At 12:43 PM +0300 5/11/2000, [EMAIL PROTECTED] wrote:
>Thanks to all for the very interesting info. For people interested, here's
>a summary of answers and ideas:
You left out my direction finding approach :( I think it has merit.
Electronically steerable antennas are quite practical at L band and
they could also be used to null-out a single point jammer.
...
>
>Some thoughts on research directions:
> ...
> Can one analyse a design which will involve communication between GPS
> receivers using local (wired or radio) communication which will provide
> `real` anti-spoofing (notice my criticism of the use only of encryption
> of the p-code for anti-spoofing)?
It seems to me that all we need is an independent, authenticated
source for the GPS satellites' ephemerides. We would then know where
the satellites are at any given time and we presumably know the exact
location of our GPS receiver. Given that knowledge, I don't think it
is possible to generate spoofing signals that produce the correct
location but the wrong time. (Even in the unlikely event that the
exact satellite constellation repeated every few orbits, we can keep
approximate time by other means well enough to avoid being fooled.)
I can think of three ways to get the ephemerides:
1. We could exchange received ephemerides with a number of other
GPS-time users on a regular basis using PKC signed e-mail. This would
delay detection of a problem until a little while after a spoof
occurred, but that might suffice in many applications.
2. We could use long range predictions of the satellites' orbits.
While these do not have full accuracy, it should be possible to
perform an error analysis that sets an upper bound on how much our
time could be distorted given the expected errors in the long term
orbital projections.
3. The U.S. government or some other trusted source could be
persuaded to broadcast GPS satellite ephemerides frequently via the
Internet and/or special radio stations (e.g. WWV) along with public
key signatures. Natural Resources Canada already does something like
this on a daily basis (unsigned, I presume). See
http://www.geod.nrcan.gc.ca/html-public/GSDproductsGuide/CACS/English/
ephem.html
> Can we reverse the roles here... and use highly secure time services
> (thru wired sources) to detect tampering with GPS signals (also for
> location???)
I am not sure you even need independent time if you have enough GPS
receivers in known locations and authenticated knowledge of the
ephemerides.
>It seems an interesting and challenging area.
>
Agreed. So OK, one more hair-brained scheme: use a moving GPS
receiver. Run a long, opaque pneumatic tube on the roof of a
building, say for several hundred feet. (Pneumatic tubes are still in
wide use and available from many sources.) Place a GPS-equipped Visor
in the pneumatic carrier and send it randomly to one of several
stations. At each station, it would read out its position and the
time via IR. Since an attacker would not know where the receiver was
at any given moment, it would be impossible to construct a correct
spoofing signal. (I has originally thought of model train technology.
More fun, but I think it would require the ability to receive signals
inside a building.)
Arnold Reinhold
