Satellite virtual atomic clock with pseudorange difference function

Satellite atomic clocks are the basis of GPS for the control of time and frequency of navigation signals. In the Chinese Area Positioning System (CAPS), a satellite navigation system without the satellite atomic clocks onboard is successfully developed. Thus, the method of time synchronization based on satellite atomic clocks in GPS is not suitable. Satellite virtual atomic clocks are used to implement satellite navigation. With the satellite virtual atomic clocks, the time at which the signals are transmitted from the ground can be delayed into the time that the signals are transmitted from the satellites and the pseudorange measuring can be fulfilled as in GPS. Satellite virtual atomic clocks can implement the navigation, make a pseudorange difference, remove the ephemeris error, and improve the accuracy of navigation positioning. They not only provide a navigation system without satellite clocks, but also a navigation system with pseudorange difference. Supported by the National Basic Research Program of China (Grant No. 2007CB815502) and the National High Technology Research and Development Program of China (Grant No. 2007AA12Z300)     

The principle of the positioning system based on communication satellites

It is a long dream to realize the communication and navigation functionality in a satellite system in the world. This paper introduces how to establish the system, a positioning system based on communication satellites called Chinese Area Positioning System (CAPS). Instead of the typical navigation satellites, the communication satellites are configured firstly to transfer navigation signals from ground stations, and can be used to obtain service of the positioning, velocity and time, and to achieve the function of navigation and positioning. Some key technique issues should be first solved; they include the accuracy position determination and orbit prediction of the communication satellites, the measuring and calculation of transfer time of the signals, the carrier frequency drift in communication satellite signal transfer, how to improve the geometrical configuration of the constellation in the system, and the integration of navigation & communication. Several innovative methods are developed to make the new system have full functions of navigation and communication. Based on the development of crucial techniques and methods, the CAPS demonstration system has been designed and developed. Four communication satellites in the geosynchronous orbit (GEO) located at 87.5°E, 110.5°E, 134°E, 142°E and barometric altimetry are used in the CAPS system. The GEO satellites located at 134°E and 142°E are decommissioned GEO (DGEO) satellites. C-band is used as the navigation band. Dual frequency at C1=4143.15 MHz and C2=3826.02 MHz as well as dual codes with standard code (CA code and precision code (P code)) are adopted. The ground segment consists of five ground stations; the master station is in Lintong, Xi’an. The ground stations take a lot of responsibilities, including monitor and management of the operation of all system components, determination of the satellite position and prediction of the satellite orbit, accomplishment of the virtual atomic clock measurement, transmission and receiving navigation signals to and from each satellite. In the north, the south, the east, the west and the center of Chinese main land, the function of CAPS demonstration system is checked and measured. In cars and on board the system is also checked and measured. The results are as follow: CA-code, horizontal positioning accuracy, 15–25 m (1 σ), vertical, 1–3 m; P-code, horizontal positioning accuracy, 8–10 m (1 σ), vertical, 1–3 m; velocity accuracy, CA-code, 0.13–0.30 m/s, P-code, 0.15–0.17 m/s; time accuracy, CA-code, 160 ns, P-code, 13 ns; determination accuracy of orbit ≤2 m. About 20 million US $ and two years are spent for the development of demonstration. A complete CAPS system is now being established. Supported by the National Natural Science Foundation of China (Grant No. 10453001), the National Basic Research Program of China (Grant No. 2007CB815500), the National High Technology Research and Development Program of China (Grant No. 2004AA105030), and the Funds of the Chinese Academy of Sciences for Key Topics in Innovation Engineering (Grant No. KGCXI-21)     

A new method of ionospheric-free hybrid differential positioning based on a double-antenna CAPS receiver

