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Excellent points, Frank!   One of the big vulnerabilities of GPS is its 
incredibly low signal strength at the receiver.  It actually has a 
NEGATIVE signal-to-noise ratio, only sophisticated signal processing can 
identify and hone in on the actual GPS signal.   In the GPS World 
article, it was noted that a jammer emitting only milliwatts of power 
could jam GPS over an area of several miles; such a transmitter would be 
almost impossible to locate -- and its low power consumption would allow 
it to run for weeks on ordinary batteries.    Aircraft navigation is 
becoming increasingly dependent on GPS; imagine putting one of these in 
a piece of checked luggage on an airliner!!

For a real-world story about the vulnerability of GPS to even 
inadvertent jamming and the difficulty of identifying the jammer, one 
has to look no further than the story of the jamming of GPS signals in 
Moss Landing Harbor in California.   
http://www.gpsworld.com/gps/system-challenge/the-hunt-rfi-776

The Monterey Bay Aquarium Research Institutes's vessel reported 
consistent loss of GPS closer than about two miles from its home dock in 
Moss Landing.  After a lot of effort by the San Francisco FCC office, 
electronics experts from the Naval Postgraduate Research Center in 
Monterey, and MBARI itself (along with a lot of sophisticated equipment) 
the jamming was identified as coming from a TV antenna preamplifier used 
by a live-aboard.   But wait, there's more.   Even after turning that 
antenna preamp off, some jamming continued.   A second TV pre-amp was 
also found to be jamming GPS!   And when that was turned off, yet a 
third jammer was identified.   The design of all three pre-amps had 
passed FCC scrutiny (there are federal standards to insure that such 
pre-amps do not emit signals on GPS frequencies), but lack of quality 
control in their manufacture had caused them to emit signals in the GPS 
band.

Anyone for eLoran??   It uses strong, terrestrial signals; jamming or 
spoofing would be almost impossible.


frankreed@HistoricalAtlas.com wrote:
> Hi John, you wrote:
> "It wasn't an issue of the conditions of the moment, but rather what I 
suspect was the build-up of crud on the receiver over time that led to a 
failure.   After two failures, I threw in the towel on the devices.  "
>
> There's no doubt that GPS receivers of all stripes do fail. And a big issue 
here is expense. If they fail with high-enough frequency, then we are thrown 
back on traditional navigation just as a matter of economic necessity. 
>
> All technology fails. Chronometers used to fail quite regularly, and in a 
way that is far more dangerous than the failure of GPS (a GPS receiver will 
almost always fail completely with an obvious indication that it is dead, 
while a chronometer will degrade to dangerous performance without any visible 
indication). But that doesn't mean we should avoid chronometers, nor does it 
mean you should abandon GPS. The trick is to use it in such a way that limits 
failures. Those dry pouches work, for example. Also, by far your best defense 
is to carry a spare. The spare should be a different make and model and 
stored in a different way from your main receiver to avoid the trap of 
identical failure modes. It should also be stored in a small metal case to 
serve as a Faraday cage.
>
> Now of course this is NavList, so we're all about traditional methods of 
navigation, but if you're out there risking life and limb, I personally would 
never suggest to anyone that they use anything but the best available 
technology, and that they should back that up by the best available 
technology. There's no reason to risk your life or anyone else's for the sake 
of an enthusiasm for traditional navigation. And other people's lives *are* 
involved whether we like it or not. If you go missing for some extended 
period of time, unless you're heading out on the water without telling a 
soul, the Coast Guard will eventually come looking for you. The lives of 
those searchers are then at risk (see yesterday's news from California 
regarding a Coast Guard C-130).
>
> So far we're talking about the failure of GPS receivers. They do fail, and 
you can be prepared for that failure by carrying a spare GPS receiver. Peter 
tells us that he has had multiple GPS receivers all fail all at once. That 
kind of astronomically improbable bad luck seems to strike certain people, 
but it's not the sort of thing that could be considered normal. Only an 
electical shock could kill multiple receivers at once under normal rules of 
"luck", and to defend against that possibility the solution is to keep your 
spare GPS in a metal box to serve as a Faraday cage (does anyone sell this as 
a standard item?). But beyond receiver failure, there's a much less likely 
yet still important risk that the GPS signal itself might fail. There are 
cases where you might have a somewhat degraded position accuracy if one or 
more satellites fails, but those are temporary conditions and apparently very 
rare. Of greater concern is the prospect that GPS signals might be jammed 
over some region, perhaps dozens of miles in extent, for a military or 
criminal purpose. I'm not talking about a direct military assault on the 
satellites or the ground stations themselves. If that happens, we're looking 
at war and we'll have bigger things to worry about. Instead consider a 
scenario where someone holds a major port hostage by planting dozens of 
concealed remote-controlled GPS jammers and then issues a demand: pay up and 
you get your GPS signals back. Tracking down jammers is not easy... good luck 
to whoever faces this scenario when it finally happens. Further, as 
governments attempt to use GPS for enforcement (tagging criminals on 
probation, e.g. or tagging ordinary vehicles for road taxes), there will be 
an inevitable rise in the use of jammers and also GPS signal spoofers. The 
latter are even more worrisome since they would yield erroneous positions in 
receivers that appear to be functioning correctly. There are always things to 
worry about, and for GPS and other such signals, the future may be worse than 
the present.
>
> The current issue of "GPS World," in which long-time NavList member Richard 
Langley has a monthly column, has a brief and interesting article about GPS 
jammers and spoofers. That's what got me thinking about criminal scenarios as 
described above.
>
> -FER
>
>
>
> >
>   


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