Rate gyros are only part of the solution.  A typical AHRS will usually
require accelerometers and magnetometers as well.  The
accelerometers/magnetometers are used as much to correct the rate gyro bias
as anything else.  In addition, the frame of reference is constantly changing
as the Earth rotates (we all remember that lovely section on Coriolis
accelerations from our Dynamics classes, don't we).  In all, you're talking
about 9 sensors (3 rate gyros + 3 accelerometers + 3 magnetometers), a
microprocessor and software.  Reliably and accurately deriving attitude and
heading, under the temperature and vibration conditions encountered in light
planes, is incredibly difficult to do.  The going price for a sensor that can
really do it is about $8000 right now.

I've tried the PC Flight Systems device.  In my opinion, it was unacceptable
for aviation use, but I'm looking for something that can be certified for a
hard IFR environment.  That may not be a requirement for others.  If you get
one, take every possible measure to isolate it from airframe vibrations.
That will help a lot.
Rick Price

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