1 Overview
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The Global Positioning System (GPS) was developed in the United States in the 1970s. It lasted for 20 years and cost 20 billion US dollars. It was fully built in 1994 and has full-scale real-time 3D navigation and positioning in sea, land and air. A new generation of satellite navigation and positioning systems. After nearly 10 years of use in China's surveying and mapping departments, GPS has won the trust of surveying and mapping workers with its remarkable features such as all-weather, high-precision, automation and high efficiency, and has been successfully applied to geodesy, engineering survey, aerial photogrammetry, Vehicle navigation and control, crustal motion monitoring, engineering deformation monitoring, resource exploration, geodynamics and many other disciplines have brought a profound technological revolution to the field of surveying and mapping.
With the continuous improvement of the global positioning system, the continuous improvement of hardware and software, the application field is constantly expanding. Now it has spread to various sectors of the national economy and has begun to gradually deepen people's daily lives.
The GPS system consists of three parts: the space part—the GPS satellite constellation; the ground control part—the ground monitoring system; the user equipment part—the GPS signal receiver.
2. Satellite and constellation
A GPS satellite constellation consisting of 21 working satellites and 3 in-orbit spare satellites is recorded as a (21+3) GPS constellation. Twenty-four satellites are evenly distributed in six orbital planes with an orbital inclination of 55 degrees, and each orbital plane is 60 degrees apart, that is, the ascending points of the orbits are 60 degrees apart. The elevation angles between the satellites in each orbital plane are 90 degrees apart, and the satellites in one orbital plane are 30 degrees ahead of the corresponding satellites on the adjacent orbital planes in the west.
At a height of 20,000 kilometers, when the Earth rotates for a week, the Earth orbits the Earth for two weeks, that is, the time around the Earth is 12 stars. Thus, for ground observers, the same GPS satellite will be seen 4 minutes in advance every day. The number of satellites above the horizon varies with time and location, with at least 4 visible and up to 11 visible. In the navigation and positioning with GPS signals, in order to settle the three-dimensional coordinates of the station, it is necessary to observe four GPS satellites, called positioning constellations. The geometric position distribution of these four satellites during the observation process has a certain influence on the positioning accuracy. For a certain place, even accurate point coordinates cannot be measured. This time period is called "gap segment". However, this time interval is very short and does not affect all-weather, high-precision, continuous real-time navigation and positioning measurements in most parts of the world. The number of the GPS working satellite is basically the same as the test satellite.
3. Ground monitoring system
For navigation positioning, the GPS satellite is a dynamic known point. The position of the star is calculated from the ephemeris transmitted by the satellite, a parameter describing the motion of the satellite and its orbit. The ephemeris broadcasted by each GPS satellite is provided by the ground monitoring system. Whether the various equipment on the satellite is working properly and whether the satellite is always operating along a predetermined orbit is monitored and controlled by the ground equipment. Another important role of the ground monitoring system is to keep each satellite at the same time standard - GPS time system. This requires the ground station to monitor the time of each satellite and find the clock difference. It is then sent to the satellite by the ground injection station, which is then sent to the user equipment by the navigation message. The ground monitoring system of the GPS working satellite includes a master station, three injection stations and five monitoring stations.
4. User equipment
4.1 GPS signal receiver
The task of the GPS signal receiver is to capture the signals of the satellites to be tested selected according to a certain satellite height cut-off angle, and track the operation of these satellites, and transform, amplify and process the received GPS signals to measure The propagation time of the GPS signal from the satellite to the receiver antenna, the navigation message sent by the GPS satellite is interpreted, and the three-dimensional position, position, and even three-dimensional speed and time of the station are calculated in real time.
In static positioning, the GPS receiver is fixed in the process of capturing and tracking GPS satellites. The receiver measures the propagation time of the GPS signal with high precision, and uses the known position of the GPS satellite in orbit to solve the position of the receiver antenna. Three-dimensional coordinates. Dynamic positioning is the use of a GPS receiver to determine the trajectory of a moving object. The moving object on which the GPS signal receiver is located is called a carrier (such as a ship in navigation, an airplane in the air, a traveling vehicle, etc.). The GPS receiver antenna on the carrier moves relative to the earth during the tracking of the GPS satellite, and the receiver uses the GPS signal to measure the state parameters (instantaneous three-dimensional position and three-dimensional velocity) of the motion carrier in real time.
