[ntp:questions] Re: Good and cheap GPS receivers (was: Reference clocks - which?)

Wolfgang S. Rupprecht wolfgang+gnus20041030T154422 at dailyplanet.dontspam.wsrcc.com
Sat Oct 30 23:01:04 UTC 2004


Bjorn Gabrielsson <bg at lysator.liu.se> writes:
> "Wolfgang S. Rupprecht" <wolfgang+gnus20041030T140650 at dailyplanet.dontspam.wsrcc.com> writes:
> > 
> > One thing to keep in mind is that Garmin GPS's which will extrapolate
> > for 30 seconds whenever it loses lock.  That tends to give a false
> > impression that things are working better than they really are.
> > Graphing the time (or positional) jitter will show you that things
> > aren't working all that well.
> 
> Does your Garmin extrapolate on the sky-plot? (Navigate is my wording
> for "at least 3-4 sats" in previous message)

The sky plot position is calculated from the almanac as far as I can
tell.  The signal strength bars at the bottom (and skyplot
highlighting) are real-time and only lightly filtered.  Covering the
antenna with my hand has them go to zero within a second or two.  The
important thing to note is that the signal strength bars on the 5-6
best satellites are only full-strength when I go outside.  Inside the
best bars are mostly "in the mud" and the rest are non-existent.  I'm
told that the bars aren't really signal strength in the classical
radio receiver sense, but are really the signal-to-noise ratio.
Having a low bar means one is seeing a very low signal to noise ratio
and the recovered data and calculated x,y,z,time will be very noisy.
I do believe that is true, given some 24hr position plots I've made
with the gps while it was inside.  None of my garmin gps's will output
time well enough to even speak of.  (They have seconds worth of time
jitter even when outside and locked to many good satellites.)

I certainly am a big fan of GPS's and think everybody should get one.
I just have heard way too many horror stories about folks buying a gps
for their computer room and then being horribly disappointed when it
didn't work when they put the antenna near a window.  They seemed to
expect it to be more like an am/fm radio where they could make it work
by just moving it near a window.

-wolfgang
-- 
Wolfgang S. Rupprecht                http://www.wsrcc.com/wolfgang/



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