With the rapid development of social economy, science and technology, the bridge-building technology has been continuously improved, and the bridge structure has gradually developed to be light and slender. At the same time, the load, span and deck width of the bridge are increasing, and the structural type is also changing. Traditional deformation monitoring methods can no longer meet the requirements of deformation monitoring, and more reliable equipment is urgently needed to monitor the deformation of bridges. At present, with the continuous maturity of GPS technology, GPS automatic monitoring system has been applied to bridges, buildings, earthquakes, dams and other industries, and achieved good benefits. The instruments of GPS automatic monitoring system are praised by experts for their excellent performance.
Working principle and application of 1.GPS
The working principle of 1. 1GPS
Global Positioning System (GPS) is a new generation of dual-use satellite navigation and positioning system developed by the US Department of Defense. The system was developed in the early 1970s, and the American government announced in April 1995 that the system had been established and put into operation. Global Positioning System (GPS) is another huge space program of the United States after Apollo moon landing program and space shuttle program. Its appearance not only revolutionized navigation technology and positioning technology, but also had a great impact on transportation, space technology, geoscience research, military affairs and all aspects of social life.
The satellite constellation of GPS is designed to consist of 24 satellites (in fact, 27 to 32 satellites are often maintained at present). They are distributed in six orbits with an interval of 60 degrees and an inclination of 55 degrees, and four satellites are evenly distributed on each orbital plane. The altitude of this satellite on the ground is 20200 kilometers. The satellite constellation distributed in this way can ensure that the receiver can receive more than four satellite signals at any time and at any position for instantaneous positioning observation.
The difference is the relative positioning between two or more observation stations, as shown in figure 1. If two points A and B observe the same group of satellites at the same time (at least four satellites). Moreover, a is a known point. The position of point B can be determined by transmitting the original correction information to point B through some data links.
The GPS receiver receives signals (electromagnetic waves) from at least four aerial GPS positioning satellites on the ground. According to the time difference between the positioning signal and the GPS receiver, the GPS receiver can calculate its precise distance from the satellite. Because the position of GPS positioning satellite in the sky is known, the position of GPS receiver on the ground (latitude, longitude, altitude, etc. ) can be converted from this position and the distance obtained by the formula just now.
1.2 Necessity of implementing GPS monitoring
Deformation monitoring is a key project in bridge safety monitoring system. The traditional deformation monitoring system has played an important role in ensuring the normal operation of the project, but it has the following defects:
(1) A large number of manual data collection methods are used, with few automatic measuring points, low degree of automation, heavy workload, and observation is easily affected by external conditions such as climate, and it is easy to miss important and dangerous signals;
(2) The deformation of each monitoring point is not synchronous in time;
(3) collecting plane displacement and vertical displacement data at different measuring points and at different times;
(4) The precision leveling network has a long route.
Using GPS monitoring system to monitor bridge deformation can overcome the defects of traditional monitoring system, and the accuracy can meet the specification requirements. Moreover, we can fully understand the changes of bridges in various periods, even instantaneous changes, and realize continuous observation and automatic data processing. It can effectively grasp the running state of the bridge, find problems in time, ensure the safety of the bridge and provide more reliable safety monitoring data for the bridge.
The mainstream observation method of 1.3GPS -RTK technology
From the research and application at home and abroad, it can be seen that GPS is a very effective bridge monitoring technology, and it has become a trend to combine GPS with other sensors for bridge health monitoring. Its observation methods are mainly divided into two schools: single base station RTK mode and real-time unified calculation mode of control center. At present, the common accuracy of GPS monitoring methods in single base station RTK mode in bridge monitoring is 1-3cm, and the data sampling frequency is generally 1Hz.
RTK measurement technology integrates many advanced technologies such as GPS satellite positioning, fast calculation, data wireless transmission and fast tracking, and is widely used in many fields such as railways, highways, buildings, water conservancy and petroleum, mainly because its measurement mode, measurement speed and accuracy have changed greatly compared with previous measurement methods.
(1)RTK overview
RTK(Real Time Kinematic) technology is the abbreviation of GPS real-time carrier phase difference. This is a technology that combines GPS with data transmission technology, calculates and processes data in real time, and obtains high-precision position information within 1 ~ 2s.
