# Precise Orbit Determination for GNSS Maneuvering Satellite with the Constraint of a Predicted Clock
## 5 Conclusions  
The clock-constrained RPPP method is proposed for POD of GNSS satellites undertaking maneuvers, in which the precise clock estimates from the dynamic POD processing before maneuvering are modeled and predicted to the maneuvering periods to impose a constraint on the satellite clock parameters in RPPP POD during maneuvers. This method depends on the precision of predicted satellite clocks. Thus, the clock prediction models are first developed according to different clock types. The comparison of predicted clocks and GBM final clock products shows that the 2-h prediction error is 0.31 ns and 1.07 ns for GPS Rb and Cs clocks, and 0.45 ns and 0.60 ns for BDS GEO and BDS IGSO/MEO clocks, respectively. This accuracy is sufficient to support the implementation of the proposed method since the duration of maneuver events is usually much less than 2 h.  
The proposed method is implemented in the PANDA software and tested on real data. Since the precise orbit products during maneuvering periods are not available, we first evaluate the POD performance of this method with the non-maneuvering data by assuming a satellite experiences orbit maneuvers during the last 2 h of the POD arc. The comparison with dynamic POD solution shows that the accuracy of the unconstrained RPPP POD solution is 69.3 cm, 5.4 cm and 5.7 cm in the radial, cross-track and along-track directions for GPS satellites, and 153.9 cm, 12.8 cm and 10.0 cm for BDS satellites, respectively. When the satellite clock parameters are constrained with the predicted clocks, the average RMS of clock-constrained RPPP POD solution is dramatically reduced in the radial direction by a factor of 7-11, with the value of 9.7 cm and 13.4 cm for GPS and BDS satellites. Then the proposed method is further tested on seven actual maneuver cases, including two GPS maneuver events and five BDS maneuver events in the experiment period. The clock-constrained RPPP POD solution is compared with the forward and backward integration orbits that are calculated from the respective dynamic POD solutions before and after maneuvering. The orbit differences between the clock-constrained RPPP POD solution and the forward and backward integration orbits are less than 20 cm in all three directions for GPS satellite, and less than 30 cm in the radial and cross-track directions and up to 100 cm in the along-track direction for BDS satellites. The maneuver start and end time is detected from the orbit differences, which reveals that the maneuver duration of GPS satellites is less than 2 min, and the maneuver activities last from 22.5 min to 107 min for different BDS satellites.  
Conceptualization, X.D. and Y.L.; methodology, X.D. and Z.D.; software, X.D. and Y.P.; validation, C.S. and Y.L.; formal analysis, X.D.; investigation, C.H., J.Q.; resources, X.D. and C.H.; data curation, X.D. and J.Q.; writing--original draft preparation, X.D., Z.D.; writing--review and editing, Y.L., C.S.; visualization, X.D. and Y.P.; supervision, Y.L. and C.S.; project administration, X.D.; funding acquisition, Y.L. and X.D.  
This research was funded by the National Key Research and Development Program of China, grant number: 2016 YFB0501802; the Excellent Scientist Foundation of Hubei, grant number: 2018 CFA052 and the National Science Foundation for Post-doctoral Scientists of China, grant number: 2018M632918.  
The authors declare no conflict of interest.