[Fwd: [TowerTalk] GPS receivers]

[Jim Lux]

At 12:04 PM 9/20/2004 -0500, Jim Brown wrote:
>On Mon, 20 Sep 2004 07:59:59 -0700, Jim Lux wrote:
>
> >This is not correct.
>
>Huh?  I said "several meters" in plan and you said "a few meters" in plan.
>
>Jim
Sorry, I was referring to the SA (Selective Availability) stuff, not the 
horizontal accuracy... 15-20m absolute accuracy would have been doing 
fairly well with SA on. sub 10m is reasonable with modern receivers and SA 
off and good geometry.

SA went away in 2000, or thereabouts (I don't recall exactly).  It also was 
turned off during the '91 war so that consumer GPS units could be used in 
the area, but then turned back on.

The whole SA thing (why it existed and why it went away) is a fascinating 
(and totally irrelevant to TT) example of technology advancing much faster 
than anticipated, and of a huge user base being created that wasn't 
expected.  I'm sure that the original designers of GPS back in the 70's 
never really contemplated that one day, consumers would buy a $20 receiver 
with 12 channel simultaneous receive and carrier phase tracking, or that 
they would be included in cellular telephones.  Back then, tracking to a PN 
single chip (1 microsecond range delay = 300m position error) was an 
impressive feat in a box that cost many tens of thousands of dollars.