Chinese Area Positioning System (CAPS) is a transmitted satellite navigation system moved by the Chinese Academy of Sciences. Three basic modes of navigation and positioning with CAPS are given, and then a comparative analysis is made in this paper. In terms of the principle that the ionospheric delay is at an inverse ratio to the frequency square, a new ionospheric-free positioning method based on a double-antenna CAPS receiver is put forward. Then the hybrid differential observations and the solving equations and algorithms for one epoch and multi epochs are deduced according to the basic principle of the method. The method may remove the global errors in signal emission, propagation, transmission and receiving (e.g., ionospheric delay, hardware delay, and clock error). So it is very convenient for the single-epoch solution and multi-epoch navigation and positioning, and may efficiently improve the precision of real time CAPS navigation. Furthermore, the method can be used not only for the geometric orbit determination of CAPS GEO and IGSO satellites and the navigation and positioning, but also for the estimation of the tropospheric zenith delay, which is useful for the study of water vapor changes in the atmosphere. Polynomials are used in this method to express the tropospheric zenith delay and CAPS satellite orbits within the limited time interval, which reduces the number of unknown parameters and thus speeds the computation. Supported by the Knowledge Innovation Project of the Chinese Academy of Sciences (Grant No. KGCX1-21), the National Basic Research Program of China (Grant No. 2007CB815500), the National High Technology Research and Development Program of China (Grant No. 2006AA12z303), the National Natural Science Foundation of China (Grant No. 40774009), and the Special Project of Taishan Scholars of Shandong Province of China (Grant No. TSXZ0502)     

Selection of satellite constellation framework of CAPS

Based on the idea of transmitting the satellite navigation and positioning system, taking the distribution and variation of the Position Dilution of Precision factor (PDOP), which is closely related with the precision of navigation and positioning, within the China area as the primary criterion, we analyze and discuss the tentative plan of constellation configuration consisting of geosynchronous orbit (GEO) communication satellites and inclined geosynchronous orbit (IGSO) satellites for the transmitting Chinese Area Positioning System (CAPS). We emphatically consider the effect on the PDOP by the three major orbit parameters including the inclination, eccentricity and right ascension of the ascending node (RAAN) of IGSO satellites, to research the strategies of the constellation configuration of CAPS through software emulation. Various constellation configurations are analyzed and compared and the results show that the constellation configuration, consisting of three IGSO communication satellites in three orbits with the same inclination as 50°, the difference in RAAN as 120° and the same “8” shaped ground track centered near 115°E and four or five GEO communication satellites within 60°E to 150°E, can satisfy the requirement that Chinese domain is availably covered and the navigation and positioning with high precision could be obtained. Three relatively excellent constellation configurations are initially suggested and some concerned issues are discussed in this work. Supported by the National Basic Research and Development Program of China (Grant No. 2007CB815501) and the Chinese National Programs for High Technology Research and Development (Grant No. 2007AA12z343)     

Multi-life cycles utilization of retired satellites

Retired geosynchronous (GEO) communication satellites affect the GEO orbit environment in outer space. According to the new concept of modern design, the authors propose creatively a method of reusing retired GEO communication satellites, through adjusting retired GEO satellites to slightly inclined orbit geosynchronous (SIGSO) satellites. After these retired satellites are applied to the navigation and communication system, integrity of navigation system and positioning accuracy of the system is improved. Meanwhile, some transponders on these retired satellites can be used to establish a new satellite communication service, and initiate the study and utilization of the multi-life cycle for retired satellites. Experimental results show that this project has significant social value and can make remarkable economic benefit. Supported by the National Basic Research and Development Program of China (Grant No. 2007CB815501) and the National High Technology Research and Development Program of China (Grant No. 2007AA12z343)     

