From owner-freebsd-hackers  Wed May  3 20: 3:56 2000
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Date: Wed, 3 May 2000 20:03:48 -0700 (PDT)
From: Matthew Dillon <dillon@apollo.backplane.com>
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To: Warner Losh <imp@village.org>
Cc: hackers@FreeBSD.ORG
Subject: Re: GPS heads up 
References: <200005032313.QAA65552@apollo.backplane.com>  <200005031744.KAA63550@apollo.backplane.com> <20000503200006.A35116@cichlids.cichlids.com> <200005031957.NAA01354@nomad.yogotech.com> <20000503130759.A15403@orion.ac.hmc.edu> <20000503221528.A37472@cichlids.cichlids.com> <20000503151513.D337@beastie.localdomain>  <200005040230.UAA36034@harmony.village.org>
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:The SA can be averaged out, but you need to have another source of
:time as well as GPS.  GPS alone will give you a grid square you are
:in, but the nature of the pseudorandom noise is such that you don't
:get a nice sine wave (collapsing for a moment to 1 dimention).  It is
:much more distorted than that.
:
:Warner

     SA can *not* be averaged out.  People... SA is NOT NOISE.  I will
     repeat that.  SA is NOT NOISE.  SA is an intentional error 
     introduced by the satellites that looks like a drunk walking around
     the town.  It takes a lot of readings over a long period of time
     (read: at least a couple of days) for any sort of averaging to
     yield a worthwhile result.

     I know this for a fact, because NextBus has a dozen or two busses
     in SF with GPS units on them reporting their position on two minute
     intervals and they've built up over a years worth of data.

     The only way to factor out SA is to have a secondary transmitter
     at a fixed location which transmits the error coming from EACH
     satellite to the GPS units, which then correct each satellite's
     signal before running it through the positional algorithms.

     With SA on, your GPS fix winds up moving at around a mile an hour
     (the rate changes as well) in odd directions.  It is *NOT* random.  
     You can't average it out and expect to get the real position 'in the 
     middle'.

					-Matt
					Matthew Dillon 
					<dillon@backplane.com>


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