# From GPS Receiver to GNSS Reflectometry Payload Development for the Triton Satellite Mission
## 3 GNSS Reflectometry (GNSS-R) Mission Payload
### GNSS-R Payload Validation  
The prototype of the NSPO self-developed GNSS-R receiver was designed and implemented in 2016. Figure 9 shows the ground test configuration of the GNSS-R prototype, and the test location is on the bridge over the river mouth. There is a dual-frequency zenith antenna to receive the direct line-of-sight GNSS signals, and the nadir antenna (an LHCP 2 \(\times\) 2 pitch antenna) is set up to point to the surface of the river mouth to collect the reflected GNSS signals and generate the corresponding DDMs. Although several ground tests have been conducted to validate the prototype of the GNSS-R payload in the early development phase, two airborne tests have been performed with an Aerospace Industrial Development Corporation (AIDC) airplane at the end of 2016 to verify the functionality of the GNSS-R receiver [21; 22; 23; 24; 25]. Figure 10 depicts one snapshot of the aircraft position and four specular point positions (red points), as well as four corresponding DDMs (PRN6, PRN19, PRN17 and PRN 9), respectively. Each entry of the DDM (the correlation value) is indicated by the color as well as the x-axis and y-axis of the DDM are the Doppler frequency bin and the code delay, respectively. After that, the development processes of the engineering model (EM), the engineering qualification model (EQM), and the flight model (FM)  
Figure 8: The GNSS-R payload system. were completed in 2017, 2018, and 2019, respectively. The entire qualification/acceptance level environmental tests, including the thermal cycle test, the vibration test, the thermal vacuum test, and the electromagnetic compatibility (EMC) test, have been performed and passed as well. Owing to the flexibility of unmanned aerial vehicle (UAV) flight tests, several UAV flight tests with GNSS-R EM were performed during the EQM/FM development phase. The configuration of the UAV flight tests is depicted in Figure 11. Figure 12 shows one snapshot of the four DDMs, the corresponding specular point positions (red circle points), and the UAV flight path. Moreover, it is shown that the interference of the UAV engine  
Figure 10: One snapshot of the aircraft position, four specular point positions (red points) and four corresponding DDMs. Figure 9: Ground test with the GNSS-R prototype. is introduced into the DDMs. Hence, the objectives of the UAV flight tests were to verify the functionality, operation scheduling, status monitoring, and data science data volume validation of the NSPO GNSS-R receiver. Because there was no interference issue in the 2016 AIDC flight tests, the AIDC airplane was used to carry the GNSS-R EM to conduct the reflectometry experiments (Figure 13) in order to assess the data quality generated from the NSPO GNSS-R receiver. Three airborne reflectometry experiments were conducted on 2020/07/27, 2020/09/03, and 2020/11/03, respectively. In addition to the DDMs, the data collection of each reflectometry experiment includes the raw data for further post-processing. Figure 14 shows the actual flight path of the AIDC flight test on 27 July 2020. In this figure, the black line is the flight path of the AIDC airplane. In order to avoid the influence of the signal incidence angle and the different transmission power of different GPS satellites, the reflected signal of the QZSS satellite (PRN195) in the flight test is used for analysis. The colored line in Figure 14 is the specular point distribution of PRN195, and the color bar indicates the received power of the reflected signal from the area near the specular point, which is the maximum value of the DDMs (maxDDM). The significant wave height (Hs) observation from the buoy is used for comparison with the received power of the reflected signal.