The GPS system consists of 24 satellites, and any point on the earth can receive signals from at least 4 and at most 9 satellites.

　　For navigation and positioning, GPS satellites are a dynamic known point. The position of the star is calculated from the ephemeris of the satellite's launch - a parameter that describes the satellite's motion and its orbit. The ephemeris broadcast by each GPS satellite is provided by the ground monitoring system. Whether the various devices on the satellite work normally, and whether the satellite has been running along the predetermined orbit, must be monitored and controlled by ground equipment. Another important role of the ground monitoring system is to keep each satellite in the same time standard - GPS time system. This requires the ground station to monitor the time of each satellite and obtain the clock difference. Then it is 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 satellite includes a main control station, three injection stations and five monitoring stations.

　　The task of the GPS signal receiver is to capture the signals of the satellites to be measured selected according to a certain satellite altitude cut-off angle, track the operation of these satellites, and 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, the navigation message sent by the GPS satellite is interpreted, and the three-dimensional position, position, and even the three-dimensional speed and time of the station are calculated in real time.

　　The navigation and positioning signal sent by GPS satellites is an information resource that can be shared by countless users. For the vast number of users in land, sea and space, as long as the user has a receiving device that can receive, track, transform and measure GPS signals, that is, a GPS signal receiver. GPS signals can be used for navigation and positioning measurements at any time. Depending on the purpose of use, the GPS signal receivers required by users are also different. At present, dozens of factories in the world produce GPS receivers, and there are hundreds of products. These products can be classified according to principle, purpose, function, etc.

　　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 calculate the position of the receiver antenna. three-dimensional coordinates. The dynamic positioning is to use 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 flight, 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 GPS satellites, and the receiver uses the GPS signal to measure the state parameters (instant three-dimensional position and three-dimensional speed) of the moving carrier in real time.

　　The receiver hardware and in-machine 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 the geodetic receiver, the two units are generally divided into two independent parts. During observation, the antenna unit is placed on the station, the receiving unit is placed at an appropriate place near the station, and the two are connected by cables. a whole machine. Some also make the antenna unit and the receiving unit as a whole, and place it on the station during observation.

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