# Improved Multi-GNSS PPP Software for Upgrading the DEMETRA Project Time Monitoring Service
## 1 Introduction  
The global navigation satellite systems (GNSS) include the United States of America's global positioning system (GPS), Russian's global navigation satellite system (GLONASS), Europe's Galileo and China's BeiDou navigation satellite system. They are well known for their positioning services with global coverage. With the GNSS precise point positioning (PPP) techniques, the positioning accuracy can reach centimeter-level accuracy [1]. Another important function that GNSS provides to the user is the timing service, based on the accurate and reliable GNSS time references, e.g., GPS system time (GPST), and their predicted difference to the Coordinated Universal Time (UTC) [2]. GNSS are widely used for accurate time and frequency dissemination, synchronization, and remote clock comparison (called time transfer in time metrology). In the industrial domain, GNSS time synchronization is widely used in the telecom network, energy grid, and financial system [3] and has proved its advantages in various specific applications, e.g., time synchronization of the intelligent transportation systems [4] and spacecraft [5]. In the scientific domain, clock comparison with GNSS is essential for the computation of International Atomic Time (TAI), with more than 50% of time laboratories contributing to TAI and UTC equipped by GNSS receivers for their time link comparisons [6]. GNSS timing is also the primary choice for some scientific activities in which all the stations from a given network observing the same event need to be highly time-synchronized, e.g., seismic monitoring [7], deep space tracking [8], and neutrinos speed measurement [9]. Many users rely on GNSS code measurements for the time synchronization purpose, this method has a statistical uncertainty of several nanoseconds over one day [10]. Better performance of GNSS time transfer is achieved by PPP techniques, which use both dual-frequency carrier phase and code measurements from the GNSS signals and the more precise satellite products instead of the broadcast navigation message for the analysis. The statistical uncertainty of this method can reach 100 ps over one day [6; 11]. DEMETRA (_DEM_onstrator of EGNSS services based on Time Reference _Architecture) [12] is a project funded by the European Union in the Horizon 2020 program coordinated by the Italian National Metrology Institute (INRIM) that involved 16 European partners, including the Royal Observatory of Belgium (ORB). The DEMETRA project aims at providing the end-users in both industrial and scientific domains with improved and new time services, some of them being based on the European GNSS. Nine different timing services with improved accuracy or availability and any requisite as certification or redundancy were designed, developed, and tested. Most of the time, services are currently still operative through their infrastructures maintained at the INRIM premises. As an example, INRIM implemented the fiber link between the financial district in Milan and INRIM for UTC time distribution service [13]. One particular service in DEMETRA called \"Time Monitoring and Steering\", provides the user, in quasi-real-time, the time difference between the user atomic clock and the reference time scale of DEMETRA from an hourly PPP solution. This PPP solution is based on the data from the user geodetic GNSS receiver transmitted hourly to the DEMETRA platform. With the data provided by the service, the user can make sure its clock is highly synchronized to the DEMETRA reference and continue being alerted to any possible non stationarities on its clock, namely phase or frequency jumps. Meanwhile, the service offers the necessary quality monitoring data (such as satellite sky plot, multipath, cycle slips, etc.) visible directly on the personal area of the DEMETRA web page. An example of part of the user personal page is attached in the Appendix A, and all the details can be found on the homepage of the DEMETRA project [14]. In addition, some parameters are sent to the users to steer their clock to UTC if needed.