The Multi-GNSS precise orbit products are based on a global network consisting of 127 Multi-GNSS Experiment (MGEX) network with RINEX version of 3.03 or 3.04. The distribution of these stations is shown in Figure 1. The red marks indicate stations tracking BDS (BDS-3 or the regional BDS-2) satellites. Once iGMAS observations is accessible (which would be achieved within one or two months), our POD processing will also include in iGMAS observations.

Two types of GNSS orbit products are provided: the rapid products updated at 13:00 UTC every day with a latency of 37-hours, and the ultra-rapid products updated every 2-hour with 24-hour post-processed arc and additional 24-h forward-predicted arc. The flowchart of our ultra-rapid processing is given in Figure 2. To speed up the processing, multi-thread method has been used in some steps such merging hourly observations to daily ones and detecting cycle-slips by ‘turboedit’.

The satellite specific phase center offset (PCO) values released by BDS metadata are used for BDS-3 satellites. Due to the lack of receiver antenna calibrations for BDS signals, the GPS L1/L2 PCOs and phase center variations (PCVs) are also applied for BDS-3 B1I/B3I and Galileo E1/E5a, respectively. Ionospheric-free code (PC) and carrier phase (LC) combinations are used to eliminate the first-order ionospheric delays. The earth tide, ocean tide loading, phase wind-up can be corrected according to the existing models. The coordinates of MGEX stations are fixed to IGS weekly combined solution. Double-differenced (DD) ambiguity resolution (AR) is applied for each system separately. No DD AR is implemented for ambiguities from different systems. Table 1 summarizes the detailed information of the POD strategies including observation models, dynamical models, and estimated parameters.

The specific yaw-attitude laws described by Galileo and QZSS metadata are used for Galileo and QZSS satellites, respectively (Li et al. 2019). For BDS-3 satellites, the attitude models demonstrated by Li et al. (2020) are applied. In addition, the a priori box-wing and the a priori box-wing-hat model are used to BDS-3 MEO, Galileo, and QZSS-IGSO satellites (Li et al. 2019, 2020; Yuan et al. 2020) for solar radiation pressure modeling together with five-parameter ECOM. For GPS, GLONASS, BDS-2, and BDS-3/QZSS GEO satellites, purely empirical five-parameter ECOM model is used.

The rapid and ultra-rapid orbit products are named as “GRT0MGXRAP_YYYYDDD0000_01D_05M_ORB.SP3” and “GRT0MGXULA_ YYYYDDD HH00_01D_05M_ORB.SP3.gz”, where “YYYY”, “DDD”, and “HH” represent year, day of the year, and hour of the day, respectively.