## 3 Results and Analysis
### Data Description and Processing Configuration  
The Luojia-1A satellite was designed by Wuhan University, China and launched successfully on 2 June 2018. Its mission objective is to test nighttime light remote sensing and LEO-based navigation augmentation [44]. The satellite is operating on a 645 km orbit and weighs about 19.8 kg. The outlook of the Luojia-1A satellite is demonstrated in Figure 5. The Luojia-1A satellite onboard the GNSS receiver is developed by the China electronics technology group corporation (CETC). The onboard receiver is capable of tracking GPS L1/L2 and BDS B1I/B2I signals and simultaneously up to 12 satellites for each signal. It should be mentioned that this receiver only tracks the BDS regional system at present and the next mission will support the global system in the future.  
Due to the platform energy constraint of the Luojia-1A satellite, the dual-frequency GNSS receiver is only active during the mission period. Only GPS/BDS data on 8 June 2018 with a 1 s sampling interval were collected in this experiment. In this study, we used GPS L1 and BDS B1 measurements for validating the proposed weighting model and single-frequency cycle slip detection algorithm. The dual-frequency reduced-dynamic orbit determination results, of which three-dimension overlapping precision is about  
Figure 4: Flowchart of the real-time single-frequency receiver clock jump and cycle slip detection algorithm. The joint detection algorithm can handle clock steering, and the cycle slip can be detected correctly in the case of receiver clock jump.  
6.7 cm, are used as the reference orbit for evaluating the KOD accuracy using the proposed strategy [45]. Figure 6 shows the number of all visible satellites.  
Figure 6 shows that the number of visible GPS satellites ranges from 6 to 12 for most epochs. The tracked signals of the BDS region system mainly cover the Asia-Pacific region. Since there are five GEO satellites and six IGSO satellites of BDS located over this region, the Luojia-1A satellite can track more BDS satellites in this region. However, the number of visible BDS satellites tracked by the Luojia-1A satellite decreases when the GEO and IGSO satellites are invisible. Therefore, the number of visible BDS satellites fluctuates periodically. The orbit solution will not be continuous if only BDS observations are used. Combining BDS and GPS is helpful to improve the number of available observations, which may benefit orbit determination. Hence, this article studies the benefit of using BDS measurements in GPS/BDS-combined single-frequency KOD.  
The Luojia-1A satellite orbit is computed with the onboard GPS/BDS single-frequency observations in the offline processing mode. The extended Kalman filter is used for the orbit estimation where ambiguity states are passed from epoch to epoch, while LEO satellite position and receiver clock are freely adjusted at each epoch. One new ambiguity parameter is set and estimated as a float constant when a cycle slip occurs or a new arc begins. Precise GPS/BDS orbit and clock products are used for decreasing model error while the real LEO KOD for onboard applications generally works with broadcast ephemerides. The detailed data processing configuration is listed in Table 1. The GPS differential P1C1 code bias products from the center for orbit determination in Europe (CODE) analysis center are used to correct the C1 code, and the P1P2 products are used to correct the bias term  
Figure 5: Outlook configuration of the Luojia-1A satellite and the GNSS antenna.  
Figure 6: Number of visible GPS and BDS satellites.  
introduced by precise satellite clock products. The correction of timing group delays for the BDS satellite is ignored in the processing of the B1 observations since BDS B1B2 products are not publicly available at present. Although it may decrease the accuracy of kinematic positioning, the validation of the proposed weighting model and single-frequency cycle slip detection is not influenced.