On a deep analysis of the reference time of multi-GNSS final and real-time precise clock products with respect to UTC

The underlying reference time of International GNSS Service (IGS) clock products is extensively used in the characteristic analysis of on-board oscillators and precise point positioning timing services. As a reference time used in timing applications, traceability to coordinated universal time (UTC) and high-performance of frequency stability are essential. In this study, based on global navigation satellite system (GNSS) observations from timing laboratories and Circular T published by the Bureau International des Poids et Mesures, the time deviation with respect to UTC and frequency stability are evaluated for the reference time of IGS precise clock products. To comprehensively investigate the performance of the GPS, Galileo, and BeiDou-3 (BDS-3) reference time in the IGS products, five multi-GNSS experiment (MGEX) final products (COM, ESM, GBM, Information and Analysis Centre (IAC), and WUM), two real-time (RT) products (Wuhan University (WHU)-RT and CNES-RT), and the IGS final products are selected for analysis. The results indicate that the GPS reference time of most MGEX products is derived from the broadcast GPS system time (GPST), and the consistency with UTC is within 8 ns over a year. The COM, ESM and WUM define the Galileo and BDS-3 reference time by estimating the inter-system bias as a constant while the Galileo and BDS-3 reference time of GBM and IAC is derived from the broadcast Galileo system time (GST) and BDS system time (BDT). Benefited from the well-performance of GPST, the BDS-3 and Galileo reference time of COM and ESM shows the best performance for the time deviation from UTC. For the RT products, the deviation between the reference time and UTC is within 7 ns for GPS, whereas the peak-to-peak values of the time deviation of BDS-3 and Galileo exceed 15 and 30 ns for nearly two months, respectively. The frequency stability of GPS reference time in most final products can achieve 1.4-5.0 × 10⁻¹⁴, 3.2-5.9 × 10⁻¹⁵ and 3.3-5.3 × 10⁻¹⁵ with the averaging time of 120, 10 020 and 608 400 s, respectively. In addition, the frequency stability of Galileo and BDS-3 reference time is comparable to that of GPS reference time. However, the stability of the reference time of the RT products is inferior to that of the MGEX products, especially for short- and mid-term stability.