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Given three very accurate laser gyros and accelerometers I believe the Inertial Reference System (IRS) should be as accurate in determining true altitude as it is in determining lat/long, providing the aerodrome elevation was supplied at initialization.

I realize that pressure and density altitude are what is used in aviation, but could the IRS be used as a backup? Could it replace or backup a radar altimeter for approach/landing?

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IRS will drift over time. Decision altitude is 200ft for ILS CAT I approach. 0.5nm drift is not to much for navigation but it will be huge difference in altitude and you can hit obstacles if your altitude has drifted just couple of feet. Probably it could be used if backed by GPS as EGPWS does or non baro RNAV approaches do. But even GPS precision alone is not enough in this case and you need WAAS. Radio altimeter can give much better precision.

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    $\begingroup$ Another point: Just think what happens if you rely on inertial signals for altitude separation. By relying on atmospheric pressure, all planes in the area have a common reference. With an inertial altitude signal, the drift is different for each aircraft and would at least need an additional margin to keep aircraft reliably separate. $\endgroup$ – Peter Kämpf Jan 27 '16 at 16:47
  • $\begingroup$ obviously, but that wasn't what i was asking for. I had my question edited (read: changed) twice to such a question though. $\endgroup$ – user Jan 28 '16 at 10:51

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