Tuesday, July 22, 2014

Long Delay Echos A Radio Propagation Mystery


Picture 1 is taken from Villard's report[5] on long delay echoes (LDEs)

There's lots of science out there still to be done, even in age old hobby of ham radio.  In one of my recent posts, I talked about Operation Smoke Puff, and experiment that bounced radio signals off a man-made cloud of ions to a distant receiver.  The project was sponsored by the military with hopes of being able to provide longer distance radio communications by creating a made-to-order skip path. As it turns out, radio signals and the ionosphere are quite capable of providing long distance communications paths all by themselves, for folks who don't happen to own a missile launcher.  In some cases, a signal can be made to transmit around the entire planet.   Many amateur radio operators have experienced the phenomenon of round-the-world radio skips where the transmitted signal continues to bounce, completing a path that spans the globe and arrives back at the station.  The radio operator winds up hearing their own transmissions a fraction of a second after it was sent.  But, what about signals that take seconds to arrive back at their origin?  Where did they go in the meantime?

Round-the-world transmissions were well understood before the 1927, when a ham reported a propagation mode that we have yet to understand.  Carl Stormer reported hearing his own signal in his headphones about a seventh of a second after it was transmitted.  That's fairly normal, it's just an around the world signal as described above.  What Stormer didn't expect was hearing his own signal again a few seconds later.  While several hypothesis have been proposed for this phenomenon over the ensuing years, none has gained the evidentiary traction required to ring true among a majority of scientists.  A brief glance at Wikipedia will give you the five most likely culprits according to modern science.  They tend involve outer space plasma clouds combined with rather complex frequency mixing.  There's one alternative possibility that will make sci-fi fans sit up and take notice.  It could all be the result of a Bracewell probe!

While the Bracewell probe is currently listed as an 'alternative' hypothesis, as recently as 1970[5], O.G. Villard, noted Stanford electrical engineering professor thought the Bracewell explanation was as plausible as any other.

What's a Bracewell probe[3] you ask?  Dr. R. N. Bracewell worked at the Stanford RadioScience Laboratory and proposed the following thoughts about interstellar exploration:
1.  It would be difficult to send actual explorers over intergalactic distances.
2.  Automated probes on the other hand could do the job quite nicely
3.  Once an automated probe found evidence of intelligence, it would send work back to its own world and...
4.  It would attempt communication with the newly located civilization by repeating their communications back

How'd all this come up?
The Operation Smoke Puff article by Gladych, which focused attention on Dr. Marmo, peripherally mentioned Dr. Oswald Garrison Villard, Jr. of Stanford who managed the radio transmitters and receivers for the experiment.  Reading through Villard's publications is like reading a brief history of the each of the propagation modes mentioned on the amateur radio exams.  Round the world signal propagation?  Check!  Tropospheric ducting?  Check!  Meteor bounce propagation?[2]  Check!

Villard also studied long delay echoes.  And, it just so happens that he and Bracewell were at Stanford at the same time.  From Villard's article[5], (picture 1)


References:
1.  O.G. Villard on Wikipedia
http://en.wikipedia.org/wiki/Oswald_Garrison_Villard,_Jr.

2.  Villard on meteor scatter propagation
http://www.arrl.org/files/file/History/History%20of%20QST%20Volume%201%20-%20Technology/QS04-53-Peterson_opt.pdf

3.  Bracewell Probe
http://en.wikipedia.org/wiki/Bracewell_probe

4.  Bracewell Probe in Nature 1960
http://www.nature.com/nature/journal/v186/n4726/abs/186670a0.html

5.  Villard's report to the Advanced Research Projects Agency and the Office of Naval Research
http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0726735

The link on the above page will download the dtic report which contains the QST article on LDEs written by Dr. Villard.


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