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Pop quiz question about VOR and LOC receivers

We're working on a pop quiz and one of the questions is about the LOC and VOR receiver portion of, for instance, a GNS430. To what extent do the LOC and VOR use the same components such as antenna, circuitry and so forth, and to what extent do they use different logic to, for instance, differ the scale width on the CDI display?

Essentially the question is "if your VOR receiver is broken, will the LOC still work?" but we need to narrow down that question a bit more, and come up with a technically correct answer.

We know their frequency ranges overlap so it would seem logical that at least the antenna and demodulator would be shared. Can anybody confirm this?

VOR and LOC work differently but the antenna is the same. The receiver recognizes a LOC by the frequency (odd decimal) and tunes to the configured MHz. However instead of evaluating the time difference between the non directional and the directional signal (which corresponds to the radial), it will measure the difference of the 90Hz/150Hz depth of modulation. The LOC consists of two antennae on the runway end which both transmit a narrow beam with a 90Hz signal (left) modulated on top of the LOC frequency and a 150Hz signal (right) modulated on top of the LOC frequency.

This is from memory, had to learn that for the IR exam..

So if your NAV receiver is broken, it is hard to tell what will work. It's one antenna for sure.

I don't know how a GNS430 decodes the VHF signals, but the traditional (e.g. a KN72 way, which can do "interesting things") is to demodulate the VHF signal to produce a "composite" signal (not to be confused, as it often is, with a "composite video" signal which is something completely unrelated) which has components in the kHz spectrum and which is easy to decode into the various signals for

  • deviation bar
  • to/from flags
  • "valid" flags

The KN72 uses mostly separate circuitry for the LOC and VOR decoding. Note that the VOR decoding needs the OBS value (or the Course Pointer position, on an HSI) but the LOC decoding does not.

The flags are generated separately and as the above article shows, it's easy to get a failure which shows no flags but the deviation bar is reading way off and you fly into a mountain. No way could a GPS fail in such a subtle way.

The KN72 is a 1970s design. The equivalent in say a KX165A radio (I have the schematics) is much the same. The modern way to decode the stuff is using digital signal processing and then "subtle" faults should be impossible. And then the analog circuitry can be common to both.

As I said I have no idea what a GNS430 does. It's a mid-1990s design, prob99 done by ex King people, and my guess is that it doesn't use DSP, because the designer would have had a choice of old well-proven analogue decoder designs to pick from. Today, there is no question you would use DSP techniques (just to avoid all the analog components and especially the close tolerance capacitors) and in fact one could do the whole lot with a $1 micro running at say 10MHz, which is about 1% of the CPU power in a modern panel mount GPS.

Shoreham EGKA, United Kingdom


When I read your post my first reaction was "the regulatory authorities make you write enough exams with toughie trivia questions with little practical value, why on earth would you want to do that to yourself ?"

I would suggest a better question would be "what are the signs the VOR may not be functioning correctly and what would you do about it" for the next one ask the same question inserting LOC for the VOR.

Wine, Women, and Airplanes = Happy

BPF, the quiz is not for pilots, but for non-pilots who have time and ability to look things up on the internet. And now the pilots have started arguing about whether the VOR and LOC receivers are the same hardware or not. You know what it's like...

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