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     All radar altimeters have an altitude warning that can be set with the
DH (Decision Height) bug.  The unit gives an aural warning, and illuminates
a DH light.  Some units have a gear logic input so it knows when the gear is
up or down.  The best unit we've seen for small aircraft is the Bendix/King
KRA-10.  Trimble made a line of both alalog and digital radar altimeters,
but every one we tried had problems like warnings at the wrong altitude,
constantly resetting itself, total blackouts, etc.  Other owners of the
Trimble units confirmed the same types of problems.

     An acquaintance was working on a homemade radar altimeter for his
Venture that uses the distance sensor from a Polaroid camera to judge
distance to the runway, then give an aural countdown in feet above the
runway.  The system was even tested successfully, but the limitation was the
short range of the sensor.  I don't think the project went farther than
that.

     David Buckwalter
     Avionics Systems
     <http://www.avionikits.com>