GPS is a US military system designed for navigation and precise positioning. It consists of 24 earth-orbiting satellites, each with an on-board atomic clock. Accurate time combined with triangulation from a number of satellites can provide precise positioning anywhere on the globe. However, the precise timing information supplied by GPS can also be used for time and frequency applications such as computer network time synchronisation. This article discusses how the Global Positioning System can be utilised by GPS NTP server systems to provide precise network synchronisation. Additionally, it describes the equipment required in order to receive accurate time from the GPS system and to synchronise computer time.

Time is reverenced to UTC time, which does not vary with timezones, local time is calculated from UTC. The GPS satellites continuously transmit precise time information. GPS time is currently 14 seconds ahead of UTC and is not adjusted for leap seconds. UTC time can easily be calculated from GPS time and used for accurate timing purposes. The GPS satellites broadcast a very weak low-power radio signal. The signal has two frequencies, L1 and L2. L1 is the civilian GPS frequency transmitted at 1575.42 MHz. L2 is not currently in operation but promises future stronger signal transmissions. The signal travels in a straight line and can pass through clouds, glass and plastics but is blocked by objects such as metal and brickwork. Therefore, ideally, a GPS antenna requires a good view of the sky. The ideal location for a GPS antenna is on rooftop with a good view of the sky. Often, antenna installation on the side of a building or in a window can provide a good enough view of the sky to give adequate results.

GPS provides a free-to-air precision time and frequency service. Many computer networks utilise the GPS clock as an accurate timing reference for computer synchronisation. Precise GPS NTP server systems utilise GPS reference clocks as an external synchronisation source. The GPS system can supply highly precise time and frequency information accurate to a couple of nanoseconds. This accuracy is generally more than enough for most computer network timing applications.

Most GPS receivers transmit time and positioning information in a serial format using the standard NMEA protocol. Information is transmitted as sentences of character strings. Also, a precise pulse output is generated which indicates the start of each second. This timing pulse can be converted to a RS232 signal level and fed into a RS232 control line interrupt to provide a precise timing reference. When the timing pulse is combined with the NMEA output sentences, a very accurate timing reference becomes available for use by NTP servers or computer timing equipment.