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March 01, 1996 12:00AM
The satellites that are used for GPS are owned by the Department of Defense,
and they don't want just anybody to know where they are to that kind of
accuracy, so they add artificial inaccuracies to the signal. This leads to what
is known as an autonomous accuracy of 100 meters 2DRMS - this means that
you are only guaranteed to be within 100 meters of the location that is shown
on a GPS receiver without external correction. (This means that you could be
off by as much as 330 feet in any direction.)

The solution to this is something known as Differential Correction - you can
receive external signals from a transmitter at a known location telling you how
far and in what direction the government (and some atmospheric effects as
well) are lying to you. Add this correction on to the fix you get from the
satellite, and you can often get within 2 to 3 meters ( 6 to 10 feet) of the
right spot. With more expensive receivers (in the 10- to 20-thousand dollar
range) you can get even closer - some claim a foot or better.

Handheld GPS receivers are generally marketed to hunters and fishermen as
general navigational aids. 330 feet doesn't make much difference when you're
looking for your deer stand or fishing spot, so differential is usually not part
of the package.

Differential corrections are available from a variety of sources to suit just about
any application. Most services are by subscription, but the US Coast Guard
does provide free radiobeacon services along the Mississippi River, the Great
Lakes, and the coastal areas. For farmers in Montana, the choices are
somewhat limited - they usually have to pay someone for satellite or FM-radio
transmissions on a monthly or yearly basis.



Norm Nelson March 01, 1996 12:00AM


Farm Works Support March 01, 1996 12:00AM

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