Discussion on the Application Prospect of GPS Technology in Highway Surveying

1 Current Status of GPS Technology Development

The Global Positioning System (GPS) is a satellite navigation system jointly developed by the U.S. Army, Navy and Air Force. It has global, all-weather, continuous, real-time navigation positioning and timing functions, and can provide precise three-dimensional coordinates, speed and time for all types of users. Single-point navigation positioning and relative geodetic positioning are two aspects of GPS applications; for conventional measurements, relative geodetic positioning is the main application method.

Relative geodetic positioning uses L1 and L2 carrier phase observations to achieve high-precision measurements. Its principle is to use the local difference method of carrier phase measurement: calculate the primary difference between receivers, calculate the secondary difference between the receiver and the satellite observation epoch, and calculate the length of the baseline to be determined through the two differential calculations; solving the integer ambiguity is its key technology. According to the algorithm model, static, fast static and RTK operation modes are designed. Static operation mode is mainly used for high-precision measurement such as crust deformation observation, national geodetic survey, dam deformation observation, etc. Rapid static measurement is widely used in general engineering measurement with its high efficiency and centimeter-level accuracy; and RTK measurement is widely used in data collection (such as fragment measurement) and engineering layout with its fast real-time and centimeter-level accuracy. RTK technology represents the mainstream of GPS relative geodetic positioning application.

GPS geodetic receiving equipment is the basic condition for realizing geodetic positioning. The receiver is divided into single-frequency and dual-frequency. The dual-frequency machine can correct the ionospheric refraction effect with L2 observation value, which is most suitable for medium and long baseline (greater than 20km) measurement, has the function of rapid static measurement, and can be upgraded to RTK function; the single-frequency machine is suitable for short baseline measurement less than 20km, and has a good performance-price ratio for general engineering measurement. The RTK system consists of GPS receiving equipment, radio communication equipment, electronic handheld and supporting equipment. The whole set of equipment has the characteristics of light weight, easy operation, real-time reliability, centimeter-level accuracy, etc., which can fully meet the requirements of data collection and engineering layout. Given the limited number of satellites in orbit in the GPS system, normal calculation cannot be guaranteed when the line of sight to the sky is blocked, which affects the accuracy and reliability of positioning. Practice has shown that the single-frequency GPS system has great limitations due to the constraints of multiple environments. With the continuous improvement of Russia's global navigation satellite system (CLONASS), the dual-constellation system (GLONASS+GPS) that uses GLONASS to improve GPS performance has been successfully developed by Ashtech in the United States. This all-weather, all-region, high-precision system provides users with more complete receiving equipment, and the receiving equipment of the dual-constellation system has reached a new level of GPS receiving equipment.

2 Application prospects of GPS technology in highway surveying

With the implementation of the Western Development Program and the rapid growth of China's national economy, the construction of high-grade highways in our province has ushered in unprecedented development opportunities, which has put forward higher requirements for survey and design. With the development of software technology and hardware equipment in the highway design industry, highway design has been realized in CAD, and some software itself requires the support of ground digital surveying and mapping products; establishing an integrated data chain of survey, design, construction, and post-management, and reducing intermediate links such as data transfer and input, is the requirement of "integration of internal and external industries" in highway survey and design, and is also the "bottleneck" that affects the development of high-grade highway design technology. Although advanced instruments and equipment such as electronic total stations have been used in highway surveys, conventional measurement methods are limited by lateral visibility and operating conditions, with high operating intensity and low efficiency, which greatly prolongs the design cycle. The progress of survey technology lies in the introduction of equipment and technical transformation. Under current technical conditions, the introduction of GPS technology should be the first choice. At present, GPS static or fast static methods are used to establish overall control surveys along the route, providing a basis for surveying and mapping strip topographic maps, route plane and longitudinal measurements in the survey phase; and to establish construction control networks for bridges and tunnels in the construction phase. This is only the initial stage of GPS application in highway surveying. In fact, the technical potential of highway surveying lies in the application of RTK (real-time dynamic positioning) technology. The application of RTK technology in highway engineering has a very broad prospect. The following is a brief introduction to the application of RTK technology in highway surveying.

