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

From: Jeff Loughlin
Subject: Re: [gpsd-users] epx, epy values never change
Date: Fri, 21 Oct 2016 14:06:39 -0400

On Fri, Oct 21, 2016 at 10:54 AM, Eric S. Raymond <address@hidden> wrote:
> 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.

Thanks for the explanation.  But now I'm wondering what epx and epy
were showing me before (prior to the fccb36 commit), because those
values seemed to make sense and reflect the local reception conditions
- I could start gpsd with my GPS sitting on the window sill and see
epx/epy values of 5-10m, then when I moved the GPS away from the
window (but still have satellite lock) those values would immediately
jump to 30-40m. Now they don't change, which makes sense given your
explanation above, but that seems less useful.  I guess you're saying
the values I was seeing before were wrong?  But if wrong, why did they
seem to make sense, and change appropriately with the local conditions
at the receiver?  It seems to me there was useful information there
before that has been lost - unless you're saying that information
wasn't really useful and only *seemed* to make sense...

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