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#11
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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. John www.cockpitgps.com |
#12
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The problem is that the VOR is aligned to magnetic north, but is not realigned as the magnetic variation changes over time. Thus, this is really a VOR problem. However, the GPS receivers that I mention compensate for this. I often see 2 or 3 degrees difference between the Garmin 530's desired track along an IAP segment, and that on the approach chart. Of course, it doesn't matter since the IAP string is a flight plan between waypoints. I know the FAA folks who publish the IAPs consider the magnetic courses they show on the official IAP document to be "guidelines," rather than the precise magnetic bearing between IAP waypoints. |
#13
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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 Remove SHIRT to reply directly. |
#14
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Ron Natalie wrote: wrote in message ... I often see 2 or 3 degrees difference between the Garmin 530's desired track along an IAP segment, and that on the approach chart. Of course, it doesn't matter since the IAP string is a flight plan between waypoints. That's to be expected. Even if there were no magnetic variation, the track (except in certain degenerate cases) must change as you fly along any straight line. Not for a 5 or 10 mile IAP segment. I know the FAA folks who publish the IAPs consider the magnetic courses they show on the official IAP document to be "guidelines," rather than the precise magnetic bearing between IAP waypoints. Nope, they're pretty danged accurate. You just have to realize that the bearing relative to a station must change while tracking a straight line. Then, why are the chart and the Garmin often in disagreement for a 5 to 10 mile IAP segment? |
#15
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wrote in message ... Then, why are the chart and the Garmin often in disagreement for a 5 to 10 mile IAP segment? OK, you got me, it may very well be wrong for the IAP. The final is smack on. I misread the earlier post. |
#16
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#17
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Steve Coleman wrote:
I can't remember the exact clearance, but I frequently get a clearance something like this departing Salt Lake, southbound. It happens around the CMK VOR too, for (if memory serves) westbound traffic. I'd guess they reorganized traffic flow at some point recently, and have yet to add the intersection of the two relevant radials as a waypoint. Presumably, this'll happen at some point. - Andrew |
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