Xueyun Wang

A new modem for two way satellite time and frequency transfer

A new time transfer modem for two-way satellite time and frequency transfer (TWSTFT) has been developed recently at Beijing Institute of Radio Metrology and Measurement (BIRMM). The DSSS and BPSK modulation techniques are used to generate a PRN signal. A FFT fast parallel algorithm is applied to achieve fast acquisition of the PRN modulated receiving signal. A 2nd order FLL assisted 3rd order PLL is designed to keep both of the performance of loop dynamic stress and carrier phase tracking accuracy, and a 2nd order DLL is used to track and measure the code phase. A short baseline TWSTFT experiment was done with two 1.2 m VSAT earth stations and a commercial geosynchronous orbit communication satellite to evaluate the modems performance. The result shows very low noise with the standard deviation (1 σ) equal to 0.13 ns at a 2.5 MChip/s code rate.

The development of GPS/BDS time transfer system

This paper describes a time transfer system that allows timing laboratories, including metrology institutes and other designated institutions, to compare the local realization of Coordinated Universal Time (UTC) with each other. The time transfer system could calculate the difference between the local time and GPS time, and the difference between the local time and BDS time. It stores the measurement results in a format compatible with international standards, which has just extended to include BDS data. The system was designed to be easy to use, allowing recently established timing laboratories to begin tracing to UTC with both GPS and BDS.

The study of near real-time GPS/BDS time transfer

This paper introduces the design of a near realtime BDS/GPS time transfer method. A Beidou data communication module is assembled into a time transfer receiver. The clock difference computation procedure is modified and a customized protocol is designed for comparison data exchange. The time difference of zero-baseline common-clock by GPS and BDS is compared. The performance of this method is almost the same level as CGGTTS standard with an uncertainty below 2 ns. The uncertainty by BDS is a little better than GPS.

A novel real time cycle slip detection and repair method for a single dual-frequency GNSS receiver

This paper develops a novel real time cycle slip detection and repair method for a single dual-frequency GNSS receiver. This method is based on several carrier-phase linear combinations, including WL (1, −1), Ultra-WL (-3, 4), NL (1, 1), GF (60,-77). First, we use the mean code range rate which is estimated by the former 10 code range data to detect and repair the big cycle slip. In this step, the big cycle slip is detected and repaired; however, some extra small cycle slip will be introduced into the carrier phase data. Second, the variation of the WL combination is used to detect the different cycle slip on L1 and L2 and the variation of the Ultra-WL combination is used to judge which frequency the cycle slip occurs on. With respect to the same cycle slip on L1 and L2, we use the variation of NL combination when the elevation of the satellite is low since it has lower noise than GF combination and the NL cycle slip is twice as L1 or L2 and use the variation of GF when the elevation of the satellite is high since the variation of the ionosphere delay is very smooth. After that, there are still some cycle slips with the same size and sign which occurs continually in every epoch on L1 or L2. For these cycle slips, we use the mean ionosphere residual method since the ionosphere residual is the same in all the 10 epochs. Finally, we can detect and repair all the cycle slip larger than 1 cycle in real time.

Two way time comparison via optical fiber at BIRMM

Nowadays time and frequency transfer through optical fiber become more important with its further reduction of uncertainty. A two-way time comparison via optical fiber experiment that utilizes the carrier phase of the transmitted signal has been done at Beijing Institute of Radio Metrology and Measurement (BIRMM) laboratory. The principle and equipments are similar to the well-known two-way satellite time and frequency transfer (TWSTFT), but all equipments needed to lock to the external 5MHz reference which provided by H-maser. In the experiment a 1.75GHz carrier frequency is used, and the pseudo-random noise (PRN) code rate is set to 20Mchip/s. The measurement results (common clock difference, CCD) show that the method of carrier phase has uncertainty with the standard deviation (1 s) less than 21ps.

Three loops — A method for tracking a new TWSTFT signal FBOC

In this paper, a new defined modem with Flexible Binary Offset Carrier (FBOC) signal has been developed by the Beijing Institute of Radio Metrology and Measurement (BIRMM), we demonstrate our work details on receiving the FBOC signal. We realize it based on the FPGA and DSP hardware platform. A structure of three loops to solve the tracking ambiguous problem is also demonstrated. To evaluate the performance of the tracking algorithm in our modem, a local 70MHz IF TWSTFT experiment was done, and the results show that the new signal and the tracking algorithm meets our expectations.

Design of a new calibration device for Two-Way Satellite Time and Frequency Transfer station

The Calibration Device is used to measure the time delay difference between the transmission and receiving paths inside Two-Way Satellite Time and Frequency Transfer (TWSTFT) earth station, it can improve the time comparison precision. A new calibration device for TWSTFT has been developed recently at Beijing Institute of Radio Metrology and Measurement (BIRMM). The signal paths inside calibration device is bidirectional which allow both the transmission and receiving signal passed, so the delay difference of the transmission and receiving inside calibration device almost be zero. The calibration device works on Ku-band. To evaluate the performance of the BIRMM calibration device, a local TWSTFT experiments was done with SATRE modem. The measurement results show that the time delay difference was quite small instability with standard deviation (1 σ) equal to 0.17ns.

Remote time and frequency transfer experiment based on BeiDou Common View

Last year the zero-baseline common-view test using BeiDou Navigation Satellite system (BDS) is done in BIRMM and the result is better than 5 ns. In this year, we have developed our own GPS/BD common view time and frequency transfer receiver. Using this receiver we carried out two remote common view experiments in BIRM-SIMT link and BIRM-NTSC link, separately. Besides, we took out TWSTFT experiment together when we made the common view experiment to check out our common view result. The experiment and some results are shown in this paper which show that the common view precision using BD is at the same level of GPS but slightly lower than that of GPS.