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Using GPS For Accurate Timing
This article describes how the Global Positioning System can be used to provide a precise timing reference and provides an overview of the equipment required.
The GPS system can be used to provide a highly accurate timing reference for computer network timing equipment such as NTP Server systems and other time critical applications. This article describes how the Global Positioning System can be used to provide a precise timing reference and provides an overview of the equipment required. The Global Positioning System is a US military solution for navigation. The system consists of 24 orbiting satellites. The satellites each have a precise atomic clock referenced to UTC time. The highly precise atomic clocks are used to calculate transmission times for coordination triangulation. However, the timing information provided by the satellites can also be used as an accurate timing reference for computer network timing systems, such as NTP Server systems. The GPS satellites are continuously broadcasting time and position information. This information can be obtained anywhere in the world with a simple GPS receiver and antenna. There are no set up or subscription fees to utilise the GPS system. Accuracy GPS receivers provide highly accurate position and timing information. Typically, a GPS receiver can provide positioning information to an accuracy of 15m. NTP Server systems can obtain timing information from the GPS system to a resolution of a few nanoseconds. GPS Signals The GPS signals transmitted from each satellite are very weak low-power radio signals, designated band L1 and band 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 clouds, glass and plastics but are blocked by objects such as metal and brickwork. The best location for an antenna is generally on a rooftop with a unobscured view of the sky. Often, installation on the side of a building with a 180-degree view of the sky will provide adequate results, provided the horizon is not too obscured. As a rule of thumb, the better the view of the sky, the greater the likelihood of a good consistent signal lock. GPS Antenna Types The GPS antenna is effectively an amplifier that boosts the weak GPS signal for transmission along a cable to the GPS receiver. GPS antenna's provided with NTP server systems or static applications utilise a pole-mounting system. The antenna screws onto a threaded pole for installation on rooftops. This arrangement provides the antenna with a rigid mount easily able to withstand adverse weather conditions. Typically the GPS antenna is fairly small in size, measuring less than 90cm in diameter. Patch type antennas are also available that are particularly suited to mobile or vehicle applications. GPS Cabling Issues 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. Typically, a GPS antenna may provide a gain of between 20 to 40 db. Coax cable such as RG58 has an attenuation of 0.64 db/m at 1575 MHz. Therefore, a cable run of 40m can be utilised. Also allow for signal reduction through any connectors. GPS Amplifiers, Splitters and Surge Suppressors In-line GPS amplifiers provide further amplification of the GPS signal to increase the cable distance between the antenna and receiver. The amplifiers are fitted in-line on the coax antenna cable. Typically, an in-line GPS amplifier may add 20-39 dB of gain, which may add upto 30m of coax cable. Additionally, multiple in-line amplifiers may be utilised to further increase cable distance. A single GPS splitter allows multiple NTP Servers or other GPS systems to share an antenna. Multiple GPS sytems may reduce cabling costs by sharing a single antenna. The GPS splitter splits the signal received from the GPS antenna into multiple outputs for synchronizing multiple NTP servers. GPS splitters are generally available with 2, 4 or 8 outputs. Surge suppressors protect expensive NTP server 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 requires a low-impedance connection to ground so that any surge picked up at the antenna can be dumped to ground.