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    Re: Can you have a big jammer in space?
    From: Lu Abel
    Date: 2015 Mar 10, 22:08 +0000
    GPS signals, as received on earth, actually have a NEGATIVE signal-to-noise ratio.   That is, background noise in fact is stronger than the signal.   But knowing what a signal looks like allows receivers to filter out the noise.

    Before we go too much further, let's identify two forms of disabling GPS -- overwhelming (jamming) the signal and spoofing the signal.

    Cheap GPS jammers are just white noise jammers that produce a signal so strong that they overwhelm the receiver's ability to even hear the signal.

    Fascinating thought, David!  But I believe a satellite-based jammer would likely have problems working.   If solar powered, then it would produce a signal no stronger than GPS itself and therefore be easily filtered out.   I could, of course, use my solar cells to charge batteries to power a much stronger transmitter -- but I'd only be able to jam the GPS signal for a short period of time before the batteries ran down.

    Spoofing the signal (and maybe even jamming the signal from space) would clearly be taken as an act of war.  And I suspect if you have North Korea's values, you'd probably rather inflict physical harm on the US (ie, drop a nuclear bomb on it) rather than simply jam a navigation system, especially when US military aircraft and warships likely have backup systems (no, not celestial, but perhaps inertial guidance systems).

    The US has had GPS-based weapons guidance systems for at least two decades now.   I rather suspect they've taken various types of "GPS jamming" into account


    From: David Pike <NoReply_DavidPike@fer3.com>
    To: luabel@ymail.com
    Sent: Tuesday, March 10, 2015 2:46 PM
    Subject: [NavList] Can you have a big jammer in space?

    The suggestion was:
    Whether they develop the means and the motivation to launch a big jammer into orbit over the Pacific basin is something one can only speculate about.
    Can you have a big jammer in space?  The reason the GNSS signal is so weak is because it has to produce an omni directional signal from 20,000k away using only solar powered batteries.  Wouldn’t a jammer in space suffer from the same problems?  I never could understand spread spectrum transmission, especially wrt GNSS, but wouldn’t this give GNSS the advantage over a faint noise jammer.  eLoran would be a much stronger signal, but we’re still left with the power/distance equations, which favour a nearby jammer over a distant transmitter.  Dave
     


       
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