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URL:
http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
GPS, or the Global Positioning System, offers a way
to determine time to a precision of better than a few hundred nanoseconds
almost anywhere on the surface of the earth. It also offers real-time position
fix with an accuracy of 3 to 100 meters on a 24-hour per day basis. GPS is
developed and operated by the US Department of Defense. The GPS system consists
of 24 satellites, orbiting the earth at an altitude of about 10,900 miles and
at an inclination of 55 degrees. The orbits are distributed around the earth in
such a way that at least 4 satellites are always visible from virtually any
point on the surface of the earth. This provides a means of precisely
determining the position of the user in longitude, latitude, and altitude. GPS
uses Spread Spectrum signals to perform measurements.
Advantages/Strengths
GPS devices can be directly connected to equipment to
provide extremely accurate time synchronization and time-stamps. For equipment
where time synchronization and time-stamping accuracy is paramount, GPS is
preferred over the alternative of SNTP (Simple Network Time Protocol)
functionality, which is used to synchronize systems over a network by analyzing
round-trip time delays statistically.
Disadvantages/Weaknesses
GPS devices, although decreasing in price, are still
too expensive for every type of device requiring some level of time
synchronization.
Keywords:
time synchronization, Position, Spread Spectrum
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