Lenny Foner <[EMAIL PROTECTED]> writes:
> 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.
Unfortunately this doesn't help those of us without military
receivers. Indeed, I was under the impression that most people here
were interested in the integrity of the C/A signal, as that is pretty
much all most of us mere mortals can read.
As for RAIM, my Garmin GNS430 (spiffy aviation GPS) has RAIM. Luckily
I've never actually seen the RAIM warning flag. My understanding of
RAIM matches what's been said before, position information is
heuristically computed and when an anomalous position/speed occurs,
the flag is raised. Sudden changes in position, altitude, speed, etc.
would set off the flag, taking under consideration that an airplane
would generally not invoke an implulse accelleration :)
However, RAIM is not cryptographically computed (or protected). Also,
I BELIEVE that if you have a consistent (albeit incorrect) signal,
RAIM will not flag. Assume there is an impulse from correct to
incorrect signal, and then incorrect signal drifts further and further
off in a "consistent" manner. RAIM will only notice this at the impulse,
but if you turn on the receiver after the impulse, your RAIM circuitry
wont notice the difference.
Personally, I'd like to see cryptographically signed C/A data...
Maybe this is where elliptic curve would be useful? Small signature
sizes.
-derek
--
Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
Member, MIT Student Information Processing Board (SIPB)
URL: http://web.mit.edu/warlord/ PP-ASEL N1NWH
[EMAIL PROTECTED] PGP key available