Signal structure of the Chinese Area Positioning System

Proper signal structure is very important in the navigation, positioning, and time services of a satellite navigation system. In this paper, the carrier wave characteristics, ranging code functions, BOC modulation, navigation data rate, the error-correcting methods, and signal channel resource allocation are discussed in terms of the technical characteristics of the transforming satellite navigation system and the resources of communication satellites. The results show that dual-frequency of C band in the Chinese Area Positioning System (CAPS), compound ranging code, a combination of the coarse code and precise code, BOC modulation, separate-channel transmission of different users are compatible with the satellite navigation system at present. The experiments show that the current signal structure can meet the demand of CAPS. Supported by the Major Knowledge Innovation Programs of the Chinese Academy of Sciences (Grant No. KGCX1-21), the National High Technology Research and Development Program of China (Grant No. 2004AA105030), the National Natural Science Foundation of China (Grant No. 10453001), and the Major State Basic Research Development Program of China (Grant No. 2007CB815502)     

The transmission link of CAPS navigation and communication system

The Chinese Area Positioning System (CAPS) is based on communication satellites with integrated capability, which is different from the Global Positioning System (GPS), the International Maritime Satellite Organization (Inmarsat) and so on. CAPS works at C-band, and its navigation information is not directly generated from the satellite, but from the master control station on the ground and transmitted to users via the satellite. The slightly inclined geostationary-satellite orbit (SIGSO) satellites are adopted in CAPS. All of these increase the difficulty in the design of the system and terminals. In this paper, the authors study the CAPS configuration parameters of the navigation master control station, information transmission capability, and the selection of the antenna aperture of the communication center station, as well as the impact of satellite parameters on the whole communication system from the perspective of the transmission link budget. The conclusion of availability of the CAPS navigation system is achieved. The results show that the CAPS inbound communication system forms a new low-data-rate satellite communication system, which can accommodate mass communication terminals with the transmission rate of no more than 1 kbps for every terminal. The communication center station should be configured with a large-aperture antenna (about 10–15 m); spread spectrum communication technology should be used with the spreading gain as high as about 40 dB; reduction of the satellite transponder gain attenuation is beneficial to improving the signal-to-noise ratio of the system, with the attenuation value of 0 or 2 dB as the best choice. The fact that the CAPS navigation system has been checked and accepted by the experts and the operation is stable till now clarifies the rationality of the analysis results. The fact that a variety of experiments and applications of the satellite communication system designed according to the findings in this paper have been successfully carried out confirms the correctness of the study results. Supported by the National Basic Research and Development Program of China (Grant No. 2007CB815504) and the Technology Research and Development Program of China (Grant No. 2007AA12z343)     

COMPASS time synchronization and dissemination—Toward centimetre positioning accuracy

In this paper we investigate methods to achieve highly accurate time synchronization among the satellites of the COMPASS global navigation satellite system (GNSS). Owing to the special design of COMPASS which implements several geo-stationary satellites (GEO), time synchronization can be highly accurate via microwave links between ground stations to the GEO satellites. Serving as space-borne relay stations, the GEO satellites can further disseminate time and frequency signals to other satellites such as the inclined geo-synchronous (IGSO) and mid-earth orbit (MEO) satellites within the system. It is shown that, because of the accuracy in clock synchronization, the theoretical accuracy of COMPASS positioning and navigation will surpass that of the GPS. In addition, the COMPASS system can function with its entire positioning, navigation, and time-dissemination services even without the ground link, thus making it much more robust and secure. We further show that time dissemination using the COMPASS-GEO satellites to earth-fixed stations can achieve very high accuracy, to reach 100 ps in time dissemination and 3 cm in positioning accuracy, respectively. In this paper, we also analyze two feasible synchronization plans. All special and general relativistic effects related to COMPASS clocks frequency and time shifts are given. We conclude that COMPASS can reach centimeter-level positioning accuracy and discuss potential applications.     

