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 way.
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 that response.
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
Advanstar Communications, 859 Willamette Street, Eugene
OR 97401, USA
click here to download PDF version, posted with
permission
