What is the global positioning system?

GPS, also known as Global Positioning System (GPS), was developed by the United States in the 1970s.1March 1994 spent 20 years and 20 billion dollars to complete the overall deployment, realizing its all-weather, high-precision and global coverage. Now, with the combination of GPS and modern communication technology, the method of measuring the three-dimensional coordinates of the earth's surface has greatly expanded its application breadth and depth from static to dynamic data post-processing to real-time positioning and navigation. Carrier phase difference GPS technology can greatly improve the relative positioning accuracy. The accuracy of centimeter level can be achieved in a small range. In addition, because GPS measurement technology is more flexible and convenient than conventional measurement methods in visibility and geometry, it can be used to measure various levels of control networks. The reverse development of GPS total station is in topographic and land survey, as well as various engineering, deformation,; Surface subsidence monitoring has been widely used, showing great advantages in accuracy, efficiency and cost.

Composition of (1)GPS system

The GPS system includes three parts: the space part-GPS satellite constellation; Ground control part-ground monitoring system; User equipment part --GPS signal receiver.

GPS satellite constellation:

A GPS satellite constellation consisting of 2/kloc-0 working satellites and 3 in-orbit standby satellites is called (2 1+3) GPS constellation. These 24 satellites are evenly distributed on six orbital planes, with the orbital inclination of 55 degrees and the orbital planes separated by 60 degrees, that is, the ascending intersection points of the orbits are separated by 60 degrees right ascension. The elevation distance between the satellites in each orbital plane is 90 degrees-the satellites in the orbital plane are 30 degrees ahead of the corresponding satellites in the adjacent orbital planes in the west.

For a GPS satellite at an altitude of 20,000 kilometers, once the earth rotates for a star, it turns around the earth twice, that is, the time to turn around the earth is 12 stars. In this way, ground observers will see the same GPS satellite four minutes in advance every day. The number of satellites above the horizon varies with time and place. At least 4 satellites can be seen, and at most 1 1 satellite can be seen. When navigating and positioning with GPS signals, in order to determine the three-dimensional coordinates of the station, four GPS satellites must be observed, which are called positioning constellations. The geometric position distribution of these four satellites in the observation process has certain influence on the positioning accuracy. For a place where even the precise point coordinates can't be measured at a certain moment, this time period is called "gap section". But this time difference is very short, which does not affect the all-weather, high-precision, continuous and real-time navigation and positioning measurement in most parts of the world. The number of GPS working satellites is basically the same as that of test satellites.

Ground monitoring system:

For navigation and positioning, GPS satellite is a dynamic known point. The position of the constant star is calculated according to the ephemeris transmitted by the satellite, which describes the motion and orbit parameters of the satellite. The ephemeris broadcast by each GPS satellite is provided by the ground monitoring system. Whether all kinds of equipment on the satellite work normally and whether the satellite has been running along the predetermined orbit should be monitored and controlled by the ground equipment. Another important function of the ground monitoring system is to keep all satellites in the same time standard-GPS time system. This requires the ground station to monitor the time of each satellite and find out the clock difference. Then it is sent to the satellite by the ground injection station and then sent to the user equipment by the navigation message. The ground monitoring system of GPS working satellite includes a main control station, three injection stations and five monitoring stations.

GPS signal receiver:

The task of the GPS signal receiver is to capture the signals of the satellites to be measured selected according to a certain cut-off angle of the satellite height, track the operation of these satellites, transform, amplify and process the received GPS signals, thus measuring the propagation time of the GPS signals from the satellites to the antenna of the receiver, interpreting the navigation messages sent by the GPS satellites, and calculating the three-dimensional position, even the three-dimensional speed and time of the station in real time.

The navigation and positioning signal sent by GPS satellite is an information resource that can be enjoyed by countless users. For the vast number of users on land, sea and space, as long as users have receiving equipment that can receive, track, convert and measure GPS signals, that is, GPS signal receivers. GPS signals can be used for navigation and positioning measurement at any time. According to the purpose, different users need different GPS signal receivers. At present, dozens of factories around the world have produced hundreds of GPS receiver products. These products can be classified according to their principles, uses and functions.

In static positioning, the GPS receiver is fixed in the process of capturing and tracking GPS satellites. The receiver measures the propagation time of GPS signal with high precision, and calculates the three-dimensional coordinates of the receiver antenna position by using the known position of GPS satellite in orbit. Dynamic positioning is to measure the trajectory of a moving object with a GPS receiver. The moving objects where the GPS signal receiver is located are called carriers (such as ships sailing, planes in the air, vehicles, etc.). ). The GPS receiver antenna on the carrier is relative to the earth in the process of tracking GPS satellites, and the motion receiver uses GPS signals to measure the state parameters (instantaneous three-dimensional position and three-dimensional speed) of the moving carrier in real time.

The receiver hardware, internal software and GPS data post-processing software package constitute a complete GPS user equipment. The structure of GPS receiver is divided into two parts: antenna unit and receiving unit. For geodesic receivers, these two units are generally divided into two independent parts. During observation, the antenna unit is placed on the station, the receiving unit is placed in an appropriate place near the station, and the two units are connected into a whole machine by cable. Some also make the antenna unit and the receiving unit into a whole and put them on the test site during observation.

GPS receivers generally use batteries as power supply. At the same time, two kinds of DC power sources inside and outside the machine are used. The purpose of setting the internal battery is not to interrupt the continuous observation when replacing the external battery. In the process of using the external battery, the internal battery will automatically charge. After shutdown, the battery in the machine supplies power to the RAM memory to prevent data loss.

