RTK (RealTimeKinematic) technology is a combined system composed of GPS measurement technology and data transmission technology, which was formed in the mid-1990s. At present, RTK technology is mainly divided into conventional RTK technology and network RTK technology. Conventional RTK is based on the strong correlation between the error of the rover and the reference station. When the rover is close to the reference station, the error is strongly correlated. At this time, the centimeter level can be obtained by using the observation data of one or several epochs. Accurate positioning results. However, with the gradual increase of the distance between the rover and the reference station, this error correlation is getting worse and worse, and the positioning accuracy drops rapidly. When the distance between the rover and the reference station is greater than 50km, the conventional RTK single epoch solution can generally only reach Decimeter-level accuracy (Li Zhenghang et al., 2002). In order to achieve better accuracy, network RTK technology came into being. The network RTK is composed of a base station network, a data processing center, a data communication link and a mobile station. The reference station needs to be equipped with a dual-frequency full-wavelength GPS receiver. The station coordinates of the reference station should be accurately known and continuously observed at the specified sampling rate, and the observation data should be transmitted to the data processing center in real time through the data communication link. The center calculates error correction information based on the approximate coordinates sent by the rover, and then broadcasts the information to the rover. Compared with conventional RTK technology, network RTK covers a wider range, higher accuracy and reliability, wider application range, and broad prospects.
1. Basic principles of network RTK
In the conventional RTK working mode, there is only one base station, and the distance between the rover and the base station cannot exceed 10km-15km, and there is no redundant base station. In the network RTK, there are multiple reference stations, and users do not need to establish their own reference stations. The distance between the user and the reference station can be extended to hundreds of kilometers. The network RTK reduces error sources, especially those related to distance.
Generally speaking, network RTK can be divided into 3 basic parts. They are data collection at the reference station; the data processing center performs data processing to obtain error correction information; and broadcasts the correction information (Cruddace et al., 2002). First, multiple reference stations collect observation data at the same time and transmit the data to the data processing center. The data processing center has a master computer that can control all reference stations through the network. All the data transmitted from the base station is first eliminated by gross errors, and then the main control computer performs networked calculation of these data. Finally broadcast the correction information to the user. In order to increase reliability, the data processing center will install backup computers to prevent host failures that affect system operation.
Network RTK must have at least 3 base stations to calculate the correction information. The reliability and accuracy of the correction information will be improved as the number of reference stations increases. When there are enough base stations, if a base station fails, the system can still operate normally and provide reliable correction information (El-Mowafy, 2005).
2. Comparison of network RTK technology
Currently, network RTK mainly has the following according to the type of technology and software
Several types: single reference station network mode, virtual reference station technology (VRS), area correction parameter method (FKP), master and auxiliary station technology (MAX), and comprehensive error interpolation method (CBI).
2.1 Single reference station network mode
In principle, this mode is not much different from the reference station during normal GPS operation. Each reference station serves all GPS users within a certain radius. For users who post-processing static tracking data for a long time, with the help of receiving FM subcarriers, broadband fast network communications, and other data communication methods, the DGPS pseudorange differential correction information is provided, for users engaged in quasi-real-time positioning or real-time precision navigation. Said that the service radius can reach tens of kilometers, hundreds of kilometers, or even longer. As for users who need to provide centimeter-level positioning accuracy in real time, the service radius of a single reference station can currently reach 30km.
The advantages of this model are: the initial investment is small; it can be upgraded and expanded at any time; the system is flexible, safe, reliable, and stable; it does not require any additional devices, does not require two-way data communication equipment to report the location of the mobile station, and the construction period is short.
2.2 Virtual Reference Station Technology (VRS)
VRS (virtualreferencestation) is researched and developed by Trimble, USA. In the VRS network, each reference station does not directly send any correction information to the mobile user, but sends all the original data to the control center through the data communication link. At the same time, the user sends a rough coordinate to the control center before working. After the control center receives this position information, the computer automatically selects the best set of fixed reference stations based on the user’s location. According to the information sent by these stations, the overall Correct GPS orbit errors, errors caused by ionosphere, troposphere and atmospheric refraction, and send high-precision differential signals to users. The effect of this differential signal is equivalent to generating a virtual reference base station next to the mobile station, thereby solving the limitation of RTK operating distance and ensuring user accuracy (Wang Ping, 2001).
The virtual reference station method is to try to establish a virtual reference station near the rover station, and calculate the virtual observation value on the virtual reference station based on the actual observation values on the surrounding reference stations. Since the virtual reference station is very close to the rover, it is generally only a few meters to tens of meters apart. Therefore, dynamic users only need to use conventional RTK technology to perform real-time relative positioning with the virtual reference station and obtain more accurate positioning results. If the data processing center of the network RTK can broadcast the observation values and station coordinates of the virtual reference station in the data format used in the conventional RTK, then the dynamic users in the network RTK can use the original conventional RTK software for data processing. In the virtual reference station method, dynamic users also need to perform single-point positioning based on pseudorange observations and broadcast ephemeris to obtain the rough position of the rover and transmit them to the data processing center in real time. The data processing center usually sets the virtual reference station P at this point. At this time, the virtual station P may be about 20m-40m away from the real rover position. The key to virtual reference station technology is how to construct virtual observations. Once a virtual observation value is constructed, it can be treated as a general reference station during data processing.
