How it works

The GPS satellites transmit low power radio signals on multiple frequencies. L1 and L2 are the two basic carrier frequencies that contain the navigation signals. The L1 frequency is 1575.42 MHz in the UHF band while the L2 frequency is 1227.6 MHz. The signals travel by line of sight (the signal passes through clouds, glass, plastic and other lightweight materials but doesn’t go through buildings, mountains or other enclosed spaces). The L1 signal contains two pseudorandom signals: the P (Protected) code and the C/A (Coarse/Acquisition) code. The P code is also modulated on the L2 signal. A receiver can identify the signals because each GPS satellite transmits a unique code. The P code can be scrambled to prevent unauthorized access (=Anti-spoofing), it is then referred to as P(Y) code or Y code. A GPS signal contains a pseudorandom code, ephemeris data and almanac data. The first code identifies the satellite that is transmitting information. Ephemeris data contains information about the status of the satellite (healthy or unhealthy), current date and time. The almanac data informs the receiver where each GPS receiver should be at any time throughout the day.

Three satellites are used to triangulate a position (longitude, latitude and height), and a fourth satellite needs to be in line of sight in order to calculate also time. The more satellites in view, the more accurate the positioning will be.