Peter wrote:

Scott Moore wrote

I don't have
the ability to evaluate GPS jamming, nor VOR jamming,

Then you should read up on the subject, and the Volpe report on GPS
vulnerability is a good start. I did have a URL for it but the
institute has moved it somewhere else. I have it archived (PDF) so if
you email me I can give you a private URL for it or email you a copy.
Or someone can post a new URL if they can find it; I am 100% sure it
is online.


Fine, you have a study. What we need is an unbiased comparision of
the vulnerabilities of GPS to the vunerabilities of VOR. That is all
that really makes the decisions here, because (again) there is no point
claiming that GPS is more vulnerable than VOR without such a study.

As others have explained, GPS can be jammed very easily, over a wide
area (of the order of 500 miles radius) whereas jamming more than one
VOR is a lot harder and the effect would be only localised; it would
need large amounts of power and most probably separate transmitters to
jam multiple VORs. This is just elementary electronics and signal
processing.

GPS jamming is localized as well. A ground based transmitter has perhaps
30 miles of horizon to jam (derived from a tangent line from the earth
and math). More altitude can jam farther (hence the "jammer on a ballon"
theory), but lets save time here -- both services are line of sight, so
they are both going to have similar jamming profiles. Everyone agrees
that it would take more power to jam VOR, but all this really does is
make the "$100 radio shack ballon" jammer appear shocking. There is no
material barrier to making a VOR jammer.


As I mention elsewhere, there are techniques to make a GPS receiver
relatively immune to jamming - especially if one is working on the
assumption that the jamming signal is just a crude carrier-wave
transmitter and that the transmitter is ground (not space) based. For
example, combining a GPS receiver with an INS (e.g. FOG based) enables
the receiver to cope with a much poorer GPS signal. You can read about
these in the above reference. These techniques are not AFAIK
commercially available, presumably for the same reasons that all
commercial GPS receivers are required to be limited to 60,000 feet
altitude in their firmware.

And, as another poster pointed out, GPS antennas are pointed at the sky,
whereas VORs cannot be so directional.

Lets back up a step. The widest deployment of GPS is in light aircraft.
Airliners don't really use it yet, and even when they do, they have
INS redundancy. The bottom line is that the most attractive target for
terrorists, the airlines, is the most unlikely one to be affected by
any jammer, VOR or GPS.

It is that fundamental fact that I believe causes the "GPS haters" to
constantly flip back and forth between talking about terrorists and
talking about other types of interference. The terrorist idea makes good
(shocking) headlines, but would be a stunningly ineffective method
if actually put into use.

Nobody in this thread ever asked me if I am for shutting VORs down.
I am not. I have a 430, which has VOR built in, and another VOR besides
that, and I am perfectly happy with it as a backup method.

But what seems to happen here as regular as clockwork is that someone
reads a "shocking" headline about radio shack GPS jamming and runs
around ranting about GPS "insecurities". I am all for a *rational*
comparision about the reliability and risks of GPS vs. VOR or any
other service. I am all *against* the chicken-little rantathon
that is getting so much play in the press and here as well. If someone
comes running down here yelling about GPS jamming, be prepared to talk
about jamming of VOR or other services as well. Otherwise its just
a **** flinging contest.


Peter.
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