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Re: [gpsd-users] epx, epy values never change

From: Eric S. Raymond
Subject: Re: [gpsd-users] epx, epy values never change
Date: Fri, 21 Oct 2016 10:54:32 -0400
User-agent: Mutt/1.5.24 (2015-08-30)

Jeff Loughlin <address@hidden>:
> That last paragraph leaves me wondering if I have a bug in my expectations
> - as I understood it, epx and epy should reflect the current reception
> conditions (which can change depending on receiver location, tree cover,
> etc), and not just the satellite configuration.  Is that an incorrect
> assumption?


The error estimates are computed from satellite geometry and a scalar
multiplier called UERE, an empirical constant ralated to the physics
of delay jitter in the ionosphere.  They tell you your uncertainty
of location *if you have sat lock*.

Local consitions like tree cover don't uncrease or decrease this uncertainty,
they control whether or not you can get sat lock.  One minor exception: if
you can see three sats but not four, you get a 2D lock that doesn't give
you altitude.
                <a href="";>Eric S. Raymond</a>

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