[time-nuts] Thunderbolt self-survey results...
Björn Gabrielsson
bg at lysator.liu.se
Sat Aug 30 13:42:35 UTC 2008
Hi Dave,
On Sat, 2008-08-30 at 11:09 +0100, David Ackrill wrote:
> Michael Baker wrote:
>
> > However, after completing its self-survey it thinks
> > its elevation is 11.2 meters when the actual elevation
> > of its antenna (on my house roof) is 28.4 meters.
>
> I've not found any GPS receiver, available to the general pubic anyway,
> that correctly reports its height above sea level. So, I tend to ignore
> that reading, personally.
Due to geometric reasons the GPS altitude/height-accuracy is worse by a
factor ca 1.8, than the horizontal (2d) accuracy.
This gives the basic reason the vertical is worse.
Other factors contributing
- the earth is flat (!?) ie heights from maps of sea levels etc can be
very accurate.
- user confusion of ellipsoid height (that GPS works in) and geoid
height (mean sea level height). You need to know/check which height is
given by the GPS in each "message" you are looking.
- inaccurate geoid model in the GPS to translate ellipsoid height to
Geoid height.
- modern high sensitivity receivers will happily track signals that
have bounced a couple times between floor and ceiling, which of cause
gives even worse height accuracy.
> Dave (G0DJA)
--
Björn
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