NTP Server - Static GPS Antenna Installation

Overview - Using GPS for Accurate Time

The Global Positioning System is a US military system for worldwide navigation. The system consists of 24 orbiting satellites, each satellite has a highly accurate atomic clock on-board synchronised to UTC time. The satellites continuously broadcast time and position information. The time and position information can be obtained worldwide with a GPS receiver and antenna. GPS works continuously in any whether conditions, anywhere in the world. Additionally, there is no set up fee or subscription charges to utilise the GPS systems.

GPS 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 GPS to a resolution of a few nanoseconds.

GPS Signal

The transmitted GPS signal is very weak low-power radio signal, designated L1 and L2. L1 is the civilian GPS frequency 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. Therefore, the ideal location for a GPS antenna is on rooftop with a full 360-degree view of the sky. However, often installation on the side of a building or in a window can provide adequate results. As a rule of thumb, the better the view of the sky, increases the likelihood of a good consistent signal lock.

GPS Antenna's

The GPS antenna acts as an amplifier to boost the GPS signal for transmission along a cable, usually coax, to the GPS receiver. The timing antenna's provided with TimeTools NTP Server systems utilise a pole-mounting GPS antenna for use with its NTP server systems. The antenna screws to a threaded pole mount for installation on rooftops. This arrangement provides the GPS antenna with a rigid mount easily able to withstand high winds without damage. Typically the GPS antenna is fairly small in size, measuring less than 90cm in diameter. Low-cost patch type antennas are also available, but these are generally better suited to vehicle applications.

Antenna Cabling

The cable distance that can be utilised by a GPS antenna depends mainly on the amplification of the GPS antenna and the quality of coax used in the installation. A typical GPS timing antenna may have a gain of 35 db, relatively low-quality coax such as RG58 has an attenuation of 0.64 db/m at 1575 MHz. Therefore, a cable run of 55m can be obtained using RG58 cable. With very high quality coax cable, such as LMR400, an unaided cable run of 200m can be achieved. However, very high quality coax can be expensive. A good price-performance compromise is LMR200 cable, which can be run to 80m.

In-Line GPS Amplifiers

In-line GPS amplifiers provide further amplification of the GPS signal to increase the cable distance between the GPS antenna and receiver. GPS amplifiers are fitted in-line with the antenna cable and obtain power from the receiver via the coax cable. No external power-supplies are required. Typically, a GPS amplifier may add a further 20 dB of gain, adding 30m of low-quality RG58 coax, 40m of LMR200 coax or 100m of high quality LMR400 coax. Additionally, multiple in-line amplifiers may be utilised to further increase cable distance.

GPS Splitters

GPS splitters allow a single GPS antenna to be utilised by two or more NTP server systems. 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 Lightning Arrestors

Surge suppressors protect expensive NTP server equipment from electro-static discharges, such as lightning, that may be picked up by an externally mounted GPS 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 low-impedance ground, to discharge any received surge. The surge suppressor requires no power-supply or additional cabling.

Copyright 2007 Andy Evans
9 Jan 2007

Resources:

http://www.TimeTools.co.uk NTP Server and GPS timing information and systems.