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Using GPS As An Accurate Time Reference

The Global Positioning System (GPS) is primarily intended to provide a worldwide positioning and navigation system. However, as a by-product of providing precise positioning information, the GPS system can also provide very accurate time. This article describes how GPS can be utilised as a precise source of time for computer and network timing applications, such as NTP time servers.

The GPS system is a US Defence department military system for worldwide navigation and positioning. The system consists of 24 orbiting satellites encircling the Earth. Each satellite contains a highly precise atomic clock synchronised to UTC time. The clocks are used to calculate minute transmission delays to provide positioning information by using triangulation. The precise timing information provided by the satellites can also be used as a reference for computer timing applications.

The GPS system continuously transmits time and position information from its orbit around the Earth. This information can be obtained anywhere on the face of the planet with a simple GPS receiver. Typically, a modern GPS receiver can provide positioning information to an accuracy of 15m. Time servers and computer applications can utilise timing information from the GPS system to a resolution of a few nanoseconds.

The signals transmitted from each GPS satellite are very weak radio signals, designated band L1 and L2. The L1 frequency is the civilian GPS signal transmitted at 1575.42 MHz. The signals travel by line of sight and can pass through materials such as clouds, glass and plastics but are blocked by dense objects such as metal and brickwork. Therefore, the ideal location for a GPS antenna is where a clear view of the sky can be obtained. As a guide, the better the view of the sky, the greater the likelihood of a consistent satellite signal lock.

A GPS antenna is essentially an amplifier that boosts the weak GPS signal for transmission along a cable to a receiver. Computer time servers or static applications generally utilise a rugged marine pole-mounting antenna. The antenna screws down onto a threaded pole for installation on rooftops and other exposed areas. This arrangement provides the antenna with a rigid mount able to withstand high winds and other adverse weather conditions. Typically, pole-mounting GPS antennas are fairly small in size, measuring less than 70mm in diameter.

The cable distance that can be utilised by a GPS system depends mainly on the amplification of the antenna and the quality of coax used in the installation. A typical antenna may have a gain of 40 db. Coax cable such as RG58 has an attenuation of 0.65 db/m at 1575 MHz. Therefore, a cable run of 50m or more can be utilised. It is however a good idea to allow for some margin of error and also to account for signal loss through connectors and any ancillary equipment such as surge suppressors.

GPS amplifiers provide further amplification of the GPS signal to increase the cable run between the antenna and receiver. The amplifiers are fitted in-line on the coax antenna cable. Typically, a GPS amplifier may add a further 20 dB of gain, adding a potential 30m of cable. Multiple in-line amplifiers may be utilised to further increase cable distance.

GPS splitters allow a single GPS antenna to be utilised by two or more GPS receivers. For example, multiple time servers may share a single antenna, reducing installation and cabling costs. The GPS splitter splits the signal received from the GPS antenna into multiple outputs for synchronizing multiple receivers. Splitters are generally available with 2, 4, 8 or even 16 outputs.

Surge suppressors protect expensive equipment from electro-static discharges, such as lightning, that may be picked up by an externally mounted antenna. Surge suppressors are installed in-line on the coax cable between the antenna and receiver, ideally, where the cable enters the building. Surge suppressors require a connection to a low-impedance ground, to discharge any surge picked up at the antenna.

To summarise, GPS is an ideal reference for network time servers and computer timing applications. It is available worldwide and is a highly accurate source of time. Additionally, it can be referenced with relatively low-cost equipment.

Article Source: http://impactdns.com

This article was written by David Evans, a technical author with a wealth of experience in field of computer time and frequency solutions. Click here to visit TimeTools web-site for more articles and other information on GPS NTP servers and time server systems.

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