SIS accuracy and service performance of the BDS-3 basic system

On December 27,2018,the basic system of the third-generation BeiDou navigation satellite system(BDS-3)completed the deployment of its constellation of 18 MEO networking satellites as well as the construction of the operation control system(OCS)and began to provide basic navigation services to users worldwide.Compared with BDS-2,BDS-3 aims to offer users better navigation signals and higher precision with a series of new technologies.For example,the spaceborne atomic clock of BDS-3 is upgraded for higher performance,the Ka-band inter-satellite link is adopted for inter-satellite ranging and communication,and new B1C and B2a signals are broadcast in addition to B1I and B3I signals(compatible with BDS-2).In addition,a 9-parameter model based on a spherical harmonic function is employed for ionospheric delay corrections.Using the observation data from 18 satellites of the basic system,this paper conducts a comprehensive evaluation of the pseudorange measurement characteristics,signal-in-space(SIS)accuracy of navigation messages and global service capability of BDS-3.The results indicate that the pseudorange measurement multipath effect and observation noise of BDS-3 satellites are better than those of BDS-2;additionally,with the support of inter-satellite links,the user range error(URE)of the BDS-3 satellite broadcast ephemeris is better than 10 cm,the precision of the broadcast clock parameter is better than 1.5 ns,and the SIS accuracy is better than 0.6 m overall.Different from the traditional Klobuchar model,the BeiDou global broadcast ionospheric delay correction model(BDGIM)can provide ionospheric delay corrections better than 70%for worldwide single-frequency users.The service capability evaluation of the basic system consists mainly of the accuracy improvement of the B1I and B3I signals according to BDS-2 as well as the global positioning accuracy of the new signals.These results prove that the BDS-3 basic system has achieved the design goal;that is,both the horizontal and the vertical global positioning accuracies are better than 10 m(95%).In the future,6 MEO satellites as well as 3 GEO satellites and 3 IGSO satellites for regional enhancement purposes will be deployed for full operation;consequently,BDS-3 will definitely provide a higher SIS accuracy and better service capability.     

Analysis on the positioning precision of CAPS

As a newly developed satellite positioning system, the Chinese Area Positioning System (CAPS) is a typical direct sequence spread spectrum ranging system like GPS. The positioning precision of such navigation signals depends on many factors, including the pseudo-code rate, the signal to noise ratio, the processing methods for tracking loops and so on. This paper describes the CAPS link budget, the solution approach for CAPS positioning, focusing on the autocorrelation function feature of C/A code signals. The CAPS signal measurement precision is studied by the software approach together with theoretical analysis of the range resolution. Because the conventional Delay Lock Loop (DLL) is vulnerable to the impact of noise, a narrow correlator and multiple correlators as well as the corresponding discrimination methods of phases are proposed, which improves the robustness of DLL and the code-phase resolution of the measurement. The results show that the improvement of the DLL structure and the discrimination method are the most important way to improve the ranging resolution. Theoretical analysis and experimental results show that a CAPS receiver could reach a 20-m positioning precision by using three satellites with a supported height from an altimeter. Supported by the National Key Basic Research and Development Technology Plan (Grant No. 2007CB815500) and the National Natural Science Foundation of China (Grant No. 60802018)     

Functions of retired GEO communication satellites in improving the PDOP value of CAPS

This paper briefly introduces the maneuverable feature of the slightly inclined geosynchronous orbit (SIGSO) satellites under a new control model degraded from the geosynchronous orbit (GEO) communication satellites which will retire as most of the fuel in these satellites has been consumed. Basing on the transmitting Chinese Area Positioning System (CAPS), the authors, by analyses, indicate that such satellites can make an improvement to CAPS constellation configuration, especially to the PDOP value from simulation. The results show that the use of SIGSO satellites can (1) actualize three-dimensional (3D) navigation and positioning compared with the situation, which, only using GEO satellites, cannot be carried out, and improve navigation and positioning accuracy to some extent; (2) reuse the communication services of these satellites for more years, and GEO communication satellites will be retired at a later time and delay their time to become space debris and reduce their pollution of the space environment, so that valuable space resources are maximally used. As for the use of these satellites in the transmitting positioning system, the authors present some views and suggestions in this work. Supported by the National Basic Research and Development Program of China (Grant No. 2007CB815501) and the Chinese National Programs for High Technology Research and Development (Grant No. 2007AA12z343)     

