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NTP Server - Static GPS Antenna Installation
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
antenna's used by http://www.TimeTools.co.uk 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 TimeTools Andy Evans 9 Jan 2007
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