Long delayed echo

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. [http://www.arrl.org/news/features/2003/10/31/1/ ARRL, "Radio Ghosts", "Long-Delayed Echos Again"]

History

These echos were first observed in 1927 by civil engineer Jorgen Hals from his home near Oslo, Norway. Hals had repeatedly observed an unexpected second radio echo with a significant time delay after the primary radio echo ended. Unable to account for this strange phenomenon, he wrote a letter to Norwegian physicist Carl Størmer, explaining the event:

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 effect only occurs sporadically and that the time-delay of the echos varies dramatically. One would expect that, if these were echoes off some region of the atmosphere or something in space (e.g. the moon) the echoes would exhibit a predictable time delay (the time taken to travel to, and back from, the deflecting entity). [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.]

Long delayed echoes have been heard sporadically from the first observations in 1927 and up to our time.

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. [http://www.arrl.org/news/features/2003/10/31/1/ ARRL, "Radio Ghosts", "Long-Delayed Echos Again"]

References

ee also

*Propagation mode

External links

* [http://www.df5ai.net/Material/articles5.html#ArticlesLDE Propagation Studies, DF5AI.NET]
* [http://www.vhfdx.net/discuss/lde.html Discussion on Long Delayed Echos]