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GPS Time Servers For Network Time Synchronisation
The GPS system is a satellite navigation system operated by the United
States military. The system is also available for civilian use without
any subscription requirement. The GPS system is primarily intended as a
highly accurate global positioning system. However, each GPS satellite
also has onboard a highly accurate atomic clock, ideal for providing a
precise time reference for computer network time synchronisation.
This article discusses how GPS time server systems obtain precise timing
information to provide a highly accurate timing reference for network
time synchronisation.
GPS Time Servers
The GPS system consists of a constellation of 24 orbiting satellites,
each with a precise atomic clock timing reference. The system covers the
entire globe. Precise time information is continuously broadcast from
each of the satellites. The broadcast timing information can be easily
received with a relatively low-cost GPS antenna and receiver.
GPS time is broadcast as Universal Coordinated Time (UTC), which is
similar to Greenwich Mean Time (GMT). UTC time is the same worldwide; it
does not vary with time zone or daylight saving time. UTC is a high
precision atomic time standard maintained by atomic clocks located in
national standards laboratories. UTC has uniform seconds defined by the
International Atomic Time (TAI) institute.
GPS Radio Transmissions
The GPS satellites transmit information as very low-power radio frequency
transmissions. Two designated frequencies are used, one for civilian use
coded L1, and one for military use coded L2. The civilian L1 frequency is
transmitted at 1575 MHz. The weak radio transmission can easily pass
through plastics and glass but is blocked by metal and brick.
GPS Antenna Location
The GPS antenna needs to have a good view of the sky in order to receive
transmissions from as many satellites as possible. Ideally, a
roof-mounted antenna is best with a full 360-degree view of the sky.
However, quite often an antenna located on the side of a building with a
180-degree view of the sky is adequate provided the horizon is not too
obscured.
GPS Antenna Types
The GPS antenna is essentially a signal amplifier. The antenna boosts the
received GPS signals for transmission along a cable to a receiver for
decoding. Coax cable is generally used to transfer signal information
between the GPS antenna and receiver. GPS antennas are available in a
range of shapes and sizes. The most common antennas types being
pole-mounting dome shaped antennas and small patch type antennas. The
pole-mounting antenna screws onto a threaded pole for mounting. While the
patch type antenna is a small flat-bottomed device ideal for mounting on
a windowsill.
GPS Receivers
GPS receivers decode the GPS transmission received from the antenna into
a useable format. There are a number of protocols utilised by GPS
receivers, the most common being NMEA. The NMEA protocol consists of a
number of sentences. Each NMEA sentence provides a packet on information
consisting of time, date and positioning information. The protocol also
provides information indicating visible satellites and satellite location.
GPS time servers may utilise a specific GPS timing receiver. Timing
receivers have additional functions to ensure a highly accurate reference
time. They can also perform an automated survey and compare satellite
atomic clocks to check for synchronicity. GPS time servers may also
utilise an accurate pulse per second (PPS) output generated by the
receiver. A PPS output provides a highly accurate reference trigger for
GPS timing.
GPS Antenna Installation
The maximum cable distance that can be utilised by a GPS antenna and
receiver depends on the gain of the antenna and the coax utilised. A
typical antenna used with a GPS time server may have 35dB gain. RG58 coax
has an attenuation of 0.64dB/m at 1575MHz. A cable run of 35/0.64 = 55m
could therefore be utilised. Higher quality coax has a much lower
attenuation value allowing longer cable runs. Also GPS amplifiers and
updown converters can be utilised to increase cable length still further.
Surgelightening arrestors are also recommended on externally mounted GPS
antennas to protect expensive network equipment from potential damage by
lightening strikes.
Summary
The Global Positioning System provides a highly accurate reference clock
ideal for network time server systems and computer time synchronisation.
Accuracies of a few nanoseconds can be reasonably achieved with
relatively low-cost GPS receivers and antennas.
About the Author
D. Evans is a technical author and development engineer. Dave develops
GPS time server solutions that synchronise time on computers and computer
networks. Find out more about NTP Server and GPS time server solutions
at: http://www.timetools.co.uk/time-servers/gps-time-t1000.htm
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