GPS approach discrepencies

I wonder if the FAC for an approach is a database item, OR only the lat/long of the FAF and MAP are in the database and the unit calculates it (resulting in a DTK for the FAC).
 
deadstick said:
I wonder if the FAC for an approach is a database item, OR only the lat/long of the FAF and MAP are in the database and the unit calculates it (resulting in a DTK for the FAC).
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It probably varies from system to system, but I think for most systems it's the second. I think that's what that paragraph from Air Force Manual 11-217 that I posted above is trying to get at.

Basically, the GPS takes the lat/longs from the updated database, and calculates a true course. It then takes the Magnetic variation from the non-update-able database (at least in many systems), and it applies that variation to the calculated true course to come up with the desired track. Therefore, the magnetic course of the desired track that it calculates will differ from the charted magnetic course by whatever the difference is between the magnetic variation the chart designer used and the magnetic variation in the non-update-able database. We use a KLN 900 GPS system, and this effect accounts for most of the error we see when the computed course doesn't match the charted course.

The other source of error is probably the difference between the magnetic variation at your present position (which is what the GPS is probably using) versus the magnetic variation at the VOR, which is what the chart designer was probably using. Probably only a factor in GPS overlay approaches, though.

Rhumb line versus Great Circle probably doesn't play much of a factor because of the short distances involved.
 
woodreau said:
For charting purposes, the FAA updates the magnetic information every 5 years. Whether the FAA decides to realign the VOR when the magnetic variation changes, it depends - if it changes the magnetic variation at the VOR - it has to rechart all the radials on the charts, and flight test them - so I believe the FAA waits until the change in variation is significant before it realigns the VORs. That's my understanding of why the courses are different between the printed chart and a GPS receiver. The GPS receiver computes it "real-time" whereas the chart is very "static."
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Couldn't figure out how to edit my previous post to put in a link. Jeppesen explains why there's a difference between charted courses and the avionics computed DTK. It's a bit old from 2001.

http://www.jeppesen.com/download/aopa/may01aopa6.pdf
 
fish314 said:
You can't make the GPS do anything different, but it doesn't really matter. All I do is check to make sure that they are within 5 of what the plate says, and then fly the approach like any other.

Of course, I'm using AF rules for this entire discussion.
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I know this is a necropost, but wanted to bounce something off this post. I know the "5 degree" rule as common knowledge, but just last night went to source it and couldn't find a reference. Google led me here. Hopefully this info isn't only found in an Air Force manual - any one know of an FAA source?
 
It has to do with how the GPS unit calculates and applies magnetic variation. The cartographer for the charts is utilizing actual mag variation for the GPS course or the declination of the VOR (if it is an overlay approach) however, the GPS is calculating where it is on a map in its head and what the mag variation that is programmed into it is at that point.

It is buried somewhere in the G430 users manual, but I have long since stopped caring. My explanation might not be perfectly correct, but functionally it is close enough for government work.
 
[edit: whoops, looks like this is quoted on the first page of this thread.... ohwell, incase anybody missed it, read on...]

This is in chapter 1 of the AIM in the GPS section:

l. Conventional Versus GPS Navigation Data

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.

Due to the GPS avionics' computation of great circle courses, and the variations in magnetic variation, the bearing to the next waypoint and the course from the last waypoint (if available) may not be exactly 180° apart when long distances are involved. Variations in distances will occur since GPS distance-to-waypoint values are along-track distances (ATD) computed to the next waypoint and the DME values published on underlying procedures are slant-range distances measured to the station. This difference increases with aircraft altitude and proximity to the NAVAID.
 
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Keep in mind RNAV Approaches are designed based on true course. So as stated above, the FMS will apply the calculated mag var that will result in slight differences. The true course will never change, but the mag will.
 
Ian J said:
I know this is a necropost, but wanted to bounce something off this post. I know the "5 degree" rule as common knowledge, but just last night went to source it and couldn't find a reference. Google led me here. Hopefully this info isn't only found in an Air Force manual - any one know of an FAA source?
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Ian,

I went looking pretty hard a couple of days ago for an FAA source, and like you, I also couldn't find one. Checked out the IFH, IPH, AIM, FARs and they all said the same thing: "a small discrepancy is normal" and "the paper takes priority." But nothing specifically about the 5 degree rule. I wonder if that's just a rule in the military, and has been adopted on the civil side as a common technique?
 
fish314 said:
Ian,

I went looking pretty hard a couple of days ago for an FAA source, and like you, I also couldn't find one. Checked out the IFH, IPH, AIM, FARs and they all said the same thing: "a small discrepancy is normal" and "the paper takes priority." But nothing specifically about the 5 degree rule. I wonder if that's just a rule in the military, and has been adopted on the civil side as a common technique?
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I'm guessing that two knuckle draggers like us equal one semi smart person and if we can't find it, it's probably not out there.
 
I've never heard of the "5 degree rule" of which you speak. I can only assume it's a mil-only rule of thumb.

On another note, guess I've been out that long now.
 
Guess so, Ed. The Tanker wasn't approved for GPS approaches when you left it, but it has been for a while now. And the T-6 is as well, so the new students are all learning them. Obviously, they weren't a factor back in the Tweet when you and I went through pilot training.

But on a side note, if you've "never heard of the 5 degree rule" that's one more data-point that this is probably a mil-only rule.
 
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