John Bell wrote:
Do you mean to say they compensate for the lack of realignment by using

the

navaid's published "declination" instead of the actual alignment of

magnetic

north? I am skeptical.

My understanding is they use an approximation like a best-fit polynomial

or

similar to derive the magnetic declination at any given location.


Dave,

From what I can tell through experimentation with the Garmin 196, GPS 400
simulator, and 295 it appears that these GPS receivers use the navaid's
published variation instead of the approximation that you mention second.
The GPS III Pilot does not use the published variation.

If you want to do some experimentation, use SWL (Snow Hill on the east coast
of Maryland). Try the OBS mode and notice that it does not match the COURSE
or DTK. Try creating a user waypoint at the same point and you will notice
that the OBS mode works differently.

I have an example in my book on page 138 of the .pdf file under
Miscellaneous. It is page 9-14 of the printed copy.

I am always open to corrections.

There is a thread "VOR versus GPS" on rec.aviation.ifr where this is discussed
at some length. I remembered Julian Scarfe's comments about how this method of
operation raises some interesting inconsistencies, including the bahavior you
note above, where the bearing to the station (using published declination) does
not match the desired track to the station (using interpolated declination). I
had forgotten that the consensus was that's the way it works, with the
aforementioned inconsistencies. You're right. I'm wrong. Thanks.

Dave

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