| Nowadays GPS is most commonly known for vehicle | | | | these have the drawback of only providing a few |
| navigation solutions. However, the GPS system can | | | | meters of cabling to the host NTP time server or |
| be used for many other useful purposes. For | | | | computer system. Marine antennas are designed to |
| instance, many computer network time | | | | withstand the worst of the weather, they typically |
| synchronisation solutions, such as NTP time servers, | | | | screw onto a threaded pole to provide a secure |
| use the atomic clocks on board each GPS satellite for | | | | mounting system. These antennas provide the best |
| accurate timing. The GPS system can be used as an | | | | solution for static timing applications. |
| accurate reference clock for synchronising time-critical | | | | The GPS receiver is generally embedded into the |
| applications on computers and networks. This article | | | | NTP time server or encapsulated within an enclosure. |
| describes how computer systems can utilise GPS | | | | A GPS receiver is a small elecronic device that |
| time and discusses what equipment is required to | | | | receives amplified GPS signals from an antenna via a |
| provide a precision timing reference. | | | | coaxial cable. The receiver decodes these signal and |
| The GPS system is a United States military system | | | | provides a computer readable output usually via a |
| intended for global navigation. The GPS system is a | | | | RS232 or USB interface. Most receivers provide |
| space-based system, consisting of a network of 24 | | | | decoded information such as: position, visible satellites, |
| orbiting satellites. Precise navigation and positioning is | | | | precise time and an accurate pulse output timing |
| achieved by utilising precision timing signals and | | | | reference. |
| triangulation between multiple satellites. Each orbiting | | | | Coaxial cable is utilised to provide a connection |
| satellite has an integrated atomic clock timing | | | | between the GPS antenna and receiver. The cable |
| reference to provide ultra high precision timing. Each | | | | distance can be critical. The antenna must amplify the |
| satellite broadcasts timing information to within a few | | | | GPS signal sufficiently to overcome any cable losses |
| nanoseconds of the correct time to provide | | | | associated with the cable run utilised. Better quality |
| positioning to an accuracy of better than 10m. This | | | | antennas generally provide more amplification (gain) |
| precise timing information can be used by NTP time | | | | additionally higher quality coax cables can be used |
| servers and other computer timing equipment as a | | | | with much smaller signal losses per meter allowing for |
| highly accurate time reference. The great thing about | | | | relatively long cable runs between a antenna and |
| GPS is that it works anywhere in the world and is | | | | receiver. In order to extend cable runs still further |
| entirely free to air service. | | | | GPS amplifiers can be used to boost the signal |
| Each GPS satellite transmits a low-power radio signal | | | | transmitted down the coax cable. Typically, a GPS |
| down to the surface of the Earth. Two separate | | | | amplifier sits in-line on the coax cable and is powered |
| frequencies are utilised, designated L1 and L2. The L1 | | | | from the receiver. Multiple amplifiers can be used for |
| band is the civilian signal; transmitted at 1575.42 MHz. | | | | extremely long cable runs. |
| L2 is the military frequency, intended for ultra-high | | | | A roof mounted GPS antenna can be prone to |
| precision positioning. The radio signals are transmitted | | | | lightning strikes and voltage surges. It is |
| from the satellites by line of sight. The transmitted | | | | recommended that surge suppressors are utilised to |
| signal is easily powerful enough to pass through | | | | protect expensive NTP time server and timing |
| clouds, glass and most plastics but is absorbed by | | | | systems from potential damage. A surge suppressor |
| more substantial material such as brick, roofing | | | | is essentially a barrier that sits in-line on the coax |
| materials and metals. The ideal location for a GPS | | | | cable between the antenna and receiver that |
| antenna tends to be outdoors and ideally on a | | | | protects the receiver from any potential damage due |
| rooftop with the best possible view of the sky. | | | | to voltage surges. |
| With the advent of GPS navigation equipment for | | | | To summarise, the GPS system provides a highly |
| the mass market, GPS antennas and receivers can | | | | precise timing reference for NTP time servers and |
| be obtained at very reasonable cost. A GPS antenna | | | | other computer timing equipment. It is a reliable free |
| is utilised to receive and amplify the GPS radio signals. | | | | to air service that can be used anywhere in the |
| While a GPS receiver decodes the information and | | | | world. Additionally, with the boom in vehicle navigation |
| presents it in a computer readable format. Many | | | | systems the technology required to receive GPS |
| antennas are available with integrated receivers but | | | | broadcasts is continually falling in price. |