Ron
There are two fairly easy ways to measure altitude, air pressure and radar.
A doppler radar unit (eg security movement sensor) would only work over a range
of 30 ft or so
and possibly only 10ft reliably.
If you want a warning tone to alert you to the fact that your undercarriage is
not down then one solution would be the
following.
1) Height sensor based on semiconductor pressure sensor(quiet inexpensive).
2) Sensor to indicate state of undercarriage (up/ down)
3) Indicators / tone generator
Disadvantages
1) The unit would work on barometric pressure and would need to be zeroed for
the airport you wish to land at.
2) It would probably need a digital readout so you could check it against your
altimeter.
3) Electronics can fail !, you would need some way of verifying that it is
functioning correctly.
Price
You could probably build such a unit for around 100 - 150 uk pounds.
Summary
Really a digital altimeter with a threshold detector linked in with
undercarriage sensor.
Same disadvantages as altimeter.
If any-one wants to build one I would happilly draw up a circuit / instructions.
Rgds
Peter Chaisty
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