Long delayed echo
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Long delayed echos (LDEs) are radio echos which return to the sender several seconds after a radio transmission has occurred. Delays of longer than 2.7 seconds are considered LDEs.[1]
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[edit] History
These echos were first observed in 1927 by radio engineer Jorgen Hals while working near Oslo, Norway. Hals observed an unexpected second radio echo with a significant time delay after the primary radio echo ended. Unable to account for this strange phenomena, he wrote a letter to Norweign physicist Carl Størmer, explaining the event:
| “ | At the end of the summer of 1927 I repeatedly heard signals from the Dutch short-wave transmitting station PCJJ at Eindhoven. At the same time as I heard these I also heard echoes. I heard the usual echo which goes round the Earth with an interval of about 1/7th of a second as well as a weaker echo about three seconds after the principal echo had gone. When the principal signal was especially strong, I suppose the amplitude for the last echo three seconds later, lay between 1/10 and 1/20 of the principal signal in strength. From where this echo comes I cannot say for the present, I can only confirm that I really heard it. | ” |
The pair, joined by another physicist Balthasar van der Pol researched the echos for some years, but failed to come up with an explanation. The reason for this is that the time-delay of the echos varies dramatically. One would expect that, if these were echos off some region of the atmosphere or something in space (e.g. the moon) the echo's would exhibit a predictable time delay (the time taken to travel to, and back from, the deflecting entity). [2]
[edit] Theories
Some believe that the aurora activity that follows a solar storm is the source of LDEs. Others believe that the ionosphere is the cause. The most popular current theory is that the radio signals are trapped between two ionized layers in the atmosphere and then are guided around the world many times over until they fall out of a gap in the bottom layer. Still others believe that LDEs are double EME (EMEME) reflections, i.e. the signal is reflected by the moon and that reflected signal is reflected by the Earth back to the moon and reflected again by the moon back to the earth. A small community of people believe the LDEs are transmissions from a Bracewell probe, an artifact of aliens trying to communicate with us by bouncing back our own signals.[3]
[edit] References
- ^ http://www.arrl.org/news/features/2003/10/31/1/ ARRL, Radio Ghosts, Long-Delayed Echos Again
- ^ http://www.df5ai.net/ArticlesDL/LEchoes%28E%29.pdf V. Grassmann, Long-delayed radio echoes, Observations and interpretations, VHF Communications vol 2, pp. 109-116, 1993.
- ^ http://www.arrl.org/news/features/2003/10/31/1/ ARRL, Radio Ghosts, Long-Delayed Echos Again
