Robert G. Brown
rgb at phy.duke.edu
Thu Oct 11 15:26:04 PDT 2001
On Thu, 11 Oct 2001, Greg Lindahl wrote:
> Now if you want GOOD time synchronization (to within a microsecond)
> that's a *much* harder problem...
Yes, it starts to become very difficult and expensive right in there
somewhere. The so called "atomic clocks" one can now buy in stores like
Wal Mart for $50 or so are autosync'd to WWVB at NIST in Colorado.
Unfortunately, Colorado is about 1000 miles away, which is around 3
microseconds right off the top that is difficult to compensate for.
Also, this station broadcasts at only 60 kHz which means that time
resolution is likely to be more like milliseconds or worse. Network
jitter and so forth complicate net-based time sync connections, although
one can get to millisecond accuracy on a good day (or over several good
However, there is hope even for microsecond resolution. GPS now
provides a VERY accurate time base that is available basically
everywhere with appropriately high frequencies (necessary to localize in
space, and localization in space equals localization in time). For
offers a clock that is good (according to their spec) "to a millionth of
a second" (microsecond pulse rise time) for only $380 that can be used
(with a bit of work with a soldering gun, it looks like) to build a
really accurate local ntp server among other things. A few other
companies (like www.sofcom.com) seem to offer ready-to-run GPS solutions
but don't give their precision and may just give you the milliseconds
that the "easy" NMEA interface will give you instead of using PPS to
produce raw UTC second pulses that you can read into a PCI card and see
on an oscilloscope.
Still, even if one has to buy a really good digital input card per host,
buy some signal splitters and logic, and write some drivers, one could
probably get to 1 microsecond or better by averaging over a long time
base and successively refining the clock setting (the central limit
theorem is your friend) at a cost of maybe $1000 plus a card per machine
to pipe the signal in on. Maybe even less.
Much less than a microsecond I think would require serious money,
though. A real atomic clock sync'd with NIST at NIST and carried to
your machine and interfaced with it? Then there are the kernel hacks
one might need to start thinking about as your wall-clock time base
resolution approaches the resolution of your CPU clock. Computer
latency already kills most internal timings at >microsecond resolution.
Is this a completely crazy project or totally inappropriate for
discussion on the beowulf list (all joking aside)? I don't think so.
I keep waiting for somebody to realize that with adequate spactial
localization and time and phase synchronization (which can be provided
now by GPS) that the array of cellular phone antennae that have sprouted
all over the landscape like mushrooms have the potential to form the
world's largest and most precise radio telescope. If a computer with a
really accurate time base were set up at each precisely located antenna,
and all the computers were linked into a giant integrated beowulf by
means of the handy optical fiber data line that I imagine that goes into
each of these sites, and each computer were set so that it could write
the output of its precisely tuned radio signal input onto local disk
WITH AN ACCOMPANYING TIME BASE (to provide the essential phase shifts)
which it could then send to a "master" node for image processing and
interference/reconstruction, one could take >>bright<< radiographs of
the sky with >>very<< fine angular resolution, diffraction limited by
the fraction of the continent exposed to the source (effectively 5000 km
or thereabouts for targets overhead) and the dispersion of the turbulent
atmosphere (with the latter highly averaged).
The scientific possibilities of such an array are mind-boggling. For
one thing, it might actually make "Star Wars" a technologically feasible
proposal instead of a joke, as it would permit precise localization of
incoming radio sources or reflectors. Inverted into phase locked
transmitters, the array might be able to focus significant radiated
power in a highly directional manner. It would also let us look far,
far away and long, long ago, which physicists and cosmologists and
astronmers always like doing.
I'm not sure microsecond time resolution would suffice -- nanosecond
would be much better, opening up all of the AM bands and at least some
of the FM bands -- but it would be fun to build with large supplies of
other people's money. There's a wild idea for a "beowulf" -- an
integrated computer/radiotelescope/death ray, built out of "commodity"
components like cellular towers and computers and very accurate
Remember folks, you heard it here first...;-)
Robert G. Brown http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567 Fax: 919-660-2525 email:rgb at phy.duke.edu
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