# Evaluation of BDS-2 and BDS-3 Satellite Atomic Clock Products and Their Effects on Positioning
In addition to the improved Rb atomic clocks, some BDS-3 satellites are outfitted passive hydrogen maser (PHM) clocks, which have smaller frequency drift ([PERSON] et al., 2015; [PERSON] et al., 2015; [PERSON] et al., 2015). It is well known that broadcast ephemeris provides satellite clock quadratic polynomial models for a short span of time, which means that broadcast clock offsets are predictive values. Theoretically, the broadcast ephemeris of satellite atomic clocks with better stabilities can provide more accurate prediction of clock offsets and thus provide users with better PNT services and even precise point positioning (PPP) services. Joint BDS-2 and BDS-3 can provide sufficient observations and offer the enormous potential to enhance the accuracy, reliability and availability of PNT services ([PERSON] et al., 2015). However, a systematic bias between BDS-2 and BDS-3 products and observations were reported recently. For example, it was observed that there were around 4.0 ns between time group delay (TGD) reference datum of BDS-2 and BDS-3 in the broadcast ephemeris ([PERSON] et al., 2015; [PERSON] et al., 2015). Moreover, it was also reported that receiver-related pseudorange biases may lead to inconsistencies in satellite clock and differential code bias (DCB) estimation when different networks of receivers were processed ([PERSON] et al., 2015; [PERSON] et al., 2015). One obvious example is that the TGD values exhibit an obvious mean offset from the DCBs provided by the multi-GNSS experiment (MGEX) for BDS-2, because different correlation techniques were adopted in tracking stations before July 2017 ([PERSON] et al., 2017; [PERSON] et al., 2017). Furthermore, [PERSON] et al. (2017) found that receiver DCB differences between BDS-2 and BDS-3 were not close to zero and present systematic biases between different networks. In addition to TGD/DCB products, [PERSON] et al. (2017) indicated that the broadcast ephemeris of BDS-2 and BDS-3 are calculated separately, and accordingly, the clock datum are inconsistent. Besides, the datum of precise clock products generated by international GNSS service (IGS) analysis centers (ACs) may also exist a systematic bias for BDS-2 and BDS-3, since there are many receiver types in MGEX networks, such as TRIMBLE, SEPT and JAVAD, etc. Consequently, the inter-system bias (ISB) between BDS-2 and BDS-3 actually involved the effect of products datum discrepancy and different receiver hardware delays, and must be considered in joint applications. Prior studies of the ISB between BDS-2 and BDS-3 have mostly been based on the results of PPP and focused on the impact of positioning and time transfer ([PERSON] et al., 2015; [PERSON] et al., 2015; [PERSON] et al., 2015), while few touch upon the long-term characteristics of different satellite clock products. In this paper, we focus on evaluation of BDS different satellite clock products with an emphasis on the effect of ISB between BDS-2 and BDS-3, including BDS clock stabilities and accuracy assessment, based on Allan deviation, the long-term characteristics of ISB between BDS-2 and BDS-3 satellite clocks and their impacts on PPP. The paper is organized as follows: an introduction to the Allan deviation, clock datum comparison and ionosphere-free PPP is presented in the next section. Then, a brief description of the data is presented. Next, the performance of BDS atomic clocks is evaluated with different clock products. Thereafter, we analyze the ISB between BDS-2 and BDS-3 through the comparison of satellite clock datum and receiver clock offset. Finally, the PPP performance, based on broadcast ephemeris, are evaluated and compared for five strategies.