Article: 2668 of rec.aviation.products
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From: "Robert O. Cherney" <p00836@psilink.com>
Newsgroups: rec.aviation.products
Subject: Re: Radar altimeter
Date: Wed, 26 Oct 94 19:40:56 -0400
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>
>Does anybody know how a "radar altimeter" works?
>If I understand correctly, they are accurate to something like a foot.
>(This is why they can be used to guide a totally automated landing.)
>However, this would require nanosecond accuracy, resolution and stability
>for the circuits involved.
>Is it some form of triangularization similar to the surveying gadgets?
>If so, how does it cope with the variation in the surface that is
>reflecting?

A RADAR altimeter uses a low-power continuous-carrier microwave transmitter.
The transmitter is frequency modulated (changes frequency with time) with
a waveform that is triangular or sawtooth shaped.  The transmitter is aimed
down (natch), and the receiver (nowdays a "Gunnplexer") receives the
modulated signal shifted in time.  The RADAR altimiter uses a small portion
of the transmitted signal and mixes (hetrodynes) it with the received signal.
The resulting difference frequency is proportional to altitude.  Frequency
is much easier to measure than the nanosecond accuracy/resolution that would
be necessary with a pulse system.

Rob-
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|   Rob Cherney                            E-Mail:  p00836@psilink.com      |
|   Ellicott City, Maryland            Home Phone:  (410) 465-5598          |
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