EndRun Tempus LX GPS, User Manual

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T e m p u s L X G P S U s e r M a n u a l 1
"Smarter Timing Solutions"
Chapter
One
Introduction
The Tempus LX is a precision server of Universal Coordinated Time (UTC) that can be connected via
a 10/100Base-T ethernet port to any TCP/IP network. In its most basic operation, it sends Network
Time Protocol (NTP)/Simple Network Time Protocol (SNTP) reply packets in response to NTP/SNTP
request packets which it has received from clients. The timestamps it sends in its NTP/SNTP reply
packets are accurate to less than one-hundred microseconds. NTP/SNTP client software is available
for virtually all operating systems.
The Tempus LX is composed of a Global Positioning System (GPS) time and frequency engine inte-
grated with an IBM-PC compatible fanless, convection-cooled 133 MHz CPU with integral ethernet
interface, a graphic vacuum-fluorescent display, a keypad, and a power supply. Non-volatile stor-
age of the embedded Linux operating system and the Tempus LX application software is via FLASH
memory.
For more detailed information that is not included in this manual, and links to other sites, please visit
our website:
http://www.endruntechnologies.com
. There you can also download firmware upgrades,
the latest manuals and other documentation.
GPS Timing-How It Works
The time and frequency engine in the Tempus LX receives transmissions from satellites that are
operating in compliance with the Navstar GPS Interface Control Document (ICD) known as GPS-
ICD-200. It specifies the receiver interface needed to receive and demodulate the navigation and
time transfer data contained in the GPS satellite transmissions. The GPS navigation system requires
a means of synchronizing the satellite transmissions throughout the constellation so that accurate
receiver-to-satellite range measurements can be performed via time-of-arrival measurements made
at the receiver. For the purposes of locating the receiver, measurements of the times-of-arrival of
transmissions from at least four satellites are needed. For accurate time transfer to a receiver at a
known position, reception of the transmissions from a single satellite is sufficient.
The GPS system designers defined system time to be GPS time. GPS time is maintained by an
ensemble of high-performance cesium beam atomic frequency standards located on the earth’s
surface. GPS time is measured relative to UTC, as maintained by the United States Naval Observa-
tory (USNO), and maintained synchronous with UTC-USNO except that it does not suffer from the
periodic insertion of leap seconds. Such discontinuities would unnecessarily complicate the system’s
navigation mission. Contained in the data transmitted from each satellite is the current offset between
GPS time and UTC-USNO. This offset is composed of the current integer number of leap seconds
difference and a small residual error that is typically less than +/- 10 nanoseconds.
Each satellite in the constellation contains redundant cesium beam or rubidium vapor atomic frequen-
cy standards. These provide the timebase for all transmissions from each satellite. These transmis-