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These last few weeks I have been flying a couple of different aircraft that had inoperative glideslope capabilities, but I was able to fly LOC approaches without trouble. In my career as a pilot and mechanic, I have always had the—possibly inaccurate—understanding that a VHF NAV receiver had the capability of receiving and processing both VOR and LOC signals.

My question is prompted by this answer to a question regarding flight plan equipment codes—and particularly the comment section of that answer. In the comments of that answer it was stated that an ILS capable VHF NAV receiver is necessary to fly a LOC approach. For the purpose of this question let's overlook the inaccuracy of the ILS part of that statement and instead consider the LOC capabilities of a VHF NAV receiver.

I understand that the additional requirements needed for ILS capability include a glideslope antenna (possibly included in the existing NAV antenna), equipment to process the frequency pairing, and a glideslope display. I know that all LOC frequencies fall within the range of VOR frequencies (108.10–111.95 MHz and 108.0–117.95 MHz respectively) and the frequency spacing is compatible. I also understand that the circuitry required for VOR signal processing includes circuitry not required for LOC signal processing (that circuitry which can deteriorate over time requiring the periodic VOR accuracy checks).

Given all this, my questions are:

  • Is LOC signal processing an inherent capability of a NAV receiver: Will any given VHF NAV radio receiver be inherently capable of processing a LOC signal, or is there additional circuitry required for that capability beyond that required for VOR signal processing?

And if not,

  • Are there NAV receivers incapable of LOC signal processing: If LOC signal processing is not an inherent capability of a VHF NAV receiver, are there NAV receivers that were built without the LOC signal processing capability? In other words, is it possible that a pilot or mechanic would encounter a NAV receiver that did not have LOC signal processing capability? More precisely—from a pilot's standpoint—might I encounter a NAV radio that I could use to navigate tuned to a VOR station but not a LOC station?
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Any VHF radio can receive the carrier signal of a LOC signal but to use it, you need dedicated circuitry.

The LOC transmitter transmits the carrier wave, frequency modulated with two side lobes of 90Hz and 150Hz. In order to use the LOC signal, circuitry needs to exist which will filter out the 90 and 150Hz then measure the difference between them to determine where in the beam the receiver is located.

This circuitry is dedicated to the ILS receiver.

It's a similar story for GS which simply uses lobes above each other, rather than left and right.

So

Is LOC signal processing an inherent capability of a NAV receiver

No.

Are there NAV receivers incapable of LOC signal processing.

Yes. All of them which do not have an ILS function (e.g. the Rockwell Collins VOR-900).

[ BEGIN Mythbusters Territory]

I've just tuned my hand held airband receiver to the ILS of my local airport. I think I can make out the beat frequencies from the 90 and 150Hz although I doubt that the low pass filter is letting much 90Hz through. I guess it's optimised for voice reception so probably cuts off at about 100 or 120Hz.

The next time I drive past the end of the runway, I'm going to test whether I can detect any change in the beat frequencies as I drive through the lobes. There is a convenient hill about a mile from the threshold which will put me a couple of hundred feet above so I should be in the beam.

I guess it might just be plausible to hear when you are going left and right of the centre line.

I'll report back.

[END Mythbusters Territory]

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  • $\begingroup$ Thank you. Can you provide an example of a NAV receiver incapable of LOC signal processing? $\endgroup$ – J Walters Feb 16 '16 at 17:23
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    $\begingroup$ @JonathanWalters I'm not sure what you are asking since any nav radio that does not have an ILS capability is, by definition, incapable of processing a LOC signal. Here is just one example, out of hundreds. $\endgroup$ – Simon Feb 16 '16 at 17:28
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    $\begingroup$ Your Mythbusting may just prove that it's plausible that one could, in a Horribly Desperate Situation ™, execute an ILS landing with a standard audio radio. Of course a Good Pilot™ would never get him/herself into said horribly desperate situation. $\endgroup$ – FreeMan Feb 16 '16 at 17:47
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    $\begingroup$ From what I understand, back when approaches were first being designed, they did indeed just listen to the audio and as they moved back and forth it would change to a solid tone where the two signals met, but modulate at different audible pitches depending on whether you were left or right of course. $\endgroup$ – Lnafziger Feb 16 '16 at 20:48
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    $\begingroup$ @Lnafziger You seem to be describing the Lorenz beam system, though the modulation was dots/dashes (one- or seven-eighths of a second per second) rather than in pitch. $\endgroup$ – Jeffrey Bosboom Feb 17 '16 at 4:10

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