GPS Systems for NTP Time Servers and Timing Applications

GPS Systems for NTP Time Servers and Timing Applications
The Global Positioning System (GPS) is most widely known as
a system for vehicle and marine navigation. However, each
GPS satellite orbiting the earth also has an integrated
precise atomic clock that can be used as a reference clock
for NTP time servers and for general computer timing
applications. This article describes the components
required to utilise GPS for computer timing applications.
It also discusses how the GPS system can be used for
computer network timing by using the Network Time Protocol
(NTP) and NTP time servers.

The GPS system is a United States military system intended
for global navigation. The GPS system is a space-based
system, consisting of a network of 24 orbiting satellites.
Precise navigation and positioning is achieved by utilising
precision timing signals and triangulation between multiple
satellites. In order to provide very precise time, each
satellite has an integrated highly accurate atomic clock
onboard. Each satellite broadcasts timing information to
within a few nanoseconds of the correct time to provide
positioning to an accuracy of better than 10m. This precise
timing information can be used by NTP time servers and
other computer timing equipment as a highly accurate time
reference. The great thing about GPS is that it works
anywhere in the world and is entirely free to air service.

Each GPS satellite transmits a low-power radio signal down
to the surface of the Earth. Two separate frequencies are
utilised, designated L1 and L2. The L1 band is the civilian
signal; transmitted at 1575.42 MHz. L2 is the military
frequency, intended for ultra-high precision positioning.
The radio signals are transmitted from the satellites by
line of sight. The transmitted signal is easily powerful
enough to pass through clouds, glass and most plastics but
is absorbed by more substantial material such as brick,
roofing materials and metals. The ideal location for a GPS
antenna tends to be outdoors and ideally on a rooftop with
the best possible view of the sky.

In order to receive GPS timing and positioning information
some relatively low-cost equipment is required. A GPS
antenna is utilised to receive and amplify the GPS radio
signals. While a GPS receiver decodes the information and
presents it in a computer readable format. Many antennas
are available with integrated receivers but these have the
drawback of only providing a few meters of cabling to the
host NTP time server or computer system. Rugged GPS marine
antennas screw down on to a rigid threaded pole mount able
to withstand the worst of the weather. These antennas
provide the best solution for static timing applications.

The GPS receiver is generally embedded into the NTP time
server or encapsulated within an enclosure. GPS receivers
are small modules that have a connection to the GPS antenna
to receive amplified GPS signals. The receiver decodes
these signal and provides a computer readable output
usually via a RS232 or USB interface. Typically,
information provided by the receiver includes time and
positioning information, number of satellites in use,
number of visible satellites and whether a signal lock has
been acquired.

Coaxial cable is utilised to provide a connection between
the GPS antenna and receiver. The cable distance can be
critical. The antenna must amplify the GPS signal
sufficiently to overcome any cable losses associated with
the cable run utilised. Better quality antennas generally
provide more amplification (gain) additionally higher
quality coax cables can be used with much smaller signal
losses per meter allowing for relatively long cable runs
between a antenna and receiver. In order to extend cable
runs still further GPS amplifiers can be used to boost the
signal transmitted down the coax cable. Typically, a GPS
amplifier sits in-line on the coax cable and is powered
from the receiver. Multiple amplifiers can be used for
extremely long cable runs.

A roof mounted GPS antenna can be prone to lightning
strikes and voltage surges. It is recommended that surge
suppressors are utilised to protect expensive NTP time
server and timing systems from potential damage. A surge
suppressor is essentially a barrier that sits in-line on
the coax cable between the antenna and receiver that
protects the receiver from any potential damage due to
voltage surges.

To summarise, the GPS system provides a highly precise
timing reference for NTP time servers and other computer
timing equipment. It is a reliable free to air service that
can be used anywhere in the world. Additionally, with the
boom in vehicle navigation systems the technology required
to receive GPS broadcasts is continually falling in price.

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The author, David Evans, is a technical author in the field
of computer network timing systems. David has a number of
years experience of writing articles and white papers about
NTP and GPS time synchronisation. Please visit our web-site
if you would like more information on NTP Time Server
systems:
http://www.timetools.co.uk/ntp-servers/ntp-s5500.htm