[simpits-tech] Real VOR indicator

Wed Feb 13 16:45:17 PST 2013

That was a great read.... but I am now pretty sure that Matt knows more
about VOR signals than is actually healthy ;-)

Fantastic work, and it is now quite obvious that I need to do a
sim-seeing tour over your side of the pond sooner rather than later!

SeanG

-----Original Message-----
From: simpits-tech-bounces at simpits.org
[mailto:simpits-tech-bounces at simpits.org] On Behalf Of Rene Koopman
Sent: Thursday, 14 February 2013 11:52 a.m.
To: Simulator Cockpit Builder's List
Subject: Re: [simpits-tech] Real VOR indicator

Nice Matt,

Must have taken some hours study ;)

Biggles.

-----Oorspronkelijk bericht-----
Van: simpits-tech-bounces at simpits.org
[mailto:simpits-tech-bounces at simpits.org] Namens dabigboy at cox.net
Verzonden: woensdag 13 februari 2013 17:54
Aan: Simulator Cockpit Builder's List
Onderwerp: Re: [simpits-tech] Real VOR indicator

---- geneb <geneb at deltasoft.com> wrote: 
> On Tue, 12 Feb 2013, dabigboy at cox.net wrote:
> 
> > Thanks! I used my sound card. :)
> >
> Ok, that all by itself requires a lengthy explanation. :)
> 
> g.

LOL! OK, here's more or less what happened:

So I had been wanting to get my hands on a real VOR indicator for a
while, with the basic plan that I could gut it and use the electrical
bits to read OBS setting, and use PWM pins on the Arduino to drive the
needle (since, as I understood it, the needle was just a simple
D'Arsonval meter). I eventually purchased this KI-201C on ebay for like
$30 shipped, mostly just to see what I could do with it. When I got to
looking at the instrument and the documentation for it (found on the
'net), I realized that the signal driving the gauge only required three
pins, and two of them were actually *output* for an autopilot
connection....so only one pin was used for signal input. Intrigued, I
started hitting the internet for more info........

Having sat through ground school years ago, I knew that the signal
coming from a VOR station was "two different signals that vary based on
your orientation to the station"....and that was about as far as a
typical ground school instructor would go. It turns out, the composite
VOR signal is pretty interesting. Each ground station puts out an
omni-directional signal which is composed of a 30hz sine wave, plus the
Morse stuff on a different frequency. The "rotating antenna" (actually a
circular array of antennae on the ground that is electronically
"rotated") puts out another 30hz signal, but is composed of a 9960hz
carrier signal. The phasing of the two signals determines what radial
you are on, and the 9960hz signal helps keep them "separated", so to
speak.The 9960hz signal also allows the signal to be differentiated, if
the two 30hz tones are 180* out of phase, for instance (otherwise there
would be no difference between the 0* radial and 180*).

The complex thing is that the 9960hz carrier signal is
frequency-modulated at 30hz, and where the 9960hz signal is at depends
on where the 30hz signal is at in its phase. This is the hardest
part.......I was getting the right idea, but wasn't sure exactly how to
make it work. One thing I knew, that 9960hz was well within the ability
of a computer's sound card to produce, so I *should* be able to actually
generate this signal myself.

Then I come across a post on a message board from 2006 where some folks
were discussing VOR signals. One poster was a ham radio operator and
mentioned that he had actually written a program that generates all the
signals for a VOR gauge. I emailed him and, thankfully, his address was
still active. He generously sent me his entire program, source code and
all! Unfortunately it was written as a Visual Studio project, and there
was so much Microsoft fluff involved, I wasn't sure exactly how to read
the code and apply it to my Python/Linux work. So, I fired up my
barely-used copy of VS 2010, migrated his project, made some tweaks to
get it to run (his program was from 2001, so it was written in an old
version of VS), and actually got it to output the tones for each radial.

I then used Audacity to record each radial, 0*-359*, and saved them as
WAVs. After that, I wrote software that calculates the current radial
from X-Plane, then plays the appropriate WAV file.

The really tricky thing is that the calibration is funky....depending on
what radial I'm at, the error is anywhere from no error at all, to
around 30*. So, I had to write a calibration table to send the correct
WAV to the sound card. I'm not sure if this is a problem with the
instrument, weirdness with my sound card (the phasing is actually messed
up by the soundcard, but it's supposed to be a constant error that you
can allow for in software), or something funky caused by playing
pre-recorded WAVs instead of generating the tone in real time. I've got
it pretty well working within 4 degrees accuracy or so, but there are a
few radials that are off by more than 20* still..........I think it's a
problem with my radial-calculating code (the X-Plane SDK does not seem
to have a dataref for current VOR radial, I have to subtract relative
bearing to the VOR from current heading, then apply corrections to keep
the resulting value within 0-360). But it is working, which is the main
thi
   ng. I don't think the calibration errors will be hard to fix.

If I wrote my software to generate the 30hz tone and the 9960hz
modulated at 30hz, I'm pretty sure I could fill in the gaps of my
knowledge through experimentation and study of the other guy's
code.....but what I'm doing now with the WAVs appears to work just fine,
so I will probably leave it.

Now, the astute observer might note that the signal coming from a real
VOR station is actually an AMPLITUDE-modulated signal between 108mhz and
119.95mhz, and you would be correct! That's the carrier signal to
broadcast to your plane. The NAV receiver in the plane de-modulates that
AM signal to yield the "clean" VOR composite signal, which is what goes
to the indicator. Theoretically, if I designed a device that takes my
current signal and amplitude-modulates it at the above frequencies
(basically, half of an AM radio transmitter), I could run the signal
right into the antenna input of a real NAV radio and actually be able to
use all the tuning and functionality of the real radio! This is overkill
for me though, I already have nice Gables control heads doing my nav/com
stuff (plus good NAV radios are over $1,000 a piece.........)

So what is all this magic inside the VOR indicator itself? It is simply
a phase-shifter, and the VOR needle indicates how far out of phase the
two signals are....turn the OBS knob until the shifted signals are in
phase, and the needle will be centered.

Matt
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