computers Article

The Global Positioning System (GPS) is a US military system intended for global navigation and positioning. is a satellite based system that consists of 24 earth orbiting satellites, each transmitting a weak radio signal that can be used to provide accurate positioning information anywhere on the globe. Each satellite has an integrated synchronised highly accurate atomic clock that can be utilised by ground based timing and synchronisation equipment.

This article describes how the 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 satellite. time is not affected by leap seconds. Therefore, time is currently about 14 seconds ahead of UTC time. However, offset information is available to readily convert time to UTC time, which is used in NTP server and computer time server systems.

Each 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 radio transmissions can easily pass through plastic, glass and clouds but is blocked by metal and brick structures.

An antenna is utilised to boost the signal and pass the signal along a coax cable. For consistent operation a antenna needs to have a view of the sky. 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 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. For extended cable runs, a amplifier can be placed in-line on the coax cable to amplify the weak signals.

GPS timing systems also utilise a receiver in order to decode the received transmission and convert it to a computer readable format. The receiver processes the transmitted positioning information and provides a readable serial transmission as output. NTP servers and computer time servers may also utilise a precise pulse per second output generated by the receiver to provide ultra-precise timing. The timing pulse output can typically provide synchronisation down to a few hundred nanoseconds.

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