The normal consecutive observing model in Chinese Area Positioning System (CAPS) can only supply observations of one GEO satellite in 1day from one station. However, this can’t satisfy the project need for observing many GEO satellites in 1day. In order to obtain observations of several GEO satellites in 1day like GPS/GLONASS/Galileo/BeiDou, the time-sharing observing model for GEO satellites in CAPS needs research. The principle of time-sharing observing model is illuminated with subsequent Precise Orbit Determination (POD) experiments using simulated time-sharing observations in 2005 and the real time-sharing observations in 2015. From time-sharing simulation experiments before 2014, the time-sharing observing 6 GEO satellites every 2h has nearly the same orbit precision with the consecutive observing model. From POD experiments using the real time-sharing observations, POD precision for ZX12# and Yatai7# are about 3.234m and 2.570m, respectively, which indicates the time-sharing observing model is appropriate for CBTR system and can realize observing many GEO satellites in 1day.

介绍了在广义相对论框架下建立的微米级卫星激光测距时延模型.为考察新模型对于参数化后牛顿系数β和γ解算精度的影响,将该模型用于定轨软件Utopia对Ajisai,Lageos 1,Lageos 2,Etalon 1四颗卫星的模拟观测资料进行批处理,并假设大气、固体潮等观测模型的效应以及动力学模型的误差已精确扣除.结果表明,在当前的仿真条件下采用微米级时延模型解算的参数化后牛顿系数解算精度比采用现有的标准时延模型的解算精度高2个量级.

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%.

Geostationary Earth Orbit (GEO) satellites play a significant role in the space segment of the Chinese Area Navigation System. The C-Band transfer ranging method developed by the National Time Service Center (NTSC) has been widely used in the Chinese Area Positioning System (CAPS), with its advantages of separating satellite ranging from time synchronization and being unaffected by weather. The explicit ranging correction models for the C-Band transfer ranging method are introduced in detail in this article for the first time. Precise Orbit Determination (POD) using C-Band pseudo-range observation of GEO satellite 2010-001A in July 2012 has been conducted. The residual Root Mean Square (RMS) of each site and POD are analysed with orbit difference over overlaps of adjacent orbit arcs. Moreover, the orbit of the GEO satellite has been evaluated by Satellite Laser Ranging (SLR) data from both domestic and foreign SLR sites for the first time. The residual RMS of POD using C-Band observation is better than 0·1 m, and the orbit difference over overlaps of adjacent orbit arcs is better than 3 m. In addition, the residual RMS in line-of-sight for a SLR site in China are better than 1 m, while the RMS for the Yarragadee site in Australia is about 3·4 m. It has been shown that the GEO satellite orbit accords very well with the C-Band observation. Also, the distribution of CAPS stations affects the orbit precision. All sites in CAPS are now located in China with low and medium latitudes. The residual RMS of the SLR site in the southern hemisphere is larger than that of the site in China.

In order to establish a continuous GEO satellite orbit during repositioning maneuvers, a suitable maneuver force model has been established associated with an optimal orbit determination method and strategy. A continuous increasing acceleration is established by constructing a constant force that is equivalent to the pulse force, with the mass of the satellite decreasing throughout maneuver. This acceleration can be added to other accelerations, such as solar radiation, to obtain the continuous acceleration of the satellite. The orbit determination method and strategy are illuminated, with subsequent assessment of the orbit being determined and predicted accordingly. The orbit of the GEO satellite during repositioning maneuver can be determined and predicted by using C-Band pseudo-range observations of the BeiDou GEO satellite with COSPAR ID 2010-001A in 2011 and 2012. The results indicate that observations before maneuver do affect orbit determination and prediction, and should therefore be selected appropriately. A more precise orbit and prediction can be obtained compared to common short arc methods when observations starting 1day prior the maneuver and 2h after the maneuver are adopted in POD (Precise Orbit Determination). The achieved URE (User Range Error) under non-consideration of satellite clock errors is better than 2m within the first 2h after maneuver, and less than 3m for further 2h of orbit prediction.

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.

In order to more restrict the transverse orbit error, a new method named "differenced ranges between slave stations by transfer", similar to Very Long Baseline Interferometry (VLBI) observation, has been developed in the Chinese Area Positioning System (CAPS). This method has the number of baselines added, the baseline length increased and the data volume enlarged. In this article, the principle of "differenced ranges between slave stations by transfer" has been described in detail, with the clock offset between slave stations and system error which affects the precision of the differenced ranges observation being discussed. Using this method, the differenced observation of the SINOSAT-1 satellite with C-band between slave stations from 6 to 13 June 2005 was conducted. Then a comparison was made between the accuracy of orbit determination and orbit prediction. A conclusion can be drawn that the combination of pseudo-range receiving the own-station-disseminated signal and the differenced range observation between slave-slave stations has a higher orbit determination and prediction accuracy than using only the former.

