> Although satellite steering is becoming 
> very popular for planting! 
> Not sure what the resolution for that is, 
> but its got to be in the foot range to work well.

Then they are combining inertial sensors with GPS. 

Precision of a GPS reading (number of digits to the right of the decimal
point) has very little to do with the accuracy of that reading, something
that one learns the first time one uses GPS to dive on a specific wreck or
feature. 

So, 4 Meters "RMS" means 7.8 meters at a 95% Confidence Interval
I call it "8", as I only have so much room for trivia that could kill me if
I did not memorize it, and only so much tolerance for math as a survival
skill.
And that's just horizontal accuracy.
Vertical is worse, see the spec:

http://www.gps.gov/technical/ps/2008-SPS-performance-standard.pdf

The only time this issue really mattered to me was when navigating the
narrow dredged/blasted channel through the coral on the way from Key West to
the Dry Tortugas and back, as a sailboat with a keel is not the best choice
of boat for that run.  Back then we did NOT have the (civilian) GPS accuracy
we have today.  But a fondness for both sailboats and diving reefs is a
combination that is more likely to end in tears and recriminations than not.

If it GPS gets better, I think it will require more than 24 birds in the sky
at a time, and receivers that will take longer to acquire more birds.

But it has eliminated the problem of forgetting where those hives were
placed, and eliminated the old "abandoned" hives problem in forgotten
holding yard positions, which used to be more common.

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