The "why" about VORs

Discussion in 'Instruments / Avionics / Electrical System' started by JMillar, May 18, 2008.

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  1. May 18, 2008 #1

    JMillar

    JMillar

    JMillar

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    Why is it, that VORs are set up the way they are? I'm not (yet) a pilot far less instrument rated, so I don't really understand it all that well (not having seen one working "in the flesh"), but it seems wierd to me.

    As I understand it, you set the angle from the station to you that you want to achieve (radial), and then fly on a course that intersects that radial. You know when you reach it from the movement of the needle. Then you follow the course, adjusted for wind. If you need an intersection, you aim to hit one of them so you know which side of the other one you're on, and then follow it to the intersection.

    On the other hand, why not set up the dial to actually read out the radial you're CURRENTLY intercepting? With a "bug" to mark what you're aiming for? Would make it a lot easier to know where you are, in my opinion. I suppose you could do the same thing by just setting the dial till the needle centers, but why should you have to?

    I know it doesn't really classify as homebuilt-related, but if this makes sense, my airplane will have one of my own design :D.
     
  2. May 18, 2008 #2

    PTAirco

    PTAirco

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    A digital VOR does just that - simply tells you what radial you happen to be on.
     
  3. May 18, 2008 #3

    Midniteoyl

    Midniteoyl

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    One word:

    GPS :)
     
  4. May 18, 2008 #4

    Joe Fisher

    Joe Fisher

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    When they first invented the VOR systom in the 1940s they used a syncronus motor to detect the faze angle it ran o pointer that displayed the radial that the airplane is on. And if you fly a hi$ airplane with an HSI you will find the radial indicator built into the DG and the course deviation indicator that you are learning about all in one insterment . In the little traning planes and most home builts it is better that the engine coust more than one of the airplanes insterments.
    Joe
     
  5. May 18, 2008 #5

    JMillar

    JMillar

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    Thanks guys. Well, PTAirco, your signature line sure did fit with that one.

    Oyl, I know that they're not really important any more. Tell that to the FAA. I guess you could call it somewhat of a historical question if you want. I'm working on a back-of-the-napkin design for a integrated nav system (uses a combination of GPS, VOR/NDB, and INS, using the radio nav to backstop the GPS, and the INS to double-check for sanity, smooth the results, and provide a certain amount of short-term data if you lose the GPS).

    Incidentally, I just found an interesting page about VORs that seems to make a lot of sense http://www.campbells.org/Airplanes/VOR/vor.html.
     
  6. May 19, 2008 #6

    Dana

    Dana

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    VOR is what it is because it was the best solution available with the technology of the day when it was implemented... and it was far better than the ADF technology it replaced (are there even any ADF's in service any more? When I was learning to fly in the 70's the main use of ADF was to tune it to an AM broadcast station and have music while flying.)

    Rather than using VOR/INS for redundancy, why not simply use dual GPS receivers?

    -Dana

    I only drink to make other people more interesting.
     
  7. May 19, 2008 #7

    JMillar

    JMillar

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    Um, cause it's there, and it's cool to do?? Good question.
    My theory is that if something external interferes with the GPS, it'll interfere with both of them equally. I suppose there aren't any realistic reasons for this to happen - USA decides to turn it off or degrade it (they can't cause so many people depend on it now...), someone jams it somehow (Yeah, that'll happen... and still leave me alive and flying), the system just fails for reasons unknown possibly including sabotage, atmospheric disturbance - none of them are very convincing.

    Since GPS does have a certain element of error, and signal loss isn't really that unusual (mind you I'm used to ground applications where things get in the line of sight! :D), plus refresh rate is limited, the INS provides interpolated and smoothed data.


    Oh - maybe this can justify it - to legally fly IFR, you have to be able to use VORs do you not? Might as well build it in.
     
  8. May 19, 2008 #8

    RacerCFIIDave

    RacerCFIIDave

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    Just an aside...a bit of technical info most might not know...

    The "nuts and bolts" of how VOR works...

    There are 2 co-located transmitters...

    1 a sweeping focused beam...that rotates every 1/60 sec...

    2 an omnidirectional flash...that flashes every time the sweeping beam passes North...

    So...if your reciever picks up the paired signals together...you are due North of the Station...

    If there is 1/120 sec between flash and sweep...you are due South of the Station...etx...

    Kinda cool especially for the day...


