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Substituting GPS for localizer DME distances

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General Aviation

The IFH (Chapter 9, page 9-27) states that GPS can be used to determine the aircraft position over a fix defined by an NDB/compass locator bearing crossing a VOR/LOC course, a typical IAF for a localizer approach.

Is GPS also approved as a DME replacement during the final phase of a localizer approach to determine the distance from the missed approach point? If so, in what FAA document is this stated (please reference paragraph/sentence)?

Does the answer apply also to VOR approaches?


5 Answers

  1. raajtram on Jan 17, 2017

    Yes, GPS can be used in lieu of a DME.

    The AC 90-108 confirms the same – https://www.faasafety.gov/files/notices/2013/Sep/AC_90-108.pdf.

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  2. John D Collins on Jan 17, 2017

    A slightly later version of guidance is also available in the AIM in Chapter 1, Section 2. You can read the entire section to find the reference. You need to know this information anyway.

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  3. Charles22 on Jan 17, 2017

    Thanks Raajtram and John.

    Section 7 Item a. of AC 90-108 and AIM, Chapter 1, Section 2, Paragraph ‘1−2−3. Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes’, Item c1 state that operators may use GPS to determine the position of the aircraft in relation to various navigation aids, but do not include localizers.

    Since lateral navigation on localizer−based courses without reference to raw localizer data is explicitly prohibited, some instructors assume that the use of GPS for determining the distance from the localizer is also prohibited.

    Any clarification on this will be appreciated.

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  4. Best Answer

    Russ Roslewski on Jan 18, 2017

    I don’t know, I think it’s pretty spelled out pretty well. AIM 1-2-3:

    c. Uses of Suitable RNAV Systems. Subject to
    the operating requirements, operators may use a
    suitable RNAV system in the following ways.

    1. Determine aircraft position relative to, or
    distance from a VOR (see NOTE 6 below), TACAN,
    NDB, compass locator, DME fix; or a named fix
    defined by a VOR radial, TACAN course, NDB
    bearing, or compass locator bearing intersecting a
    VOR or localizer course.

    It says you can determine position relative to a DME fix. That’s what the MAP is, as are any other fixes on a localizer final defined by DME. Note 2 disallows lateral navigation on a Localizer, but that doesn’t prohibit your ability to determine your distance from the DME fix using the GPS.

    The DME antenna for a LOC/DME facility is often located at the LOC antenna, but isn’t always. So you’re not measuring your distance to the LOC antenna itself, but to the DME antenna. (For example, KPDX, where runways 10L and 28R share the same DME antenna which is located at the 28R localizer antenna. They switch which LOC is on when they switch runways, but the DME antenna stays in the same location, the distances are just calculated to be correct for each direction.)

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  5. Charles22 on Jan 18, 2017

    I was thinking of the LOC/DME facility as a single unit that you either replace with GPS or you don’t. This misled me. After reading your explanations I am fully convinced that DME can be replaced with GPS in a localizer approach.
    Thank you very much.

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