Yang Xuhai

Chinese Area Positioning System With Wide Area Augmentation

The Chinese Area Positioning System (CAPS) is a regional satellite navigation system; its space segment consists of some Geostationary Earth Orbit (GEO) satellites and 2∼3 Inclined Geo-Synchronous Orbit (IGSO) satellites. Only a few satellites are needed to provide good area coverage and hence it is an ideal space segment for a regional navigation system. A time transfer mode is used to transmit navigation signals, so no high-precision atomic clocks are required onboard the satellites; all of the transferred navigation signals are generated by the same atomic clock at the master control station on the ground. By using virtual clock technology, the time of emission of signals from the ground control station is transformed to the time of transfer of signals at the phase centre of the satellite antenna; thus the impact of ephemeris errors of satellite on positioning accuracy is greatly decreased, enabling the CAPS to have the capability of wide area augmentation. A novel technology of orbit determination, called Paired Observation Combination for Both Stations (POCBS), proposed by the National Time Service Centre, is used in CAPS. The generation and measurement of ranging signals for the orbit survey are carried out in the ground station and the instrument errors are corrected in real-time. The determination of the clock offset is completely independent of the determination of satellite orbit, so the error of the clock offset has no impact on orbit determination. Therefore, a very high precision of satellite orbits, better than 4·2 cm (1 drms) can be obtained by the stations under regional distribution.

Satellite orbit determination and time transfer based on TWSTFT

Two-way satellite time and frequency transfer (TWSTFT) is one of the most precise and accurate long-distance time transfer techniques nowadays. A C band multi-station satellite tracking system is developed based on TWSTFT in China. This system can not only measure the distance between a satellite with transponders and earth stations, but also realize the time synchronization between all earth stations. The satellite orbit can be determined by the distances. Five earth stations have been constructed in China. The master station is placed at the National Time Service Center (NTSC) in Lintong. There are a very small antenna terminal (VSAT) of 3.7 meters, up/down converters, a modem and an atomic clock in each station. The up and down link frequencies are 6GHz and 4GHz. The spread spectrum signal generated by the modem is 20MHz chip rate. The transmitter power is less than 1 W. The results show that the accuracy of ranging, orbit determination and time synchronization between all earth stations is about 1cm, 10cm and 0.1ns respectively.

Two-Way Satellite Time and Frequency Transfer Experiment via IGSO Satellite

In order to study the instability introduced by path non-reciprocity due to satellite motion in an earth fixed frame in Two-Way Satellite Time and Frequency Transfer (TWSTFT), we do TWSTFT experiment between National Time Service Center (NTSC) and Urumchi Observatory (UO) via APSTAR-1 inclined geosynchronous satellite orbit (IGSO) satellite for the first time. The satellite is with the inclination of 2.2 degree and it can moves up to plusmn1600 km in the direction of perpendicular to equator plane, while SINOSAT-1 geostationary (GEO) satellite moves only up to about 30 km in that direction. To improve the quality of observation, the PN-code with chip rate of 20 MChip/s is chosen in SATRE modem in TWSTFT. Each observation period includes 3 hours: APSTAR-1 satellite is observed with the two TWSTFT stations in the first hour, and SINOSAT-1 satellite is observed with the same stations in the second hour, and nothing is done in the third hour. We repeat the periodic observation for about two months in winter in 2006. Our analysis is as follows, for a given satellite, we first calculate the ionosphere delay with IGS TEC map data and deduce it from TWSTFT result, for our TWSTFT comparison is with C-band. Then we calculate and deduce Sagnac effect. After that, the variation of difference between the TWSTFT result via SINOSAT-1 satellite and that via APSTAR-1 satellite mainly represents the influence introduced by APSTAR-1 satellite motion in the earth fixed frame. Experiment result and analysis indicate that the path non-reciprocity due to satellite movement in the earth fixed frame in TWSTFT between NTSC and UO via APSTAR-1 satellite varies in an approximate sinusoidal pattern everyday, and the amplitude of the daily variation is about a few tenths of a nanosecond. The experiment is helpful for studying TWSTFT via non-geostationary satellite.

