Using GPS to Synchronise Computer Systems

By: David Evans

The GPS system is global navigation system introduced by the US military. The system provide navigation and positioning information anywhere on the face of the Earth. The GPS system also provides a highly accurate and precise time and frequency signal ideal for computer timing applications. The GPS system is a orbiting constellation of 24 satellites, each broadcasting time and position information around the globe.

This article describes how the GPS system can be used to provide an accurate time and frequency reference for NTP servers and computer network time synchronisation.

GPS timing information is continuously broadcast by each GPS satellite. GPS time is not affected by leap seconds. Therefore, GPS time is currently about 14 seconds ahead of UTC time. However, offset information is available to readily convert GPS time to UTC time, which is used in NTP server and computer time server systems.

Each GPS satellite transmits information as very low-power radio frequency transmission. Two frequencies are used, one for civilian use coded L1, and one for military use coded L2. The civilian L1 frequency is the most widely used and is transmitted at 1575 MHz. The broadcast GPS signal can easily penetrate less dense material such as plastic, but cannot penetrate higher density materials such as brick.

An antenna is utilised to boost the GPS signal and pass the signal along a coax cable. Ideally, a GPS antenna needs to have as good a view of the sky as possible to receive from as many satellites as possible. Ideally, it should be located outdoors on a rooftop with a good 360-degree un-obscured view of the sky.

GPS antenna systems transmit signals down a coax cable and are sensitive to coax quality and cable length. The coax type and antenna gain determines the cable length that can be utilised by a GPS antenna. Higher quality coax cable generally have much lower attenuation figures and hence provide much longer cable runs between the receiver and antenna. Typically, a low-quality coax can run to 20-30m, while a high quality coax can be used in excess of 100m cable runs. To increase these distances still further, a GPS amplifier can be used to amplify and boost the received GPS signal.

GPS timing systems also utilise a GPS receiver in order to decode the received GPS transmission and convert it to a computer readable format. The GPS receiver processes the transmitted GPS positioning information and provides a readable serial transmission as output. GPS NTP servers and computer time servers may also utilise a precise pulse per second output generated by the GPS receiver to provide ultra-precise timing. The pulse per second timing pulse on most receivers is accurate to within a few milliseconds of UTC.

To summarise, the GPS system is ideal for synchronising NTP server and computer time servers. Accuracies of a few hundred nanoseconds can be reasonably achieved with low-cost GPS receiving equipment.

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