# From GPS Receiver to GNSS Reflectometry Payload Development for the Triton Satellite Mission
## 2 GPS Receiver (GPSR) Design/Development  
Figure 5 shows the block diagram of the NSPO GPSR. The MAX2769 IC [11] is the main chip of the radio frequency front-end (RFFE), which is designed for GPS L1 signal conversion and digitalization. The unpacking and filtering of the digitalized RF signal, as well as the interface between the digital signal processor (DSP) and other onboard devices, are implemented into the field programmable gate array (FPGA) unit to reduce the computational load of the DSP. The main processor (the DSP) is in charge of the fast search algorithm engine, signal tracking, almanac/ephemeris decoding, raw measurement forming, navigation solution, orbit propagation, host command and message decoding, and user data exchange. It is noteworthy that one key feature of the common space-grade GPSRs is its capability to overcome the high Doppler shifts and Doppler shift rates of the high dynamic environment of LEO missions. Figure 6 depicts that the Doppler shift caused by a LEO  
Figure 4: Triton satellite configuration. Figure 5: The National Space Organization (NSPO) GPS receiver (GPSR) block diagram. satellite at an altitude of 550 km is about \(\pm\)40 kHz and the contact period is only around 40 min. Due to the implementation of the fast search algorithm, the cold start of the NSPO GPSR is within 90 s. It is faster than some of the space-grade GPS receivers, such as the SSTL SGR-07 [12], the New Space NGPS-01-422, and the Astrium MosaicGNSS single-frequency receiver. The key specifications of the NSPO GPSR are listed in Table 1. Two types of performance tests have been performed for the NSPO GPSR, with one being a terrestrial test and the other being a high dynamic environment test. In the 4-day terrestrial test, the 3D position and velocity errors of the static GPSR performance are (4.9576, 7.9232, and 5.3995) (m) and (0.0466, 0.0602, and 0.0396) (m/s), respectively. In order to verify the capability of the NSPO GPSR, the end-to-end test platform is required to reproduce the RF signal that the receiver will experience in the high dynamic environment. There are two approaches to reproduce the designated GPS signal via Spinet GPS simulator for the high dynamic tests. One is by using pre-loaded motion data files or pre-determined scenarios in the Spinet GPS simulator. The other approach is to construct an attitude control and LEO mission dynamic environment software simulator platform, which is implemented on the NI PXI real time system to generate a corresponding satellite attitude, which is fed into the Spinet GPS simulator in a real time manner [13]. The first high dynamic test was conducted with a predetermined scenario of the orbit of altitude set at 780 km and the inclination set at 45 deg. The position and velocity errors of the nine orbits navigation performance test were (1.7637, 1.6696, and 1.30) (m) and (0.0283, 0.0277, and 0.0239) (m/s), respectively. The average time to first fix (TTFF) of the NSPO GPSR cold start for 30 times with the same high dynamic scenario was about 66 s. The other high dynamic test was conducted with the satellite attitude at a rate of 1 deg/sec to validate the NSPO GPSR's tracking stability during satellite tumbling. The upper two sub-figures of Figure 7 show the position and velocity error, respectively. The third sub-figure depicts the number of tracked GPS signals (blue line) and the number of tracked GPS signals used for navigation (red line). The lowest sub-figure indicates the information concerning the attitude of the LEO satellite, which means that the satellite is tumbling. Hence, the NSPO GPSR maintains the navigation solution (after 150 s), even if the satellite is in a tumbling situation [14]. In addition to the performance test, the environmental tests are essential to verify that the NSPO GPSR is compliant with its specification and is ready to fly with LEO missions. The NSPO GPSR has passed the entire qualification test campaign, undergoing such tests as a thermal cycle  
Figure 6: Doppler shift simulation of a low Earth orbit (LEO) mission.