## 4 Results and Discussion
### Quality of BDS-3/BDS-2 Real-Time Precise Products  
Figure 6 presents the epoch-wise data availability information of the BDS-3/BDS-2 real-time precise satellite products. The epoch is filled with specific colors for different constellations when the real-time product is available at this epoch, or the corresponding product is unavailable. The data missing can be divided into two types. One is the real-time product missing at the distinct time spans for almost two-thirds of the BDS-3/BDS-2 satellites, and the other one is the real-time product missing at the same time spans for almost all the BDS-3/BDS-2 satellites (e.g., DOY 128 and 147 of 2020). It is seen that the real-time products of IGSO and GEO satellites miss epochs more frequently compared with MEO satellites. The real-time products of several BDS-3 MEO satellites have more data gaps, especially for C29, in comparison with BDS-2 MEO satellites. The operational problems of NTRIP Caster, the scheduled maneuvers, the unstable real-time data streams broadcasted by the ground tracking stations, and other potential factors may result in the unavailability of real-time products. Figure 5: Global maps of the maximum, minimum and average position dilution of precision (PDOP), values for BDS-3/BDS-2 and GPS on DOY 140–146 of 2020. The availability statistic of BDS-3/BDS-2 real-time precise satellite products is shown in Figure 7. The theoretical epoch number is divided by the number of the received epoch-wise real-time corrections to calculate the statistical availability of real-time precise satellite products. There is a satisfied availability for all BDS-3/BDS-2 MEO satellites, which exceeds 95.0%, except for C29, the availability of which is only 80.0%. The availability still exceeds 90.0% for all BDS-2 GEO and IGSO satellites. The average value of availability is computed for each orbit type. The BDS-2 MEO satellites achieve the largest availability of 97.0% on average, followed by the BDS-3 MEO satellites with an average availability of 95.1%. Smaller availability is detected for the BDS-2 GEO and IGSO satellites, which is 93.4% and 94.6% on average, respectively. For comparison, we also calculate the average availability of GPS real-time precise satellite corrections derived from CLK93, and the obtained statistic with a value of 99.3% is found to be larger than the BDS-3/BDS-2 ones. Figure 6: Epoch-wise data availability of BDS-3/BDS-2 real-time precise satellite products. Figure 7: Availability statistics of BDS-3/BDS-2 real-time precise satellite products. Figures 8-10 show the BDS-3/BDS-2 real-time precise satellite orbit errors in the three directions. Figure 11 presents the corresponding satellite clock errors. Different colors represent different satellites. The final precise satellite products from WHU are taken as references. Both real-time satellite orbit and clock offset errors have frequent spikes, indicating that the stability of BDS-3/BDS-2 real-time precise products should be further enhanced. The along-track and cross-track orbit quality are lower than the radial one. Compared with non-MEO satellites, the error magnitudes of MEO orbit and clock products are smaller, and the qualities of BDS-2 MEO products are comparable with respect to those of BDS-3 MEO satellites. Except for the larger orbit errors on DOY 131-136 of 2020 for BDS-3 MEO satellites as well as the spikes, the orbit errors vary in a similar range for the BDS-3/BDS-2 MEO satellites, which is \(-0.3\) to \(0.3\), \(-0.3\) to \(0.3\), and \(-0.1\) to \(0.1\) m in the cross-track, along-track and radial directions, respectively. The varying range of orbital errors for the BDS-2 IGSO satellites is increased by a factor of 1.5 to 2.5 times in comparison with the MEO satellites. As to the BDS-2 GEO satellites, they achieve the largest noise level of orbit errors, which is \(-6.0\) to \(6.0\), \(-10.0\) to \(10.0\), and \(-1.5\) to \(1.5\) m in the three directions, respectively.