| The GPS system is a worldwide navigational system | | | | signals down a coax cable to a GPS receiver. |
| operated by the US military. The system is maintained | | | | The cable length that can be utilised between a GPS |
| by USNO, the United States Naval Observatory. The | | | | antenna and receiver is critical. The maximum distance |
| Global Positioning System is implemented from space | | | | that can be utilised is dependant on the gain of the |
| by 24 satellites in high orbit. Each satellite has precise | | | | antenna and the quality of the coax cable used. High |
| timing equipment that can be used to triangulate | | | | antenna gain and high quality coax allows for much |
| position for navigational purposes. Low-cost GPS | | | | longer cable runs. |
| antennas and receivers are now widely available and | | | | GPS signal amplifiers can be fitted inline on the coax |
| provide precise time and frequency information | | | | cable to boost the signal and increase cable length. |
| worldwide. | | | | GPS amplifiers are generally powered from the coax |
| This article presents an introduction to using the GPS | | | | cable. The higher the amplification, or gain, provided by |
| system for computer timing applications. It describes | | | | the antenna, the longer the cable length that can be |
| the equipment that can be utilised to provide a precise | | | | utilised. |
| time resource for computers and computer networks. | | | | Roof-mounted antennas can be prone to lighting |
| Each GPS satellite has a highly accurate atomic clock | | | | strikes or other voltage surges. A lightning strike up to |
| timing reference, which can be used by NTP servers | | | | a quarter of a mile away can damage electronic |
| to synchronise computer networks. Each satellite | | | | components. Therefore, it is recommended that a |
| continually broadcasts accurate time and positioning | | | | surge suppressor be fitted. Lightning or surge |
| information. This information can be received by a | | | | suppressors are installed inline on the GPS cable and |
| GPS receiver and externally mounted GPS antenna. | | | | redirect any surge to earth. Various types of surge |
| The timing information received is referenced to UTC | | | | suppressor are available, from gas discharge devices |
| time. The system is a free to air service requiring no | | | | to solid-state multi-strike devices. |
| set-up or subscription fees. | | | | GPS receivers decode the received signal from the |
| Many Network Time Protocol (NTP) server systems | | | | antenna and provide easily readable information in a |
| utilise GPS time as an external timing reference to | | | | serial format. Most operating systems, including |
| synchronise Windows, LINUX, UNIX and other | | | | Microsoft Windows, LINUX and UNIX as well as time |
| operating systems and network infrastructures. GPS is | | | | server systems can utilise the time output provided by |
| an ideal time and frequency source because it can | | | | the GPS receiver for accurate synchronisation. Many |
| provide highly accurate time anywhere in the world | | | | different GPS protocols are utilised, the most popular |
| using relatively cheap components. | | | | being NMEA. The NMEA protocol was developed by |
| Each GPS satellite transmits a very low-power radio | | | | the National Marine Electronics Association. The |
| signal at two frequencies; L1 and L2. L1 is the civilian | | | | protocol specifies a set of sentences that contain time |
| frequency band transmitted at 1575 MHz, while L2 is | | | | and positioning information obtained by the GPS |
| used for military positioning purposes. The radio signal | | | | receiver. The sentences can be parsed by a PC or |
| transmitted by each satellite can easily pass through | | | | time server to obtain accurate time. |
| material such as glass and plastics but are blocked | | | | To summarise, the GPS system is an excellent source |
| more substantial materials used in buildings. The signals | | | | of precise time for NTP server systems or for |
| travel by line of sight. The ideal location for a GPS | | | | synchronising stand-alone Windows, LINUX and UNIX |
| antenna is on a rooftop with a good view of the sky. | | | | PC's. By using relatively low-cost components, the |
| The radio signals can be received by a low-cost GPS | | | | Global Positioning System can provide time sensitive |
| antenna. The antenna is essentially a high gain amplifier, | | | | applications with precise timing information. |
| which is used to amplify and transmit the weak radio | | | | |