VOR check

[rotorman]

While flying directly toward a VOR I compare my KN53/KI204 against my GTN650 in the VOR mode, and with the 650 course indicator centered, the KI204 is off considerably. The station ID is solid. The to/from indicator is correct. Would this be a KI204 indicator problem or a KN53 receiver problem?  

[RLCarter]

Do a ground check to see which is correct 

[Jerry 5TJ]

Compare the GPS to/from the VOR waypoint for a deciding vote.  

[carusoam]

If you fly from VOR to VOR...

The digital magenta line from the GPS will be very straight...

The analog course between the VORs often will show a scalloped path...

I believe the width of the corridor may have something to do with this non-linear error...

So how far off course were you, and how far from the VOR were you when you made your observation?

PP thoughts only, not an analog signal expert...

Best regards,

-a-

[kortopates]

Unless the difference is huge it could simply be just the difference in the amount of magnetic variation the GPS is using versus the VOR. The latter is much further out of date but the supplement will be able to tell you precisely what your local VOR is using.


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[Bob - S50]

This may be what you did but I can't tell.  For a proper inflight VOR check, tune your old equipment and the GTN650 VOR to the same station.  On the GTN650, hold down the Home button to get to the default page.  Change the source from GPS to VOR.  You'll see VLOC at the bottom of the GTN.  Now center both needles and compare the depicted course.  They must be within 4 degrees of each other.

You can't use a GPS course and compare it with a VOR course.  It has to be VOR and VOR.

Another way to check is to find a published ground reference point and fly over it.  Or find a prominent ground reference point that lies under a published airway.  Fly over the point and compare your displayed radial with the published radial.  Has to be within 6 degrees.

See AIM 1-1-4.

[Jerry 5TJ]

44 minutes ago, Bob - S50 said:

You can't use a GPS course and compare it with a VOR course.  It has to be VOR and VOR.

You may not use GPS to accomplish the airborne VOR check.   But you may use GPS as a diagnostic test to see which of your VOR receivers is the more correct.  

[Bob - S50]

11 hours ago, Jerry 5TJ said:

You may not use GPS to accomplish the airborne VOR check.   But you may use GPS as a diagnostic test to see which of your VOR receivers is the more correct.  

I'll respectfully disagree.  Each VOR is set with a given magnetic variation.  GPS units do not know what that variation is.  Instead, they use an algorithm to calculate magnetic variation for their current location.  It can differ from actual magnetic variation by several degrees.  That's why the course to steer displayed by the GTN650 is usually a few degrees different than that indicated on Low Charts.

If two VOR's do not agree within 4 degrees and we are trying to figure out which is right, the best thing to do would be to: 1. Go someplace with a VOT or 2. Go someplace with a ground check point or 3.  Fly over a published airborne ground check point.  We can find a list of all three in the back of the Chart Supplement.

That's also one of the reasons why we are prohibited from building approaches using radial/DME fixes.  The radial could be off by several degrees.  So if it is off by 3 degrees and we are going to a fix that is 30 miles from the VOR, we will miss the actual fix by about 1.5 miles.

[flyingcheesehead]

1 hour ago, Bob - S50 said:

That's also one of the reasons why we are prohibited from building approaches using radial/DME fixes.  The radial could be off by several degrees.  So if it is off by 3 degrees and we are going to a fix that is 30 miles from the VOR, we will miss the actual fix by about 1.5 miles.

Huh? There are still approaches using radial/DME fixes...

[Jerry 5TJ]

Hi Bob

Your argument is that the GPS receiver’s isogonic interpolation is off by several degrees only 30 miles distant from a surveyed VOR site’s magnetic variation?  

And that value is greater than the typical pair of GA VOR receivers’ errors?   

Doubt field data will support that finding.  

Rather than delve into GDOP and RF scalloping and suchlike amusing thread discussions—I am only claiming the GPS data make a great quick check of the VOR radio performance. 

Just sayin:   If Nav 1 reads 322 TO, Nav 2 says 309 TO, and the GPS says 320, I would pay my Avionics guy to look at #2 radio first.  

You’re right that R-Theta fix accuracy degrades with distance.  That’s probably why few VOR approaches have legs beyond 10 miles away.  

There used to be more approaches where the VOR was not on the airport but those seem to be mostly gone.  Good riddance.  

I well remember flying those with my Instrument instructor.  I’d get to the MAP with the needle centered, take off the hood, and look left and right to find the field.  Sometimes I spotted it.  

Give me a WAAS RNAV GPS approach any day.   

Or for a true wake-up-screaming nightmare—how about a back ADF approach up a valley?  Call it a game of You Bet Your Life your magnetic compass is correct.  

[Bob - S50]

1 hour ago, Jerry 5TJ said:

Just sayin:   If Nav 1 reads 322 TO, Nav 2 says 309 TO, and the GPS says 320, I would pay my Avionics guy to look at #2 radio first.   

If it's off by that much, then yes, the GPS would work well for troubleshooting.

[Jerry 5TJ]

The FAA surveys VOR and ILS approaches every 6 months. The survey aircraft’s validation reference is GPS.