How a GPS Tracker locate

- Apr 18, 2018-

The GPS receiver can receive accurate to nanosecond time information that can be used for timing; forecast ephemeris for predicting the satellite's approximate location in the coming months; broadcast ephemeris for calculating satellite coordinates needed for positioning , Accuracy is a few meters to tens of meters (various for each satellite, changing at any time); and GPS system information, such as satellite status.


    The GPS receiver can measure the code to get the distance from the satellite to the receiver. Because it contains the satellite satellite clock error and atmospheric propagation error, it is called pseudorange. The pseudorange measured on the 0A code is called the UA code pseudorange, and the precision is about 20 meters. The pseudorange measured on the P code is called the P code pseudorange, and the accuracy is about 2 meters.


    After the GPS receiver decodes the received satellite signal or uses other techniques to remove the information modulated on the carrier, 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 carrier phase of the satellite signal affected by the Doppler shift and the phase of the signal generated by the local oscillator of the receiver. Generally measured at the epoch of the receiver clock to determine, keep track of the satellite signal, you can record the value of the phase change, but the initial phase of the receiver and the satellite oscillator when the observation is not known, The phase integer of the initial epoch is also unknown, ie, the integer ambiguity can only be solved as a parameter in data processing. The precision of the phase observations is as high as millimeters, but the precondition is that the integer ambiguities are solved. Therefore, the phase observations can only be used when there is a relative position and a continuous observation value, and the position accuracy better than the meter level is only achieved. 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 based on a receiver's observation data to determine the location of the receiver, it can only use the pseudo-range view measurement, can be used for the general navigation and positioning of vehicles and ships. Relative positioning (differential positioning) is a method of determining the relative position between observation points based on the observation data of two or more receivers. It can use both pseudo-range and phase-view measurements. Geodetic or engineering measurements should be performed. Use phase observations for relative positioning.


    The GPS observations include satellite and receiver clock errors, atmospheric propagation delays, and multipath effects. They also suffer from satellite broadcast ephemeris errors in positioning calculations. Most of the common errors are caused by relative positioning. Offset or weakened, so the positioning accuracy will be greatly improved, dual-frequency receivers can be based on the observation of the two frequencies to offset the main part of the ionospheric error in the atmosphere, in the high precision requirements, the distance between the receiver is farther (atmosphere there are significant differences ), should use dual-frequency receiver.


    In positioning and observation, if the receiver moves relative to the surface of the earth, it is called dynamic positioning, such as the pseudo-range single point positioning with a precision of 30 to 100 meters for the general navigation and positioning of vehicles and ships, or for the navigation and positioning of urban vehicles. Meter-level precision pseudo-range differential positioning, or centimeter-level phase differential positioning (RTK) for measurement stakeout, etc. Real-time differential positioning requires the data chain to transmit observation data from two or more stations in real-time to calculate together. In positioning observations, if the receiver is stationary with respect to the surface of the earth, it is called static positioning. In the control network observation, this method is generally adopted by several receivers to observe simultaneously, and it can exert its limit to the maximum extent of GPS. Positioning accuracy, a receiver dedicated to this purpose is called a geodetic receiver and is the best performing receiver. At present, GPS has been able to meet the accuracy requirements for crustal deformation observations. IGS's perennial observation stations have been able to form a millimeter-scale global coordinate frame.