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The Global Positioning System (GPS) is a US military system intended for global navigation and positioning. GPS 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 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 GPS 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 GPS signal and pass the signal along a coax cable. For consistent operation a GPS 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 antennas are very sensitive to cable types and distances. The maximum cable length that a GPS antenna can utilise depends on the quality of coax utilised and the gain of the 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 receiver decodes the GPS time and positioning information and converts it into a regular serial transmission that can easily be read. 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 timing pulse output can typically provide synchronisation down to a few hundred nanoseconds.
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.
David Evans is a technical author with many years experience in the time and frequency industry. David has written a number of technical articles providing information on NTP servers. Please click here, for more information on computer time server systems.
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