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The GPS system is a global navigation system primarily intended to provide accurate positioning information. Developed by the US military, the GPS system consists of 24 Earth orbiting satellites. A GPS receiver calculates accurate positioning information by using precise time from satellites and triangulation. However, the highly precise time information received from GPS satellites can also be used for timing applications. Each GPS satellite has an integrated atomic clock and can provide nanosecond timing. This article discusses how the GPS system can be utilised by NTP server systems for precise network timing. It also describes the hardware required to utilise the GPS system for timing 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. 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. 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.
D. Evans is a technical author with many years experience in the field of computer network time synchronisation. Click here to find out more about GPS NTP Server systems.
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