双星光学观测体系的目标定位误差分析

为提高双星光学观测体系的定位精度,构建了新型双星光学定位系统。通过对卫星、光电观测平台的建模,构建了地惯系下平台与目标间的观测矢量模型。利用几何定位算法,推导出了地惯系下的目标定位模型与定位误差模型,并利用蒙特卡罗法获得了定位误差分布。在此基础上,引入了小波理论进行误差的优化重构,以提高双星光学观测体系的定位精度。利用测量数据进行仿真,结果表明,引入小波理论对目标定位误差进行降噪重构后,可以使目标定位精度提高30%,为工程上减小目标定位误差提供了新的思路。     

空间弥散X射线对脉冲星导航精度的影响

基于RXTE卫星天基数据,建立了时空坐标系转换、时间修正及历元折叠方法,构建了用于提取导航信息的Crab脉冲星轮廓,剖析了该天基载荷结构及特性,对卫星运行空间背景辐射进行了模拟计算。结果表明,在设定导航条件下,空间弥散X射线对航天器单星定轨及多星定位影响在km量级以上。同时阐述了实用化脉冲星导航探测中,改进导航定位精度急需注意的技术问题。     

激光测距卫星的质心改正模型

卫星质心改正值(简称CoM,Center-of-Mass)是卫星激光测距系统中的一个重要参量,准确估算该参量有助于提高卫星激光测距精度.本文以Lageos卫星为例,应用概率理论以及卫星上的角反射器对光子的反射概率与反射器的光学截面成正比,建立了球形激光卫星的CoM数学模型,并计算了几颗球形卫星的CoM值,还对一般的情况进行了讨论.     

Barometric altimetry system as virtual constellation applied in CAPS

This work describes the barometric altimetry as virtual constellation applied to the Chinese Area Positioning System (CAPS), which uses the transponders of communication satellites to transfer navigation messages to users. Barometric altimetry depends on the relationship of air pressure varying with altitude in the Earth’s atmosphere. Once the air pressure at a location is measured the site altitude can be found. This method is able to enhance and improve the availability of three-dimensional positioning. The difficulty is that the relation between barometric pressure and altitude is variable in different areas and under various weather conditions. Hence, in order to obtain higher accuracy, we need to acquire the real-time air pressure corresponding to an altimetric region’s reference height. On the other hand, the altimetry method will be applied to satellite navigation system, but the greatest difficulty lies in how to get the real-time air pressure value at the reference height in the broad areas overlaid by satellite navigation. We propose an innovational method to solve this problem. It is to collect the real-time air pressures and temperatures of the 1860 known-altitude weather observatories over China and around via satellite communication and to carry out time extrapolation forecast uniformly. To reduce data quantity, we first partition the data and encode them and then broadcast these information via navigation message to CAPS users’ receivers. Upon the interpolations being done in receivers, the reference air pressure and temperature at the receiver’s nearby place is derived. Lastly, combing with the receiver-observed real air pressure and temperature, the site’s altitude can be determined. The work is presented in the following aspects: the calculation principle, formulae, data collection, encoding, prediction, interpolation method, navigation message transmission together with errors causes and analyses. The advantages and shortcomings of the technique are discussed at the end. Supported by the National Basic Research Program of China (Grant No. 2007CB815500), the National High Technology Research and Development Program (Grant No. 2004AA105030), the Pilot Project of the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KGCX1-21), and the National Natural Science Foundation of China (Grant No. 10453001)     

Orbit determination and prediction for Beidou GEO satellites at the time of the spring/autumn equinox