In recent years, many types of GPS geodetic receivers have been introduced in China. When various types of GPS geodetic receivers are used for precise relative positioning, the accuracy of dual-frequency receivers can reach 5MM+ 1PPM. The accuracy of single frequency receiver can reach 10MM+2PPM. Within a certain distance. The accuracy of differential positioning can reach sub-meter level to centimeter level.

At present, various types of GPS receivers are getting smaller and lighter, which is suitable for field observation. GPS and GLONASS compatible global navigation and positioning system receivers have come out.

(2) the positioning principle of 2)GPS

The basic positioning principle of GPS is that satellites constantly send their own ephemeris parameters and time information. After receiving this information, the user calculates the three-dimensional position, three-dimensional direction, movement speed and time information of the receiver.

(3) Characteristics of 3)GPS system

GPS system has the following main characteristics: high precision, all-weather, high efficiency, multifunction, simple operation and wide application.

The application practice of high positioning accuracy proves that the relative positioning accuracy of GPS can reach 10-6 100-500KM,10-71000-9. In the precise positioning of 300- 1500M project, the position error of the calculated plane observed over 1 hour is less than1mm. Compared with the side length measured by ME-5000 electromagnetic wave range finder, the maximum side length difference is 0.5 mm, and the correction error is 0.3 mm. ..

The observation time is short. With the continuous improvement of GPS system and the continuous update of software, the relative static positioning within 20KM only takes15-20min. In the fast static relative positioning measurement, when the distance between each mobile station and the reference station is within 15KM, the observation time of the mobile station is only 1-2 minutes, and then it can be positioned at any time, and the observation of each station takes only a few seconds.

Interstation intervisibility is not required. GPS survey does not require intervisibility between stations, but the sky above the station is very wide, which can save a lot of bidding fees. Because it is not necessary to see the position of points between points, the density can be carried out as needed, which makes the selection of points very flexible and can also save the measurement of transmission points and transition points in the classical geodetic network.

Classical geodesy can provide three-dimensional coordinates, and measure plane and elevation respectively by different methods. GPS can accurately determine the three-dimensional coordinates of the site at the same time. At present, GPS leveling can meet the accuracy of fourth-class leveling.

Simple operation: with the continuous improvement of GPS receiver, the degree of automation is getting higher and higher, and some have reached the level of "fool"; The receiver is getting smaller and lighter, which greatly reduces the work tension and labor intensity of the measurement workers. Make field work easy and enjoyable.

All-weather operation At present, GPS observation can be carried out at any time within 24 hours a day, which is not affected by cloudy and dark days, heavy fog and strong winds, rain and snow, and has many functions and wide applications.

From these characteristics, we can see that GPS system can be used not only for measurement and navigation, but also for speed measurement and time measurement. The accuracy of speed measurement can reach 0. 1M/S, and the accuracy of time measurement can reach tens of nanoseconds. Its application fields are constantly expanding. The application prospect of GPS system The main purpose of designing GPS system is to be used for military purposes, such as navigation and intelligence gathering. However, the later application and development show that the GPS system can not only achieve the above goals, but also use the navigation and positioning signals sent by GPS satellites to carry out static relative positioning with centimeter or even millimeter accuracy, dynamic positioning with meter to sub-meter accuracy, speed measurement with sub-meter to centimeter accuracy and time measurement with nanosecond accuracy. Therefore, GPS system shows a very broad application prospect.

The use of global positioning system

GPS was originally established to provide accurate positioning for the military, and it is still controlled by the US military. Military GPS products are mainly used to determine and track the coordinates of soldiers and equipment traveling in the field, provide navigation for warships at sea, and provide position and navigation information for military aircraft.

At present, the application of GPS system will be very extensive. We can use GPS signals to carry out guided geodesy, precise positioning, time transmission and speed measurement of sea, land and air navigation missiles. In the field of surveying and mapping, GPS satellite positioning technology has been used to establish a high-precision national geodetic control network to determine global geodynamic parameters; It is used to establish land and marine geodetic datum and carry out high-precision island-land joint survey and marine surveying and mapping; Used to monitor the motion state of the earth plate and crustal deformation; Used in engineering survey has become the main means to establish urban and engineering control network. The rapid mapping of aerial survey with little or no ground control through the camera position used to determine aerial photography time has led to the technological revolution of geographic information system and global environmental remote sensing monitoring.

Many commercial and government organizations also use GPS devices to track the position of their vehicles, which usually requires the help of wireless communication technology. Some GPS receivers integrate radios, wireless phones and mobile data terminals to meet the needs of fleet management.

Due to the emergence of diversified space resource environment, systems such as GPSGLONASSINMARSAT have navigation and positioning functions, forming a diversified space resource environment. This diversified space resource environment has prompted the formation of a consistent international strategy, that is, on the one hand, making full use of existing systems, on the other hand, actively building civil GNSS systems. By 20 10, a purely civil GNSS system will be built all over the world, which will form a three-legged trend of GPS/GLONASS/GNSS, fundamentally get rid of the dependence on a single system, and form an internationally available and internationally shared security resource environment. The world can enter the highest application realm of satellite navigation as a single navigation means. In turn, this international and non-governmental strategy has influenced and forced the United States to make more open adjustments to its GPS use policy. In a word, the establishment of diversified space resources and environment has created an unprecedented good international environment for the development and application of GPS.