Network Time Synchronisation using GPS

Nowadays GPS is most commonly known for vehicle navigation solutions. However, the GPS system can be used for many other useful purposes. For instance, many computer network time synchronisation solutions, such as NTP time servers, use the atomic clocks on board each GPS satellite for accurate timing. The GPS system can be used as an accurate reference clock for synchronising time-critical applications on computers and networks. This article describes how computer systems can utilise GPS time and discusses what equipment is required to provide a precision timing reference.

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. 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. 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. 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. Any GPS antenna has to amplify received signals enough to overcome cable losses over the intended cable run. 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.