# Analysis of BDS-3 Onboard Clocks Based on GFZ Precise Clock Products
## 6 Discussion  
According to the above analysis, there are pronounced bumps near 13,600 s in the OADEV plots for both BDS-3 Rb clocks and PHMs. This feature may be the response to the 12.88-h periodic signal lying in the clock series, which is further confirmed in the spectrum analysis. Some researchers have noticed a clear relationship between the orbital dynamics and the periodic signals [16; 30]. The orbit errors would be mapped to clock offset estimates when solving for the clock offset in conjunction with orbit parameters due to the correlations between the clock offset and the radial component of the orbit [32]. Therefore, if the influence of orbital errors can be eliminated while solving for the clock offset, the \"bump\" could be flattened and the OADEV could be reduced. The BDS-3 satellites are equipped with inter-satellite link (ISL) payloads. The realization of BDS-3 ISL technology provides an opportunity for the analysis of the clock with ISL observations [33; 34]. The subtraction of bidirectional ISL observations can eliminate the orbit parameters; thus, the estimation of clock offset could mitigate the influences of orbit errors. Compared to the  
Figure 13: Averaged amplitude spectra of BDS satellite clock offsets.  
ground-only clock estimation, the OADEV of the clock estimation using ISL observations is better for averaging intervals longer than 5000 s, and the \"bump\" is notably flattened [30]. However, ISL observations are access limited and the analysis centers (AC) cannot use them in the production of precise orbits and clocks.  
Although orbit error is absorbed in the precise clock, there are some useful conclusions. The results of short-term stability and fitting residuals show that the precise clock of PHMs aboard satellites in different types of orbits has great differences in accuracy. Clock product quality for IGSO and GEO satellites are obviously lower than for MEO satellites. However, this does not prove that clocks equipped on MEO satellites are better than those equipped on IGSO and GEO satellites. As mentioned above, orbit errors may be absorbed into the clock estimates. The GEO satellites have the characteristics of geostationary and higher orbit height, and they can only be tracked by ground stations located in the Asia-Pacific region. The situation of IGSO satellites is also roughly similar. Therefore, the orbit determination accuracy of IGSO and GEO satellites is inferior to that of MEO satellites. The 3D orbit errors of BDS-3 MEO satellites estimated using ground-tracking observation are about 7 cm, which are better than the 3D orbit errors of 15 cm for IGSO satellites and 40 cm for GEO satellites [35]. As a result of that, the estimated clock accuracy of GEO and IGSO satellites is inferior to that of MEO satellites.  
The comparison and analysis of long-term clock bias, frequency, drift rate, and noise for BDS-2 and BDS-3 satellites prove that BDS-3 satellites have greatly improved in clock quality. There are two reasons for the improvement of BDS-3 onboard clocks. First, BDS-3 satellites are equipped with more advanced Rb clocks and PHMs. Another reason is that the on-orbit operation time of BDS-2 satellites has been longer than that of BDS-3 satellites, thus the aging of their satellite hardware facilities has been greater.