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Using GPS for Computer and NTP Server Timing
The Global Positioning System is a global navigation system operated by the United States Naval Observatory (USNO). The GPS system consists of 24 satellites orbiting the Earth. By utilising relatively low-cost GPS receivers, accurate time and positioning information can be received anywhere in the World. This article presents an introduction to using the GPS system for computer timing applications. It describes the equipment that can be utilised to provide a precise time resource for computers and computer networks. Each GPS satellite has a highly accurate atomic clock timing reference, which can be used by NTP servers to synchronise computer networks. Each satellite continually broadcasts accurate time and positioning information. This information can be received by a GPS receiver and externally mounted GPS antenna. The timing information received is referenced to UTC time. The system is a free to air service requiring no set-up or subscription fees. Many Network Time Protocol (NTP) server systems utilise GPS time as an external timing reference to synchronise Windows, LINUX, UNIX and other operating systems and network infrastructures. GPS is the system of choice as a timing reference since it can provide time to within a few nanoseconds of UTC (Universal Coordinated Time) at a low-cost. Each GPS satellite transmits a very low-power radio signal at two frequencies; L1 and L2. L1 is the civilian frequency band transmitted at 1575 MHz, while L2 is used for military positioning purposes. The radio signal transmitted by each satellite can easily pass through material such as glass and plastics but are blocked more substantial materials used in buildings. The signals travel by line of sight. The ideal location for a GPS antenna is on a rooftop with a good view of the sky. The radio signals can be received by a low-cost GPS antenna. The antenna is essentially a high gain amplifier, which is used to amplify and transmit the weak radio signals down a coax cable to a GPS receiver. The cable length that can be utilised between a GPS antenna and receiver is critical. The maximum distance that can be utilised is dependant on the gain of the antenna and the quality of the coax cable used. High antenna gain and high quality coax allows for much longer cable runs. GPS signal amplifiers can be fitted inline on the coax cable to boost the signal and increase cable length. GPS amplifiers are generally powered from the coax cable. The higher the gain provided by the amplifier, the longer the cable can be extended. Roof-mounted antennas can be prone to lighting strikes or other voltage surges. A lightning strike up to a quarter of a mile away can damage electronic components. Therefore, it is recommended that a surge suppressor be fitted. Surge suppressors sit in-line on the coax cable and dump any voltage surge to a low-impedance earth. Various types of surge suppressor are available, from gas discharge devices to solid-state multi-strike devices. GPS receivers decode the signal received from the amplifier and generally make the received information available in a RS232 serial format. Microsoft Windows, Linux and Unix PC's or NTP Time Servers can utilise the timing information from the receiver for synchronisation purposes. Many different GPS protocols are utilised, the most popular being NMEA. The NMEA protocol was developed by the National Marine Electronics Association. The protocol specifies a set of sentences that contain time and positioning information obtained by the GPS receiver. The sentences can be parsed by a PC or time server to obtain accurate time. To summarize, the GPS system is an excellent source of precise time for NTP server systems or for synchronising stand-alone Windows, LINUX and UNIX PC's. Utilising relatively low-cost equipment the GPS system can be used to synchronise any time critical application. |
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