###### Abstract
[13] evaluated the impact of different post-processed precise products from three analysis centers, namely the Center for Orbit Determination in Europe (CODE), Wuhan University (WHU), and GeoForschungsZentrum Potsdam (GFZ), on the PPP time transfer performance of multi-GNSS combinations. The results showed that the PPP with precise products provided by CODE and GFZ had better time transfer performance than that with WHU. [PERSON] et al. [14] also investigated the PPP time transfer performance of the GPS/GLONASS/BDS-2/Galileo quad-system combination and verified that constraining PPP with a priori inter-system bias (ISB) could improve the time transfer accuracy. As of 31 July 2020, the construction of the full BDS-3 constellation was completed [15]. The nominal BDS-3 constellation includes 3 geostationary orbit (GEO) satellites, 3 inclined geosynchronous orbit (IGSO) satellites, and 24 medium earth orbit (MEO) satellites [16]. Some researchers evaluated the PPP time transfer performance of BDS-3 [17; 18]. It is worth noting that most of the atomic clocks carried by the BDS-3 satellites are hydrogen masers. Compared with the satellite clocks of the BDS-2 satellites, those of the BDS-3 satellites are more stable. In addition, the quality of BDS-3 observations is better. Therefore, it is generally believed that there is also an ISB between BDS-2 and BDS-3. [PERSON] et al. [19] adopted four strategies (i.e., estimating ISB as constants, random walk process, and white noise process, as well as ignoring ISB) to deal with the ISB between BDS-2 and BDS-3, and compared the influence of the four strategies on PPP time transfer performance. The result showed that the four strategies, the frequency stability with the legacy signals B1I and B3I was improved by 23.89%, 23.96%, 11.46%, and 20.18%, respectively, compared with the time transfer results using only BDS-2 observations. For the frequency stability with B1C and B2a, which are newly broadcast by BDS-3, the corresponding enhancements of the four strategies were 20.22%, 17.53%, \(-\)3.69%, and \(-\)50.77%, respectively. This provides a foundation for the selection of ISB estimation strategies in BDS-2/BDS-3 PPP time transfer. With the modernization of GPS, the gradual revival of GLONASS, the completion of the full constellation construction of BDS-3, the rapid progress of Galileo, and the vigorous development of Quasi-Zenith Satellite System (QZSS), more and more in-orbit GNSS and Regional Navigation Satellite System (RNSS) satellites that are capable of providing precise services are available and the observation quality is continuously improved. It is foreseeable that the performance of multi-system integrated PPP time transfer will be improved accordingly. The previous studies on PPP time transfer based on multi-constellation combinations could not fully utilize all available satellites, as the satellites were continuously launched with the modernization of GNSS and RNSS. The derived conclusions might be obsolete. In order to fully exploit the advantages of multi-constellation combinations in PPP time transfer under the current constellation status, this contribution focuses on the BDS/GPS/GLONASS/Galileo/QZSS five-system integrated PPP time transfer, in which the ISB parameter between BDS-2 and BDS-3 is carefully considered, and a rigorous multi-system PPP time transfer mathematical model is established. Although some previous studies tried to enhance the multi-system PPP time transfer performance by introducing the a priori ISBs into the observation model [14], the day-boundary jumps of ISB estimates due to the effects of satellite clock datum were ignored. Therefore, the five-system integrated PPP time transfer with a priori ISB constraints considering satellite clock datums is also investigated in this study. This paper starts with the materials and methods, including the five-system integrated PPP time transfer model and the proposed PPP time transfer model with a priori ISB constraints considering satellite clock datums.