Huijun Zhang

High-Resolution and Multi-Channel Time Interval Counter Using Time-to-Digital Converter and FPGA

A high-resolution and Multi-Channel time interval counter is designed using Time-to-Digital Converter(TDC) and(Field-Programmable-Gate-Array)FPGA. The core of counter is the TDC chip which is interfaced to and controlled by FPGA . The Counter has 1 start channel and 8 stop channels and can be provided with four different modes, and thus four different resolutions. In the highest resolution mode, the counter demonstrates a statistical standard deviation of 18.6 ps. In the lowest resolution mode, that is 82ps, the counter can work with 8 channels and be endless measurement range by internal retrigger of start. The measurement data can be read from the two FIFOs which are part of TDC as a form of real-time transfer or block. Therefore, the counter is communicated with personal computer by serial port RS232. This configuration provides an simply and efficient way of using a computer not only to control and operate the counter, but also to store and process measured data.

Measurement of the time delay of gps timing receiver based on UTC(NTSC)

GPS provides a method for accurate timing. The measurement of the time delay of timing receiver is a difficult task with the existing method, high in cost and complex in operation. A new and simpler measurement method is presented in this paper, based on the time difference between UTC(NTSC) which is the time scale kept in National Time Service Center and the output of 1pps signal by the GPS timing receiver. On the basis of the international comparison link of UTC(NTSC) and UTC, and the time difference between UTC and GPST, a new method for the measurement of the time delay of GPS timing receiver is given. This method is easy to do. The error of with this method is also analysized.

Research on GNSS System Time Offset Monitoring and Prediction

With the progress of Global Navigation Satellite System (GNSS), which mainly include GPS, GLONASS, GALILEO as well as BeiDou, multi-GNSS joint working mode has become a development and application trend. The System time offset between two satellite navigation systems is one of the most important aspects of their interoperability and compatibility. This paper introduces the research progress of GNSS System Time Offset Monitoring and prediction at NTSC. The reasons that GNSS system time offset monitoring is done by means of Signal-in-Space Reception are analyzed and the measurement principle is discussed in detail. Multi-GNSS receiver calibration is the key technology of GNSS system time offset monitoring with Signal-in-Space Reception. The causes of Inter-System biases and inter-frequency biases are researched and so does their calibration method. The calibration results are showed and analyzed. The key technologies of BDs system time offset monitoring are researched which include the algorithms related to satellite equipment group delay and dual-frequency ionosphere delay correction whose measurement result is illustrated. And then, the system time offset monitoring results of UTC(NTSC)-GPST and UTC(NTSC)-GLONASST by Signal-in-Space Reception for 6 months long are showed and their precisions is up to 2.10 and 3.24 ns respectively. Furthermore, the experiment results about BDS system time offset UTC(NTSC)-BDT calculated from all visible BDs satellites from 1 to 12 are displayed. Finally, the intentional application and development directions of GNSS system time offset monitoring are put forward.

A method of GNSS system time offset monitoring

With the progress of Global Navigation Satellite Systems(GNSSs), which mainly includes GPS, GLONASS, GALILEO as well as BeiDou, multi-system joint working mode has become research focus and application trend. But, Each satellite navigation system has its own independent system time, such as GPST, GLONASST, GST and BDT. The System time offset between the two satellite navigation systems is one of the most important aspects of their interoperability. It will cause combined navigation position solution bias and time solution. Therefore, it is necessary and significant to monitor and research system time offset. A method of GNSS System Time Offset monitoring which is called Signal-in-Space Reception method is put forward and its principle is described. In this method, UTC(NTSC) time scale is regarded as reference time scale on account of its status of standard time and frequency resources. Then, The time offset between each satellite navigation system time and UTC(NTSC) which include UTC(NTSC)-GPST, UTC(NTSC)-GLONASST, UTC(NTSC)-BDT, UTC(NTSC)-GST can be acquired. Thus, system time offset among satellite navigation systems, such as GPST-GLONASST, can be acquired with about 5ns (1 σ) precision. On the basis of above described method, GNSS System time offset monitoring system has been set up since 2011 and it is gradually improved and perfected. A great deal of monitoring data has been collected. Addition to common error sources, some special error sources related to GNSS timing receiver such as receiver delay, inter-system hard delay bias, inter-frequency biases and so on are analyzed, as they can effectively affect monitoring results and precision. The monitoring data and IGS precision orbit as well as corresponding data published by BIPM Circular T Bulletin are combined together to determine above error values. The results of the error items and GNSS system time offset monitoring results are to be showed.

GNSS system time offset monitoring at NTSC

GNSS System time offset between two satellite navigation systems is one of the most important aspects of their interoperability and compatibility. This paper introduces the research work of GNSS System Time Offset Monitoring at NTSC. The method of GNSS System Time Offset monitoring by Signal-in-Space Reception is described. Multi-GNSS receiver calibration is the key technology of GNSS system time offset monitoring. The causes of Inter-System biases and Inter-Frequency biases are researched and so does their calibration method. The calibration results are showed and analyzed. The key technologies of BDs system time offset monitoring are researched which include the algorithms related to satellite equipment group delay and dual-frequency ionosphere delay correction whose measurement result is illustrated. And then, the system time offset monitoring results of UTC(NTSC)-GPST and UTC(NTSC)-GLONASST and UTC(NTSC)-BDT for several months long are showed respectively.

