>Sounds almost perfect to what I need, except for one thing. They state,
>"As the microwave radiation penetrates many materials, such as plastics
>and ceramics which are opaque to light, the sensor modules can for many

Have you considered optical (laser) ranging?

Two laser transmitters set to converge at a spot at the far end of your
expected range.

Use a CCD to accurately measure the distance between the dots.  This is
linearly proportional to the distance.

Simple geometry --> simple code.

Andy

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Andy Kunz               Life is what we do to prepare for Eternity
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andy@rc-hydros.com      http://www.rc-hydros.com     - Race Boats
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