GPS Antenna Array
GPS Antenna Array
Abstract.
This paper provides an overview of the results of a continuous, 2
month experimental evaluation of all timing data provided by several
GPS receivers. The primary purpose of this experiment was to
provide measurement data facilitating fault modeling in our project
SynUTC, which aims at external clock synchronization in
fault-tolerant distributed real-time systems. As expected, the GPS
receivers under test exhibited a wide variety of failures, ranging
from transient omissions up to considerable deviations of the timing
signals provided. Whereas those findings justify the appropriateness
of our basic failure assumptions, it became nevertheless apparent that
rerunning the experiment for a longer duration and with
new brands/models of GPS receivers is advisible.
Abstract.
In January 1995 the research project SynUTC was started at the
Department of Automation at the Technical University of Vienna.
This project is dedicated to fault-tolerant synchronization
of computer-clocks in distributed real time systems.
In contrast to former approaches which established only
a global time base without any relationship to
international accepted time scales (like UTC [Coordinated Universal Time]),
the synchronicity to such time scales is a major issue in SynUTC.
This approach certainly needs an efficient access to
the time scale. A highly attractive mean w.r.t.
accuracy and availability is the satellite navigation
system NavSTAR GPS (Navigation Satellite Timing and Ranging Global
Positioning System), which provides worldwide and
continously accurate position and time
(accuracy depending on expense from 10 ns to 1 µs)
relative to the time scale UTC [Actually only
relative to the real-time representation of UTC, namely
UTC(USNO, MC) which is realized by the master clock of
the United States Naval Observatory
(USNO)]. The availability of cheap
GPS-Receivers (some 1000 ATS) finally justifies why the GPS is considered
the primary means for accessing the international time-scale UTC.
Fault-tolerance --- a major issue in the project ---
requests a deeper study of the potential failure modes
of GPS receivers. For this purpose 6 different GPS receivers
(prices from 5,000 ATS to 60,000 ATS) were evaluated
in a 2 months experiment. Among other things,
it was interesting to find out whether the often
propagated opinion that the time is provided
by GPS receivers is highly reliable, is true. Surprisingly it turned
out that only 3 of the 6 receivers
worked satisfactory. Several different failure modes, including
sporadic misbehaviour or an extremly offlying time signal (some µs),
were observed for the 3 other receivers.
