The GPS system is a global navigation and positioning system. It is most commonly known for providing navigation systems for cars and marine vehicles. However, the GPS system also has a less well-known feature - precision time. Each GPS satellite orbiting the Earth has onboard a high precision atomic clock. This clock can be used by NTP time servers and other computer network timing systems as a highly precise source of time. This article provides an insight into how the GPS system can be used to synchronise networks of computers and also discusses the equipment required to receive accurate time from the GPS system.

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. Each orbiting satellite has an integrated atomic clock timing reference to provide ultra high precision timing. 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. The radio signals are transmitted from the satellites 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. The ideal location for a GPS antenna tends to be outdoors and ideally on a rooftop with the best possible view of the sky.

In order to receive GPS timing and positioning information some relatively low-cost equipment is required. 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. Marine antennas are designed to withstand the worst of the weather, they typically screw onto a threaded pole to provide a secure mounting system. 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. GPS receivers are small modules that have a connection to the GPS antenna to receive amplified GPS signals. 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.

Coaxial cable is utilised to provide a connection between the GPS antenna and receiver. The cable distance can be critical. The antenna must amplify the GPS signal sufficiently to overcome any cable losses associated with the cable run utilised. Better quality antennas generally provide more amplification (gain) additionally higher quality coax cables can be used with much smaller signal losses per meter allowing for relatively long cable runs between a antenna and receiver. For longer cable runs, an amplifier can be used to further amplify GPS signals to overcome cable losses. Typically, a GPS amplifier sits in-line on the coax cable and is powered from the receiver. Multiple amplifiers can be used for extremely long cable runs.

A roof mounted GPS antenna can be prone to lightning strikes and voltage surges. It is recommended that surge suppressors are utilised to protect expensive NTP time server and timing systems from potential damage. A surge suppressor is essentially a barrier that sits in-line on the coax cable between the antenna and receiver that protects the receiver from any potential damage due to voltage surges.

To summarise, the GPS system provides a highly precise timing reference for NTP time servers and other computer timing equipment. It is a reliable free to air service that can be used anywhere in the world. Additionally, with the boom in vehicle navigation systems the technology required to receive GPS broadcasts is continually falling in price.