Date: 09 May 2000 11:14:28 -0400
From: Derek Atkins <[EMAIL PROTECTED]>
I think the answer you will find is that standard GPS is fairly easy
to jam. The satellite transmissions are fairly weak, which means that
a strong ground-based transmitter on the same set of frequencies could
effectively jam GPS reception. Indeed, the US military has stated
that they can jam GPS in a region if necessary.
To decrease the jamming power required (this -is- spread spectrum,
after all), it's helpful to have your jammer hop the same way your
receiver will be hopping. This is pretty easy to do, since your
jammer can trivially figure out the hops by observing the satellites
you can see. Note also that any outfit that makes GPS's typically
buys test equipment that generates valid but fake GPS signals, which
they use for prototyping, etc. If one were to put this signal through
an amplifier and an antenna instead of an attenuator and a coax, one
could presumably cause large amounts of mischief with no technical
skill whatsoever.
However, it's pretty easy to be inadvertently jammed, too---standing
anywhere near an active radar will typically raise the noise floor
sufficiently that GPS won't work. [There is a harbor cruise in Boston
that passes fairly close to Logan's main radar; GPS's lose lock for
about 1km of that passage, -unless- you're standing on the far side of
a large metal structure, such as the chimney for the engines...] I've
seen reports that as little as 5W, properly applied, can jam GPS for
hundreds of square kilometers (there was a report in Risks a while
back about a test lab that accidentally left their transmitter on for
a few weeks and figured it out from reports of the resultant GPS
outage in their area; I can dig this up if someone cares).
Btw, an excellent book on how GPS works, which goes very in-depth on
technical details, is _GPS Satellite Surveying_, by Alfred Leick.
As for corrupting the signal, I think that would require a bit more
work, but I personally think it could be done. I don't think there is
any kind of cryptographic integrity protection of GPS signals.
Not correct. (And finally relevant to this list!) Military receivers
make use of the P(Y) code, which can be -jammed- but not -spoofed-
unless you already have the relevant keys. These keys can be rotated
weekly (they use the concept of "GPS week"). Civilian receivers
-sometimes- have warnings that all is not right---RAIM alerts on civil
aviation receivers may warn you that the computed fix seems way off,
even if the receiver cannot determine what it -should- be, and there
is a fair bit of research out there for making civilian receivers more
jamproof.
As an example, consider this message sent to Risks Digest, several
years ago; I remembered it and dug this one specifically out of the
archives, but there may be others which are relevant.
- - - Begin forwarded message - - -
Date: Tue, 3 Sep 1996 20:54:42 -0700
From: [EMAIL PROTECTED] (Jim Easton)
Subject: Warning on the use of GPS
Over the last few weeks I have experienced a series of GPS navigation errors
ranging from minor (triggering RAIM) to as large as 20 miles horizontally
and calculated GPS altitudes of below NEGATIVE 5000' MSL. I have never been
given a NOTAM telling me to expect this performance.
Reading the current issue of AOPA Pilot, I now understand.
* First: Kudos to AOPA for finally telling us what is going on.
The military has been conducting GPS jamming exercises in Southern
California affecting at least the Los Angeles and San Diego areas (that I
have observed) lasting for times up to some 15 minutes (again that I have
observed).
You should note that the vast majority of GPS units flying do NOT have RAIM
and will NOT automatically flag an erroneous GPS position.
I would seriously warn pilots against trusting VFR GPS navigation in
Southern California without cross-checks. Should a RAIM flag go on in an IFR
GPS do NOT assume that because you are receiving lots of healthy satellites
with good signal strength that you can ignore the warning. This is exactly
what you will see when you are receiving jamming. Look at the calculated
GPS altitude and calculated position error and cross check with any other
available navigation source.
Note that the government has decided to take down LORAN and VORs which will
leave you dead when GPS is jammed. In spite of the absolutely predictable
loss of airplanes and lives that this decision will cause, it is apparently
cast in concrete. I believe that the plan may be to have a multibillion
dollar fix to GPS after all alternative means of navigation have been shut
down and a thousand or so people have been killed by GPS failure.
Jim Easton 4364 Bonita Rd., No. 166 Bonita, CA, 91902-1421
Tel: (619) 548-0138 FAX: (619) 470-8616
- - - End forwarded message - - -
