Petabits/sec, and the like

Robert G. Brown rgb at phy.duke.edu
Wed Nov 5 19:18:26 EST 2003


On Thu, 6 Nov 2003, Chris Samuel wrote:

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> On Thu, 6 Nov 2003 09:52 am, Robert G. Brown wrote:
> 
> > On a second thought, I suspect that the OF link is going to be
> > delivering real time analog data.  This is very similar to a plan for an
> > even bigger radiotelescope that I've had for years -- one that spans a
> > continent, or even continents.  The key to making a radiotelescope is
> > being able to deliver realtime traces of the received signals with very
> > precise time/phase delay information to a centralized location where the
> > traces can be recombined and used to create an interference projection
> > of the sky.
> 
> LOFAR is an interferometer in its own right, and it'll be the only 'scope 
> going down to those frequencies (AFAIK) and so there won't be anything else 
> to combine it with. :-)

I meant the "outlying stations" that give it a large baseline for high
resolution.  My idea has (for years now) been to transform all the cell
phone towers in a country into a gigantic radiotelescope.  You lose the
single receiver directionality that LOFAR has with a tight array of
parabolic receivers, but it is potentially SO cheap and there are SO
many stations with SUCH a large baseline that overall brightness and
resolution should be quite satisfactory.  The north american continent,
for example, would have an aperture of what, roughly 5000 km with towers
strung in irregular networked distributions -- every few km along major
highways and in dense clusters near cities and increasingly near even
small highways and small towns.  There must be tens to hundreds of
thousands of towers by now, with interference brightening of 10^8 or so
along the selected direction.

I actually have a student working on this idea to a limited extent at
this very moment -- sort of a preliminary feasibility study.  In
fundamental terms this means determining if the cell tower owners are
willing to permit a dual public use of their receivers (which should be
passive and utterly irrelevant to their function as cell phone
antennae).  Otherwise, I expect that all of the towers have fiber to
them already; it is just a matter of piggybacking...;-)

Using GPS and/or atomic clocks to establish a precise time base, local
PC's should be able to record a generalized radio trace at a particular
frequency.  The same GPS can be used to precisely locate the towers.
With a precise physical map of the receivers and a precise signal
against a common time base, recombining the signals with various delays
to assemble an image is then straightforward.  In fact, with the time
base, one could even do (I think) Hanbury-Brown-Twiss correlation
studies, which I imagine is also a goal of LOFAR via its outlier
stations although I haven't read far enough to find out.

In this context I don't know whether or not the traces from the
individual towers would be best sent digitized or not.  In the LOFAR
context they probably are.  Alas, I'm a theorist and so I'm not
sufficiently familiar with the hardware requirements one has to work
with to capture, save, send, and ultimately recombine the signals,
although I can visualize the math easily enough.

I'll see if I can get my student to join the LOFAR discussion group.  I
think he's a bit behind on this anyway, with all the work he has this
semester.

   rgb

-- 
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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