# Validating Precise Orbit Determination from Satellite-Borne GPS Data of Haiyang-2D
## 3 Results and Discussion
### Orbit Determination Strategies and Analysis of Accuracy
#### 3.2.2 Carrier-Phase Residuals Analysis  
The carrier-phase residuals are an important indicator for testing the orbit determination method, reflecting the degree to which observations and the mechanical model are adapted to the actual situation [38]. The quality of the observations, the length of the POD arc, and the parameters to be estimated are all important factors influencing the carrier-phase residuals [39]. The carrier-phase residuals of RD orbits were calculated over 7 days starting from DOY 262 in 2021, and summaries are shown in Table 4.  
As shown in Table 4, the carrier-phase residuals fluctuated from \(-\)0.0601 to 0.0691 m, with more than 99.4% of the residuals distributed within \(\pm\)0.025 m, and the fluctuation was relatively gentle. The 7 day RMS values of the carrier-phase residuals fluctuated between 7 and 8.1 mm, and the fluctuation range was only 1.1 mm, which indicated that the strategy and dynamics model we used in POD were reliable and could determine RD orbits with high accuracy. The sequence of carrier-phase residuals depending on the elevation angle is plotted in Figure 2.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Data** & **Organization** & **Address** \\ \hline Satellite-borne GPS data & NSOAS & [[https://osdds.nsosa.org.cn](https://osdds.nsosa.org.cn)]([https://osdds.nsosa.org.cn](https://osdds.nsosa.org.cn)) (accessed on 15 March 2022) \\ GPS precise ephemeris & CODE & [[ftp://ftp.aitb.unibe.ch/CODE/2021](ftp://ftp.aitb.unibe.ch/CODE/2021)]([ftp://ftp.aitb.unibe.ch/CODE/2021](ftp://ftp.aitb.unibe.ch/CODE/2021)) (accessed on 15 March 2022) \\ GPS precise clock bias & CODE & [[ftp://ftp.aitb.unibe.ch/CODE/2021](ftp://ftp.aitb.unibe.ch/CODE/2021)]([ftp://ftp.aitb.unibe.ch/CODE/2021](ftp://ftp.aitb.unibe.ch/CODE/2021)) (accessed on 15 March 2022) \\ Earth rotation parameter & CODE & [[ftp://ftp.aitb.unibe.ch/CODE/2021](ftp://ftp.aitb.unibe.ch/CODE/2021)]([ftp://ftp.aitb.unibe.ch/CODE/2021](ftp://ftp.aitb.unibe.ch/CODE/2021)) (accessed on 15 March 2022) \\ SLR tracking data & ILRS & [[https://cddis.nasa.gov/archive/slr/data/npt_crd/hy2d/2021/](https://cddis.nasa.gov/archive/slr/data/npt_crd/hy2d/2021/)]([https://cddis.nasa.gov/archive/slr/data/npt_crd/hy2d/2021/](https://cddis.nasa.gov/archive/slr/data/npt_crd/hy2d/2021/)) (accessed on 15 March 2022) \\ DORIS data & ILS & [[ftp://doris.ign.fr/pub/doris/data/h2d/2021](ftp://doris.ign.fr/pub/doris/data/h2d/2021)]([ftp://doris.ign.fr/pub/doris/data/h2d/2021](ftp://doris.ign.fr/pub/doris/data/h2d/2021)) (accessed on 15 March 2022) \\ SLR station data & ILRS & [[https://ilrs.gsfc.nasa.gov/archive/slr/products/resource/](https://ilrs.gsfc.nasa.gov/archive/slr/products/resource/)]([https://ilrs.gsfc.nasa.gov/archive/slr/products/resource/](https://ilrs.gsfc.nasa.gov/archive/slr/products/resource/)) (accessed on 15 March 2022) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Sources of data.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Model/Parameters** & **Description** \\ \hline HY-2D satellite-borne GPS data & Sampling interval 1s \\ Global Gravity Feld Model & XGM2019 (120 \(\times\) 120 degree) \\ N-body & JPL DE405 \\ Relativity & IERS2010 XY \\ Solid-earth tides & TIDE2000 \\ Ocean Tides & FES2004 \\ Elevation cutoff & \(5^{\circ}\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Reduced-dynamic orbit determination strategy for HY-2D.  
As shown in Figure 2, the RMS of the residuals was 7.8 mm, indicating the high accuracy of the carrier observations of HY-2D. The carrier-phase residuals were large when the elevation angles were less than 20\({}^{\circ}\); the main reason for the phenomenon was that the receiver's ability to capture the signal was poor when the elevation angles were too low, resulting in poor quality of observations. Therefore, in the process of POD, the low-elevation angle data with poor quality could be deleted by setting the cut-off elevation angle. The cut-off elevation angle set in the process of orbit determination in this study was 5\({}^{\circ}\).