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Posting-Version: version B 2.10.2 9/18/84; site watcgl.UUCP
Path: utzoo!watmath!water!watcgl!dmmartindale
From: dmmartindale@watcgl.UUCP (Dave Martindale)
Newsgroups: net.aviation
Subject: Re: Need technical info for VOR
Message-ID: <2006@watcgl.UUCP>
Date: Mon, 10-Jun-85 09:04:42 EDT
Article-I.D.: watcgl.2006
Posted: Mon Jun 10 09:04:42 1985
Date-Received: Tue, 11-Jun-85 02:27:58 EDT
References: <2965@hplabsb.UUCP> <106@ulose.UUCP>
Reply-To: dmmartindale@watcgl.UUCP (Dave Martindale)
Organization: U of Waterloo, Ontario
Lines: 40

Some additional information:
First, why expect 360 antennas?  A VOR doesn't generate 360 radials,
it generates a radio signal that changes continuously as the position
of the receiving antenna relative to the ground station changes,
and there are thus an infinite number of possible "radials" to fly.

The actual position determination is done by comparing the phases of
two 30Hz signals.  The phase reference is provided by a 9960Hz subcarrier
on the main signal that is frequency modulated at 30 Hz; this signal
is broadcast by the omnidirctional centre antenna.  The four other
antennas are fed signals that are directly amplitude-modulated
by a 30Hz signal whose phase varies 90 degrees from one antenna to the next.
The phasing is so arranged that if an aircraft is directly (magnetic) north
of the VOR, the phase of the AM modulation matches that of the FM reference
signal extracted from the subcarrier.  As the aircraft's angular position
with respect to the VOR changes, the phase difference changes by the
same amount.

In the aircraft, one of the two received signals is shifted in phase by
0, 90, 180, and 270 degrees and then these four signals are fed to
a phase-shift circuit in the VOR head that provides a continuous
phase shift of from 0 to 360 degrees as the course selection knob is
rotated.  This shifted signal and the other received signal
are then fed to phase-comparing circuits that drive the course deviation
indicator and the TO/FROM indicator.

Thus, the phase difference in the received signal depends on your position,
and the amount of phase difference that causes the CDI needle to null is
determined by the setting knob on the VOR head.  The connections to the
CDI galvanometers are arranged so that when the needle does deflect from
centre, it does so in the direction that you must fly, and the gain is
adjusted so that full-scale deflection occurs at the proper off-course
angle.  The TO/FROM indicator is really just a second CDI meter driven with
one of the two signals shifted 90 degrees so its nulls are 90 degrees
away from that of the main needle.

Of course, the signal broadcast is also modulated with the Morse code
identification for the VOR plus, sometimes, voice.
The modulation depths of the various signals are controlled to
allow them to coexist simultaneously.