GPS approach discrepencies

[bLizZuE]

Why will the GPS show a DTK that is different than the published course? The chart will show a 070 course, but when activated in the GPS it will show a 068 DTK.

[ppragman]

Originally Posted by bLizZuE

Why will the GPS show a DTK that is different than the published course? The chart will show a 070 course, but when activated in the GPS it will show a 068 DTK.

Chart Courses are rhumb lines, GPS courses are great circle distances. That's the first thing that comes to my mind.

[bLizZuE]

rhumb?

[ppragman]

Originally Posted by bLizZuE

rhumb?

http://en.wikipedia.org/wiki/Rhumb_line

[bLizZuE]

Interesting. Thanks.

[nutz4life]

Not sure if rhumb line explains it since the charts show courses that are very short in distance. It's the longer distances that matter. My guess would be the FAA is just running behind like always to upgrade the charts to the current database.

[BrewMaster]

Let's say you fly VOR to VOR on a published Victor Airway. Going away from ABC VOR on the 090 radial into the XYZ VOR on the 270 radial aren't the same thing. You can't curve a radial out of a VOR, so assuming you aren't on the equator they will actually cross each other instead of lead into each other. A GPS constantly corrects for this and will change during the course of your flight.

[29.92inHg]

Thats true, and I see that, but how does that make a GPS give you a DTK that is 2 degrees off the published course, on a VOR approach that only goes 10 miles from the VOR. Also, It does it on GPS approaches, you would think this would be on the plate if that is the approach. However, barring that the Rhumb Line Explanation is by far the best reasoning I have heard so far. I hadn't thought of that

GOOD Question

[BrewMaster]

I guess that explanation was for nutz4life.

I kind of misunderstood the real question. Because the GPS satellites are in space(a vacuum) they work perfectly, but as they enter the earth's atmosphere, the signal gets pushed and bounced around. This bouncing around can change or move around due to what it is, and I think the GPS may be correcting for this, and changing the DTK accordingly. I can't remember if this is for a WAAS unit only, but I think it may be a built in algorithm that tells the GPS to display a certain desired track according to what it believes the atmosphere is doing to the signal.

The Navdata on the cards is the same data on the approach plates, so I don't think it's anything like that.

If I'm wrong or if there is a simpler explanation please correct it. I've been awake forever so I may have screwed that up.

[fish314]

I don't think any of the explanations above are correct.

The following is from the Air Force's AFMAN 11-217 (Instrument Flight Procedures). It is basically the Air Force bible on instrument flight and is like a combination of the FARs and the AIM for civilian pilots. In other words, some of the information is regulatory (if it is in bold italics) and the rest is just informational.

Anyway, here goes. The key information is in the last paragraph, but the whole thing talks about differences between the GPS display and the charted procedure. These paragraphs come from the section about GPS overlay procedures, but most of the same information is found in the RNAV approach section, including the last paragraph.

Originally Posted by AFMAN 11-217 Vol. 1

7.13.5.3. In the event of differences between the terminal procedure chart or approach chart and database, the published approach chart, supplemented by NOTAMs, holds precedence and the database may not be used to fly terminal area or instrument approach procedures except as noted below.

7.13.5.3.1. In some cases, waypoints in the navigation database may differ from the charted instrument procedure. The differences listed below are acceptable and do not preclude use of the database procedure.

7.13.5.3.1.1. Step down fixes depicted on the approach chart may not be contained in the aircraft database. Pilots are responsible for ensuring compliance with applicable step down fixes regardless of whether or not they are in the aircraft database.

7.13.5.3.1.2. The database may contain some waypoints (capture fixes,
and a point in lieu of a FAF for non-FAF overlay approaches) that are not depicted on the approach chart.

7.13.5.3.2. For GPS overlay approaches, certain unnamed points and fixes appearing on a chart are assigned a database identifier.

7.13.5.3.2.1. NOTE: These database identifiers should not be used for pilot/controller communications or on flight plans.

7.13.5.3.3. Small differences may exist in distances between waypoints. Differences less than 0.3nm are acceptable for GPS overlay approaches. For stand-alone GPS and RNAV approaches, the maximum allowable difference is 0.1nm. If distance information varies by more than these tolerances, the procedure shall not be flown.

