GPS is a US military system for global navigation; it is widely used for vehicle tracking and marine navigation. The system consists of 24 orbiting satellites, which can provide positioning information anywhere on Earth to within a few meters. The satellites achieve this by utilising highly accurate atomic clocks and triangulation. However, the on-board atomic clocks can also be used as relatively inexpensive time references for computer time synchronisation applications. This article describes how the GPS system can be utilised as a time reference for computer and computer network time synchronisation.

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.

Each GPS satellite continously transmits positioning and timing data. 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. The only costs involved in using the GPS system is equipment costs, there are no ongoing subscription fees. 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. Ideally, a GPS antenna should be located with the best view of the sky possible. Quite often, however, an antenna installed with a restricted view of the sky can still provide a regular signal lock. 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. Most GPS antennas are compact devices which can easily be mounted in an unobtrusive manner. Patch type antenna's are a cheaper alternative, but they are generally of a inferior quality and can be more difficult to mount securely.

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 typical GPS antenna may have 36 db of amplification; 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 conclude, the Global Positioning System provides a precise and flexible solution to computer network time synchronisation.