Mart,
For airspace infringement purposes pressure altitude should be used
as that is how the airspace levels are defined. That seems to be the
thrust of the linked article.
If you are really good and lucky it will be within about +/-100 feet.
Up to +/-200 feet is possible when all error sources are accounted
for. That's AFTER you apply the most recent calibration.
The author is however wrong by claiming that GPS altitude error is
reduced in Europe by the WGS 84 ellipsoid being above MSL. The
GPS GGA message (the only place altitude is reported) is already
corrected for that even though that difference is reported in the
next number in the frame. This could of course be got wrong by the
GPS module manufacturer or in the application processor.
I've never seen GPS altitude out by anything like 500 feet due to
poor signal. Maybe possible in a hang glider with structure above the
receiver. Not in a sailplane with an antenna on top of the instrument
panel cover. I've been using GPS altitude for final glides since
shortly after SA was permanently turned off in May 2000. With 30
satellites it seems to work out. Before SA was turned off 500 feet
was about the usual sort of error.
I got one trace today that had NO missing fixes or anomalies. Maybe
the antenna is well located but that's the standard we work on in
the B2000/B2500. Can't say I've ever noticed any anomalous GPS
altitude fixes. GPS receivers vary greatly. The better ones give you
control over the optimisation of the software. The ones we use let
you set "aviation" mode. Some commercial receivers don't let the end
user do this and mostly these are optimised for surface navigation.
These may not work all that well in a sailplane. There are g limits
and rate of change of g limits. How the receiver handles dropouts is
another thing you can set in the better receivers. We don't do any DR.
GPS altitude is geometric altitude and that's what your glider cares
about for final glides.
Regards
Mike
At 07:25 PM 2/09/2011, you wrote:
Thanks Mike for the article on your website.
Here is another article on barometric vs gps altitude.
http://bhgc.wikidot.com/tutorials:differences-between-pressure-and-gps-altitude
The hang gliding community has been writing extensively about this
subject and ended up with using barometric altitude. Firstly because
that is the usual way to measure height in aviation, but secondly and
more important, it was found out that GPS altitude can be out by as
much as 500ft in extreme cases due to poor signal from the satellites.
regards,
Mart
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