The Global Positioning System was developed by the U.S. Department of Defense for navigation. It comprises some 24 satellites, each of which has an atomic clock synchronized to GPS Time (offset by a constant from International Atomic Time (TAI)) and transmitting time codes. A station receiving transmissions from several satellites can determine its position (latitude, longitude and height) and the current time. International clock comparisons are now routinely performed via GPS with accuracy on the order of 50 nanoseconds.

GPS Receivers and Modules

Many GPS receivers are designed primarily for navigation and provide only approximate time output, accurate to perhaps one or two seconds. Many receivers output standard data messages over a serial (RS232C) interface at 2400 and higher bits per second. These messages include navigation and timing data.

The more accurate receivers provide a precise one pulse-per-second output signal, which can be used for clock synchronization and for disciplining precision frequency standards. Complete receivers typically cost on the order of $1000 and up. Examples include the Trimble Scoutmaster with the precision timing option, and the CNS Systems clock.

Receiver modules such as the Garmin GPS-20 series and the Motorola Oncore, cost several hundred dollars and provide serial data and 1 PPS outputs. See the Tucson Amateur Packet Radio (TAPR) GPS page

GPS Synchronization Shareware

PC software accepts the GPS messages and can synchronize PC clocks.

• TAC-32 from CNS Systems