An algorithm for a near real-time data processing of GPS common-view observations

Abstract Near real-time transfer of GPS common-view data is no longer a problem, but near real-time data processing of the data still calls for study, because it is not yet achieved by the usual smoothing and filtering techniques. Based on the characteristics of the GPS common-view data, a Kalman filtering algorithm is designed for estimating the time difference between two sites, while greatly reducing the observational noise. The algorithm is applied to the time difference between the National Time Service Center (NTSC) of China and the Communications Research Laboratory (CRL) of Japan (Over 2000 km apart), and to that between the CRL and the Korean Research Institute of Standards and Sciences (KRIS) (over 1000 km apart). The root mean square errors of the results obtained by the Kalman filter relative to those obtained from the Circular T of BIPM are less than 2.9 ns and 2.6 ns, in the two cases. Further, it is pointed out that, when multi-site data within a common-view network are available we can further improve the accuracy of the time comparisons by indirect observation adjustment. This statement was justified by application to the data from all three stations, i.e. NTSC, CRL, and KRIS.

Signal Biases Calibration for Precise Orbit Determination of the Chinese Area Positioning System using SLR and C-Band Transfer Ranging Observations

In C-Band transfer measuring systems, the Precise Orbit Determination (POD) precision of Geostationary Earth Orbit (GEO) satellites is limited by signal biases such as the station delay biases, transponder delay biases, the ionospheric delay model bias, etc. In order to improve the POD precision, the signal biases of the Chinese Area Positioning System (CAPS) are calibrated using Satellite Laser Ranging (SLR) and C-Band Transfer Ranging (CBTR) observations. Since the Changchun SLR site and C-Band station are close to each other, the signal biases of the Changchun C-Band station are calibrated using the co-location comparison method. Then the signal biases of the other two CAPS C-Band stations, located in Linton and Kashi, are calibrated using the combined POD method, with the signal biases of the Changchun C-Band station being fixed. After the signal biases are calibrated, the RMS of the line-of-sight residuals of the Changchun SLR observations decrease by 0·4 m, with the percentage improvement being 75·19%.

Effect of temperature on precision of TWSTFT clock comparison in Chinese Area Positioning System

In CAPS, the temperature changes greatly in one day, especially for master station with continental climate by 10 °C. Moreover, there is periodic tendency in temperature and clock offset residuals. The relation between temperature and clock offset residuals should be researched to improve TWSTFT precision. However, there is no explicit model for effect of temperature on clock offset. In order to test the effect of temperature on clock comparison, experiment has been conducted using C-B and observation on 17 June, 2005. There are two TWSTFT links: Shanghai-Linton, Changchun-Linton. The RMS for clock offset residuals between Shanghai and Linton has decreased from 0.6174ns to 0.2771ns, with that being from 0.6445ns to 0.4050ns. The TWSTFT precision has increased by 55% for Shanghai-Linton link and 37% for Changchun-Linton link. Therefore, TWSTFT precision has been improved after temperature compensation. The temperature of each station should be set constant, which is significant for TWSTFT in CAPS.

The study on centralized disposing mode timekeeping algorithm of autonomous running of navigation constellation

The building algorithm of system time is one of the research focus of auto-navigation. This paper bases on centralized processing mode of auto-timekeeping, gives a kind of building algorithm of system time. On-board clocks weight are determined by the clock quantity which referring to the computation, change of clock velocity can reflect change of clock frequency, so it uses clock velocity instead of clock frequency for determining weight; this algorithm uses two kinds of method of predicting initial velocity: a. with the former two epochs each satellite of each day predict initial velocity, b. with the former two epochs each satellite of first day predict initial velocity. Choosing one clock as the main clock, according to defined sample interval, getting all the available clock error relative to the main clock. This paper gives the computation model of system time, testing the algorithm with IGS precise clock error(system time is IGST). The result shows: comparing to the equal weighted algorithm, the weighted algorithm has better stability. Adding prediction has a significant improvement on the system times stability, method a: the difference between TA(weight) and TA(IGST) (International GNSS Service Time) is about 45ns, method b: the difference be-tween TA(weight) and TA(igst) (International GNSS Service Time) is about 65ns. The algorithm is in reason, it can be used for the building algorithm of system time of auto-navigation.

