Chinese Area Positioning System Research Articles

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.

CAPS satellite spread spectrum communication blind multi-user detecting system based on chaotic sequences

Multiple Path Interference (MPI) and Multiple Access Interference (MAI) are important factors that affect the performance of Chinese Area Positioning System (CAPS). These problems can be solved by using spreading sequences with ideal properties and multi-user detectors. Chaotic sequences based on Chebyshev map are studied and the satellite communication system model is set up to investigate the application of chaotic sequences for CAPS in this paper. Simulation results show that chaotic sequences have desirable correlation properties and it is easy to generate a large number of chaotic sequences with good security. It has great practical value to apply chaotic sequences to CAPS together with multi-user detecting technology and the system performance can be improved greatly.

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.

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.

Precise orbit determination of a maneuvered GEO satellite using CAPS ranging data

Wheel-off-loadings and orbital maneuvers of the GEO satellite result in additional accelerations to the satellite itself. Complex and difficult to model, these time varying accelerations are an important error source of precise orbit determination (POD). In most POD practices, only non-maneuver orbital arcs are treated. However, for some applications such as satellite navigation RDSS services, uninterrupted orbital ephemeris is demanded, requiring the development of POD strategies to be processed both during and after an orbital maneuver. We in this paper study the POD for a maneuvered GEO satellite, using high precision and high sampling rate ranging data obtained with Chinese Area Positioning System (CAPS). The strategy of long arc POD including maneuver arcs is studied by using telemetry data to model the maneuver thrust process. Combining the thrust and other orbital perturbations, a long arc of 6 days’ CAPS ranging data is analyzed. If the telemetry data are not available or contain significant errors, attempts are made to estimate thrusting parameters using CAPS ranging data in the POD as an alternative to properly account for the maneuver. Two strategies achieve reasonably good data fitting level in the tested arc with the maximal position difference being about 20 m.

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.

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.

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.

A Positioning System based on Communication Satellites and the Chinese Area Positioning System (CAPS)

The Chinese Area Positioning System (CAPS) is a positioning system based on satellite communication that is fundamentally different from the 3``G'' (GPS, GLONASS and GALILEO) systems. The latter use special-purpose navigation satellites to broadcast navigation information generated on-board to users, while the CAPS transfers ground-generated navigation information to users via the communication satellite. In order to achieve accurate Positioning, Velocity and Time (PVT), the CAPS employs the following strategies to overcome the three main obstacles caused by using the communication satellite: (a) by real-time following-up frequency stabilization to achieve stable frequency; (b) by using a single carrier in the transponder with 36 MHz band-width to gain sufficient power; (c) by incorporating Decommissioned Geostationary Orbit communication satellite (DGEO), barometric pressure and Inclined Geostationary Orbit communication satellite (IGSO) to achieve the 3-D positioning. Furthermore, the abundant transponders available on DGEO can be used to realize the large capacity of communication as well as the integrated navigation and communication. With the communication functions incorporated, five new functions appear in the CAPS: (1) combination of navigation and communication; (2) combination of navigation and high accuracy orbit measurement; (3) combination of navigation message and wide/local area differential processing; (4) combination of the switching of satellites, frequencies and codes; and (5) combination of the navigation message and the barometric altimetry. The CAPS is thereby labelled a PVT5C system of high accuracy. In order to validate the working principle and the performance of the CAPS, a trial system was established in the course of two years at a cost of about 20 million dollars. The trial constellation consists of two GEO satellites located at E87.5° and E110.5°, two DGEOs located at E130° and E142°, as well as barometric altimetry as a virtual satellite. Static and dynamic performance tests were completed for the Eastern, the Western, the Northern, the Southern and the Middle regions of China. The evaluation results are as follows: (1) land static test, plane accuracy range: C/A code, 15~25 m; P code, 5~10 meters; altitude accuracy range, 1~3 m; (2) land dynamic test, plane accuracy range, C/A code, 15~25 m; P code, 8~10 m; (3) velocity accuracy, C/A code, 0.13~0.3 m s−1, P code, 0.15~0.17 m s−1; (4) timing accuracy, C/A code, 160 ns, P code, 13 ns; (5) timing compared accuracy of Two Way Satellite Time and Frequency Transfer (TWSTFT), average accuracy, 0.068 ns; (6) random error of the satellite ranging, 10.7 mm; (7) orbit determination accuracy, better than 2 m. The above stated random error is 1σ error. At present, this system is used as a preliminary operational system and a complete system with 3 GEO, 3 DGEO and 3 IGSO is being established.