The constraint in the transverse direction of satellite orbit with differenced ranges between master station and slave stations by transfer as an angular observation data is explained in theory. Differenced ranges in combination with C-band ranging by transfer were used in satellite orbit determination. The position error of overlapped orbit differences for combining is less than that for ranging only. The residual of predicted orbit forward 5.5 days for combining is 3.1762 m, while the residual for ranging only with the same duration is 3.5380 m. Both the orbital overlapping and orbit prediction experimentations can testify the constraint in the transverse direction of satellite orbit with differenced ranges, and the results show that the accuracy of orbit determination and orbit prediction is improved by combining differenced ranges and C-band ranging.

For analyzing the correlation between the clock offset and the environmental temperatures of stations for TWSTFT(two-way satellite time frequency transfer),the correlation coefficients between the clock offset and the environmental temperatures of stations for TWSTFT are calculated and analyzed,and a fitted function that relates the residual of clock offset with the environmental temperatures of stations is constructed by using the observation data of TWSTFT(in C-band,for Shanghai-Xi′an link,during the June of 2005).The results show that the environmental temperatures obviously affect the residuals of clock offset.The residuals of clock offset are revised with the fitted function and the RMS(root-mean-square) values of the revised residuals of clock offset reduce by 25%~50%.The observation devices should be kept in an environment with constant temperature,which is of special significance to the time synchronization between the stations for TWSTFT in C-band.

Combined with two-way satellite time and frequency transfer(TWSTFT),a Sagnac effect solution about satellite motion is proposed.The TWSTFT technique is based on geostationary satellite.As a result of the perturbation forces,the geostationary satellite is not absolutely stationary,but it does the diurnal motion with small amplitude.Sagnac effect is closely related to both satellite and earth observation station positions.The satellite' motion makes Sagnac effect have diurnal variance characteristic.The Sagnac effect value is calculated by using the high accuracy orbit data of geostationary satellite.It is shown that the Sagnac effect value is variable with variety of satellite positions.The variety value is about several hundred picoseconds.It improves the Sagnac effect correction accuracy comparing with the conventional method assuming that the geostationary satellite is absolutely stationary.It has the important application value for all kinds of time transfer methods and satellite navigation community.

It is almost reciprocity of the Two-way satellite time and frequency transfer(TWSTFT) communication links. The path delays are almost canceled out due to the symmetry. It is the advantage of TWSTFT. But as the technology development, the higher accurate is required for time comparison. The incomplete reciprocity must be studied in detail for TWSTFT. Satellite motion is a kind of the incomplete reciprocity and can result that the uplink geometry paths aren't equal to the downlink geometry paths. Satellite motion's correction depends on the satellite velocity and time difference between the signal from two stations arrive at the satellite.

The original idea of orbit determination called “determination of satellite orbit by transfer” was proposed by the National Time Service Center, Chinese Academy of Sciences. It shows that the system is very stable and the orbit determination accuracy is improved greatly. A new observation mode called “differenced ranges between master station and slave stations by transfer” is introduced. It is the development of “determination of satellite orbit by transfer”. In principle, the differenced ranges between master station and slave stations have a high angular resolution, and strongly constrains in the transverse direction of satellite orbit, perpendicular to the line-of-sight. The principle of “differenced ranges between master station and slave stations by transfer” is discussed in detail. And differenced ranges in combination with ranging data were used to determine satellite’s orbit and orbit prediction under different arc observations. It shows that orbit determination accuracy for orbit prediction can be improved with differenced ranges in combination with ranging data.

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.

This article describes a visualization software of GEO satellite orbit determination,which is developed in the Windows system by adopting the Microsoft Visual Studio 2005 software platform and using the Visual Basic.NET programming technique.This software can pre-process observation data,compute GEO satellite orbit with high accuracy,analyze and plot the calculated results of satellite orbit,etc.Furthermore,this software has virtues such as friendly interface,high operability,convenience and time-saving,so as to improve the efficiency of GEO satellite orbit determination.