(2) the working principle of RTK
The real-time dynamic positioning system consists of a reference station and a mobile station. Establishing wireless data communication is the guarantee of real-time dynamic measurement. Its principle is to take the first control point with high accuracy as the reference point, and place a receiver as the reference station to continuously observe the satellite. The receiver on the mobile station receives the satellite signal and the observation data on the reference station through the radio transmission equipment, and the random computer calculates and displays the three-dimensional coordinates and measurement accuracy of the mobile station in real time according to the relative positioning principle. In this way, the data observation quality of the point to be measured and the convergence of the baseline calculation results can be monitored in real time, and the observation time can be determined according to the accuracy index of the point to be measured, thus reducing redundant observation and improving work efficiency. (The application of VRS system improves work efficiency.
High-precision real-time dynamic (RTK)GPS positioning has brought great changes to the field of measurement and engineering, and brought unprecedented efficiency. But up to now, using RTK system to carry out high-precision survey also means that you must first establish a control point near the survey area, and then establish a reference station on the control point. However, with the maturity and perfection of technology and the birth of virtual reference station VRS system, it is possible to start GPS measurement at any point in the survey area without setting up a reference station.
This brand-new RTK positioning method fundamentally improves the work efficiency and measurement quality. It no longer needs to establish a reference station, thus saving the measurement time and the cost of buying another reference station receiver. In VRS network, common control points have been established, so there is no need to worry about error propagation caused by inaccurate control points. And the initialization of the receiver will be faster, so you can be sure that all the data have been strictly checked before you get the result. It can be said that the generation of VRS system greatly facilitates RTK measurement.
2.GPS is applied to dynamic detection of bridge deformation.
2. 1 Dynamic detection of bridge deformation based on GPS
When designing a bridge, it is necessary to consider the ability to resist external forces, such as wind, traffic, temperature, tides and some unpredictable loads, such as earthquakes, floods and typhoons. These external factors are the main factors to be considered in the design stage and operation stage of the bridge. Different from the settlement of bridge foundation which is easy to be measured by conventional measuring instruments, the dynamic deformation characteristics or deflection deformation of bridges are difficult to achieve good measurement results with conventional instruments, which is the main monitoring content of bridge monitoring. Typical dynamic deformation of large flexible bridges includes lateral vibration caused by wind and vertical movement caused by traffic or environmental temperature changes.
In recent years, many large bridges have been damaged due to overuse. Obviously, the operation time of the bridge and the increasing traffic volume in recent decades are the main reasons. According to the statistics of the Federal Highway Administration (FHWA), nearly half of the bridges are damaged due to structural damage or functional abandonment. Therefore, there is no doubt that the design, construction and operation of bridges, especially suspension bridges, need necessary inspection to reduce the accident rate of bridges. This engineering requirement also urges the need to provide new monitoring systems and methods to ensure the safe operation of bridges. Based on this, this paper puts forward the bridge deformation detection and analysis research based on GPS, aiming at studying the application scope, methods and related technologies of GIS in engineering survey.
2.2 environmental factors affecting the bridge and the importance of GPS detection system
Generally speaking, the external environment that affects the bridge is very bad, which makes maintaining the safety of the bridge a very complicated task. The main environmental factors affecting the bridge are: (1) high-speed tide and wind; (2) It may collide with a moving ship; (3) Bridge corrosion damage caused by air humidity and salinity; (4) Near the edge of the earthquake zone; (5) high-density traffic load; (6) Structural damage caused by material fatigue and time. At present, the traditional monitoring tools include displacement sensor, accelerometer, tilt sensor, laser interferometer, total station, precision level and so on. These methods have achieved certain results, but there are also many shortcomings. For example, the accelerometer can do nothing about the slow displacement of the bridge caused by temperature changes and the large displacement under the influence of strong wind; Laser interferometer, total station and precision level are seriously affected by climate, and the sampling rate is difficult to meet the requirements of dynamic measurement. In recent years, due to the continuous development of GPS technology, especially RTK technology, the sampling rate of its receiver has generally reached tens of hertz, and the positioning accuracy has reached centimeter level or even millimeter level, making it possible to use it for bridge monitoring. The application of GPS in bridge monitoring is not affected by the climate, and it can automatically measure at any time, provide the given result (IK) in real time, and conveniently realize the time synchronization of each measuring point. Therefore, it is of great value to carry out bridge detection based on GPS in current projects.
The application of GPS technology in bridge detection has the advantages of high precision, no limitation of working environment and distance, and high degree of automation, which greatly reduces labor intensity, reduces field workload, greatly improves work efficiency and quality of results, brings great changes to the traditional bridge deformation monitoring operation mode and significantly improves the bridge detection level.
In addition, at present, most bridge monitoring systems have realized automatic data transmission, automatic calculation and processing, quasi-real-time measurement results and graphic display of measurement results. Although it is difficult to use the monitoring method of real-time unified solution mode of control center, its accuracy is good. This monitoring method will be the direction of future research.
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