# Validating Precise Orbit Determination from Satellite-Borne GPS Data of Haiyang-2D
###### Abstract  
Haiyang-2D (HY-2D) is the fourth satellite in the marine dynamic satellite series established by China. It was successfully launched on 19 May 2021, marking the era of the 3-satellite network in the marine dynamic environment satellite series of China. The satellite's precision orbit determination (POD) and validations are of great significance for ocean warning and marine altimetry missions. HY-2D is equipped with a laser reflector array (LRA), a satellite-borne Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) receiver, and a satellite-borne dual-frequency GPS receiver named HY2 that was independently developed in China. In this paper, the quality of GPS data collected by the HY2 is analyzed based on indicators such as the multipath effect, cycle slips, and data completeness. The results suggest that the receiver can be used in POD missions involving low-Earth-orbit (LEO) satellites. The precise orbits of HY-2D are determined by the reduced-dynamics (RD) method. Apart from POD, validation of orbit accuracy is another important task for LEO POD. Therefore, two external validation methods are proposed, including carrier differential validation using one GPS satellite and inter-satellite differential validation using two GPS satellites. These are based on space-borne carrier-phase data, and the GPS satellites used for POD validation do not participate in orbit determination. The results of SLR range validation cannot illustrate the orbit accuracy in \(\chi\), y, and \(\mathit{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{ \text{\text{\  
HY-2C form a large-scale global marine dynamic environment monitoring system that provides accurate marine dynamic environment information for early warning of marine disasters, sustainable development of marine resources, effective response to global climate change, and marine scientific research [1]. HY-2D adopts an orbit with a regression period of 10 days in the early stage and will changes to an orbit with a regression period of 400 days in the later stage. HY-2D is equipped with a laser reflector array (LRA), a satellite-borne Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) receiver, and a satellite-borne dual-frequency GPS receiver named HY2 that was independently developed in China. Due to the high speed and low orbit of low-Earth-orbit (LEO) satellites, observations of LEO satellites by ground tracking stations have the shortcomings of short continuous periods and unsustainable observation [2]. Therefore, one station has fewer observations of SLR and DORIS than that of GPS for LEO satellites. The DORIS system was successfully developed by the National Centre for Space Studies (CNES) and other institutions after nearly 10 years of hard work. It can provide high-precision orbital products in the full arc and is mainly used for precise satellite orbit determination and precise point positioning. DORIS observation has the advantages of a short time frame, all-weather operation, more data, and an even distribution of beacon stations. The satellite-borne GPS data have the characteristics of high accuracy, round-the-clock coverage, and continuity. Since [PERSON] et al., (1994) successfully applied the satellite-borne GPS receiver on the TOPEX/Poseidon satellite in 1992 [3], the satellite-borne GPS receiver has been installed on several LEO satellites and become a common technique for LEO satellite orbit determination [4; 5; 6; 7; 8]. At present, satellite-borne GPS data are used in precise orbit determination (POD) for Jason [9], SWARM [10], GOCE [11], GRACE [12], and CHAMP, and the GPS POD technique has achieved high-precision orbit results. Accuracy evaluation of orbits is an important task of LEO POD. The commonly used methods are mainly divided into internal validations and external validations. The internal validations mainly include carrier-phase residuals and overlapping orbit validation.