# GNSS Timing Performance Assessment and Results Analysis
## 1 Introduction  
Global Navigation Satellite Systems mainly include the Global Positioning System (GPS) of the United States, Global Navigation Satellite System (GLONASS) of Russia, the Global Satellite Navigation System (Galileo) of the European Union, and the BeiDou Navigation Satellite System (BDS) of China, which provide global users with services, such as positioning, velocity measurement, and timing. Timing is a basic service provided by a GNSS to realize time transmission by GNSS signal in space (SIS). Its coverage is wide, the propagation attenuation is small, and it is not limited by the number of users. GNSS timing is widely used in the time synchronization of the military digital communication network, the telecommunication network, electricity, etc. [1, 2]. With the rapid development of global integration, all industries around the world have become more dependent on high-precision GNSS timing. When GNSS timing services are abnormal, the global distribution industries of communications, transportation, finance, and others cannot achieve a unified time, which will have a huge impact. For example, from 12 July 2019 to 17 July 2019 of UTC, the Galileo system service was degraded and interrupted for nearly 117 h. The technical incident originated from an equipment malfunction in the Galileo ground infrastructure, affecting the calculation of time and orbit predictions that generate the navigation message. The malfunction affected different elements on the ground facilities. During this period, Galileo signals were unavailable, and users underwent services outage [3, 4]. Therefore, it is very important to assess the GNSS timing performance to provide a reference for users while using time services. With the development of GNSS, optimization and standardization of the GNSS timing performance by GNSS providers have gradually been carried out. Currently, GPS, GLONASS, Galileo, and BDS have published open service performance specifications [5; 6; 7]. However, the indicators of timing performance are different across GNSSs. In [8], timing accuracy was revealed as a key indicator of BDS timing performance, with a result better than 20 ns. In [7; 9], the indicator of GPS and GLONASS timing performance was defined as a time transfer accuracy less than 30 ns and 40 ns, respectively. In [10], the performance of the Galileo time service, also called the time dissemination service, was indicated by a UTC time dissemination accuracy of less than 30 ns. Performance reports of open service are published periodically by GNSS providers according to GNSS open service performance specification. Unfortunately, due to different timing performance indicators, the assessment results in reports are not comparable. In addition, GNSS timing is monitored and timing performances are assessed by international organizations and research institutions. For example, the International GNSS Service (IGS) set up a Multi-GNSS Experiment (MGEX) to track, collate, and analyze all available GNSS signals [11]. These products contain postprocessed monitoring results in real time, rather than service performance assessment results. The GNSS service performance analysis report provided quarterly by the William J. Hughes Technical Center of Federal Aviation Administration (FAA) only contains GPS and Galileo, and the timeliness is poor [12]. The International GNSS Monitoring and Assessment System (iGMAS) regularly publishes service performance assessment results for BDS, GPS, GLONASS, and Galileo, but no assessment results of timing performance [13]. The authors of [14] referred to the role of GNSS in disseminating time and presented the relations of UTC with predictions of UTC(k) disseminated only by GPS and GLONASS. They did not focus on the methodology of GNSS timing performance assessment or a comparison of time transfer accuracy assessment results across GPS/GLONASS/BDS/Galileo. At present, there is no uniform assessment methodology of timing performance and its outcomes.