Symmetricom S100 - Manual (Page 11)

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S100 User Guide – Rev. D – June 2005 3
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Product Details
Details about the physical description and operating environment of the S100 are found in
Appendix A,
“S100 Specifications” on page 119 of this User Guide.
Details about S100 operations are in “The Web-Based Interface” on page 53 , as well as
“Chapter 3” on page 13 , and “Operations & Time-Protocols” on page 87 .
Time Standards
The international time standard is called Coordinated Universal Time or, more commonly,
UTC. This standard was agreed upon in 1972 by worldwide representatives within the
International Telecommunications Union; today, the Internet Engineering Task Force (IETF)
sets standards based on the 1972 work. Today UTC is coordinated by the world’s
International Bureau of Weights and Measures, or BIPM. (The designations “UTC” and
“BIPM” were chosen as a compromise among all the countries’ abbreviations for the terms.)
The global availability and precision of UTC time makes it the ideal source of time for Time.
The S100 uses UTC as its time standard.
Global Positioning System (GPS)
The U.S. Department of Defense Global Positioning System (GPS) is a constellation of
approximately 29 satellites that orbit Earth twice a day. Their orbits are inclined 56 degrees to
the equator. The GPS satellites signals are used by a GPS receiver to precisely determine its
own position and time.
The orbits of these satellites and the offset (relative to international standard time, UTC) of
their on-board Cesium atomic clocks is precisely tracked by the U.S. Air Force control
network. Position and time correction information is uplinked from the ground control stations
and maintained in the satellites in what are termed ephemeris tables, or tables of data that
describe the satellite’s position when compared to specified coordinates. Each satellite
transmission reports the satellite’s current position, GPS time, and the offset of the satellite’s
clock relative to UTC, international standard time.
The “S100 GPS” model uses GPS to obtain time. (The “S100 ACTS” model obtains time by
dialing NIST's Automated Computer Time Service (ACTS).)