Aimin Zhang

Performance evaluation of NIM GPS receivers in use for time transfer with PTB

Two self-developed GPS receivers of NIM (National Institute of Metrology), which are based on a Javad E_GGD receiver and a NovAtel OEMV3 receiver, respectively, have been installed and operated at PTB (Physikalisch-Technische Bundesanstalt) for a time transfer link calibration between both institutes. During the operation of the traveling receivers at PTB for several weeks, their performances have been thoroughly evaluated. A subset of the results of general interest is presented. From the results we decided on the most suitable mode of operation of the NIM GPS traveling receivers. The uncertainty of the link calibration will potentially be influenced by the temperature sensitivity of the receivers, which was thus investigated. Finally, we studied the receiver internal delay as a function of time and its installation in the host laboratory.

Study and development of a New GNSS receiver for time and frequency transfer

Time and frequency transfer techniques based on GPS and GLONASS were studied. On the basis of Topcon GNSS receiver module, one new type of GNSS time and frequency transfer receiver named NIMTFGNSS-1 has been developed and its scheme has been accounted for. Compared with several popular GPS time and frequency transfer receivers made aboard in the world by the common clock difference experiments and the long baseline experiments, the performance of the receiver has been tested and verified. Its time and frequency transfer noise level can match those of the several commercial receivers and the time stabilities from the GPS code and carrier phase results in the common clock difference experiments using two NIMTFGNSS-1 receivers during a few days are separately within 1 ns and 100 ps and the GLONASS code results is with 4 ns.

Reconstruction of UTC (NIM)

NIM has reconstructed UTC (NIM) since 2010 at new campus, and the official UTC (NIM) was switched to the new campus in October 2012. The new clock ensemble includes 6 Hydrogen masers and 7 Cesium clocks, with 1 Hydrogen maser and 2 Cesium clocks placed at the old campus. The master clock is a Hydrogen maser at new campus, and UTC (NIM) is generated from a micro phase stepper steered by a new algorithm and UTC, the frequency of the Hydrogen maser is predicted and rapid UTC data is used to steer UTC (NIM). A dual mixer time difference measurement system is used to measure the time difference between the clocks and UTC (NIM) at new campus, the clocks at old campus are linked to UTC (NIM) by GPS. GPS and TWSTFT data are reported to BIPM everyday automatically. The GPS receivers at new campus are calibrated with reference to one GPS receiver calibrated by BIPM. The UTC (NIM) has been operated normally at new campus; the accuracy and stability are improved. In this paper the generation of new UTC (NIM) is described and the performance of UTC (NIM) is presented.

Summary of the link calibration between NIM and PTB using a traveling GPS receiver

The time transfer link between NIM and PTB was for the first time calibrated using a travelling GPS receiver developed by NIM The calibration uncertainty was estimated to about 2 ns. The result obtained could be compared to the result derived from differential calibrations made by BIPM in previous years. Both results agree well within 1 ns.

Preliminary time transfer through optical fiber at NIM

TWOTFT has been studied and implemented according to the similar principles to those of TWSTFT. We initially constructed the experiment system for TWOTFT and done some experiments such as time and frequency transfer through the laboratory optical fiber and the real optical fiber links including the real link with 109 km length at NIM. We can get the time stability of less than 6 ps/s and 0.9 ps/100s, and the standard uncertainty of less than 200 ps for time transfer.

Developing of one time link calibrator with GNSS at NIM

Since 2014 NIM has also started to design and develop one kind of homemade calibration system for the time link calibration on the basis of NIMTFGNSS-2 receiver. The system has been constructed preliminarily and calibrated, and in the near future, with this calibrator we might implement the calibration campaigns of some TAI links under the APMP(Asia Pacific Metrology Programme) scheme.

UTC time link calibration in the frame of APMP — Campaigns in 2014-2016

The experiments and results of these calibation campaigns in the frame of APMP from 2014 to 2016 have been reported and the experiment setups and the corresponding delay measurements have been discussed, and the uncertainty has been evaluated for the campaigns.

Enhanced predictability of hydrogen maser using random pursuit strategy

The predictability of atomic clock is generally used to characterize its overall performance. Anomalous clock behaviors lead to greater prediction error in traditional prediction method, such as polynomial fitting predictor and Kalman filter predictor. Our previous work developed random pursuit strategy (RPS) to avoid or minimize above the problem. It was further applied to real data of three Hydrogen masers to verify its effectiveness. Compared to Kalman filter predictor worked on suitable condition, the RPS showed a certain advantage, particularly for clock data with phase or frequency jumps. It could be helpful for steering and weighting algorithms in time keeping.

Further results of time transfer through the optical fiber at NIM

The performance analysis of TWOTFT with the two terminal equipments at the two remotes sites has been implemented and one way time transfer through optical fiber has been designed and the performance evaluation, demonstration, and verification experiments in short distance have been realized and its uncertainty has been evaluated as roughly 2.1 ns.

Experimental research on BeiDou time transfer using the NIM made GNSS time and frequency receivers at the BIPM in Euro-Asia link

A new time and frequency transfer system compatible with BDS has been developed at NIM, and two of them are in operation at the BIPM since Jan 2017. Experiments with this equipment have been implemented to characterize the time and frequency transfer by BDS. This includes analysis of the satellites signal coverage statistics in the BIPM area, the error correction, the measurement noise of the NIM time and frequency transfer receivers, the short and long-term instabilities in the European and Asian baselines, the time transfer link calibration and the accuracy of the time transfer and and AV and CV modes.