The emergence of VRS is the result of the rapid development of modern technology. Its representative software is GPSNetwork (Trimble). VRS is not only a GPS product, but a system that integrates Internet technology, wireless communication technology, computer technology and GPS positioning technology. The emergence of VRS reduces the user's positioning cost. Users do not need to set up a reference station by themselves, and only need a simple GPS receiver to achieve centimeter-level positioning. VRS expands the application field of GPS and represents the development direction of GPS. But VRS is not perfect, and there are flaws. In VRS technology, the effects of the ionosphere and troposphere can only be corrected with the help of correction models. The correction effect is easily affected by the outside world, and VRS cannot eliminate the influence of orbital errors.
2.3 Area Correction Parameter Method (FKP)
The regional correction parameter (FKP) method was first proposed by Geo++GmbH in Germany. This method is based on the state space model, adopts the overall network solution, performs non-difference processing on the data with Kalman filtering, and transmits the synchronized observation values collected by all reference stations at each observation instant without differential processing to the data in real time The processing center performs real-time processing to generate a FKP network area correction parameter, and then this FKP parameter is sent to all mobile stations in the service area through the extended RTCM information.
This technology has the same defects as the VRS technology. The ionosphere and troposphere can only be corrected by models, and they are susceptible to external influences. The orbit error cannot be eliminated, and other methods can only be used. In VRS, all reference stations are used to calculate the correction information, while in the FKP method, only the three reference stations closest to the rover station are required (Wang Yanmei et al., 2005).
2.4 Master and Auxiliary Station Technology (MAX)
Master and Auxiliary Station Technology (MAX) is a new generation of reference station network software launched by Swiss Leica based on the "master and auxiliary station concept". The basic idea of MAX technology is to calculate the dispersive and non-dispersive differential correction number of the auxiliary station relative to the main station, and use the correction number of the main station and the relative correction number of the auxiliary station to calculate the error of the rover, and the observation value of the rover is After correction, high-precision positioning is performed.
The basic requirement of the primary and secondary station technology is to simplify the phase distance of the reference station to a common level of unknowns for the whole week. If relative to a certain satellite and receiver "pair", the whole week unknown of the phase distance has been eliminated or adjusted, then when the double difference is formed, the whole week unknown is eliminated. At this time, We can say that 2 reference stations have a common whole-week unknown level. Main network processing software
The task is to reduce the whole-week unknowns of the phase distance of all reference stations in the network (or sub-network) to a common level. Once this task is completed, it is then possible to calculate the dispersive and non-dispersive errors for each satellite-receiver pair and for each frequency (Wu Xinghua et al., 2005).
2.5 Comprehensive Error Interpolation (CBI)
The Comprehensive Error Interpolation Method (CBI) was proposed by the Satellite Navigation and Positioning Technology Research Center of Wuhan University. In the process of GPS observation, due to the influence of ionosphere, tropospheric delay, and satellite orbit errors on observations, the obtained observations inevitably contain many errors. In the process of network RTK differential algorithm research, it is difficult to distinguish the effects of many errors and make a single accurate calculation or correction, but their comprehensive effects can be uniformly calculated or eliminated by simple and accurate methods. Based on the above reasons, the concept of comprehensive error was proposed. The comprehensive error here, to be precise, refers to the comprehensive effect of all system errors in GPS observations except observation noise. Because the observation noise is generally small and random, it can sometimes be said that the combined error is the combined effect of all errors in the observation value.
The synthetic error interpolation method is based on the strong correlation of multiple system errors in a certain area, and a certain algorithm is used to directly interpolate the data of any rover in the area through the known errors of multiple reference stations. Comprehensive error. Using the integrated error interpolation method only needs one epoch of data to well eliminate the double-difference integrated error of the rover.
CBI technology utilizes the advantages of double-difference combination. It does not distinguish errors caused by ionosphere and tropospheric delay when calculating correction information at the reference station. Instead, each monitoring center centralizes the selection, calculation and broadcast of all reference station observation data to users. Comprehensive error information. Because a variety of errors have a strong linear correlation between the main and auxiliary stations, the combined error is used to represent the combined effect of all system errors in the double-difference observation equation. This technology uses the correlation of satellite positioning errors to calculate the combined effect on the reference station Error, and interpolate the synthetic error of the user station. Research shows that the weakest point of the accuracy of the synthetic error interpolation method is located in the middle of the baseline of the reference station, for errors that vary linearly with different positions, such as ionospheric delay and orbital errors , Can basically completely eliminate its influence, and at the same time eliminate most of the errors that do not conform to linear changes, such as system errors such as tropospheric delay. The representative software of the comprehensive error interpolation method is PowerNET (Wuhan University GNSS Center).
Compared with the network RTK technology discussed above, the CBI method has greater advantages. Aiming at the impact of ionospheric and tropospheric delays, the CBI method does not use the model to correct, but directly corrects the error. The correction effect is less affected by the outside world. Other methods are needed to eliminate or weaken the influence of track errors.
3. Conclusions and prospects
At the time when network RTK technology is booming, many provinces and cities in China have established a complete CORS system based on network RTK technology. Network RTK technology is improving day by day, and the scope of application continues to expand. However, because the technology used in network RTK is not very mature, there are still many problems in application. There are no corresponding standards for network RTK technology in the world, and systems built in various places The data processing methods and data formats are different, resulting in poor system compatibility; there are still many problems in the generation of error models. The errors that occur under the conditions of strong ionospheric and tropospheric activities are still a problem that affects practical applications; in the network RTK Network stability is the main factor that affects positioning accuracy. Therefore, network stability should be ensured as much as possible. The future network RTK technology will develop in the direction of long distance, high precision, multi-frequency and multi-mode, and high stability.