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4 Answers

GPS…based on true or magnetic?

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Aircraft Systems, General Aviation, Student Pilot

Does GPS have magnetic variation built in...or is it based on true course?

If it IS based on true course...and you flew an appropriate heading to maintain the "desired track"...wouldn't you end up being OFF COURSE if magnetic variation is not taken into account by the GPS?

4 Answers

  1. Tracy Rhodes on Dec 17, 2010

    GPS units have the magnetic variation included in their database and, knowing it’s position, will apply the appropriate value to the true track that it has calculated. 
    A GPS will calculate a desired track between any two points and will display that line on a map or through a course deviation indicator(CDI). As long as you stay on the line or center the CDI you will always end up at the point on the end of the line. The heading that you need to fly to maintain that track is determined by the winds that you are flying through as you progress along the track. 

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  2. Kent Shook on Dec 17, 2010

    The GPS doesn’t really care – It knows where it is, and it knows what the variation is there, so it knows both true and magnetic.
    As for what is on the display, that depends on the GPS – On the popular Garmin 430W, you can set it to True, Magnetic, or even User (where you define your own variation)! You do this by going to the Aux 3 page (“Setup 1”) and choosing “Units/Mag Var”. For other models of GPS, check your user manual! There’s lots of other useful knowledge in there too.
    Finally – You would not be off course. You’re flying “an appropriate heading to maintain the desired track.” So, basically, matching TRK to DTK. By doing so, you are automatically compensating for magnetic variation, deviation, and even winds aloft – By matching the TRK to the DTK, you eliminate all the potential errors from the equation! Pretty slick.

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  3. Wesley Beard on Dec 17, 2010

    Here is the FAA reference:
    AIM 1-1-19 (I)  Conventional Versus GPS Navigation Data  (page 550)
    There may be slight differences between the course information portrayed on navigational charts and a GPS navigation display when flying authorized GPS instrument procedures or along an airway.  All magnetic tracks defined by any conventional navigation aids are determined by the application of the station magnetic variation.  In contrast, GPS RNAV systems may use an algorithm, which applies the local magnetic variation and may produce small differences in the displayed course.  However, both methods of navigation should produce the same desired ground track when using approved, IFR navigation system.  Should significant differences between the approach chart and the GPS avionics’ application of the navigation database arise, the published approach chart, supplemented by NOTAMs, holds precedence.

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  4. Skyrunner on Sep 05, 2013

    Hi. If I wish to fly outbound 041(T) from a point, on an OBS course on the GPS what will the GPS be displaying?

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