7.13.5.3.4. Computation of the GPS final approach course is based on the station magnetic variation retrieved from the aircraft magnetic variation database. Many aircraft have a non-updateable magnetic variation database, thus will be almost guaranteed to be different from actual magnetic variation. This will cause a difference between the displayed GPS final approach course and the charted final approach course in the IAP. The discrepancy between displayed and charted magnetic variation will depend on discrepancy between aircraft magnetic variation database (age of database) vs. magnetic variation upon which charted approach course is based (date of magnetic variation survey). Variation between charted final approach course in the IAP and the final approach course computed by the aircraft should be no more than 5 degrees. If the two differ by more than 5 degrees, the procedure is not authorized.

[fish314]

Here is another possible source of the error (also from AFMAN 11-217)

Originally Posted by AFMAN 11-217

7.12.2.5. Conventional vs. GPS Navigational Database Information. There may be slight differences between the heading information portrayed on navigational charts and the GPS navigation display. All magnetic tracks defined by a VOR radial are determined by the application of magnetic variation at the VOR. GPS equipment may apply the magnetic variation at the current position, resulting in small differences in the displayed course. Additionally, due to the use of great circle courses, the bearing to the next waypoint and the course from the last waypoint (if available) may not be exactly 180 degrees apart when long distances are involved. In any event, the resulting ground track should be the same. Distance information may also vary since GPS distances are along track values, while DME measures slant range distance. For a complete discussion of database issues, see paragraph 7.13.

[deadstick]

Originally Posted by ppragman

Chart Courses are rhumb lines, GPS courses are great circle distances. That's the first thing that comes to my mind.

But the final on a GPS is about 5nm while the airway can be over 100. I'd think the difference, if any, would be in 10ths of degrees.

[bLizZuE]

I thought it to be magnetic variation of some sort, but couldn't think of how it affected the approach.

Next question: If there is a difference, and I should use what the chart says as a course, how do I get the GPS to do that? When the final segment activates, I can't change the DTK on the 430.

[fish314]

Originally Posted by bLizZuE

I thought it to be magnetic variation of some sort, but couldn't think of how it affected the approach.

Next question: If there is a difference, and I should use what the chart says as a course, how do I get the GPS to do that? When the final segment activates, I can't change the DTK on the 430.

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.

On a GPS overlay (rather than a straight RNAV approach), I still have the underlying NAVAIDs displayed, and then I'll crosscheck that the underlying NAVAID course is still set to the course on the plate (and still lined up). In that case, the GPS course and the underlying NAVAID course will read different numbers, but they should still line up. So in other words, you may have a 170 VOR course dialed in, and the GPS may compute 168, but when you are on centerline both the GPS and the VOR should line up.

Of course, I'm using AF rules for this entire discussion. Not sure if the civilians have anything different, but then I don't fly much civilian and then usually only in good weather!

[ppragman]

Originally Posted by deadstick

But the final on a GPS is about 5nm while the airway can be over 100. I'd think the difference, if any, would be in 10ths of degrees.

Ok, maybe the agonic line that they used to draw the chart goes right between the two fixes. Honestly, that's the best I've got, and in all actuality, there's probably no real reason why.

[BrewMaster]

I think what I explained about the GPS correcting was just a variation or another way of saying it, I think the date of the surveys is probably a good answer to, its probably just a combination of a couple things. Nothing I've seen here has been really black or white.

[woodreau]

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."

[Minuteman]

Originally Posted by bLizZuE

Why will the GPS show a DTK that is different than the published course? The chart will show a 070 course, but when activated in the GPS it will show a 068 DTK.

I don't mean to imply there is user error, only to investigate it. Could you provide the specific points/routes involved in noticing the discrepancy?

It would be interesting to see if others (or me) could reproduce it.

[Mike H]

I think the discrepancy comes from the fact that the GPS knows exactly where you are on the approach at any given time, and where you want to end up (e.g. the next waypoint). DTK is what will get you there from where you are

[bLizZuE]

Originally Posted by Minuteman

I don't mean to imply there is user error, only to investigate it. Could you provide the specific points/routes involved in noticing the discrepancy?

It would be interesting to see if others (or me) could reproduce it.

Sure, any approach in Florida.

[deadstick]

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).

[fish314]

Originally Posted by deadstick

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).

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]

Originally Posted by woodreau

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."

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

[bLizZuE]

Thank you!