The receiver hardware and in-flight software as well as post-processing software packages for GPS data form a complete GPS user equipment. The structure of the GPS receiver is divided into two parts: an antenna unit and a receiving unit. For geodetic receivers, the two units are generally divided into two separate components. The antenna unit is placed on the station during observation. The receiving unit is placed in the appropriate place near the station, and the two are connected by cable. A whole machine. Some antenna units and receiving units are also made into a whole, and they are placed on the test site during observation.
GPS receivers typically use a battery as a power source. At the same time, two kinds of DC power sources are used inside the machine. The purpose of setting the internal battery is to not interrupt continuous observation when replacing the external battery. In the process of using the battery outside the machine, the battery inside the machine is automatically charged. After shutdown, the internal battery powers the RAM memory to prevent loss of data.
In recent years, many types of GPS geodetic receivers have been introduced in China. When various types of GPS geodesic receivers are used for precise relative positioning, the accuracy of the dual-frequency receiver can reach 5mm+1PPM.D, and the accuracy of the single-frequency receiver can reach 10mm+2PPM.D within a certain distance. It is used for differential positioning with an accuracy of sub-meters to centimeters.
At present, various types of GPS receivers are getting smaller and smaller, and the weight is getting lighter and easier to observe in the field. GPS and GLONASS compatible global navigation and positioning system receivers have been introduced.
4.2 Classification of GPS receivers
The navigation and positioning signal transmitted by the GPS satellite is an information resource that can be shared by countless users. For users of land, sea and space, as long as the user has a receiving device capable of receiving, tracking, transforming and measuring GPS signals, ie a GPS signal receiver. GPS positioning signals can be used for navigation and positioning measurements at any time. Depending on the purpose of use, the GPS signal receivers required by the user also vary. At present, there are dozens of factories in the world that produce GPS receivers, and there are hundreds of products. These products can be classified according to principles, uses, functions, and the like.
4.2.1 Classification by receiver usage
Navigation receiver
This type of receiver is mainly used for navigation of motion vectors, which can give the position and speed of the carrier in real time. Such receivers generally use C/A code pseudorange measurement, and the single-point real-time positioning accuracy is low, generally ±25mm, and ±100mm when there is SA influence. These receivers are inexpensive and widely used. Depending on the field of application, such receivers can be further divided into:
Vehicle type - used for vehicle navigation and positioning;
Nautical type - used for navigation and positioning of ships;
Aviation type - used for aircraft navigation and positioning. Due to the fast speed of the aircraft, the receivers used in aviation are required to adapt to high-speed motion.
Spaceborne type - used for navigation and positioning of satellites. Since the speed of the satellite is as high as 7km/s or more, the requirements for the receiver are higher.
2. Geodetic receiver
Geodesic receivers are mainly used for precision geodesy and precision engineering measurements. Such instruments mainly use carrier phase observations for relative positioning, and the positioning accuracy is high. The instrument is complex in structure and expensive.
3. Timing receiver
This kind of receiver mainly uses the high-precision time standard provided by GPS satellites for timing, and is often used for time synchronization in observatory and radio communication.
4.2.2 Classification by receiver carrier frequency
Single frequency receiver
The single-frequency receiver can only receive the L1 carrier signal and measure the carrier phase observation for positioning. Single-frequency receivers are only suitable for precision positioning of short baselines (<15km) due to the inability to effectively eliminate ionospheric delay effects.
2. Dual frequency receiver
The dual-frequency receiver can receive L1, L2 carrier signals simultaneously. The use of dual-frequency is different for the ionospheric delay, which can eliminate the influence of the ionosphere on the delay of the electromagnetic wave signal, so the dual-frequency receiver can be used for precise positioning of thousands of kilometers.
4.2.3 Classification by receiver channel number
The GPS receiver can simultaneously receive signals of multiple GPS satellites. In order to separate the received signals of different satellites to achieve tracking, processing and measurement of satellite signals, a device having such a function is called an antenna signal channel. According to the type of channel that the receiver has, it can be divided into:
Multi-channel receiver
2. Through-channel receiver
3. Multi-channel multi-channel receiver
4.2.4 Classification by receiver operating principle
Code related receiver
The code-dependent receiver obtains pseudorange observations using code correlation techniques.
2. Square receiver
The square type receiver uses the square technique of the carrier signal to remove the modulated signal to recover the complete carrier signal. The phase difference between the carrier signal generated in the receiver and the received carrier signal is measured by the phase meter to determine the pseudorange observation value.
3. Hybrid receiver
This kind of instrument combines the advantages of the above two kinds of receivers, and can obtain the code phase pseudorange and the carrier phase observation value.
4. Interferometric receiver
This type of receiver uses a GPS satellite as a radio source and uses an interferometric method to measure the distance between two stations.
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