RTK positioning technology is the use of global satellite navigation system (Global Navigation Satellite System, GNSS) due to the real-time dynamic relative positioning technology, RTK positioning technology has good real-time, high positional accuracy terrain mapping, construction sampling and other fields indispensable equipment.
When the traditional RTK positioning technology bit technology, the first need to manually set up the reference station, and then in the form of a data chain between the reference station and the mobile station to transmit the difference data [2]. However, the effective operating distance is usually only 10 km At the same time, it is blocked by trees and buildings during the operation, which leads to unstable data transmission and reduces the positioning accuracy.
With the improvement of national informatization and the rapid development of computer network and communication technology, many cities have established Continuously Operating Reference Station (CORS) system [3]. This paper combines the shortcomings of traditional data transmission methods CORS system to realize a high-precision system RTK positioning technology. This technology uses 3G network communication technology to establish a data transmission link, making data transmission more stable and reliable, transmission distance is farther, network equipment with low power consumption, small structure, easy to operate.
The basic principle of RTK technology is based on the carrier phase of two measuring stations processed in real time. First, the reference station sends the observed carrier phase measurements and its own coordinate information to the mobile station in real time through the data chain. The mobile station receives the carrier phase of the GPS satellite signal and the carrier phase measurement value of the reference station to utilize the spatial correlation of the observation error between the reference station and the streaming station to reduce most of the errors in the observation data of the streaming station to form a real-time processed phase difference observation value to solve the coordinates of the observation point [4].The schematic block diagram of the RTK localization technology is shown in Figure 1.
2 System structure design and function overview
According to the RTK system structure in the three core modules are: microprocessor module, GPS module, 3G network module. The power supply module is mainly to provide safe and reliable power supply guarantee for each module of the system. The connection of each module is shown in Figure 2.
2.1 S5PV210 microprocessor
The department launched the S5PV210 application processor [5], S5PV210 uses the ARM CortexTM-A8 core, ARM V7 instruction set main frequency up to 1 GHz, with ** 32 / 32-bit internal bus KB512 KB secondary cache can be realized 2 000 DMIPS (200 million instruction sets per second) high-performance operation capability. Terminal to make the Tiny210 core board equipped with S5PV210 processor, 256 MB capacity DRAM, 1 GB of NAND Flash memory chip, its rich peripheral interfaces to facilitate data processing and human-computer command interaction.
2.2 GPS Module
The system's GPS high-performance module acquisition BD970 board with good raw data quality, high positioning accuracy [6], mainly used to receive satellite signals, the output of the received satellite calendar data and observation data, to meet the user's requirements for multi-system multi-frequency data processing. The hot start time is less than 8 s, the recapture time is less than 0.1 s, the power consumption is only 215 mW, the structure is small, the performance is excellent, and it is very suitable for use in embedded systems with high requirements on power consumption and size.
2.3 3G Network Module
The key part of the RTK positioning technology that realizes the data chain system also known as data transmission system. Its main function is to complete the RTK terminal and CORS center data interaction. Now the mobile Internet coverage is very wide, MU based on the low-power design concept of 709 network module, sleep mode current is only 1 mA, and internal integration of TCP / IP protocol and extended TCP / IP protocol, with AT commands are easy to operate [7]. Installed in the network module SIM card, using 3G network communication technology to achieve with CORS wireless communication center.
3 Software system design
The system is designed to function as a 3G network transmission technology replaces the traditional radio transmission technology, ensuring the stability and remote transmission of data and improving the positioning accuracy. The system runs in a high stability and real-time embedded system Linux mainly includes: the main control program, GPS data processing software design, differential data processing software design, data interaction software design.
3.1 Main Control Program
The main control program begins to execute after the system is powered on and started, the system is initialized first. the initialization of the GPS module is mainly the serial port UART0 initialization, that is, set the UART0 transmission frame format and baud rate. According to BD set 970 board hardware characteristics UART0 baud rate of 11 520 bit/s, 8 data bits, no parity verification bit, 1 stop bit. If GPS data is received process; otherwise, reset the serial port and receive data.
In Linux when the system starts, read the script program to start the 3G implementation of the network module ppp dial-up Internet connection CROS At this point, the system center server needs to verify the account and password of the user information and establish a connection after success, otherwise the next connection will be reconnected. After the initialization is completed, enter the next differential data processing section. The flowchart of the main control program is shown in Figure 3.
3.2 GPS Data Processing Module
The purpose of this module is to realize the analysis of positioning information. the BD970 board outputs positioning information in accordance with the NEMA-0813 communication standard format, and the output data is in ASCII code, sentence classification contains $GPGGA, $GPGSA, $GPGSV and so on. Among them, GPGGA sentence is one of the most widely used data, including 17 fields: sentence flag header, world time, latitude, latitude hemisphere, longitude, longitude hemisphere, etc., and the final markers are the carriage return character and the line feed character, which are separated by 14 commas, respectively [9]. The system mainly uses GPGGA statements to contain the information required for system localization, so it is only necessary to read the GPGGA statements.
Receiving the microprocessor BD through the serial port if the raw data of the 970 is saved to the buffer, then the GPGGA is used to begin and end with the beginning and the end, and the data is verified in the correct position, which can be judged as complete and valid data information, and then proceed to the next step of data analysis and processing. If it does not meet the above provisions, then discard the judgment as invalid data, and continue to judge the received data.GPS is shown in Figure 4.
