At 03:56 AM 19/02/06, you wrote:
To get around the altitude inaccuracy problem, has anybody tried looking at
a combination of GPS and baro-altitude, similar to what is done with
embedded GPS/INS technology. The GPS could update the altitude on a
long-term basis and the baro-altitude could "fill in the gaps" by
integrating the rate of climb or descent which should be more accurate than
GPS altitude on a short term basis.
I'm not a technical guy so I don't know if that's possible within FLARM but
it alleviates the weakness of both altitude sources and the technique is
currently used in fighter aircraft to accurately determine altitude for the
bombing computations.
Over to the technical people to work out if it's feasible...
Patrick Barfield
Patrick,
In an article I read in "Aviation Week and Space Technology" a few
years ago the GPS/INS sensor fusion was using a 50 HZ update INS and
a 10 HZ GPS. As a fighter plane can fly at speeds in excess of 1000
kts (~500m/s) the distance between GPS updates can be as large as 50
meters and the high dynamics (>6g, large rate of change of g, 3 axis
manoueverability) means that predictions of position and velocity
between GPS fixes aren't good. The INS fills these in by measuring 3
axis accelerations and attitudes. INS however suffers from drift
with time. It won't tell you how fast you are going or in what
direction unless it knows the starting conditions and the GPS fixes
provide these and minimise INS drift. I'm sure the operational
system is more complicated than this to allow for the odd bad GPS
fix or dropout but the concept is quite simple. There is also the
assumption that the GPS fixes are accurate or at least good enough
for the purpose and the INS is locked to these.
GPS/pressure altitude is a different problem. You have two signals
subject to random errors of about the same magnitude but unknown sign
and you want the position relative to another of these systems with
the same errors but the errors on the second system ARE NOT
necessarily the same at the same time - satellite constellation may
be different and the position errors on the static pressure are
almost certainly different (even assuming perfect pressure sensors).
Add in the fact that you need to do this in real time, not by post
processing. I guess you could simply average the two numbers and
compare the results from the two installations. This doesn't,
however, prevent worst case scenarios from happening where the
pressure error and GPS in one are both in the same direction and and
on the other are the same in the opposite direction at the same time.
Mike
Mike
Borgelt Instruments - manufacturers of quality soaring instruments
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