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FAA requires every aircraft to have a
"sensitive" altimeter ..... By sensitive, IIRC, they mean capable of detecting altitude
changes of +/- 10 ft.
They must also be adjustable to the local sea
level barometric pressure, and when so adjusted they read height
above sea level, to be certified for use, I think they have to be
within +/- 20 ft. when within about 2,000 ft of the ground
relative to their neighbors... is vitally important when
the outside conditions are lousy--what the Brits call "instrument
meteorological conditions"--that is, when you can't see anything
outside and have to rely on the aircraft's instruments for flying and
navigating.
Aircraft equipped with a fully functional radar transponder (I doubt
you can buy otherwise today, but you used to be able to)
have a pick-off from the pressure altimeter
that sends the altitude
information to the radar control center to a precision of +/- 500
feet.
Unless this precision has been changed recently, and it well
might have been, the recent FAA proposal to reduce the altitude
separation between aircraft under their control bothers me.
Aircraft
whose altimeters may be off by a few hundred feet (not too uncommon)
can easily end up at the same altitude and not know of each other's
presence, unless the altitude separation used by the controllers is
such that such errors would still keep them vertically separated.
Radar altimeters have more restricted use, and are primarily used by
military aircraft, although commercial airlines are commonly equipped
with radar altimeters to help them with ground clearance. Radar
altimeters, by their nature, give elevation above the ground and are
primarily to keep aircraft from running into the ground when they
don't want to.