What signal does GPS use

There are currently 27 global positions system (GPS) satellites in orbit around the earth. Of these, 24 are active and the other three act as backups. Each satellite transmits a regular GPS signal that is carried by radio waves in the microwave part of the electromagnetic spectrum.

GPS imageEach GPS satellite continuously broadcasts a navigation message at 50 bits per second on the microwave carrier frequency of approx 1600 MHz. FM radio, for comparison, is broadcast at between 87.5 and 108.0 MHz and wi-fi networks operate at around 5000 MHz and 2400 MHz. More precisely, all satellites broadcast at 1575.42 MHz (this is the L1 signal) and 1227.6 MHz (the L2 signal).

The GPS signal gives the precise "time-of-week" according to the satellite's onboard atomic clock, the GPS week number and a health report for the satellite so that it can be discounted if it is faulty. Each transmission lasts 30 seconds and carries 1500 bits of encrypted data. This small amount of data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. GPS receivers know the PRN codes for each satellite and so can not only decode the signal but distinguish between different satellites.

Second GPS imageThe transmissions are timed to begin precisely on the minute and the half minute as indicated by the satellite's atomic clock. The first part of the GPS signal tells the receiver the relationship between the satellite's clock and GPS time. The next chunk of data gives the receiver the satellite's precise orbit information.