How the GPS tracker works

The GPS tracker is a terminal with a built-in GPS module and a mobile communication module. It is used to transmit the positioning data obtained by the GPS module to a server on the Internet through the mobile communication module, so that the location of the terminal can be checked on a computer.

How the GPS tracker works
Tracking principle:
The GPS receiver can receive accurate time information to the nanosecond level that can be used for timing; a forecast ephemeris for forecasting the approximate location of the satellite in the next few months; a broadcast ephemeris for calculating the satellite coordinates required for positioning , The accuracy is a few meters to tens of meters (each satellite is different and changes at any time); and GPS system information, such as satellite status.
The GPS receiver can measure the code to get the satellite-to-receiver distance. Because it contains the receiver's satellite clock error and atmospheric propagation error, it is called pseudorange. The pseudo-range measured for the 0A code is called a UA-code pseudo-range with an accuracy of about 20 meters, and the pseudo-range measured for the P code is called a P-code pseudo-range with an accuracy of about 2 meters.
The GPS receiver decodes the received satellite signals or uses other technologies to remove the information modulated on the carrier, and then the carrier can be recovered. Strictly speaking, the carrier phase should be referred to as the carrier beat phase, which is the difference between the received satellite signal carrier phase affected by the Doppler shift and the receiver's local oscillation-generated signal phase. Generally measured at the epoch time determined by the receiver clock and keeping track of the satellite signal, the phase change value can be recorded, but the initial phase values of the receiver and satellite oscillator at the beginning of the observation are unknown. The phase integer of the initial epoch is also unknown, that is, the ambiguity of the whole week can only be solved as a parameter in data processing. The accuracy of the phase observations is as high as millimeters, but the premise is to resolve the ambiguity of the whole cycle. Therefore, the phase observations can be used only when they are relatively positioned and have a period of continuous observations. Phase observations can be used.
According to the positioning method, GPS positioning is divided into single-point positioning and relative positioning (differential positioning). Single-point positioning is the way to determine the position of the receiver based on the observation data of a receiver. It can only use pseudo-range observations and can be used for rough navigation and positioning of vehicles and ships. Relative positioning (differential positioning) is a method to determine the relative position between observation points based on the observation data of two or more receivers. It can use both pseudo-range and phase observation. Geodetic or engineering measurements should be used. Relative positioning using phase observations.
GPS observations include errors such as satellite and receiver clock differences, atmospheric propagation delays, and multipath effects. They are also affected by satellite broadcast ephemeris errors during positioning calculations. Most of the common errors are affected by relative positioning. Cancel or weaken, so the positioning accuracy will be greatly improved. The dual-frequency receiver can cancel the main part of the ionospheric error in the atmosphere according to the observation of two frequencies. When the accuracy is required and the distance between the receivers is far (the atmosphere has a significant difference) ), Should choose dual frequency receiver.
During positioning observation, if the receiver moves relative to the earth's surface, it is called dynamic positioning, such as pseudo-range single-point positioning with an accuracy of 30 to 100 meters for rough navigation positioning such as vehicles and ships, or for urban vehicle navigation positioning. Meter-level pseudo-range differential positioning, or centimeter-level phase differential positioning (RTK) for measuring stakeout, etc. Real-time differential positioning requires a data link to transmit the observation data of two or more stations in real time for calculation. In positioning observation, if the receiver is stationary relative to the earth's surface, it is called static positioning. In the control network observation, this method is generally used to observe by several receivers at the same time, which can maximize the use of GPS. Positioning accuracy, receivers dedicated to this purpose are called geodetic receivers, and are the best performing receivers. At present, GPS has been able to meet the accuracy requirements for crustal deformation observations, and IGS annual observation stations have been able to form millimeter-scale global coordinate frames.

How the GPS tracker works 1
How GPS positioning trackers work
The GPS system consists of 24 satellites. At any point on the earth, it can receive signals from at least 4 and up to 9 satellites.
For navigation and positioning, a GPS satellite is a dynamically known point. The position of the star is calculated based on the ephemeris emitted by the satellite-parameters that describe 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 properly, and whether the satellite has been running along a predetermined orbit, must be monitored and controlled by ground equipment. Another important role of the ground monitoring system is to keep the satellites at the same time standard-the 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, and the satellite is then sent to the user equipment by the navigation message. The ground monitoring system of the GPS working satellite includes a main control station, three injection stations and five monitoring stations.
The task of the GPS signal receiver is to be able to capture the signals of the satellites to be selected selected according to a certain satellite altitude cut-off angle, and 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, interpret the navigation message sent by the GPS satellite, and calculate the 3D position, position, and even the 3D speed and time of the station in real time.
The navigation and positioning signals sent by GPS satellites are an information resource that can be shared by countless users. For land, sea and space users, as long as the user has a receiving device capable of receiving, tracking, transforming, and measuring GPS signals, that is, a GPS signal receiver. You can use GPS signals for navigation and positioning measurements at any time. According to the purpose of use, the GPS signal receiver required by the user also varies. At present, dozens of factories in the world have produced GPS receivers, and there are hundreds of products. These products can be classified according to principles, uses, functions, and so on.
In static positioning, the GPS receiver is fixed during the process of capturing and tracking GPS satellites. The receiver accurately measures the propagation time of the GPS signal, and uses the known position of the GPS satellite in orbit to calculate the position of the receiver antenna. Three-dimensional coordinates. For dynamic positioning, a GPS receiver is used to determine the trajectory of a moving object. The moving object where the GPS signal receiver is located is called a carrier (such as a ship in flight, an airplane in the air, a moving vehicle, etc.). The GPS receiver antenna on the carrier moves relative to the earth during the process of tracking GPS satellites. The 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 and in-flight software and GPS data post-processing software package constitute a complete GPS user equipment. The structure of a GPS receiver is divided into an antenna unit and a receiving unit. For a geodetic receiver, the two units are generally divided into two separate parts. When observing, the antenna unit is placed on the measuring station, and the receiving unit is placed in an appropriate place near the measuring station. The two are connected by a cable. A whole machine. Some also make the antenna unit and the receiving unit as a whole, and place it on the measurement site when observing.

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