The following "Directions 2001" article by David W. Allan appeared in the
December 2000 issue of GPS World magazine, p. 36.
It has been posted with the publisher's permission to
As we look to the future, I suggest that there are two major timing
opportunities that could greatly benefit systems like GPS and Galileo, as
well as many other timing-sensitive systems. These suggestions have not been
fully utilized to date, one being a breakthrough invention. I propose a
better utilization of clock ensembles and a new invention called
"Synchronistic Modulation Detection (SMD)."
As an example, both in the application of GPS and in the planning of
Galileo, one of the most significant concerns for their use is integrity. In
the case of GPS, integrity has been primarily worked from the top down
rather than from the bottom-up. By "top down" I mean that integrity is
primarily a system responsibility in guaranteeing that all signals
transmitted by the system are consistent with specifications as to both
position and timing. By 'bottom-up" I mean that the user could also take
responsibility for integrity in a very inexpensive, robust, and reliable
Those working integrity with GPS have brought the response time down to six
seconds, during which interval the user would be informed of faulty data
coming from the system. Galileo design calls for a one-second response time.
Of course, the goal is to have the response time as short as possible while
having the highest integrity reasonably attainable as to the validity of
We have a great opportunity to increase the integrity and to shorten the
response time by utilizing inexpensive, quartz-crystal oscillator ensembles
in the user sets. Clock ensembles have the great advantage that they are
self-characterizing; hence, they can sense statistical outliers very quickly
and with great integrity. This concept was introduced a year ago using
theory and simulations demonstrating significant cost-effectiveness. The
concept has not been implemented to date. Such ensembles could be made very
small as well as inexpensively. We can easily envision high integrity
response times well under a second using this "bottom-up" approach.
By working the "top-down" and the bottom-up" approaches together, we can
produce both a great improvement in integrity as well as a significant
decrease in the response time. In a cooperative mode, these two approaches
could be integrated and made totally compatible. Legal issues may arise as
to where to place responsibility for the data's integrity -- with the system
or with the user -- but in a well-designed system that responsibility could
be clearly be delineated. The benefits versus the costs for this combined
approach are anticipated to be very significant and well worth the effort to
bring it about.
The first simulations of the new "Synchronistic Modulation Detection" have
affirmed the theory allowing effective transfer of time and frequency over
various media. In its theoretical limit, SMD shows that the transfer medium
does not limit synchronization accuracy and precision. Application
opportunities for this approach exist in mobile telephony, synchronization
of satellite clocks, and new research experiments.
Copyright 2000 GPS World magazine
December 2000 issue, p. 36
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