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Using the GPS System for Accurate Computer Time

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.

GPS time can easily be adjusted to provide UTC time for computer network time synchronisation. The GPS satellites broadcast a very weak low-power radio signal. The signal has two frequencies, L1 and L2. L1 is intended as a civilian GPS band broadcast 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.

If it is impractical to locate a roof-mounted antenna, installation on the side of a building can be adequate. The GPS system provides a subscription-free accurate timing resource. 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.

Typically, GPS can provide timing information to within a few nanoseconds of UTC. 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. Additionally, a highly accurate timing pulse is generated which marks the beginning 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.

Dave Evans provides a technical authoring service to the time and frequency and network time synchronisation industries. Click here for more information on GPS NTP Servers.

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