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On Sat, 28 Sep 1996, Bill Stewart wrote:
[Quoting sombody else]
> >2. The DoD is doing a study right now on how to make GPS useless to the
> >enemy at wartime.
> >I think figuring out a way to turn off A/S and getting accurate GPS on
> >commercial equipment at all times would make a nifty Cypherpunks project
> >- if it really involves breaking some encryption.

> Except for takeoff/landing, airplanes don't much need differential GPS;
> you shouldn't be flying within a hundred meters of other planes anyway,
> and if you're doing cropdusting or barnstorming you'd better be able
> to see what you're doing or have good radar anyway - most topographic
> maps don't have tall trees marked on them.

It may be considerably more than 100 meters if you have the ill fortune 
to get caught in one of the GPS jamming tests the Air Force is currently 
conducting. AOPA Pilot reports in their September issue that only now the 
Air Force has at least agreed to clasify their GPS-ECM (Electronic 
Countermeasures) as distance notams, meaning they'll warn pilots in advance.

Anyway, encryption has nothing to do with these ECM.