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.
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GPS satellite constellation
The GPS working satellite and its constellation consist of a constellation of 21 satellites and three in-orbit spare satellites, which are recorded as (21+3) GPS constellations. 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 it does not affect the all-weather, high-precision, continuous real-time, real-time, and high-speed, continuous real-time GPS satellites. The number of the GPS working satellites is basically the same as that of the test satellites.
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.
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. When observing, the antenna unit is placed on the station, 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.
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