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3 Answers

Why is it that when using GPS for DME, they put an I in front of the localizer identifier?

Asked by: 858 views Instrument Rating

Why is it that when using GPS for DME, they put an I in front of the localizer identifier? For example, for SNA localizer, they enter into the GPS "ISNA" to get the distance to the SNA localizer?   Is this documented anywhere in AIM?


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3 Answers

  1. Russ Roslewski on Dec 12, 2016

    You put in ISNA because the DME is (usually) collocated with the localizer and that IS the identifier of the localizer (actually “I-SNA”). You can see that on the approach chat in the upper left corner.

    If you entered just “SNA” it would be ambiguous – do you mean the airport or the localizer? Since the two have different coordinates, you need to have the right one.

    Consider an even better example – San Francisco Intl. The runway 28L ILS identifier is I-SFO. The airport is KSFO. The VOR is SFO. If you entered the identifier “SFO”, you’d get the distance to the VOR instead of to the Localizer, and would therefore descend about a mile early at each fix along the way. Not good!

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  2. John D Collins on Dec 13, 2016

    It is also the localizer ident that you listen to to identify the station is tuned properly.

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  3. Mark Kolber on Dec 15, 2016

    I’m with John on the simple answer. The”I” is there because a localizer identifier begins with an “I”. The GPS just uses the navaid or waypoint name, whether it’s KDEN for Denver International Airport, MOD for Modesto VOR, DREAR for that named intersection just east of the Madison, WI airport.

    If you look at the chart that shows the localizer, what does it say?

    It is not any more complicated than that.

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