Using the GPS System for Accurate Computer Time

By: David Evans

The Global Positioning System (GPS) consists of a number of orbiting satellites that provide precise positioning information for navigation purposes. Positioning is calculated by using accurate timing information and triangulation. However, the accurate timing system can also be used for computer network time synchronisation purposes. Each orbiting satellite has an onboard atomic clock that can provide highly precise time. This article describes how precise GPS timing information can be utilised to provide network timing via GPS NTP servers. It also discusses the type of equipment required to process GPS signals for time and frequency applications.

Time is referenced to Universal Coordinated Time (UTC), which is the same worldwide and does not vary with time zones. The GPS satellites continuously transmit precise time information. GPS time is not affected by leap seconds and is currently 14 seconds ahead of UTC time. 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 should provide stronger signal transmissions in the future. 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. 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. 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.

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