Precision analysis of non-continuous two-way satellite time comparison

As one of the high accuracy time comparison methods, Two-way Satellite Time and Frequency Transfer (TWSTFT) is one of the important methods for Bureau International des Poids et Mesures (BIPM) to organize international comparison, calculation of the International Atomic Time (TAI) and Coordinated Universal Time (UTC). Due to the interference of emitting device of TWSTFT to co-located devices of Very Long Baseline Interferometry (VLBI) or International GNSS Service (IGS) and the limitation of satellite transponder resources, the comparison is not proceed continuously. Ten consecutive days of data from C-band TWSTFT Network of National Time Service Center (NTSC) was used for analyzing the precision of non-continuous TWSTFT. The raw data of non-continuous TWSTFT was dealt with linear interpolation method. Then the result was compared with continuous TWSTFT which was seen as the true value. Finally, the influence of interval to the precision of non-continuous TWSTFT was analyzed. The comparison shows that: when the interval time is less than 2.5 days, the RMS of the difference between non-continuous and continuous TWSTFT is better than 1 ns; when the interval time is 0.5 day, the RMS is better than 0.5 ns.

A method for determination of clock onboard satellite combining L-band phase smoothed code and C-band TWTRS measurements

Yang Xuhai (2011) presented a new method for determining the offset of navigation satellite onboard clock that processes ion-free code and phase combinations together with simultaneous TWSTFT measurements in the mode of transmitting and receiving with the same station (TWTRS for short) (signal flow: ground station-satellite-own ground station), which was called combination solution. The estimation error of the radial component of the orbit is not correlated with the clock estimation error. But there are still some details to analyze further. A successful determination of clock offset with respect to the master clock depends to a large extent on the correct determination of the inter-system bias. Two aspects of this paper are as follow: on the one hand, code measurement accuracy is limited as some minor errors are submerged in the overall trend and not easy to find. So the carrier phase smoothed code measurement can be used to improve the code measurement accuracy. On the other hand, clock offset onboard can be calculated independently from individual ground station by the combination solution. Taking difference for the same clock offset onboard during the same period can assess the combination solution. The RMS of residual value is less than 2 ns.

Method of common-view time transfer with transfer mode based on geostationary satellite

A new method of Common-view time transfer with transfer mode(TCV for abbreviation) via GEO telecommunication satellite is put forward, with which we can transfer the standard time kept in a time keeping laboratory, such as UTC (NTSC), to many users in the local area covered by the GEO satellites signal. The time keeping laboratory is equipped with transmitting device and receiving device, and their external reference is from the main clock of the lab. The pseudo-code ranging signal is generated by the transmitting device in the lab, and is transmitted to the GEO satellite via a paraboloidal antenna, and then is broadcasted to the earth by the satellite. The pseudo-range from transmitter to GEO satellite and to the receiving device is measured in the lab. And at the same time the pseudo-range from transmitter to GEO satellite and to the user time receiver is also measured. By processing the pseudo-range measured by the user time receiver and that measured in the lab with paraboloidal antenna in common-view method, we can get the time difference between the user receiver clock and the main clock in the lab, carrying out the common-view time transfer with transfer mode (TCV). The precise coordinates of paraboloidal antenna in NTSC and the user time receiver, as well as precise orbit of the GEO satellite should be known in advance in this method. Chinese national standard time, UTC (NTSC) is kept in National Time Service Center (NTSC), Chinese Academy of Science. Based on the device of Two-Way Satellite Time and Frequency Transfer with C-band (TW(C) for abbreviation) in NTSC and Xinjiang Astronomical Observatory (XAO), we did TCV experiment. Both the transmitting and receiving units of the TW(C) device in NTSC are used, and only the receiving unit of the TW(C) device in XAO is used as a user time receiver. The main clock of UTC (NTSC) is a HP5071A Cs atomic clock, and an OSA5585 PRS Cs atomic clock is equipped in XAO. SATRE MODEM made by Timetech Company in Germany is used in our TW(C) devices and the code rate is 20MChips. The GEO satellite used in the experiment is Sinosat-1 telecommunication satellite (110.5°E). For the data processing in TCV method, the precise coordinates of the transmitting station and the receiving station are known in advance, the satellite orbit is provided by the Chinese Area Positioning System (CAPS) of Chinese Academy of Sciences, orbit precision is on the level of meter. And the system errors including Sagnac effect, ionosphere delay, troposphere delay etc, are taken out during the data processing, but the device delay is not deducted. We compare the results of TCV and TW(C), and it shows that ignoring the device delay (almost constant), the RMS of the difference between TCV and TW(C) is about 1ns for five consecutive days, and such result is very better than that in GPS Common-view time transfer with code.