[time-nuts] The Demise of LORAN (was Re: Reference oscillator accuracy)
Mike Monett
xde-l2g3 at myamail.com
Sun Nov 15 16:37:11 UTC 2009
"J. Forster" <jfor at quik.com> wrote:
>> I've read and heard from this forum as well as a number of other
>> sources that GPS can be easily jammed. What makes GPS so
>> vulnerable? How can it be jammed?
> Signal strength.
> LORAN transmitters put out multi-hundred KW to MegaWatt class
> pulses. Wiki has a list. I would think a GPS bird puts out less
> than 100 Watts CW.
> Also, GPS birds are a LOT farther away, especially measured in
> wavelengths (much higher path loss)
> Those factors combine to make a huge difference in received power.
> It could well be over 100 dB.
>> From what I've heard a GPS jammer smaller than a deck of playing
>> cards can easily wipe out GPS w/in a mile or more for a week or
>> longer.
> John
It should be easy to locate a jammer. Go to the area where the GPS
signal is being jammed. Drive in some direction until the signal is
regained. Repeat to find three locations where the signal is lost.
Three points define a circle. The diameter tells the strength of the
jamming signal. The center defines the location.
Once you are near the center, ordinary DF techniques should quickly
identify the source.
For faster response, have a number of GPS receivers report the
status of the GPS signal to a central location. This would identify
a moving jammer.
It should also be possible to develop a GPS antenna with one or more
nulls in the horizontal direction. Rotate the antenna until the GPS
signal is regained. The null points to the jammer. Multiple
receivers would remove the ambiguity from antennas with more than
one null.
These techniques should identify a jammer very quickly, perhaps in
hours or minutes instead of weeks. I'm sure the military has some
more advanced methods, as well as effective methods of dealing with
the threat.
Mike Monett
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