Study on Integrated Testing and Evaluation Method of GNSS Timing Performance

GNSS timing is the process for users to obtain the UTC time from the satellite navigation signal. At present, each GNSS has the timing function, but timing performance characterization of different navigation systems is not unified as well as the published data not comparable, which makes the users great inconvenience. According to the users’ using characteristics, the paper proposes the conception and method to test the bias between UTC broadcasted by the satellites and actual UTC, which studies the timing testing methods of single satellite and multiple satellites, and referring to the timing performance of GPS, GLONASS, and BDS we develop the integrated testing and evaluation experiments. The result shows GPS timing error is better than 20 ns, and GLONASS timing performance is less than GPS with timing error relatively higher fluctuation; compared with GPS and GLONASS, timing evaluation of BDS has still a certain distance that sometimes the maximum timing error can be up to 120 ns, thus from the users’ perspective the UTC precision obtained by the BeiDou satellites is less than GPS and GLONASS.

Analyzing Prediction Methods and Precision of GNSS System Time Offset Using End-Point and Kalman Filter

GNSS compatibility and interoperability is one of the most significant problems in multi-system navigation and position. The precise prediction of system time offset is one of the most important problems for application of GNSS compatibility and interoperability. To increase the prediction precision of system time offset and meet the requirement of multi-system users, this essay studies the prediction algorithm of system time offset. First, based on research on the prediction principle of end-point (EP) and Kalman filter, the feature of GNSS system time offset is analyzed. The initial values of Kalman filter parameters are determined to reduce the uncertainty and error caused by it. In addition, the real-time measurement data is provided by a platform that monitors GNSS system time offset at National Time Service Center (NTSC). It verifies the applicability of two methods to predict the system time offset and compares the precision of EP and Kalman filter. The result shows that for any type data of system time offset, when real-time precision prediction is required, Kalman filter provides higher precision and will be influenced less significantly by other factors: For GPS data, precision is about 1 ns and for GLONASS data, it is 2 ns. It is 0.5 times higher than that of EP. In other cases, EP is more precise.

Research on Testing Method and Influence of GlONASS Inter-frequency Biases

GLONASS adopts FDMA signal mechanism, some inter-frequency biases generated at the receiver end make difficult to the implementation and application of multi-mode navigation, which is an important subject having to be considered and resolved. First, in this paper, GNSS simulator was implemented based on the study on the testing approach for GLONASS inter-frequency biases, the experimental data from two kinds of brand receiver (Septentrio PolaRx 4 and Novatel OEMV_3G_L1L2s) was obtained and analyzed in details. The results from receivers show that there is a difference in inter-frequency bias between the two, whose increasing presents a linear trend. Second, the multi-station measured data comparison was experimented, influence of GLONASS inter-frequency biases on the results from GLONASS independent positioning, GPS/GLONASS integrated positioning and system time offset monitoring are analyzed. The experiment result indicates that correction in GLONASS inter-frequency bias during location solution and system time offset monitoring can improve the accuracy of positioning and the precision of system time offset monitoring significantly, the performance is related to the type of receiver.

Measurement and Correction Method of the System Time Offset of Multi-mode Satellite Navigation

Multi-mode satellite navigation is an important development direction of Global Navigation Satellite Systems (GNSS). Because of each satellite navigation system owing an independent and stable operating system time scale, one of key issues that must be solved to implement multi-mode navigation is to determine the system time offset between two satellite navigation systems. National Time Service Center (NTSC) keeps our country’s standard time (UTC (NTSC)). It is an available resource for us to monitor the system time offset of satellite navigation systems by means of receiving signal-in-space using the geodetic time receiver. The monitoring principle and main measurement errors are discussed. The correction method of system time offset measuring results is studied with the IGS precise orbit ephemeris. In order to test rationality of the measurement method, circular T bulletin data published by Bureau International des Poids et Mesures (BIPM) is applied to compare with the monitoring data and revised data. Data Processing results are given and shown that this monitoring method is practical and can be applied to multi-mode navigation.

Galileo in-orbit satellite clocks performance assessment at NTSC

National Time Service Center has developed the system time offset monitoring and forecast system based on UTC(NTSC) standard time scale in order to support time interoperability of BDS relative to other GNSSs since 2013. The monitoring results include UTC(NTSC)-GPST and UTC(NTSC)-GLONASST and UTC(NTSC)-BDT. With the development of Galileo satellite navigation, it is imperative to take Galileo system time offset monitoring into consideration. At present, the Galileo in-orbit satellites include 4 IOV (In-Orbit Validation) satellites and 8 FOC (Full Operational Capability) satellites. The complete Galileo constellation is expected by 2020. Therefore, the experiments of Galileo In-Orbit Satellite clocks assessment at NTSC have implemented for several months. This paper will present a scheme of Galileo in-orbit satellite clock assessment. The short-term performance specifications of all Galileo in-orbit satellites clocks are assessed by receiving the signal-in-space, which includes frequency stability, the consistency of the frequency stability over time. The method and process of absolute delay calibration of the receiver and associated antennas are discussed. The calibration results are given with which more accurate Galileo system time offset with respect to UTC(NTSC) are obtained preliminarily. The evaluation results show that in normal condition, frequency stabilities of Galileo satellite clock are all close to the level of e-13 and consistent over time when average times equal to 60s and 300s respectively. After satellite clock correction, the time difference between Galileo System Time(GST) and UTC(NTSC) which derived from each satellite change from -35 ns to -5 ns. The satellite clock stabilities are slightly improved by broadcasted clock error correction. The performance of two abnormal FOC satellites(E14 and E18) are also analyzed. The results will be very significant not only for Galileo system time offset monitoring but also for Galileo standalone positioning, multi-GNSS combination positioning, Integrity monitoring, the Galileo system time interoperability, etc.