GPS uses the ranging principle to fix the ship. The GPS concept is based on time and the known position of specialized satellites.
The satellites carry very stable atomic clocks that are synchronized to each other and to ground clocks. Any drift from true time maintained on the ground is corrected daily. Likewise, the satellite locations are known with great precision. GPS receivers have clocks as well; however, they are not synchronized with true time, and are less stable.
GPS satellites continuously transmit their current time and position. A GPS receiver monitors multiple satellites and solves equations to determine the precise position of the receiver and its deviation from true time.
The time difference between data transmitted from the satellite and its reception at the receiver is measured (which is in Nano-seconds) and integrated with speed of light (3 X 10 meters per second).
i.e. d= s x Δt
At a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and clock deviation from satellite time).
The GPS satellites transmit signals on two carrier frequencies.
- The L1 carrier is 1575.42 MHz and carries both the status message and a pseudo-random code for timing.
- The L2 carrier is 1227.60 MHz and is used for the more precise military pseudo-random code.