters in the equation are defined in [27]. Therefore, Figure 17 shows the flow chart of the theoretical model SNR calculation for link budget analysis [28]. In the upper part of Figure 17, the input parameters, the GNSS satellite position, and the GNSS-R receiver position are provided by the GNSS-R EM in order to calculate the reflection point. Then, some geo-physical parameters, such as wind speed and GPS equivalent isotropically radiated power (EIRP), are assumed to be at a reasonable value according to CWB broadcast wind speed and the GNSS interface control document (ICD). Moreover, some supplemental information (such as AIDC airplane/UAV attitude, antenna gain pattern, and LAN gain/noise figure look-up table) is provided to calculate the received power and thermal noise over each grid. Finally, each grid is accumulated and processed in a non-coherent integration to obtain the theoretical model DDM SNR. The comparison of 2020/09/03 airborne reflectometry experiment is illustrated in Figure 11.1 The trending and value are similar so that good confidence is provided for the signal reception of the Triton GNSS-R mission based on the link budget analysis method. However, the average error of the comparison is about 0.5 dB. The reason for this is that some uncertain parameters are difficult to obtain for link budget analysis (for example, real wind speed, GNSS-R receiver/LNA temperatures, etc.). Figure 11.2 A signal-to-noise ratio (SNR) calculation of DDM.