# Accuracy Analysis of GNSS Hourly Ultra-Rapid Orbit and Clock Products from SHAO AC of iGMAS
## 3 Assessment and Analysis
### Accuracy Analysis of Different Methods for Orbit Prediction  
In order to access and analyze the performance of the 6H and 1H products, the orbit and clock products from another source with high accuracy should be chosen as a reference. The IGS provided final combined products, which are well known with high accuracy. Thus, the IGS final combined products can be used as the reference to access accuracy of the GPS orbit and clock and GLONASS orbit. However, the combined orbit and clock products for Galileo and BDS are not provided by IGS yet. Fortunately, several ACs provided the GNSS products for MGEX. The orbit and clock products from GFZ have been assessed and showed better performance [8; 14]. Thus, in this study, GFZ's orbit and clock products are used as the reference to access the accuracy of orbit and clock for Galileo and BDS, and for GLONASS, because the combined GLONASS clock products is not provided by IGS [27].  
[PERSON] et al. (2014) analyzed the impact of the one-day and three-day arc length on orbit accuracy. [PERSON] et al. (2019) assessed orbit accuracy of the two-day ultra-rapid solutions. [PERSON] et al. (2017) analyzed the accuracy of the 25-h solution. Their results show the long-arc solutions will improve the accuracy and disclosures of the orbit [8; 11; 28]. However, the long-arc solutions will increase time consumption, which means the products will not be  
Figure 4: Computational efficiency of 6H and 1H products for GPS/GLONASS/Galileo/BDS. The 6H and 1H are on behalf of 6-hourly and 1-hourly products, respectively.  
provided to users on time. Thus, a new method to generate the prediction session of the orbit is discussed in this section, before accessing the accuracy of 6H and 1H products.  
Obviously, the accuracy and the computational efficiency of the orbit and clock products is an important issue for users, which should be considered together, especially for ultra-rapid products. The new method is addressed for integrating the forecast orbit by the fitted orbit, from 24 h, and adjacent afore 24 h-orbits, which were determined by observation data values. The results are analyzed with respect to the old method, which integrated forecast orbits from only 24-h orbits.  
Figure 5 and Table 2 show the accuracy of the orbit for the new and old method for GPS/GLONASS/Galileo/BDS from day of the year (DOY) 041 st to 043 rd in the year 2019, with respect to IGS/MGEX products. We can see the accuracy of the 24-h observation orbit is close between new and old methods. The root mean square (RMS) of GPS prediction orbit in along, cross, and radial directions is from 3.5 cm to 4.4 cm for the 1 st-8 th prediction hour for the new method, which is improved from 9% to 32% versus the old method. The RMS of the GLONASS prediction orbit is from 5.7 cm to 7.1 cm for the new method, which is improved from 13% to 19% relative to the old method. The RMS of the Galileo prediction orbit is from 4.4 cm to 5.0 cm for the new method. The accuracy for the new method is improved from 12% to 67% relative to the old method, which shows an obvious improvement. Similar to Galileo, a distinct improvement can be seen from the BDS prediction orbit for the new method, especially for MEO and IGSO. The RMS is from 4.7 to 8 cm for the BDS MEO prediction orbit for the new method, which is improved from the 20% to 73% versus the old method. Moreover, the RMS is from 18 cm to 28 cm for prediction BDS IGSO for the new method, improved from 54% to 83% versus the old method. However, accuracy of BDS GEO prediction orbit for the new method is close to the old method. Thus, the results show the new method can improve the accuracy of the prediction GNSS orbit, especially for Galileo and BDS.