3.3 Differential Data Processing Module
Ntrip (Networked Transport of RTCM via Internet Protocal) is the current CORS the system's main data transmission protocol can be transmitted over the Internet RTK based data HTTP protocol of the general protocol is also based on the network of the TCP/IP protocol based on the protocol, HTTP object for the GPS data stream [ 3].
Ntrip network mainly consists of three parts: client, server and data processing center. Here Ntrip clients and Ntrip servers are only equivalent to clients in HTTP connection and Ntrip processing center is the service processor that responds to them. In CORS network, Ntrip client is the mobile station device Ntrip configuration processing center IP that receives the RTK usage data stream connected to the address Ntrip processing center through the Internet. The flow chart of differential data processing is shown in Figure 5. First, the user needs to send the account and password to the control center for user access, the mobile station can request differential data. Then analyze whether the differential data is valid from the buffer to determine whether the verification location is correct. If it meets the requirements, save the location information, otherwise, request the differential data from the former CORS center again.
3.4 Data Interaction Module
This part mainly adopts Bluetooth communication technology to realize the data interaction between RTK terminal and user [10]: the terminal sends the final positioning results to the user in real time, and the user can also send operation commands to the terminal via Bluetooth.
Since the rate at which the bit board solves the output positioning results is different from the data output rate of Bluetooth, a multi-threaded model is designed to cache the data in combination with a circular queue with locks in order to avoid data overwriting and loss. When data arrives, the write thread acquires a mutual exclusion lock at the end of the queue and writes the data to the cache, releasing the mutual exclusion lock as soon as a complete positioning count knot is written. At this time, the read thread will obtain the mutual exclusion lock at the head of the queue, read the complete data, and then send it via Bluetooth and release the mutual exclusion lock. As shown in Figure 6.
4 Result Analysis
The Yifu Science and Technology Building of Chongqing University of Posts and Telecommunications was chosen as the terminal's availability and stability in the actual application environment. It is divided into two cases: without shade and with shade. Placing the terminal to work in the open area without shading will be carried out on the roof of the Yifu Science and Technology Building. Placement of the terminal next to a large tree in front of the Shaw Tech building as a shade. Measure and save the final positioning data in outdoor positioning and then analyze the accuracy of the positioning data in the host computer. To ensure the independence of each time, shut down the computer before starting the next time.
4.1 Realize fixed time analysis
RTK When the positioning result of the terminal reaches the fixed solution, the output data is valid, so the time required to realize the fixed solution is an important index to measure the performance of the terminal. The sixth field in the final positioning result indicates the GPS status, with 0 indicating no positioning, 4 indicating a fixed solution, and 5 indicating a floating point solution.
The terminal observes a set of GPS data corresponding to the moment called calendar element, every 1 time or so s a set of GPS can be observed in the actual measurement process, the number of observed calendar elements to indicate the working time of the terminal. Figure 7 shows the time state output valid data required to reach a fixed solution without masking and always in a fixed solution state. Fig. 8 shows the time required for fixed solution, where the terminal will be in the floating-point solution state for a certain period of time (around 10 s) and then reach the fixed solution state, output valid data, and always be in the fixed solution state. The results show that when the satellite signal and data transmission chain are blocked, the terminal can also realize fixed solution in a short time to meet the actual demand.
4.2 Analysis of positioning accuracy
In terms of positioning accuracy, it is necessary to analyze the positioning data output by the receiver after fixation solution. The average value of all the fixation solutions of each group of data is taken as the theoretical value of the position, and then the deviation value and the maximum deviation value of the positioning result and the theoretical value of each group of data are calculated MATLAB After analyzing the data, the results shown in Fig. 9 and Fig. 10 are obtained.
In different environments in different environments, the acquisition time of each group of data is about 5 min (about 300 calendar elements). The results were statistically analyzed without masking through Table 1 and with masking through Table 2.
From analyzing the deviation averages in Table 1 and Table 2, it can be seen that the overall localization results in both cases are relatively stable and the errors are very close to each other, both being 1 cm within the allowable error range.
By analyzing the maximum deviation value in Table 1, the fluctuation is small and the positioning accuracy is high, the maximum deviation value is about 2 cm, but the error of the calendar element is mostly centimeter level within the allowable error range. By analyzing the maximum deviation value in Table 2, it can be seen that when there is shielding, the differential data transmission is affected part of the time due to the fact that it affects the received satellite signals, leading to fluctuations in the positioning error. Therefore, there is sometimes a great fluctuation in the process, and the maximum deviation value is about 2.9 cm, and the fluctuation range is within the permissible range of accuracy.
In general, both unshielded and shielded environments will have some fluctuations in the process of positioning results, but the overall experimental results show that the RTK positioning system based on the BD970 board can achieve centimeter-level positioning accuracy, and the positional accuracy required for earth measurements is also at centimeter level.
5 Conclusion
Aiming at the geographical environment, effective distance, meteorology and other factors that can easily affect the radio transmission of differential data, this paper realizes practical RTK technology based on the Linux embedded platform, and develops and completes the RTK terminal system by combining the GPS positioning technology with the 3G wireless communication technology to improve the reliability and stability of the differential data transmission, and to improve the positioning accuracy. Bluetooth communication technology is used to provide good user experience and realize visualization function. The use of high-performance embedded technology realizes miniaturization and integration, improves the intelligence of the system, and facilitates operation. Through the analysis of different environments, this paper realizes that the resultant RTK positioning system is feasible for the application scenarios with high stability without obstruction or shade, and can provide cm-level high-precision positioning results to meet the needs of practical applications such as earth surveying. The device is small in size, low power consumption, easy to install in small UAVs and other carriers for positioning and navigation, the application prospect is broad.
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