The previous two chapters introduced two methods of high-precision positioning: ground-based differential and satellite-based differential. Both methods are based on the principle of differential positioning. The system includes a base station and a mobile station. The base station generates differential corrections and sends them to the mobile station through a communication link. The mobile station receives the ordinary ephemeris and satellite clock errors broadcast by the satellite, and then corrects the broadcast ephemeris and satellite clock errors with differential corrections to eliminate the residual errors in the ephemeris and satellite clock errors, thereby obtaining high-precision positioning results.
Is there a way to obtain high-precision positioning results without the need for a base station?
Yes, this is non-differential single-machine positioning (PPP).
What is non-differential single-machine positioning?
Key technologies for non-differential single-machine positioning
Accuracy of non-differential single-machine positioning
Comparison with differential positioning
Conclusion
What is non-differential single-machine positioning?
Non-differential single-machine positioning only requires a GNSS receiver, without a radio station or the assistance of other base station systems, to obtain high-precision positioning results with positioning accuracy up to the cm level.
In ordinary single-machine positioning, the main sources of positioning errors are satellite orbit errors broadcast by satellites, satellite clock errors and ionosphere-troposphere errors. The accuracy of satellite broadcast ephemeris data and clock error data directly affect the accuracy of Beidou single-point positioning. According to the spatial information accuracy indicator SISRE (space signal ranging error) in the Beidou Satellite Navigation System Open Service Performance Specification Version 3.0, it is ≤2m.
Key technologies for non-differential single-machine positioning
Non-differential single-machine positioning obtains high-precision satellite orbit data and satellite clock data by obtaining precise ephemeris data. When positioning, it does not directly use the satellite orbit parameters and satellite clock parameters broadcast by the satellite, but uses high-precision satellite orbit data and satellite clock data to obtain higher positioning accuracy.
Non-differential single-machine positioning also needs to eliminate errors in the troposphere and ionosphere by receiving signals from multiple satellite frequency bands.
Acquisition of high-precision ephemeris data:
Obtained through precise ephemeris server: GNSS data processing centers in various countries will regularly publish precise ephemeris and clock error data. GNSS receivers access the GNSS data center at fixed time intervals, read precise ephemeris and clock error data from the data center, and then use them for positioning calculations by GNSS receivers. This method requires networking, and has poor real-time performance for positioning. For example, the server of the IGS data center: ftp://igs.ign.fr/pub/igs/products/mgex/2198/, the server of the GNSS Center of Wuhan University in China: ftp://igs.gnsswhu.cn/
Directly obtain precise ephemeris through satellites: Some satellite navigation systems, such as Beidou and Galileo, can directly broadcast precise ephemeris data through satellite signals. The GNSS receiver on the ground only needs to parse the received navigation signals in real time to obtain these data. The advantage of this method is that it does not require networking and is easy to use. For example, high-precision ephemeris data is broadcast on the B2b band of Beidou satellites and the E5b band of Galileo
Correction of atmospheric errors using multi-frequency observations
High-precision ephemeris data can only eliminate errors in satellite orbits and satellite clocks, but cannot eliminate errors in the ionosphere and troposphere. Therefore, GNSS receivers simultaneously receive satellite signals in multiple frequency bands and use multi-frequency signals to correct the delay effects of the ionosphere and troposphere on signal propagation to obtain higher positioning accuracy.
Accuracy of non-differential single-machine positioning
BeiDou-3 satellites support precise point positioning services, which are provided by the PPP-B2b signals of the three GEO satellites in the BeiDou-3 constellation. The BeiDou system can provide precise point positioning services to China and surrounding areas (the area between 75 and 135 degrees east longitude and 10 and 55 degrees north latitude).
Beidou non-differential single-point positioning uses a multi-frequency Beidou receiver to receive signals from multiple frequency bands of satellites, and simultaneously receives precise ephemeris and clock error data broadcast on the Beidou B2b frequency band. Precise ephemeris provides high-precision satellite orbit position information, and precise clock error compensates for satellite atomic clock deviation. When solving positioning, multi-frequency signals are used to eliminate ionospheric delay errors, and precise ephemeris and clock error data are used for single-point positioning, thereby obtaining positioning results with higher accuracy than conventional single-point positioning.
According to the Beidou Satellite Navigation System Public Service Performance Specification Version 3.0, the PPP precise single-point positioning accuracy is: horizontal accuracy ≤ 0.3m (95%), vertical accuracy ≤ 0.6m (95%).
Comparison with differential positioning
| Compare Projects | Differential positioning | Non-differential single-machine positioning |
|---|
| Base Station | Need to build your own base station | No need to build your own base station |
| Types of data obtained by GNSS receivers | Differential correction information generated by the base station | Precise ephemeris and clock information |
| Use of acquired data | Differentiate the observations from the mobile GNSS receiver using the normal ephemeris and satellite clock information broadcast by the satellite. | Use precise ephemeris and clock error to replace the normal ephemeris and satellite clock information broadcast by satellite for single-machine positioning solution |
| positioning accuracy | The carrier phase differential positioning accuracy can reach cm level, and the pseudo-range differential positioning accuracy is m level. | Using the high-precision ephemeris information broadcast by satellites for single-machine positioning, the positioning accuracy is generally at the meter level;
using the high-precision ephemeris information from the GNSS central server, long-term convergence calculation, the positioning accuracy can reach the cm level |
| Convergence time | Generally less than 1 minute, short time | Generally 20min~30min, long time |
Conclusion
Non-differential single-point positioning does not require the establishment of a separate base station and does not use differential positioning solution methods. It is a non-differential positioning method. It uses multi-band receivers to eliminate atmospheric errors and uses high-precision precise ephemeris and clock errors to obtain high-precision positioning results. The disadvantage is that the real-time performance is poor and the convergence time is long.