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The GPS system is a global navigation system maintained by the United States military. The system consists of 24 Earth orbiting satellites, which can provide precise time, and positioning information anywhere on the face of the globe. This article provides an overview of how the GPS system can be used for computer time synchronisation solutions and describes the equipment required to provide a GPS time reference.
Each Earth orbiting GPS satellite has a highly accurate precisely synchronised real time clock onboard. This clock is used to calculate precise positioning through a triangulation technique. However, the GPS system clock can also be used for precise computer time synchronisation applications.
GPS satellites continuously broadcast time and position information. The time and position information can be obtained anywhere on the face of the planet with a GPS receiver and antenna. GPS works in all whether conditions, anywhere in the world. There are no subscription charges and no set up fee when utilising the GPS system, other than equipment costs. Many computer and network time synchronisation systems, such as NTP Server systems, utilise GPS as an accurate external timing reference.
GPS receivers are primarily designed to provide highly accurate positioning information. Typically, a GPS receiver can provide positioning information to an accuracy of better than 10m. Many GPS systems can also provide timing information to a resolution of a few nanoseconds.
The GPS signal transmitted from the satellites is a very weak low-power radio signal, with designated bands L1 and L2. Band L1 is designated as the civilian GPS band transmitted at 1575.42 MHz. The broadcast radio signals travel by line of sight and pass through clouds, glass and plastics but are blocked by objects such as metal and brick. The ideal location for a GPS antenna is where a full 360-degree view of the sky is visible. However, quite often installations with a fairly restricted view of the sky can provide adequate results. As a rule, the better the view of the sky, the more consistent the signal lock.
GPS antennas act as amplifiers to boost the weak GPS signal for transmission along a cable, usually coax, to a GPS receiver. GPS antennas provided with computer time synchronisation equipment generally utilise a pole-mounting system. The antenna screws to a threaded pole for installation on rooftops. This arrangement provides the GPS antenna with a sturdy mount easily able to withstand harsh weather conditions without damage. A GPS antenna is fairly small in size, measuring less than 90cm in diameter. Low-cost patch type antennas are also available, but these are generally better suited to vehicle applications.
The cable distance between a GPS antenna and receiver can be critical. Cable distance is largely dependant on the gain, or amplification, of the GPS antenna and the cable losses. A GPS antenna may have 35db gain; typical low quality coax (RG58) has an attenuation of 0.64db/m at 1575MHz. Therefore, a cable run of up to, but not beyond, 55m can be used. High quality cables with lower attenuation losses can be used for longer cable distances. Additionally, GPS amplifiers can be utilised to increase the signal gain to extend cable runs even further.
GPS surge suppressors protect expensive time synchronisation equipment from lightning strikes and other potential electro-static discharges. A suppressor is mounted in-line on the GPS cable at the point where the antenna cable enters the building. To install, a suppressor only requires a low-impedance ground connection to dump any electro-static discharge.
To summarise, utilising the GPS system can provide a highly accurate yet cost-effective solution to the problems of computer and computer network timing.
Dave Evans is a technical author and engineer that has been involved in the computer timing industry for a number of years. Please clock here, if you would like more information about PC time synchronisation.
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