In addition, the solution strategies used by MGEX ACs in calculating multi-GNSS orbit parameters are not exactly the same: CNES/CLS, CODE [20], and GFZ use the one-step method strategy; TUM use the two-step method strategy [21]; and WU use the three-step method strategy. Studies have shown that more available satellites benefit the estimation of common parameters (such as station coordinates and tropospheric parameters) when using the one-step method. Theoretically, the one-step method strategy is a more rigorous solution, which is more conductive to multi-GNSS POD [22]. IGS MGEX also encourages MGEX ACs to use the one-step method. At present, IGS provides GPS/GLONASS two-system ultra-rapid orbits, and the International GNSS Monitoring and Assessment System (iGMAS) provides four-system (GPS/GLONASS/BDS/Galileo) ultra-rapid orbits ([[http://www.igmas.org/](http://www.igmas.org/)]([http://www.igmas.org/](http://www.igmas.org/))). Besides, MGEX provides final precise orbits, and all MGEX ACs are testing their own ultra-rapid orbits. However, these ultra-rapid orbits are not fully released for GNSS users, and the published research and application of ultra-rapid orbits are mainly focused on a single or dual system [23; 24]. Two other examples also show this problem: [PERSON] et al. evaluated the prediction accuracy of IGS ultra-rapid orbit products [25]; and IGS ACs, such as CODE, introduced their solution strategies for IGS ultra-rapid orbit products [26; 27]. Recently, based on the one-step method and non-difference observations, [PERSON] et al. proposed a method to solve the five-system ultra-rapid orbit problem [28]. There is relatively little data available at present regarding the introduction and research of a multi-GNSS (especially for five systems) ultra-rapid orbit based on the one-step method and the error equation of double-difference observations. On the other hand, a diversity of observation models and processing strategies will facilitate to validate the ultra-rapid orbits offered by various POD software platforms. Because the fact that the predicted part of the ultra-rapid orbits is important for a (near) real-time precise service, it is necessary to analyze these five-system ultra-rapid precise orbits provided by the one-step method and the error equation of double-difference observations. Developing a multi-GNSS POD software platform based on this method and error equation will be the premise of the corresponding analysis. However, Bernese v5.3 has not been released publicly, and Bernese v5.2 can only handle GPS/GLONASS/Galileo three-system data. Therefore, this paper will derive the double-difference observation model of a multi-GNSS ultra-rapid POD based on the one-step method. A new prototype software platform for a five-system ultra-rapid POD will be developed on the basis of the Bernese v5.2 framework, and this model and method. The structure of this paper is as follows: First, the multi-GNSS observation model and the processing strategies of the software platform are introduced in the second section. Then, the preliminary results of this software platform are shown in the third section. Finally, the discussion and conclusions are given.