At 12:59 AM 18/02/06, you wrote:
Mike,
While I would tend to agree that two identical GPS receivers nearby each
other will tend to have a similar GPS altitude offset, this is not
guaranteed to be true, due to the fact that each receiver may in fact
see a different selection of satellites due to installation details and
the different orientation of the aircraft etc.
This is much less likely nowadays. The satellites are pretty good and
even changing constellations doesn't seems to have great effects in
our experience.
Yes, calibration and instrument errors present their own problems for
barometric altimeters.
Yes they require periodic calibration and checking which is a pain
and causes expense.
(Even errors due to static pressure variations
in the cabin due to venting etc).
These may be the largest single source of error.
I expect that using modern data fusion techniques could use both GPS
altitude and pressure altitude for a highly reliable system.
On the basis that there isn't any magic this seems to be quite
difficult in a flight vehicle.
GPS altitude errors is also highly dependent on latitude. Near the
equator is best, further away it can get quite bad. I routinely get
reports of gps altitude errors in the UK of several hundred feet and my
experience with a modern GPS engine is about 100 feet error.
Our experience around here is that 100 feet is about the max, in any
case for anticollision systems it is the difference between two
receivers that is relevant.. I'll have some more data soon as we've
just got indoor GPS by using an active antenna on the roof with a
retransmit inside the workshop.
In any case it seems we'll have Galileo in a few years which should
fix any minor shortcomings in the GPS system particularly at high latitudes.
Regards
Mike
Borgelt Instruments - manufacturers of quality soaring instruments
phone Int'l + 61 746 355784
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