EndRun Sonoma N12, Руководство пользователя

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S o n o m a U s e r M a n u a l 1
"Smarter Timing Solutions"
Chapter
One
Introduction
This chapter introduces the GPS-Synchronized Sonoma Network Time Server and gives a brief over-
view of what it is and how it works.
What It Is
The Sonoma Network Time Server is a precision server of Coordinated Universal Time (UTC) that
can be connected via an Ethernet port to any TCP/IP network. Available timing protocols include:
Network Time Protocol (NTP), Simple Network Time Protocol (SNTP), Time, Daytime, and the
optional Precision Time Protocol (PTP/IEEE-1588).
In its most basic operation, the Sonoma sends NTP reply packets in response to NTP request packets
which it has received from clients. The timestamps it sends in its NTP reply packets are accurate to
10 microseconds, typical. For an introductory paper on NTP see:
http://www.endruntechnologies.com/pdf/NTP-Intro.pdf
GPS Timing-How It Works
The GPS Subsystem in the Sonoma 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-sat-
ellite 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, recep-
tion 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 en-
semble of high-performance cesium beam atomic frequency standards located on the earth’s surface
(GPS Master Clock Ensemble). GPS time is measured relative to UTC, as maintained by the United
States Naval Observatory (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 nanosec-
onds.
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-