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

[Eric S. Raymond]

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?

Yes.

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="http://www.catb.org/~esr/";>Eric S. Raymond</a>