When we do a test on ground and someone drives underneath the antennas we can see the altimeters go down for a brief moment until he clears the antennas.
I don't see how this disproves the cone theory. The "cone" is limited in angle otherwise the antennas would pick up objects
well off to the side of the aircraft.
Neither does the system put a narrow pencil beam down to the ground, otherwise any tiny roll or pitch movement of the aircraft would produce an erroneous height due to slant angle.
Why would you see a variation during test. It feeds a (typically) fixed 40' test signal into the system.
Seems this topic has been visited before:
Radio Altimeter [Archive] - PPRuNe Forums
Now, getting back to 747-400ERs and Rad Alts... There is 160 pages in my engineering training notes on Aux Tank fuel management alone, but the highlight is...
"Management operation...
During normal operations, the FSMC sends commands to open the valves for descent (Aux Tank) pressurisation when all of the following conditions are true:
*In the air
*Aicraft altitude decreasing (Cabin Pressure Controller detects descent) and
*Valid Radio Altitude data (8190 ft).
Note: Flaps entering the Landing range provide a back-up signal for descent pressurization."
There are numerous references to 10,000' altitude, but I can't figure out if the data is from the ADCs (perhaps via the cabin pressurisation controllers) or the RA's.
I couldn't quite believe what was written, but my instructor said it was valid.