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Re: GPS question

At 02:00 PM 3/20/97 -0500, you wrote:
>DUBOVIK@hsdwl.utc.com wrote:
>> 
>> I'm in the market for a new toy, er.. I mean a GPS. Have seen
>> them for as low as $149.00. Currently, units on the market
>> do not give altitude readings. I've been told, read, (can't
>> remember when) that the satellites which transmit the
>> Lat/Long information are capable and do transmit altitude
>> bearing signals, but do to security or whatever reasons, the
>> US Govt does not allow civilian GPS units to receive the signal.
>> 
>> I've also been told, read, (geeez, my memory is shot!) that
>> *soon* they are going to relax this restriction and allow
>> civilian units to receive the "altitude" signal.
>> 
>> Anybody know the real scoop on this
>> 
><sig deleted>
>
>I have a Garmin GPS 38 and it definitely calculates an altitude reading.
>It's only accurate to within 200-300 feet and only if you are receiving
>fix information from at least four satellites.  It is my understanding
>that this is true of all GPS receivers.
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>
I used to work in defense electronics back when GPS was first being
developed, and if my memory isn't totally shot, the deal is this:  Military
users obtain position accuracy of plus/minus 3 meters in three dimensions,
which is orders of magnitude better than previous positioning technology.
To prevent adversaries from obtaining the same benefit, civilian licensees
were allowed accuracy of plus/minus 25 meters.  This was implemented through
signal processing algorithms, but size/weight/power of receivers is also a
factor in accuracy.   Of course, commercial users would love to have better
accuracy, and they are always trying to persuade the DoD to relax its
restrictions, which may or may not happen.

Tom Horton
thorton@HFAonline.com

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