A new strategy of precise orbit determination (POD) for GEO (Geostationary Earth Orbit) satellite using SATRE (SAtellite Time and Ranging Equipment) is presented. Two observation modes are proposed and different channels of the same instruments are used to construct different observation modes, one mode receiving time signals from their own station and the other mode receiving time signals from each other for two stations called pairs of combined observations. Using data from such a tracking network in China, the results for both modes are compared. The precise orbit determination for the Sino-1 satellite using the data from 6 June 2005 to 13 June 2005 has been carried out in this work. The RMS (Root-Mean-Square) of observing residuals for 3-day solutions with the former mode is better than 9.1cm. The RMS of observing residuals for 3-day solutions with the latter mode is better than 4.8cm, much better than the former mode. Orbital overlapping (3-day orbit solution with 1-day orbit overlap) tests show that the RMS of the orbit difference for the former mode is 0.16m in the radial direction, 0.53m in the along-track direction, 0.97m in the cross-track direction and 1.12m in the 3-dimension position and the RMS of the orbit difference for the latter mode is 0.36m in the radial direction, 0.89m in the along-track direction, 1.18m in the cross-track direction and 1.52m in the 3-dimension position, almost the same as the former mode. All the experiments indicate that a meter-level accuracy of orbit determination for geostationary satellite is achievable.

Precise orbit determination requires high-precision earth rotation parameter (ERP), which can be obtained from IERS. Sometimes we can simplify the calculation procedure of orbit determination without ERR Using the pseudo-range observation of SINOSAT-1 satellite with C-band, we compare the orbit with and without ERP. A conclusion can be drawn that ERP may not be used to users with km-level orbit determination, and polar motion may not be used to users with hundred meters-level orbit determination too.

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.

GEO satellites play a significant role in Compass satellite navigation system.So it has important significance using other orbit measurement system for its precision orbit determination,using itself pseudo-range and phase measurement.Observing data has been obtained through transponder ranging system build by national time service center,Chinese academy of sciences.This paper makes the orbit determination for Compass-GEO satellite using transponder ranging data,analyze the accuracy of orbit determination through measurement accuracy,orbit residuals and orbit overlap respectively.

The Chinese Area Positioning System (CAPS) is a new type of satellite navigation systems. The geostationary satellites for normal communication with 2 or 3 sets of inclined geostationary orbit (GEO) satellites are employed to construct a satellite navigation system. The navigation signals are constructed and disseminated on the ground, and the satellites only transfer the signals to the ground by transponders. Such a system is easy to construct. But for such a positioning system, it needs precise and accurate orbit determination of geostationary satellites. A GEO satellite is placed at an altitude of 36000km above the equator and its movement with respect to the station on the ground is very small, so it is very difficult to determine the orbit precisely. An original method for determination of satellite orbits based on pairs of combined observations is developed at the National Time Service Center, Chinese Academy of Sciences. A special ranging observation with the distance between two stations via a satellite, called pairs of combined observations, is proposed. The satellite behaves with such an observation is not a ball, but an ellipse ball. The Dilution of Precision (DOP) is much less than those methods such as the SLR observation. The advantage of pairs of combined observations is that clock offsets for all observing stations and on-bound are cancelled by the method itself, so, the accuracy and precision of orbit determination is not affected by the clock offsets and the DOP is greatly improved. The ranging signals are generated and measured on the ground, and the instrumentation errors can be determined in real time with a high accuracy. Therefore, such a system is very stable. A very high rate of the PN code is employed, a disseminated power is very low, and it can observe signals from remote stations with a lower power less than 1w. With the spread-spectrum technique, multi-stations can work at the same time and same carrier frequency. It can reach the accuracy of raging about 1 cm. The accuracy and precision of the orbit determination are very high, up to a few centimeters. The observing system with the model proposed above for the GEO satellites including 5 stations (one main station and 4 slave stations) placed in China has been set up. The combined observations were made between the main station and the slave stations. The number of combined observations for the 5 stations is 9. The observations are made per second. The observations for 8 sets of satellites are continuous, lasting for about 5 years. The results for the GEO observations show that the observation residuals of the satellite orbit determination for a 3-day solution are 4.0 cm. With the development of the method to determine the orbit based on the discussion above, it is very easy to correct the orbit with observations nearly in real time. Obviously, the method proposed by the National Time Service Center is suitable for precise orbit determination of GEO satellites in a satellite navigation system, and also it is suitable for satellite management and deep space tracking.

1 课题意义 随着卫星导航、飞行器编队等航天活动的深入开展,对星座自主时间同步和自主定轨产生了重大需求.在这种需求背景下,本课题开展星间双向时间同步方法的研究,并开展星座自主时间同步和定轨的仿真计算和分析.本课题研究成果对于发展我国卫星导航系统具有重要参考意义.