Timing services are an indispensable part of national economy and people’s livelihood. Currently used timing methods include shortwave timing, longwave timing, internet timing, telephone timing, fiber-optic timing, and GNSS timing. Among these methods, GNSS RTK timing stands out for its high accuracy, simplicity of implementation, and outstanding application potential in short-distance precision timing and time synchronization without relying on real-time precise ephemeris. This paper primarily utilizes observation data from three GNSS tracking stations of the National Time Service Center of the Chinese Academy of Sciences, focusing on BDS-3, GPS, and Galileo, to analyze RTK timing accuracy, convergence time, and stability.

Backgroundof RTK Timing
Global satellite navigation systems mainly include GPS, GLONASS, Galileo, BDS, etc., providing positioning, navigation, and timing services (PNT) to global users. Timing is a fundamental service provided by GNSS, achieved through the transmission of time via GNSS space signals (SIS). It has a wide coverage, minimal propagation attenuation, and is not limited by the number of users. GNSS timing is widely used in time synchronization for military digital communication networks, telecommunication networks, power systems, and more.

The core of RTK timing is applying RTK positioning technology to timing. The mobile station receives real-time observation data from the reference station through a communication link, forms a differential with the local observation data, and solves the relative clock difference between the mobile station and the reference station. When the reference station is connected to the standard time frequency signal UTC(k) in the timing laboratory, the RTK timing solution for clock difference is the relative clock difference of the mobile station’s clock relative to standard time, achieving the purpose of timing.

Current research on the timing mainly focuses on single systems such as GPS and BDS-3, with limited reports on the research of GNSS multi-system fusion RTK timing. To better verify the performance of GNSS multi-system fusion RTK timing and enhance reliability in urban environments, this paper first presents the observation equations for multi-system RTK timing. Subsequently, experiments are conducted using observation data from GPS, BDS-3, and Galileo based on the time-frequency resources of the National Time Service Center, analyzing GNSS multi-system fusion RTK timing from the aspects of timing accuracy, convergence time, and stability.

Multi-GNSS RTK Timing Principle and Model
The principle of RTK timing is similar to RTK positioning. The user receiver receives real-time pseudorange and carrier phase observation data from the reference station through a communication link, forms inter-station differential measurements with the user receiver, and solves the user receiver’s clock error and three-dimensional coordinates. When the distance between two measurement stations is short, the impact of ionospheric delay and tropospheric refraction has strong correlation, making single-differencing capable of eliminating most of the atmospheric refraction errors. In multi-system RTK timing, using the GPS system’s clock error as a reference, the system inter-bias (ISB) of other systems introduced relative to GPS is considered. The parameter estimation for the timing is carried out using an extended Kalman filter, and the simplified observation equation can be expressed as:

Performance Analysis of GNSS RTK Timing-equation

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