# From GPS Receiver to GNSS Reflectometry Payload Development for the Triton Satellite Mission
## 3 GNSS Reflectometry (GNSS-R) Mission Payload  
### GNSS-R Payload Design  
The major task of the GNSS-R receiver is to receive and process reflected GNSS signals, after which the features on the surface of the earth can be retrieved. Therefore, the retrieval of the data concerning wind speed over the ocean is the main objective of the Triton mission. The design of the NSPO GNSS-R payload is based on several applicable GNSS-R processing options, which are discussed in [16]. By examining the needs and referring to the design of the SGR-ReSI (an existing GNSS-R receiver) [17], the GNSS-R payload is designed as a scatterometer, which includes the following requirements [18]. * The GNSS-R payload shall be able to process GPS L1 reflection signals and then generate the associated delay-Doppler map (DDM), either autonomously or by schedule. * The generated DDM resolution shall be at least 128 (in code phase)
* 64 (in frequency bin) and each entry of the DDM shall be better than 16 bits. * The GNSS-R payload shall be capable of processing at least four DDMs simultaneously. * The DDM update rate shall be at least 1 s.
* The GNSS-R payload should potentially be extended to process reflected Quasi-Zenith Satellite System (QZSS) [19] and Galileo L1 signals and generate DDMs of the aforementioned space, magnitude, and time resolution in real time. * The GNSS-R payload shall be able to record direct line-of-sight and reflected GPS L1/L2 band signals in raw data format at the intermediate frequency (IF) for ground post-processing purposes. * The GNSS-R payload shall include at least 512 M bytes of RAM to store the raw data. * The GNSS-R payload shall be designed with a process-and-record mode for ground debugging purpose. * The GNSS-R payload shall facilitate meta-data to support calibration and retrieval. The photo of the GNSS-R payload system is depicted in Figure 8. The zenith antenna is a commercial dual-frequency (L1 and L2) right-hand circularly polarized (RHCP) antenna for the reception of the direct line-of-sight signals from GNSS satellites. The nadir antenna is a high-gain dual-frequency left-hand circularly polarized (LHCP) antenna that is used to receive the reflected/scattered signals from GNSS satellites. The low noise amplifier (LNA), installed close to the antenna, is used to provide the appropriate amplification to enhance the signal-to-noise ratio (SNR). Both the nadir antenna and the LNA are designed and supplied by the cooperation of the NSPO and domestic industries. There are a RFFE with two RF inputs, a navigation unit, a science unit, and a power unit inside the GNSS-R receiver. The RFFE is in charge of signal conditioning, down-conversion, and analog-to-digital conversion. The incoming signals are sampled at 16.368 M samples per second, and each sample is represented as 4-bit data (2-bit in-phase/ 2-bit quadrature). The core technology of the navigation unit is based on the NSPO GPSR, and it can process the zenith antenna signal to provide PVT information and GNSS satellite position/velocity information. The timing information is used to synchronize the generation of DDM. The PV information and the GNSS satellite PV information are used to compute the specular reflection point. Moreover, the tracking information provided by the navigation unit (such as code phase, code phase rate, and Doppler frequency) is used to determine the relative code shift and Doppler shift for the DDM processing. The telemetry/telecommand (TM/TC) interface of the GNSS-R payload is RS422 in order to operate and monitor the GNSS-R payload. The output data of the science unit (which includes generated DDM and/or stored raw data) are transmitted to the Triton satellite bus through the SpaceWire interface. The transmitted pulse per second (PPS) signal is used for the purpose of synchronization. The power unit regulates the input power from the Triton satellite bus to distribute regulated voltage to the other units.