Garmin 430/530 Questions

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

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.

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.

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?

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.

Dave Butler wrote:

wrote:

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?

Well, it might be that they are using different values for the local magnetic
declination. For example, suppose NACO uses a measured declination from an FAA
check flight, and Garmin uses a calculated value from a best-fit polynomial.



Exactly, as I said earlier in the thread.

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