The GPS navigation system includes three parts: space part—GPS navigation satellite constellation; ground control part—ground monitoring system; user equipment part—GPS signal receiver.
GPS satellite constellation
The GPS working satellite and its constellation are composed of 21 working satellites and 3 on-orbit spare satellites. The GPS satellite constellation is denoted as (21 + 3) GPS constellation. The 24 satellites are evenly distributed in 6 orbital planes, the orbital inclination angle is 55 degrees, and the orbital planes are 60 degrees apart, that is, the ascension points of the orbits differ by 60 degrees. The ascending angle distances between the satellites in each orbital plane differ by 90 degrees, and the satellites in one orbital plane are 30 degrees ahead of the corresponding satellites in the adjacent orbital plane in the west.
In GPS satellites at a height of 20,000 kilometers, when the earth rotates for a star for one week, they orbit the earth for two weeks, that is, the time for a circle around the earth is 12 stars. In this way, the ground observer will see the same GPS satellite 4 minutes in advance every day. The number of satellites above the horizon varies with time and location, with a minimum of 4 satellites and a maximum of 11 satellites. When navigating and positioning with GPS signals, in order to settle the three-dimensional coordinates of the station, four GPS satellites must be observed, called positioning constellations. The geometric position distribution of the four satellites during the observation process has a certain influence on the positioning accuracy. For a certain place at a certain time, even accurate point coordinates cannot be measured, this time period is called "gap section". But this time interval is very short, and it does not affect the all-weather, high-precision, continuous real-time, most of the world's places. The number of GPS working satellites is basically the same as the test satellites.
Ground monitoring system
For navigation and positioning, the GPS satellite is a dynamically known point. The position of the star is calculated based on the ephemeris launched by the satellite—a parameter that describes the movement of the satellite and its orbit. The ephemeris broadcast by each GPS satellite is provided by the ground monitoring system. Whether the various equipment on the satellite is working normally, and whether the satellite has been running along the predetermined orbit, must be monitored and controlled by the ground equipment. Another important role of the ground monitoring system is to keep all satellites at the same time standard-GPS time system. This requires the ground station to monitor the time of each satellite to find the clock difference. It is then sent to the satellite by the ground injection station, and the satellite is then sent to the user equipment by the navigation message. The ground monitoring system of GPS working satellites includes a main control station, three injection stations and five monitoring stations.
GPS signal receiver
The task of the GPS signal receiver is to be able to capture the signals of the satellites under test selected according to a certain satellite height cutoff angle, and track the operation of these satellites, transform, amplify and process the received GPS signals in order to measure The propagation time of the GPS signal from the satellite to the receiver antenna, interpret the navigation message sent by the GPS satellite, and calculate the three-dimensional position, location, even three-dimensional velocity and time of the station in real time.
In static positioning, the GPS receiver is fixed during the process of capturing and tracking GPS satellites. The receiver measures the propagation time of GPS signals with high precision, and uses the known position of the GPS satellites in orbit to solve for the location of the receiver antenna. Three-dimensional coordinates. In dynamic positioning, a GPS receiver is used to measure the running trajectory of a moving object. The moving object where the GPS signal receiver is located is called a carrier (such as a ship in navigation, an airplane in the air, a walking vehicle, etc.). The GPS receiver antenna on the carrier moves relative to the earth in the process of tracking the GPS satellites. The receiver uses GPS signals to measure the state parameters (the instantaneous three-dimensional position and three-dimensional velocity) of the moving carrier in real time.
The receiver hardware and on-board software and the post-processing software package of GPS data constitute a complete GPS user equipment. The structure of the GPS receiver is divided into two parts: the antenna unit and the receiving unit. For geodetic receivers, the two units are generally divided into two independent components. During observation, the antenna unit is placed on the measuring station, the receiving unit is placed in an appropriate place near the measuring station, and the two are connected by a cable A whole machine. Some also make the antenna unit and receiving unit as a whole, and place it on the measuring station during observation.
GPS receivers generally use batteries as power sources. At the same time, two DC power sources are used inside and outside the machine. The purpose of setting the internal battery is to continue observation without interruption when replacing the external battery. In the process of using the external battery, the internal battery is automatically charged. After shutdown, the internal battery powers the RAM memory to prevent data loss.
In recent years, many types of GPS geodesic receivers have been introduced in China. When various types of GPS geodetic 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. For differential positioning, the accuracy can reach sub-meter level to centimeter level. At present, various types of GPS receivers are getting smaller and lighter in weight, making them easier to observe in the field. GPS and GLONASS compatible GNSS receivers have been released.
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