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Thread: Navaids

  1. #1
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    Navaids

    Does anyone know the acceptable error for a VOR station? How about ADF?

    If you add this to the airplane's intrument error and the pilot's induced error things could get pretty bad.

    The reason I ask is that I was heading to a fix via VOR today and the GPS didn't agree with the VOR. I did a VOR check at the beginning of the day, but it was one against the other so the VOR could be the cause.
    Is it possible for the VOR and GPS database to have waypoints that differ by as much as couple of miles? We were close to the station and the radials were at 90

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    Maybe if you can give some details (e.g. what's the fix, what radial were you on, how were you identifying the fix in the GPS), there may be some clues.

    My guess is that VOR transmission is fairly accurate, but the receiver/CDI can easily be off by a few degrees, and VOR radials don't correspond with GPS bearings due to magnetic drift. It's also extremely easy to mess up a setting on the OBS, e.g. to dial in 098 when the chart says 089, or something like that.
    -harry

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    Quote Originally Posted by PaulThomas View Post
    Does anyone know the acceptable error for a VOR station? How about ADF?

    If you add this to the airplane's intrument error and the pilot's induced error things could get pretty bad.

    The reason I ask is that I was heading to a fix via VOR today and the GPS didn't agree with the VOR. I did a VOR check at the beginning of the day, but it was one against the other so the VOR could be the cause.
    Is it possible for the VOR and GPS database to have waypoints that differ by as much as couple of miles? We were close to the station and the radials were at 90
    Victor airways are designed with a primary obstacle clearance area of 4 NM based upon system accuracy of +- 4.5 degrees from the NAVAID and a secondary clearance area of 2 NM based upon system accuracy of +-6.7 degrees from the NAVAID. Based upon the accuracy of your VOR receivers, the CDIs and your distance you may be several miles from the actual Lat/Long of the intersection. The accuracy of GPS navigation verses VOR navigation is the reason that GPS MEAs have started being published on some routes.

  4. #4
    UNO Av8R Guest
    Quote Originally Posted by PaulThomas View Post
    Does anyone know the acceptable error for a VOR station? How about ADF?
    The first chapter in the AIM deals with navaids and under the section of VOR says

    "the accuracy of course alignment of the VOR is excellent, being generally plus or minus 1 degree"

    That, of course, is for the transmitting ground station.

    The accuracy of VOR recievers is a different matter, and depending on the method used to check can be as much as 4 degrees.

    14 CFR 91.171 deals with the specifics for VOR uquipment checks.

    Here's the summary:

    Ground VOT or VOR check +/- 4 degrees
    Airborne VOR check +/- 6 degrees (yikes)
    Airborne check using a Victor airway +/- 6 degrees
    Dual VOR check maximum 4 degrees difference

    You also have to keep in mind when doing airborne checks that certain prop RPM settings can cause pretty significant CDI errors.


    Because of the nature of NDB's (the frequency range they operate in) they are susceptible to a lot more interference, like lightning, precipitation, static, etc. When you combine that with the fact they don't have a flag or any way to let you know (other than continuous identification) they ar emessed up, it's easy to see why they are being phased out.

    Quote Originally Posted by PaulThomas View Post
    .
    Is it possible for the VOR and GPS database to have waypoints that differ by as much as couple of miles?
    Are you sure the GPS database is up to date?
    Last edited by UNO Av8R; 04-18-2009 at 11:31.

  5. #5
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    The intersections are SERF and JOCKS using RSW and LBV vor. The GPS is up to date and it was just on the moving map. We were not trying to identify the intersection via GPS but it was surprising to see it pass by well before the VOR agreed.

    I should have checked the DME readout.
    Last edited by PaulThomas; 04-19-2009 at 12:27.

  6. #6
    krush Guest
    GPS>>>>>>>>> VOR any day.

    I prefer to use both, and if they don't agree I'd prolly trust the GPS more.

  7. #7
    UNO Av8R Guest
    Quote Originally Posted by krush View Post
    GPS>>>>>>>>> VOR any day.

    I prefer to use both, and if they don't agree I'd prolly trust the GPS more.
    Depends on the GPS. I was under the assumption the GPS being used is panel-mounted and IFR certified, meaning it has RAIM capability. If that's the case I'd trust it. If it's VFR-only or a hand-held I'll take the VOR.

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    My handheld GPS is waas-capable, and accurate to within tens of feet.

    If you're 20 miles from the VOR (as we appear to be in this example), and your CDI is off by 4 degrees (and thus still within specs for IFR flight), your indication of the cross-radial will be off by 1.3 miles.
    -harry

  9. #9
    krush Guest
    Quote Originally Posted by UNO Av8R View Post
    Depends on the GPS. I was under the assumption the GPS being used is panel-mounted and IFR certified, meaning it has RAIM capability. If that's the case I'd trust it. If it's VFR-only or a hand-held I'll take the VOR.
    If somebody told me I had only one choice and had to choose between the two, i'd pick even the cheapest junkiest GPS over a VOR.

  10. #10
    jfitzpat Guest
    Quote Originally Posted by krush View Post
    If somebody told me I had only one choice and had to choose between the two, i'd pick even the cheapest junkiest GPS over a VOR.
    I had an experience were a phone briefing turned up nothing, but DUATS turned up a NOTAM about potentially GPS unreliability because of testing at China Lake.

    My habit is to use everything always, so we had both a VOR and a Bendix King GPS set to Filmore VOR. Under the hood, my companion was just scanning the GPS.

    I pointed out, "what's the VOR doing?" (showing station passage). "I dunno, we're still over 9 miles away, broken?" "Take off your hood and look down" (station clearly below us). "What's 9 miles ahead?" "Those mountains maybe..." The scary part was that the GPS otherwise appeared perfectly normal.

    Personally, I like having redundant navigation, using different technologies. At runup I generally do 'nearest' VOR with the GPS, then set the VORs to match and compare bearings.

  11. #11
    jfitzpat Guest
    Quote Originally Posted by hm View Post
    My handheld GPS is waas-capable, and accurate to within tens of feet.
    Subject to view and geo. of the sats., and distance and atmospheric gradient from the nearest station...

    Don't get me wrong, I love GPS. But there have been at least two times I can think of that a handheld GPS was worthless and I was @#$% glad to have a chart and a good old fashioned compass.

  12. #12
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    GPS is great, but the signal is weak and is subject to jamming, intentional and accidental. I have had a few cases of cell phones jamming a GPS signal on an airliner. I also had two cases in the Army where the navigating pilot relied on a VFR GPS for navigation that went on the blink without giving us any indication and it caused problems. The first case resulted in a fuel emergency when I realized the other pilot did not know where we were and the GPS was wrong. The other case resulted in our flying over a hostile town where there was a fire fight going on when the navigating pilot blindly followed an erroneous GPS. If it is a VFR GPS, trust but verify.

  13. #13
    krush Guest
    And ya'll have never had a VOR crap out on you or give unreliable readings?

    Nothing is perfect, and I never said anything was.

  14. #14
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    Eh, the first time I ever used the panel-mounted VFR GPS in my club's Cherokee 140, it crapped out on me in the middle of Nowhere, Ohio. Fortunately I was near the Turnpike and just followed that home.

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    Quote Originally Posted by krush View Post
    And ya'll have never had a VOR crap out on you or give unreliable readings?

    Nothing is perfect, and I never said anything was.
    Don't get me wrong. I love GPS. But I've never had a VOR let me down like GPS.


 

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