From: hopkins@gvlf9-o.gvl.unisys.com (Bill Hopkins)
Subject: Re: unreliable VOR navigation
Reply-To: hopkins@gvl.unisys.com
Organization: Paramax Systems Corp (A Unisys Company), Great Valley Labs, PA
Date: Mon, 27 Jul 1992 19:21:17 GMT

In article <19784@leadsv.UUCP> claude@leadse9.UUCP (Claude Goldsmith) writes:
>
>Signal "scalloping" is a constant problem with VOR navigation.  A
>major contributor to this is "multi-path" propagation.  That is, the
>signal arrives at a single point in space by 2 or more paths of
>different lengths.  
> [...]
>Multi-path is caused by reflections of the signal off of terrain and
>structures.  As a result of those reflections, the strongest signal
>that you receive may not actually be the signal for the radial that
>you are on.  [Strengths vary with position.]  The result is your CDI
>wandering back and forth.  [...]  It is more apparent on some some nav
>receivers because the CDI is less damped.  
>
>The manufacturer of the equipment knows that wandering CDI's, whether
>caused by noise on the signal or multi-path, are annoying to the pilot
>and a problem for some autopilots.  They include circuitry to
>integrate the signal over time and display the results.  On those
>displays the needle seems to be steady or only move slowly.

Hang on a minute, here!  Typically, multipath propagation will give
one correct signal and one or more incorrect signals.  Averaging
(damping) in these cases will guarantee an incorrect indication!
This doesn't seem to be such a great idea...

>Some manufacturers feel that seeing the needle move around gives the
>pilot important information about the quality of the received signal;
>so they include less damping.  It then becomes the pilots
>responsibility/task to smooth out (average) the course information.

There's a lot of appeal in this approach.  In addition to a quality
indicator, it gives you an opportunity to pick the right reading.  In
particular, if you're tracking a stable radial, and the needle starts
oscillating to one side, it's a fair bet that the good signal is
still centered and a bogus one is showing up to cause the deviation.
Centering the oscillation should require a substantial correction,
more at larger distances from the VOR, and that suggests GIGO (garbage
begets garbage) to me.  Wouldn't it make sense to keep the oscillation
where it appears (if it can be done without abrupt heading changes)?

This assumes that (a) you're not close to the VOR, (b) the damping
isn't interfering, (c) you're paying enough attention to identify
the good indication, and (d) nothing else is going on that might
cause CDI excursions.  This is *only* speculation, of course, but
it *is* testable if you know your position via GPS, pilotage, or
other arcane technology, when scalloping appears.

	Bill (isn't this what experimental aircraft are for?) Hopkins

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hopkins@GVL.Unisys.Com          Paramax Systems Corporation (A Unisys Company)
215-648-2854 or 363-7464        Great Valley Labs, PO Box 517, Paoli, PA 19301
MXE 016 radial, 8 nmi              Opening my mouth is not company policy.
From: mtc@hpcc01.corp.hp.com (Mark Cousins)
Date: Fri, 24 Jul 1992 00:44:49 GMT
Subject: Re: Carlsbad Caverns Fly In
Organization: HP General Systems Division