Re: No matter where you go, there they are.

[jim bell <jimbell@pacifier.com>]

At 02:36 PM 4/10/96 -0700, rick hoselton wrote:
>At 10:04 AM 4/10/96 -0400, Peter Wayner wrote:
>>
>>Hmm. Here's an interesting question.  Let's say that there are 3
>>satellites in view broadcasting signals f1(t), f2(t) and f3(t).
>>...Okay, so why can't I just tape the signals I get from each of
>>the three satellites. 
>
>>Or course, I could be completely missing some neat feature of
>>DGPS. ... Any thoughts?
>
>I have read that GPS uses encryption to place time-dependent, 
>location-dependent inaccuracies into the signals.  Innacuracies 
>small enough so they are not a problem for civilian navigation, 
>(mostly) but large enough to prevent GPS from being a useful method 
>of military targeting for anyone who does not hold the keys.

It's called "S/A" (Selective Availability) which is the NWO term for adding 
errors that "authorized" users can remove. (Not to be confused with A/S, or 
anti-spoofing)   It was originally intended to be turned on in wartime to 
deny the enemy accurate fixes, but during the Gulf War military GPS 
receivers were so scarce that the soldiers had to use commercial products, 
so the S/A actually was turned OFF then!

Since then, pressure has been building to turn off S/A, since its usefulness 
is nearly zero.  Even so, the amplitude of S/A errors are only a little 
larger than natural errors caused by satellite timing errors, atmospheric 
propagation variations, etc.  The result is that DGPS is useful, which is 
(more or less) a fixed antenna and GPS system which knows where it is, and 
subtracts where it "seems" to be by GPS every second, and broadcasts the 
resulting error data on some terrestrial system to receivers locally.  The 
result is errors down to the 1-meter level and even lower.  That system 
compensates for both natural errors and S/A, so the whole purpose of having 
S/A is negated.  Eventually S/A will probably be turned off permanently, but 
even then we'll want to continue to use DGPS systems.


>
>Perhaps Ms. Denning is suggesting that the US feral government could 
>act as a "trusted server" (and she has repeatedly suggested such trust) 
>and tell us whether a GPS that "thinks" it's at some location, right now, 
>is REALLY at some known location.  

Denning's trust for the government is apparently boundless.

Jim Bell
jimbell@pacifier.com