    Dave
     
  9. May 20, 2008 #9

    RacerCFIIDave

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    Oh yeah...BTW Dana... the 172 I teach out of most still has an ADF...and I still use it and teach it...LOL

    It is so much fun to watch someone that has learned exclusively in late model glass panel planes come and fly with me in something old...put 'em under the hood and make 'em do an NDB approach to mins...:gig:

    I used to have a Tri-Pacer that had 2 ADFs.... that was 25yrs ago though...lol

    Dave
     
  10. May 20, 2008 #10

    RacerCFIIDave

    RacerCFIIDave

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    I don't really think that the possibility of FedGov shutting down the GPS constellation is so far fetched...in the case of serious civi unrest it sure would make sense for some totalitarian scum to shut it down...

    Because...it would sure be easy for a few aeronautical engineers to build a series of lovely surface-to-surface missiles...using handheld GPS units and video cameras for guidance...

    If I think of it right off the top...don't ya think those yutzes at DHS might eventually...they arent the brightest bulbs in the box...but likely not completely stupid...???

    Dave
     
  11. May 20, 2008 #11

    Midniteoyl

    Midniteoyl

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    There was a plan at one time to 'shut down' the GPS system in time of war by using encryption. Remember the deliberate 'error' in civilian units?
     
  12. May 20, 2008 #12

    JMillar

    JMillar

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    Yeah, it is possible they might do it. But, I think that navigation is becoming so dependent on GPS that the decision to suspend it, or even degrade the signal again, would be very much a last resort and not taken lightly. There would likely be tragic consequences and a major public backlash.

    Interestingly, it turns out that during Iraq 1.0, the selective availability was turned off, because the military was using huge numbers of civilian grade systems - opposite to the original intent of SA.

    Supposedly, there is a system in place to selectively "turn off" access in certain areas, so that the global SA system is not needed. Future GPS satellites will not support SA, although we don't know what other use-control features they may have.
     
  13. Jun 1, 2008 #13

    wsimpso1

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    VOR's are actually easy, but they ain't taught that way. A centered needle with a TO indication will will give you the heading (before crosswinds correction is added) to fly to the station. A centered needle with a FROM indication is the basic heading to fly to go away from the station. If you want to fly a particular bearing, adjust course to keep the needle centered. It may be raw data flying, but it works fine, is the same in all airplanes, and remains cheap and reliable.

    Want to go direct? KNOW where you are and go deduced reckoning. If you have gotten disoriented, yeah, the GPS box will give you instant orientation. No GPS? Get bearings off of two VOR's, plot your dead reckoning course, take a swag at the cross wind correction (you did get winds aloft before departure didn't you), and set your course. If you can see the ground, pilotage should keep you updated, and you can periodically take another pair of VOR bearings to further check.

    I own part of a 1980 Archer II, and have traveled from Michigan to Florida several times, Colorado three times, Virginia (two or three times a year), Oshkosh (averaging over one time a year), Massachusetts twice, Texas twice, and Seattle through the Canadian Rockies. I have done all of that with 2 VOR's, an ILS reciever, an NDB and a frequently less than useful Loran. In Canada, I was faced with having to change my Custom's appointment mid flight or flying an approach that had two NDB's in a valley because the ILS and VOR approaches were out of service. I flew the NDB approach. I had deliberately practiced NDB approaches because they are still widely used elsewhere in the world. I have never felt under equipped with that airplane, but flying with other folks who spend their whole trip looking at the GPS, UGH.

    So, there is more than one way to look at this stuff. I like looking out the window and keeping my thumb on the chart...

    Billski


    While flying direct with an GPS is nice, everytime I fly with someone who has one of the gadgets running, they spend an inordinate amount of time looking at the thing. Jeez, if I wanted to watch an electronic gadget, I could have flown a flight simulator.

    Then, if you are flying IFR, the GPS has to be IFR certified, the subscription to keep it current costs several hundred dollars a year, you still have to have charts of some type, and now you had better not make any mistakes setting up your route or altering your destination for weather. Thank you much, I like being able to look at the chart, put the right four digits into two boxes, and KNOW in a few seconds where I am and which way to get on to my destination.
     
  14. Jun 1, 2008 #14

    PTAirco

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    The best and cheapest way to understand and use VOR's effectively: MS Flight Simulator. Seriously. Dial in some local VOR's go into slew mode (Y) and watch the needles respond, play around with it and it all makes sense after a while without burning up fuel and instructor time.
     
  15. Jun 24, 2008 #15

    djschwartz

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    VORs work this way because when they were invented the normal navigation procedure for the previous systems was to fly from station to station. Thus, they were designed to operate by allowing you to set the course from one station to the next (the radial) and then use a left/right needle indication to stay on course. The idea of using triangulation from multiple stations to get a "fix" came later.
     

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