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From: nanders@plains.NoDak.edu (Noel Anderson)
Subject: Re: Electronic Compass wanted
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Date: Tue, 26 Apr 1994 13:42:49 GMT
References: <1994Apr24.094514.1@ashley.cofc.edu> <CosK0J.4p3@news.cis.umn.edu>
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In article <CosK0J.4p3@news.cis.umn.edu>,
Brynn Rogers <roger034@gold.tc.umn.edu> wrote:

>  Actually what you get is better than that.  All you have to do is average
>10 readings or so together and you have eliminated the random signal that the
>DOD inserts.   Also I hear that they are probably going to do away with
>the random stuff because it isn't real effective at giving people a less
>accurate position.

There are two components to the DoD error. One that cycles about every 5-6
minutes and another that cycles over 5-6 hours. If only 10 readings are to
be used, they should be spread out over the 5-6 hours.

A major element of the Selective Availability ("random stuff") debate is
whether GPS should be controlled by the department of defence with military
issues having priority or the department of commerce with civilian use
having priority.

>  A buddy of mine who works with this a lot says that with 2 recievers,
>one at a known location (say an airport) you can tell the relative
>location of the second reciever down to better than a foot.  With a system
>like this you can land planes more accurately than the microwave landing
>system, at a very small fraction of the cost (of MLS).

This is differential GPS. The errors at two nearby receivers should be
about the same. If the position of the fixed receiver is known, the error
can be calculated as the difference between the true position and that
reported by receiver. This correction is then radioed to the roving receiver,

>  He ways griping because they could not get funding on a method of getting
>sub-millimeter (no misprint) positioning accuracy with the current GPS
>sattelites.  How is that possible?  If you count exactly how many waves you
>get on the carrier wave you know exactly where you are.  I think that this
>also might only work in the relative mode (2 recievers, one at a fixed, known
>location).

The most accurate receivers cost tens of thousands of dollars compared to
the $1000 for a 1-10 m unit. They also require the user to remain in the
same spot for a minute or two to process a longer portion of the GPS signal.



-- 
Noel W. Anderson                   nanders@plains.nodak.edu (Internet)
Asst Prof, Computer Engineering         "I speak for myself, not NDSU"
EE Dept, North Dakota State University         nanders@plains (BITNET)
