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Aircraft Systems, Instrument Rating

I'm curious about the difference between the two. I understand that practically, the LOC is much more sensitive and is not "radial" specific like a VOR. I would expect that given the type of signal, any NAV that can be used for VOR navigation can also be used for LOC, but I don't know the technology well enough to say for certain. Are there any certification standards or hardware (NAV radio itself, CDI, etc.) specifics that would be relevant? Can anyone answer and possibly elaborate on the reasons why or why not?

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  1. John D. Collins on Nov 26, 2012

    The two systems use different methods of modulating the signal. In the case of a VOR signal, it consists of two signals, one a reference signal and the other a variable signal that is phase shifted based on the relative radial. In other words, one signal contains a reference and the other contains the bearing information. The VOR also can transmit audio that the pilot can listen to. The audio is sometimes used for communication or weather information, but most often, it is used to identify the VOR identifier via Morse Code. The receiver decodes both of the directional signals and sends them to a box called a VOR converter. It takes the pilot selected radial from the Omni Bearing Selector (OBS) and compares it with the other two signals. It then outputs the comparison to the left-right course indicator and to the To/From flag. In this way the pilot is able to select any course to or from the VOR and use it for navigation. To fly directly from any position to the VOR location, one tunes the VOR, identifies the Morse code to verify they tuned the correct frequency, turns the OBS until the needle centers on the indicator and the To/From flag reads To. They then turn the heading to that course and follow the fly left – fly right needle indications to navigate to the VOR location.

    A localizer works differently. The signal does not contain any relative bearing information, The antenna array is located about 1000 feet past the departure end of a runway it is used to transmit two tones, one to indicate to fly left and the other to indicate to fly right. The course is the runway course and can’t be adjusted by the pilot, turning the OBS has no effect. The pilot intercepts the course and turns towards the runway heading. When the fly left and fly right signals are balanced, the course indicator will indicate on course, the needle will be in the center. If more fly right than fly left is received, the course indicator will fly right. The stronger the imbalance between the two signals, the greater the fly left or fly right will deviate from the center of the course and the greater amount the pilot will be off course. The drill is to keep the needle in the center so that it takes you to the runway.

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