## 1 Introduction  
The inter-satellite link (ISL) is not only an important feature and technological innovation of BDS, but also an important means to optimize the distribution of regional station, improve the accuracy of spatial signals and increase the frequency of message injection [1; 2; 3]. BDS-3 has provided the global services officially on 31 July 2020, and all of the satellites are equipped with ISL [4]. Based on the Time Division Multiple Access (TDMA) technique and the Ka-band technology of BDS ISL strategy, any visible satellites can achieve real-time inter-satellite measurement and communication by the multipoint-to-multipoint time-division two-way links [5; 6].  
In contrast with GPS and GALILEO systems that update ephemeris and clock offset parameters mainly through the global uniform distribution of stations [7; 8], BDS adopts the combined time synchronization strategy which determines satellite clock offset parameters by the satellite-ground link (SGL) for visible satellite and inter-satellite link (ISL) for invisible satellite [9; 10]. This strategy was first published in 2018, and usually called as the \"one-hop\" reduction method [11]. According to this method, when the satellite is visible by the Master Control Center (MCC), the clock offset is measured through the SGL L-band satellite-ground two-way time synchronization; when the satellite is invisible by MCC, the clock offset is calculated by one node satellite (which is visible to both MCC and the invisible node satellite) of the SGL L-band two-way time synchronization and these two satellites of the ISL Ka-band two-way time synchronization, thus clock offsets of invisible satellite can be calculated through the \"one-hop\" reduction mode by one node visible satellite [11; 12]. Compared with only SGL, the tracking coverage is extended by 40% by using ISL [11], the prediction error can be reduced from 3 ns to 1 ns [13], and the orbit result is also greatly improved [14; 15; 16].  
Some achievements have been published to estimate the affections of the systematic error. [PERSON] [17] and [PERSON] [18] estimate the satellite antenna phase center offsets (PCOs), phase variations (PVs) and the differential code bias (DCB) of BeiDou-3 satellites, and has the results that PCOs and PVs is different with satellites, and DCB value is related to the receiver type. Moreover, [PERSON] [19] proposes a network adjustment model by the correction of inter-satellite clock offsets to detect and analyze the closed residuals, and has the results that the random noise of the inter-satellite clock corrections is reduced by 30% to 50%. These achievements have evaluated the performance of current system and given the application conclusions that the signal in space accuracy has reached up to 0.5 m [2]. However, systematic errors of the measured SGL clock offsets and the calculated ISL clock offsets using different node satellite exist, which affect the accuracy of the time synchronization [12; 20].  
This paper further studies the whole-network adjustment method by multi-source clock offset of the satellite-ground and inter-satellite observation. Taking SGL and ISL data of all orbiting satellites as a whole, this new method is used to realize the satellite-ground and inter-satellite time synchronization in the entire constellation. The propose is to optimize the time synchronization strategy of BDS, minimize the impact of different devices, and improve the accuracy of BDS broadcast clock offset.