Actually, there are some very nifty little _rate_ sensors
available--"gyros on a chip"--used in such things as r/c helicopter
autopilots. Probably not more than $200/axis.
I'd imagine that one of these, suitably integrated, would provide good
short-term attitude output. The function of the "automatic erection
mechanism" (no jokes, please) in conventional gyros could be handled by
a pendulum-type "true vertical" sensor, suitably damped. The software
rules would invalidate inputs from this pendulum sensor whenever there
was a significant steady yaw rate (i.e., airplane turning) or whenever
an accelerometer showed the airplane pulling more than 1 g. Thus, your
electronic gyro would not erect to sustained turns the way our current
mechanical ones do.
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