Re: GPS

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

When first out, the errors in position using civilian GPS could be quite

large, particularly in terms of elevation, all due to the futzing of the

signal by the military. Now, even the standard receivers are much
better, 
as the military have relaxed the rules somewhat.

>>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.

Which is almost a given, as the fixed reference point changes with
locality 
anyway.

As for usage of GPS, when my father in law to be can plot a course for
the 
yacht using his computer, and the thing can steer itself through the 
channels in the Port of Miami and down to the Keys, or across to the 
Bahamas relying on the laptop and the boat's differential GPS system
(two 
receivers and a reference station in South Florida), then the system can
no 
longer be futzed up too badly.

John.

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