[simpits-tech] Real VOR indicator

Wed Feb 13 20:05:38 PST 2013

Definitely! Actually I just had a "sim party" a couple weeks ago with a few of the guys from Wichita. Great fun! I never thought I would know so many people who are into this crazy hobby just as badly as I am. :P You should come to CockpitFest USA if you're ever over in this direction. September was the first one here, and it was a blast. This year they're having it in conjunction with Avsim's "FanCon 2013", so we are going to have tons of vendors, guests, and exhibits. And at least 3 or 4 sims will be flying online with PilotEdge.

I'm still trying to get my hands on a Learjet cockpit....got a couple of leads working right now.

Matt

---- Sean Galbraith <Sean.Galbraith at pacificsimulators.com> wrote: 
> 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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