Re: GPS

Thomas Barclay wrote:
> I'm sure I'm labouring under a few incorrect 
> impressions, so someone around here who is in 
> the know can clear things up:
> 
> 1) Has not GPS (civilian side) been given up to 
> some sort of open system? (If so, wouldn't 
> futzing up parts of it be problematic?)

I'm not sure what you mean by "open system". The GPS system's always 
been pretty open, at least the civilian side.

> 2) Can differential GPS (multi-receiver) defeat or 
> significantly attenuate the futzing up signal? 
> (That is, can't differential GPS be used in such a 
> way as to amerliorate the effects of signal 
> uncertainty introduction)? 

Yes. However, both the mobile and stationary receivers have to have one 
or more satellites in common. The correction is on a 
satellite-by-satellite basis, so the more you have in common the better.

 > If so, the only
 > receivers that will be seriously futzed by the
 > fuzz that can be inserted (fuzz substituting for
 > any real technical understanding....) would be
 > the non-differential single receiver GPSes,
 > correct? (And yes, these are the more common
 > variety IIRC)

Yes.

> 3. If the US (or anyone) were to fuzz up a 
> signal coming from someplace and this caused 
> some manner of disaster (air crash, ship to run 
> aground and sink, etc), wouldn't that be a very 
> bad thing? (Yes, I know secondary navigation 
> methods are meant to verify positions, but 
> people ARE becoming GPS dependent).  I 
> realize when weighed against a large scale 
> offensive, these risks are minimal, however I can 
> see someone attempting some form of 
> litigation... (Land of the Free, Home of the 
> Lawyer)

I wouldn't be surprised if the legislation that funds the GPS system -- 
and pays for the civilian side -- doesn't exclude the government from 
damages. The government's funny that way...