# From GPS Receiver to GNSS Reflectometry Payload Development for the Triton Satellite Mission
## 1 Introduction  
Due to the omnipresence of global navigation satellite systems (GNSS) signals and their well-maintained infrastructure, many GNSS applications are widely explored. One of most common GNSS space applications is using the direct GNSS signal to provide the position, velocity, and time (PVT) information for orbit determination (Figure 1). The other two space applications are probing the atmospheric and ionospheric profiles with the bending angle and total electron content of the GNSS signal while passing through the atmosphere and ionosphere (GNSS radio occultation, GNSS-RO) and receiving the reflected GNSS signals to sense their surface properties (GNSS reflectometry, GNSS-R). At present, equipping a space-grade global positioning system (GPS) receiver is seen as indispensable for low Earth orbit (LEO) missions because of the convenience of autonomous onboard positioning and timing. The domestic industry in Taiwan (such as MediaTek Inc. and SkyTraq Technology, Inc.) has established a strong capability for designing and manufacturing civil GPS receiver (GPSR) modules/chipsets. However, such a civil GPSR module/chipset is not able to be applied to satellite missions due to the International Traffic in Arms Regulations (ITAR) [1]. In order to overcome export control, the National SpaceOrganization (NSPO) has the responsibility to play an important role in promoting the self-reliant technologies of the space industry so that the NSPO has developed the first space-grade GPSR in Taiwan (Figure 2) [2].  
The first GPS RO constellation mission is the FORMOSAT-3/COSMIC (simplified as FS-3 in the following descriptions) program, which is an international collaboration program between the NSPO and the University Corporation for Atmospheric Research (UCAR) of the U.S. The FORMOSAT-7/COSMIC-2 (simplified as FS-7) is the follow-up program (launched in 2019) to the successful FS-3 program (which was decommissioned in 2020) and is equipped with the upgraded RO mission payload, which itself is a collaboration between the NSPO and the National Oceanic and Atmospheric Administration (NOAA) of the U.S. Each satellite of the FS-7 constellation is equipped with a GNSS-RO payload to receive the GNSS signal from GPS, GLONASS, and Galileo satellites. The observed RO data is then downlinked to the ground station to retrieve and process into useful atmospheric and space weather data (such as temperature, pressure, water vapor content, electron density, etc.). The RO data distribution for 24 h coverage via FS-7 constellation is shown in Figure 3, which includes about 4500 soundings from GPS and GLONASS signals. Currently, the receiving ability of the Galileo signal is under development. Therefore, it greatly increases the amount of atmospheric and ionospheric observation data available at low latitudes, including in Taiwan [3]. In addition to the RO mission, the NSPO is aggressive in the development of new GNSS remote sensing technology so that the GNSS-R mission is selected for the Triton satellite. The main objective of the Triton program is to collect GNSS signals reflected off the Earth's surface from the LEO satellite [4]. The data could be used for research in several remote sensing domains, such as ocean surface wind [5], sea surface height [6], sea ice [7], vegetation [8], and soil moisture [9]. This could  
Figure 1: The space applications of global navigation satellite systems (GNSS).  
Figure 2: The first space-grade GPS receiver in Taiwan.  
be especially useful in retrieving wind speed over a sea surface for weather forecasting and typhoon path prediction, thereby benefitting people's livelihoods in Taiwan.