3 Application of RTK technology in highway surveying

3.1 Introduction to real-time dynamic (RTK) positioning technology

Real-time dynamic (RTK) positioning technology is a real-time differential GPS (RTDGPS) technology based on carrier phase observation values. It is a new breakthrough in the development of GPS measurement technology and has broad application prospects in highway engineering. As we all know, whether it is static positioning or quasi-dynamic positioning, due to the lag in data processing, it is impossible to solve the positioning results in real time, and it is also impossible to check the observation data, which makes it difficult to ensure the quality of the observation data. In actual work, it is often necessary to rework to re-measure the unqualified observation results caused by gross errors. The main way to solve this problem is to extend the observation time to ensure the reliability of the measurement data, which reduces the efficiency of GPS measurement. The

real-time dynamic positioning (RTK) system consists of a base station and a mobile station. Establishing wireless data communication is the guarantee of real-time dynamic measurement. Its principle is to take the first-level control point with higher point accuracy as the base point, place a receiver as a reference station, and continuously observe the satellite. The receiver on the mobile station receives the observation data on the base station through the radio transmission equipment while receiving the satellite signal. The random computer calculates and displays the three-dimensional coordinates and measurement accuracy of the mobile station in real time according to the principle of relative positioning. In this way, users can monitor the data observation quality of the measured point and the convergence of the baseline solution results in real time, and determine the observation time according to the accuracy index of the measured point, thereby reducing redundant observations and improving work efficiency. [page]

3.2 Application

Real-time dynamic (RTK) positioning has two measurement modes: fast static positioning and dynamic positioning. The combination of the two positioning modes can be applied in highway engineering to cover highway survey, construction layout, supervision and GIS (geographic information system) front-end data collection.

3.2.1 Fast static positioning mode.

It is required that the GPS receiver be stationary at each mobile station for observation. During the observation process, the synchronous observation data of the base station and the satellite are received at the same time, and the unknown numbers of the whole cycle and the three-dimensional coordinates of the user station are solved in real time. If the change of the solution result tends to be stable and its accuracy meets the design requirements, the real-time observation can be ended. It is generally used in control measurement, such as control network encryption; if conventional measurement methods (such as total station measurement) are used, they are greatly affected by objective factors and are difficult to implement in areas with relatively harsh natural conditions. The use of RTK fast static measurement can achieve twice the result with half the effort. Single-point positioning only takes 5-10 minutes (with the continuous development of technology, the positioning time will be shortened), which is less than one-fifth of the time required for static measurement. In highway measurement, it can replace the total station to complete the control point encryption work such as traverse measurement.

3.2.2 Before dynamic positioning

measurement, it is necessary to observe statically at a control point for several minutes (some instruments only need 2 to 10 seconds) to perform initialization work. After that, the mobile station can automatically observe according to the predetermined sampling interval, and determine the spatial position of the sampling point in real time together with the synchronous observation data of the base station. At present, its positioning accuracy can reach centimeter level.

The dynamic positioning mode has broad application prospects in the highway survey stage, and can complete topographic mapping, center pile measurement, cross section measurement, longitudinal section ground line measurement and other tasks. The accuracy can reach 1-3cm in 2-4S measurement, and the entire measurement process does not require line of sight, which has incomparable advantages over conventional measuring instruments (such as total stations).

3.3 Advantages of RTK technology

3.3.1 Real-time dynamic display of centimeter-level precision measurement results (including elevation) that have been verified for reliability.
3.3.2 Completely get rid of rework caused by gross errors and improve GPS operation efficiency.
3.3.3 High operation efficiency, each stakeout point only needs to stay for 1-2s, and the mobile station group operation, each group (3-4 people) can complete the center line measurement of 5-10km. If it is used for topographic measurement, each group can complete 0.8-1.5km3 of topographic mapping per day, and its accuracy and efficiency are unmatched by conventional measurement.
3.3.4 The center pile leveling work is completed while the center line is being laid out.
3.3.5 Wide application range - it can cover highway surveying (including horizontal, longitudinal and horizontal), construction layout, supervision, completion surveying, maintenance surveying, GIS front-end data collection and many other aspects.
3.3.6 If assisted by the corresponding software, RTK can work in conjunction with the total station to give full play to the respective advantages of RTK and total station.

3.4 Promotion suggestions

3.4.1 The method combining GPS static positioning technology and dynamic positioning technology can complete the highway plane control measurement efficiently and accurately.
3.4.2 The production process using conventional methods combined with GPS technology can greatly improve production efficiency.
3.4.3 With the development of GPS technology, various manufacturers have successively launched instruments with independent patented technologies, which have shorter initialization time, stronger tracking ability, higher accuracy, stronger reliability, and good cost performance. They have the trend of replacing total stations in survey and design units, and this factor should be considered when updating unit equipment.
3.4.4 The application of GPS technology in highway surveying is a revolutionary technological innovation in highway surveying, which will update the traditional operation concept.

4 Conclusion

The application of GPS in highway surveying has brought about a revolutionary change in the surveying means and operation methods of high-grade highways, greatly improving the surveying accuracy and efficiency. In particular, the real-time dynamic (RTK) positioning technology will have broad application prospects in highway surveying, construction, and later maintenance and management. (end)