# Accuracy Analysis of GNSS Hourly Ultra-Rapid Orbit and Clock Products from SHAO AC of iGMAS
## 1 Introduction  
The Global Navigation Satellite System (GNSS) has achieved remarkable progress in modernization and global service capacity in recent decades. The American Global Positioning System (GPS) and Russian Global'naya Navigatsionnaya Sputnikova Sistema (GLONASS) are enabling full global coverage services and are undergoing modernization. Moreover, the new global navigation satellite systems, such as the Galileo satellite navigation system(Galileo) established by the European Union (EU), and the Chinese BeiDou navigation satellite system (BDS) were launched to improve global and regional coverage, in order to provide more improved services to users [1, 2, 3, 4]. Some countries have also claimed regional navigation satellite projects, such as Japan, India, and South Korea [5, 6]. The multi-frequency signals are transmitted from the GNSS constellations, which can be received and used by global users conveniently. Moreover, there are great challenges and opportunities for satellite positioning, timing, and navigation (PNT) for more and more satellites, observation frequencies and types which can benefit users for many applications [7, 8, 9]. Some organizations have studied GNSS in past decades. International GNSS Services (IGS) was founded in 1996 and provides GNSS observation data and products to support the terrestrial reference frame, PNT, and other scientific research and engineering applications [10]. The network of IGS stations has grown rapidly in recent years, reaching more than 500 stations distributed all over the world, about two-thirds of which have been updated to track GNSS, and could upload daily, hourly, and high-rate observation data files to the IGS Data Center (DC) [10]. The IGS Analysis Center (AC), which retrieves the observation data from DCs, can generate products to IGS, for combination, and then provide to the users. The IGS provides official final, rapid, and ultra-rapid products of GPS and GLONASS to users (www.igs.org). However, the combined orbit and clock products of new constellations, such as BDS and Galileo, are still missing in the IGS official products. IGS launched the Multi-GNSS Experiment (MGEX) project in 2011, and then started a pilot project in 2016 after the experimental phase ended. The MGEX station network was established or updated from the IGS reference network, which could receive radio signals transmitted from BDS, Galileo, and the Japanese Quasi-Zenith Satellite System (QZSS), as well as legacy GPS and GLONASS. More MGEX observation data files were named (by longname) with RINEX version 3, which can be downloaded from IGS/MGEX DCs. Many ACs, such as the Center for Orbit Determination in Europe (CODE), GeoForschungsZentrum (GFZ), Shanghai Astronomical Observatory (SHAO), and Wuhan University etc., can provide GNSS orbit and clock products for MGEX. However, the combined MGEX products have not been available until now by MGEX [2]. The international GNSS Monitoring and Assessment System (iGMAS), which was initiated by China, is an open technology platform for the collection, storing, analysis, and managing observation data, and can provide relative products and evaluation performance of GNSS [1; 7]. iGMAS consists of 30 global tracking stations, three DCs, and more than seven ACs. The ACs of iGMAS can provide GNSS products with high precision, including orbits, clock, station coordinates, earth rotation parameters (ERP), zenith troposphere delay (ZTD), and global ionosphere delay. These products include three types: final, rapid, and ultra-rapid (www.igmas.org). The ultra-rapid orbit and clock products of IGS or iGMAS comprise of observation and prediction sessions. The predicted session, especially the predicted orbit, can be used to estimate real-time clock offset for real-time precise point positioning (PPP). The predicted clock can also be used as an initial value to estimate the real-time clock. However, the lack or latency of observation data in the predicted session decreases the accuracy of the predicted orbit and clock [11].