Geostationary(GEO) satellites form an indispensable component of the constellation of Beidou navigation system(BDS). The ephemerides, or predicted orbits of these GEO satellites(GEOs), are broadcast to positioning, navigation, and timing users. User equivalent ranging error(UERE) based on broadcast message is better than 1.5 m(root formal errors: RMS) for GEO satellites. However, monitoring of UERE indicates that the orbital prediction precision is significantly degraded when the Sun is close to the Earth's equatorial plane(or near spring or autumn Equinox). Error source analysis shows that the complicated solar radiation pressure on satellite buses and the simple box-wing model maybe the major contributor to the deterioration of orbital precision. With the aid of BDS' two-way frequency and time transfer between the GEOs and Beidou time(BDT, that is maintained at the master control station), we propose a new orbit determination strategy, namely three-step approach of the multi-satellite precise orbit determination(MPOD). Pseudo-range(carrier phase) data are transformed to geometric range(biased geometric range) data without clock offsets; and reasonable empirical acceleration parameters are estimated along with orbital elements to account for the error in solar radiation pressure modeling. Experiments with Beidou data show that using the proposed approach, the GEOs' UERE when near the autumn Equinox of 2012 can be improved to 1.3 m from 2.5 m(RMS), and the probability of user equivalent range error(UERE)2.0 m can be improved from 50% to above 85%.     

基于STK的BDS星座仿真和性能分析

利用卫星仿真工具包(STK)设计和建立了北斗卫星导航系统(BDS)的全球星座。在此基础上,选取南北半球不同经纬度的11个城市作为观测点,分别获得了BDS和全球定位系统(GPS)的可见星数和几何精度衰减因子(GDOP)的值,并按照5°×5°的空间分辨率,仿真了全球范围内BDS和GPS的可见星数和GDOP值。仿真结果表明,全球星座的BDS具有与GPS同样优良的导航性能,其在亚太地区的性能甚至更优于GPS,完全可以为用户提供高性能的满足所需导航性能(RNP)的导航服务。     

机载北斗/GPS/SINS组合导航系统软硬件设计

针对单一导航导航系统在导航精度、稳定性、设备成本以及导航信息完备性等方面的局限性,设计了卫星导航/惯性导航组合导航系统。针对GPS导航系统受制于人及北斗导航系统发展尚不完善的特点,提出了基于北斗/GPS/SINS的军用机载组合导航系统软硬件设计。搭建了北斗/GPS/SINS组合导航系统硬件平台,采用基于不确定度的加权平均数据融合算法提高组合导航系统的导航可靠性和准确性。仿真结果表明,该组合导航系统稳定性好,可靠性高,定位准确。     

Orbit determination and time synchronization for a GEO/IGSO satellite navigation constellation with regional tracking network

Aiming at regional services,the space segment of COMPASS (Phase I) satellite navigation system is a constellation of Geostationary Earth Orbit (GEO),Inclined Geostationary Earth Orbit (IGSO) and Medium Earth Orbit (MEO) satellites.Precise orbit determination (POD) for the satellites is limited by the geographic distribution of regional tracking stations.Independent time synchronization (TS) system is developed to supplement the regional tracking network,and satellite clock errors and orbit data may be obtai...     

基于GPS/LBS双模定位人体监护系统的研究

据报道每年中国丢失的儿童、老人、智障患者共有80多万,只有极少数可以被安全找回,再者当今社会经常出现老年人摔倒在地而无人搀扶的情况。为解决此类问题,本文设计了一款基于GPS/LBSS的人体定位监护系统。该系统以ARMSCortex-M3S系列的STM32F103ZE为核心处理器,结合GPS卫星定位模块、无线通信模块、三轴加速度计和三轴陀螺仪以及在VC++平台开发的GIS上位机构成监护系统。该监护系统可根据用户的需求实现实时定位、紧急求救、跌倒检测、电子围栏等功能。定位模式采用GPS/LBS双模式定位,提高定位精度;通信方式采取GSM/GPRS双通信模式,无通信盲区,数据传输稳定。