Method and performance of time holdover for RT-PPT receivers utilizing on-line estimation of clock parameters

Real Time Precise Point Timing (RT-PPT) receivers can achieve sub-nanosecond accuracy through precise clock offset estimation and receiver clock discipline. However, when satellite signals are lost, the receiver clock drifts, causing timing errors due to clock noise and environmental factors. Strict autonomous holdover is therefore essential for high-precision timing receivers. This paper proposes a time holdover method based on online clock modelling. At the timing stage, the frequency bias compared to the time reference is estimated and eliminated by the closed control loop. Meanwhile, the recursive least square algorithm is used to estimate the parameters of the clock temperature and ageing effects online. In the event of signal interruption, the initial frequency bias is reduced by one-step prediction, while the clock offset caused by temperature and ageing is compensated for using the estimated parameters. To verify the validity of the method, real experiments were carried out using an RT-PPT receiver with different types of oscillators. The results show that with the method proposed in this paper, we can achieve holdover accuracy better than 1ns in 1h using a rubidium clock after correcting for the initial frequency bias, with an improvement of more than 67% over the free-running clock. And the holdover accuracy of the receiver using an OCXO after correcting for temperature and ageing is better than 1us in 24h, with a performance improvement of more than 70% over the free-running clock.