## 2 Precise MGEX Orbit and Clock Products  
Currently, multi-GNSS precise orbit and clock products are generated by seven MGEX ACs, including CODE, GFZ, CNES/CLS, JAXA, SHAO, TUM and WHU. In addition, the European Space Operations Centre of the European Space Agency (ESA) has been providing multi-GNSS orbit and clock products since the end of 2017 ([[http://navigation-office.esa.int/products/gnss-products/](http://navigation-office.esa.int/products/gnss-products/)]([http://navigation-office.esa.int/products/gnss-products/](http://navigation-office.esa.int/products/gnss-products/))) [27]. In this section, an overview on these products is given in terms of product availability and processing strategies. Table 1 summarizes the precise orbit and clock products provided by the seven MEGX ACs and the ESA. As of November 2019, all the seven MEGX ACs utilize the long filename convention for their products ([[http://mgex.igs.org/IGS_MGEX_Products.php](http://mgex.igs.org/IGS_MGEX_Products.php)]([http://mgex.igs.org/IGS_MGEX_Products.php](http://mgex.igs.org/IGS_MGEX_Products.php))). The initial three-character field of each filename specifies the analysis center and thus is used to denote the orbit/clock products from different ACs throughout the rest of this article (see \"ID\" in Table 1). Specifically, \"COD\", \"ESM\", \"GFZ\", \"GRG\", \"JAX\", \"SHA\", \"TUM\" and \"WUM\" are used to denote orbit/clock products from CODE, ESA, GFZ, CNES/CLS, JAXA, SHAO, TUM and WHU, respectively. For now, COD, ESM, GFZ, JAX and TUM orbit products are provided with 5-min sampling, while the GRG, SHA and WUM orbit products are provided at 15 min intervals. The complementary clock products are made available by seven ACs (excepting TUM). In 2017, only GRG and GFZ clock products were generated with 30 s sampling [9], but now 30 s clock products are generated by six ACs (excepting SHAO). The availability of the MGEX products is depicted in Figure 1. Since day of year (DOY) 001 of 2018, COD and ESM orbit and clock products were provided without interruption. Nevertheless, TUM products were interrupted for more than five months from DOY300 2018 to DOY088 2019, and frequent interruptions were observed for SHA products as well. As for the new constellations, Galileo was included in COD, ESM, GFZ, GRG, SHA, TUM and WUM. BDS-2 was included in COD, ESM, GFZ, SHA and WUM, as well as TUM, since DOY217 2018. Please note that BDS-2 geostationary orbit (GEO) satellites were excluded in COD and ESM products. BDS-3 was only included in ESM and WUM since DOY340 2018 and DOY001 2019, respectively. SHA included BDS-3 products for four weeks from DOY196 2019 to DOY223 2019, but BDS-3 satellites are excluded in SHA products for now. QZSS satellites were included in COD, ESM, GFZ, JAX, TUM and WUM. The JAX products only included J01, while the other five products included all the QZSS IGSO satellites (i.e., J01, J02 and J03). Note that the JAX products once included J02 and J03 for a short time from DOY070 2019 to DOY083 2019. In addition, GFZ, TUM and WUM products also included the GEO satellite J07. The main strategies of the MGEX POD and PCE processes are summarized in Table 2. Different software packages were employed by different ACs: Bernese by CODE and TUM; NAPEOS by ESA; EPOS by GFZ; POD GINS by CNES/CLS; MADOCA by JAXA; and PANDA by SHAO and WHU. CODE used double differenced (DD) observations for the POD process, while un-differenced (UD) observations are commonly used by other ACs. The POD arc durations ranged from 24 h to several days. CNES/CLS used an additional 3 h of data from the neighboring days (3 + 24 + 3 h) to get smaller discontinuities at the day boundaries [10]. The satellite PCO and PCV values of Galileo and QZSS published by the EGSC and CAO were updated into the Antenna Exchange (ANTEX) format file [28; 29; 30; 31] and were commonly used by most ACs. As for BDS-2 satellites, MGEX conventional PCO values [32] were used by most ACs, while ESA and GFZ adopted PCO and PCV values estimated by [PERSON] et al. [33].