Jim Donohue wrote:
"Steven Shelikoff" wrote in message
...
On 20 Dec 2004 07:58:48 -0800, wrote:
It does get locally turned off once in a while for tests, military
exercises, etc. Usually they give plenty of warning. For instance,
here's an AP line about an outage back in June for the Carolinas to
Florida:
----------
Military testing system designed to jam GPS signals off coast
WILMINGTON, N.C. (AP) - Military officials are testing a system for
jamming signals from navigational satellites - an experiment that could
disable boaters' GPS devices and force them to rely on dead reckoning.
The test began Friday and will run through June 20, as part of a NATO
training exercise along the Atlantic Coast.
The Coast Guard said GPS signals will be unreliable or unavailable in
waters up to 60 miles off North Carolina and along the coast of northern
and central Florida. Officials said the testing may affect cell phones
as well as GPS navigation devices in boats, airplanes or cars.
Coast Guard stations are trying to get word out about the interference.
"What we kind of foresee happening is some confusion on the mariners'
end," said Petty Officer 1st Class David Christensen.
----------
And mo Would anyone want to be in a commercial or private airplane
when making an initial approach in low ceiling weather to landing and
having GPS suddenly go off? This may well be an "end."
No, that won't be the end either... unless the plane doesn't have a
pilot and it's being flown automatically, directed by GPS.
Steve
Actually Steve I think the military jammed the signal rather than turning it
off or degrading it. I also believe that there are antennas that would make
Right. That's what the article says, that the military was jamming GPS
signals from 60 miles off North Carolina and along the coast of northern
and central Florida. Of course, it doesn't really matter why it's
unusable ... whether it's jammed or turned off, it's still unusable.
it difficult or impossible to jam a GPS particularly from a boat a few miles
away from the jamming site. I doubt the military will talk about it but
If you increase "few" miles to a hundred miles or more, I'd agree. That
also depends on how they jam it. I.e., whether it's from the air or
ground. You can possibly get directional antennas and point the null in
the direction of the jammer. But that assumes there's only one and that
it's not somewhere above you. There's also ways of decreasing the
effectiveness of a jammer via signal processing. Of course, most
boaters don't go out equipped to handle possible jamming.
reasonably simple shielded antenna with deliberate limitation of the sky
viewed can probably provide pretty good reception even in the face of heavy
jamming.
It's got to be a little better than that. It has to have a pretty good
null specifically in the direct of the jammer. That's because GPS
signals are so weak (actually, they're below the level of background
noise and signal processing is needed to dig them out) that just about
any interference can cover them up.
On the other side jamming GPS in a relatively close in situation like a
harbor or a city is a piece of cake. I am sure the hacker community already
has a couple of desings on the internet.
You want a link? I've seen several make your own designs, complete with
schematics and parts lists with sources, design considerations, etc. To
take out GPS up to maybe 50 miles or more you only need about 4 watts
ERP, same as a CB walkie talkie. If you're equipped to handle the
jamming, you can overcome that and limit the range of the jammer to
maybe only a few miles. But again, most boaters aren't equipped to deal
with jamming and it would render the GPS of most boaters useless.
None of this in any way applies that the gov can't easily screw around with
the signal to any desired extent. I would also point out it would be
relatively trivial for the gov to take out Russian or European sattellites
as well. Probably construable as an act of war but why else would the gov.
do it?
It's just as trivial for another sophisticated government, one with
space capability, to take out our GPS satellites. Also probably an act
of war.
Steve