Abstract
Thanks to the international GNSS service (IGS), which has provided multi-GNSS precise products, multi-GNSS precise point positioning (PPP) time and frequency transfer has of great interest in the timing community. Currently, multi-GNSS PPP time transfer is not investigated with different precise products. In addition, the correlation of the receiver clock offsets between adjacent epochs has not been studied in multi-GNSS PPP. In this work, multi-GNSS PPP time and frequency with different precise products is first compared in detail. A receiver clock offset model, considering the correlation of the receiver clock offsets between adjacent epochs using an a priori value, is then employed to improve multi-GNSS PPP time and frequency (scheme2). Our numerical analysis clarify how the approach performs for multi-GNSS PPP time and frequency transfer. Based on two commonly used multi-GNSS products and six GNSS stations, three conclusions are obtained straightforwardly. First, the GPS-only, Galileo-only, and multi-GNSS PPP solutions show similar performances using GBM and COD products, while BDS-only PPP using GBM products is better than that using COD products. Second, multi-GNSS time transfer outperforms single GNSS by increasing the number of available satellites and improving the time dilution of precision. For single-system and multi-GNSS PPP with GBM products, the maximum improvement in root mean square (RMS) values for multi-GNSS solutions are up to 7.4%, 94.0%, and 57.3% compared to GPS-only, BDS-only, and Galileo-only solutions, respectively. For stability, the maximum improvement of multi-GNSS is 20.3%, 84%, and 45.4% compared to GPS-only, BDS-only and Galileo-only solutions. Third, our approach contains less noise compared to the solutions with the white noise model, both for the single-system model and the multi-GNSS model. The RMS values of our approach are improved by 37.8–91.9%, 10.5–65.8%, 2.7–43.1%, and 26.6–86.0% for GPS-only, BDS-only, Galileo-only, and multi-GNSS solutions. For frequency stability, the improvement of scheme2 ranges from 0.2 to 51.6%, from 3 to 80.0%, from 0.2 to 70.8%, and from 0.1 to 51.5% for GPS-only, BDS-only, Galileo-only, and multi-GNSS PPP solutions compared to the solutions with the white noise model in the Eurasia links.
Highlights
The global positioning system (GPS) has become an effective tool for time and frequency transfer since its first application in the 1980s [1]
S represents GPS, GLONASS, BDS, and Galileo satellites in this study. dr,IF and dSIF refer to the clock offsets of the receiver and satellite in meters, respectively. dtrop represents the slant troposphere delay in meters. λIF is the ionosphere-free carrier wavelength on the frequency band; and NIF is the float ambiguity in cycles. dr,IF and dSIF refer to the uncalibrated code delay (UCD) of the IF combination at the receiver and the satellite end in meters, respectively; br,IF and bSIF represent the uncalibrated phase delay (UPD) of the IF combination at the receiver and satellite end in cycles, respectively; εSr,PIF and εSr,LIF represent the code and phase observation noise
This section starts with the performance of multi-GNSS precise point positioning (PPP) time transfer with different precise products released by GFZ and Center for Orbit Determination in Europe (CODE), as well as the advantage of multi-GNSS PPP with respect to a single satellite system
Summary
The global positioning system (GPS) has become an effective tool for time and frequency transfer since its first application in the 1980s [1]. Thanks to the international GNSS service (IGS), which provides precise orbit and clock products, another technique, called the all-in view (AV), is employed for time transfer [3]. The AV method allows a direct time comparison of any station on Earth with respect to the GPS system time (GPST) or the international GNSS service time (IGST) in the case where the IGS final ephemeris is used to compute the satellite orbits and clock offsets. It provides better results than the CV technique and was added to the Bureau International des Poids et Mesures (BIPM) software in September 2006.
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