GNSS is the abbreviation of Global Navigation Satellite System,which refer to all satellite navigation system, including the global, regional, enhanced. Currently global positioning navigation system have American GPS, Russian Glonass,European Galileo and Chinese Beidou satellite navigation system. International GNSS system is a complicated combined system with multi-system ,multi-level, multi-module. All GNSS are adopt the same basic working principle, just the methods of achieving navigation positioning can be various. GPS is a pioneer of GNSS technology, and the earliest fully functional GNSS that has been put into full operation.
The working principle of GNSS is same to use the distance between the known location satellite and user receiver to obtain the receiver’s specific location. Simply, each satellite had been installed the high precision atomic clock to ensure the on-orbit satellite and ground time synchronization, at the same time, satellite will constantly transmit its location and time information to the ground, the time of signal by receiver receive is again compare to the the time of satellite transmit signal, at the same time combine the location information of satellite will get the the distance between user receive and satellite.
As our living environment is a three-dimensional space, so if you want to confirm the specific of any one point, and then need to three satellite to obtain the receiver’s location. The specific location of the receiver is determined by the intersection of sphere the distance three satellite each with receiver as radius. Due to the atomic clock on the satellite and ground time ate will always have time error,so there will be 4 variable in the three-dimensional space, 4 variable require 4 equation, but the set of equations obtained from 4 mutual independent satellite. This could be achieved so long as there are at least 4 observable satellite on any part of earth.
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