The Global Positioning System (GPS) is most widely known as a system for vehicle and marine navigation. However, each GPS satellite orbiting the earth also has an integrated precise atomic clock that can be used as a reference clock for NTP time servers and for general computer timing applications. This article describes the components required to utilise GPS for computer timing applications. It also discusses how the GPS system can be used for computer network timing by using the Network Time Protocol (NTP) and NTP time servers.

The GPS system is a United States military system intended for global navigation. The GPS system is a space-based system, consisting of a network of 24 orbiting satellites. Precise navigation and positioning is achieved by utilising precision timing signals and triangulation between multiple satellites. In order to provide very precise time, each satellite has an integrated highly accurate atomic clock onboard. Each satellite broadcasts timing information to within a few nanoseconds of the correct time to provide positioning to an accuracy of better than 10m. This precise timing information can be used by NTP time servers and other computer timing equipment as a highly accurate time reference. The great thing about GPS is that it works anywhere in the world and is entirely free to air service.

Each GPS satellite transmits a low-power radio signal down to the surface of the Earth. Two separate frequencies are utilised, designated L1 and L2. The L1 band is the civilian signal; transmitted at 1575.42 MHz. L2 is the military frequency, intended for ultra-high precision positioning. Broadcast radio signals travel through the air by line of sight. The transmitted signal is easily powerful enough to pass through clouds, glass and most plastics but is absorbed by more substantial material such as brick, roofing materials and metals. Ideally GPS antennas require a good unobscured view of the sky. Therefore, ideally the antenna should be located high-up with a good all-round view of the sky.

With the advent of GPS navigation equipment for the mass market, GPS antennas and receivers can be obtained at very reasonable cost. A GPS antenna is utilised to receive and amplify the GPS radio signals. While a GPS receiver decodes the information and presents it in a computer readable format. Many antennas are available with integrated receivers but these have the drawback of only providing a few meters of cabling to the host NTP time server or computer system. Rugged GPS marine antennas screw down on to a rigid threaded pole mount able to withstand the worst of the weather. These antennas provide the best solution for static timing applications.

The GPS receiver is generally embedded into the NTP time server or encapsulated within an enclosure. A GPS receiver is a small elecronic device that receives amplified GPS signals from an antenna via a coaxial cable. The receiver decodes these signal and provides a computer readable output usually via a RS232 or USB interface. Most receivers provide decoded information such as: position, visible satellites, precise time and an accurate pulse output timing reference.

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