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SETI pauses/shuts down

Discussion in 'Science & Technology' started by El_Machinae, Apr 27, 2011.

  1. Blue Emu

    Blue Emu GroFAZ

    Joined:
    Oct 21, 2006
    Messages:
    294
    Location:
    New Brunswick, Canada
    Radar? Isn't the O'Hare International Airport radar beacon brighter than the Sun, at the specific narrow wavelength that it uses?
     
  2. emzie

    emzie wicked witch of the North

    Joined:
    Jul 5, 2004
    Messages:
    20,526
    Location:
    Ottawa, Ontario
    Correct me if I'm wrong, but radar bounces off the atmosphere, no? If so, I'm not really sure the point you're making. We're not talking about radio waves no longer being used, but radio waves that an alien civilization could detect not being used.
     
  3. Blue Emu

    Blue Emu GroFAZ

    Joined:
    Oct 21, 2006
    Messages:
    294
    Location:
    New Brunswick, Canada
    Only the very lowest frequencies (longest wavelengths) of RADAR will reflect off the ionosphere. Shortwave RADAR has too rapid a cycle time for the electrons in the ions to react to the signal and absorb/re-emit it.

    The ionosphere reflectance centers around 30 MHz. Airport RADARs (in the USA, for example) use X-band RADAR centered around 10 GHz... more than 300 times more penetrating. Ballistic Missile Early Warning RADARs center around 750 MHZ, and satellite transponders center around 6 GHz. Ground-mapping RADARs center around 30 GHz. None of those systems would work properly if the ionosphere reflectance reached such short wavelengths.

    There are a few high frequency windows that are blocked by absorbtion from atmospheric gasses (Oxygen, Water Vapor)... but those frequencies obviously aren't used for long-range RADARs.
     

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