How GPS track Position

- Mar 16, 2018-

The basic principle of the GPS navigation system is to measure the distance from the known position of the satellite to the user's receiver, and then integrate the data of multiple satellites to know the specific location of the receiver. To achieve this goal, the position of the satellite can be detected in the satellite ephemeris according to the time recorded by the satellite clock. The distance from the user to the satellite is obtained by recording the time it takes the satellite signal to travel to the user and multiplying it by the speed of light (due to atmospheric ionospheric interference, this distance is not the actual distance between the user and the satellite, but Pseudorange (PR): When GPS satellites work normally, they will continuously transmit navigation messages with pseudo-random codes consisting of 1 and 0 binary symbols (pseudo-codes). There are two kinds of pseudo-codes used by GPS systems. It is civilian C/A code and military P(Y) code, C/A code frequency is 1.023MHz, repetition period is one millisecond, code spacing is 1 microsecond, equivalent to 300m, P code frequency is 10.23MHz, repetition period is 266.4 days, The code spacing is 0.1 microseconds, which is equivalent to 30m, and the Y code is formed on the basis of the P code, and the security performance is better.The navigation message includes satellite ephemeris, work status, clock correction, ionospheric delay correction, atmospheric refraction correction Information, etc. It is demodulated from the satellite signal and transmitted on the carrier frequency with 50 b/s modulation.The navigation message contains 5 sub-frames in each main frame, each frame is 6 seconds long, and the first three frames each have 10 words; Repeat every thirty seconds, every hour Once, the last two frames total 15000b. The contents of the navigation message mainly include telemetry code, conversion code, and data blocks 1, 2, and 3. The most important one is ephemeris data, which is extracted when the user receives the navigation message. The time of the satellite is compared with its own clock to know the distance between the satellite and the user, and then the satellite ephemeris data in the navigation message is used to calculate the location of the satellite transmission message. The user is in the WGS-84 geodetic coordinate system. The location speed and other information can be known.