Rob Saecker wrote:

> Ummm... I don't want to get into a pissing match here,

not a good idea with a computer freak :)

> but all GPS units triangulate;

Not surprising, as that's how they work.

> And since the signals of all the satellites are distorted,

Once again, I belive a certain number of bits are encrypted. This means
instead of (we'll use decimal to make it seem more normal) getting a
number like:

1234567

your unit gets

12345xx

i.e (in this example) 5 digits precision rather than 7.

> more triangulation doesn't translate into better accuracy on the ground.
> More expensive units are more accurate,

Hey, you contradict yourself with no intervening words! First you say
they can't be more accurate, then you say they are!

> but the military induced fuzz is
> the same for all (i.e. about 100m).

Since the low-order bits are encrypted, this is certainly true.

So, given your scenario, just how, then, do the more expensive units -
not, I assure you, coincidently - which query more satellites achieve
more accuracy?

My understanding is that each less-precise signal describes a slightly
different circle (with its radius of 100m or whatever it is) and that
the more expensive units can calculate, presumably, the intersection of
many of them giving a greater precision. Think Venn diagrams...

Undoubtably a simplistic analog, but this may help you understand how
integrating results from a bunch of satellites can, theoretically at
least, lead to a more accurately calculated position.

They're not triangulating, though, I give you that, if they're
integrating results from 10 satellites, they're "decangulating".
I suspect the implementers use some heuristics, too, nothing as
simplistic as I suggested above.

Of course, the drawback is if you can only raise three, you're still
left with the originally designed accuracy. Military units - which
know the key (undoubtably changed fairly frequently) - achieve the
highest designed accuracy once they get three satellites.

--
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