Towards Clock Ties in GNSS: A Real-Time Phase Calibrator for Receiver Instability Mitigation

The increasing demand for Earth science applications presents challenges in enhancing geodetic reference frames. Systematic errors currently restrict the accuracy of these frames, as traditional geometric connections between various space-geodetic techniques are inadequate. To tackle this issue, the DFG-sponsored project FOR5456 aims to reduce systematic errors by utilizing clock ties, including the integration of optical clocks into Space Geodesy.

Recent developments in optical clocks have achieved frequency instabilities below 1×10⁻¹⁵ at 1 s integration time. While this level of short-term stability is beyond the immediate needs of space-geodetic instruments, optical clocks offer substantial benefits for the long-term stability of timing signals.

In particular, the long-term coherence of GNSS time transfer can be improved by calibrating receiver-induced phase instabilities. GNSS carrier-phase measurements do not directly represent the phase of the input clock signal, as they are affected by variable delays inside the receiver. This calibration is enabled by an optical delay-stabilized timing system developed at the Geodetic Observatory Wettzell, which provides a highly stable and well-defined phase reference. Based on this infrastructure, we have developed a real-time GNSS phase calibrator that generates a pilot signal synchronized with the phase of the input clock. This pilot signal is then used terrestrially to remove receiver-induced phase instabilities.

GNSS is currently the only continuously operating space-geodetic system capable of continuous comparison of clocks at the 10⁻¹⁸ level and beyond. However, achieving this requires careful mitigation of receiver instabilities is essential. This contribution presents the design of the GNSS phase calibrator, its synchronization procedure with the clock signal, and an analysis of its performance in terms of long-term time and phase stability, enabling future high-precision clock comparisons over extended time scales.