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  • 620102

    1)Any update as to what the examiner is after with this one?

    EDIT: ALSO....

    Q 620183

    The DME Line Of Position is a circle with radius:

    "The ground distance and centre the DME-station"

    2)Is this definitely the answer the examiner is after? Thanks

    Last edited by Jakegoat; 02-11-2013, 21:39.

  • #2
    Re: 620102

    The question would help................

    As for the other one, a DME Line of Position is a circle with the DME at the centre and the SLANT RANGE the radius.

    This question has always been a pain and was amended several years ago by the then CAA Examiners to 'Slant Range', however, we have no idea what the current examiners have as the "correct" answer.

    I tell our students to select Slant Range and let me know if the question comes up so we can qualify it with the CAA.

    Best Regards,

    Tony Pike

    Give Sergei back his dignity......Simples!

    Comment


    • #3
      Re: 620102

      Okay great. Sorry I had the question number as title of the thread, probably wasn't clear! It's: 620102

      "Two aircraft are located on the same radial but at (arbitrary) different distances from a VOR-station. Which statement is true?

      --At a certain moment of time, the phase of the reference signals is equal and the phase of variable signals is unequal for both aircraft.
      --At a certain moment of time, both the phase of the reference signals and of the variable signals are equal for both aircraft.
      --At a certain moment of time, the phase of the reference signals is unequal and the phase of variable signals is equal for both aircraft.
      --At a certain moment of time, both the phase of the reference signals and of the variable signals are unequal for both aircraft."

      This is the answer I will most likely put however I have seen on a separate bank they have "both signals equal" as the answer.

      Thanks Tony I appreciate your help especially considering it's the weekend

      Comment


      • #4
        Re: 620102

        Back soon - this is a pain to explain as the examiners have definitely got it wrong this time (not that they'll admit it!!)
        Best Regards,

        Tony Pike

        Give Sergei back his dignity......Simples!

        Comment


        • #5
          Re: 620102

          Thanks Tony

          Comment


          • #6
            Re: 620102

            Before you read this please be aware that this is one of two terrible questions, both of which serve absolutely no logical purpose for the required knowledge for any pilot, let alone professional pilots, and which have, in my humble opinion, been badly constructed by the authors.

            Also, I am only using the CVOR as a reference for the following explanation.

            The VOR operates in the VHF frequency band, using carrier wave (CW) frequencies between 108.00 MHz and 117.95 MHz, spaced at 50 kHz intervals.
            There is a portion of that frequency range which is shared with the Instrument Landing System (ILS); that is 108.00 MHz – 112.00 MHz and within that range the VOR system uses those frequencies where the 100 kHz digit (the first number after the decimal point) is an even number.
            For example, 108.10, 108.15, 111.90 and 111.95 MHz are all ILS frequencies whereas 108.20, 108.25, 111.80 and 111.85 MHz are all VOR frequencies.
            Anything from 112.00 – 117.95 MHz is a VOR frequency.

            Equipment
            There are three main components to the VOR:
            ?Transmitter
            ?Receiver
            ?Display Equipment

            Transmitter
            The transmitter transmits two signals, referred to as the:
            ?Reference Phase
            ?Variable Phase (Variphase)
            These are each transmitted on a sub-carrier wave, +/- 9,960 Hz from the main carrier wave.
            Each of these sub-carrier waves is modulated with an additional signal, modulated to differentiate one from the other. In the case of a Conventional VOR (CVOR) these are:
            ?Reference Phase = Frequency Modulated (FM)
            ?Variable Phase = Amplitude Modulated (AM)
            In both cases, the frequency of the modulation is 30 Hz, which means that each modulated signal completes 30 complete cycles or waves in one second. Each complete wave passes through 360° of phase; as such, the modulated signal passes through 360° of phase 30 times per second.

            Calculation of Bearing
            The majority of VOR transmitters are aligned with Magnetic North.
            A magnetic bearing from a VOR is referred to as a RADIAL. Magnetic North is 000°M or the R-000 radial.
            The transmitter provides information which allows the VHF Nav receiver to determine which radial the aircraft is on at any given point in time.
            That calculation is made by calculating the difference in phase between the Reference and Variable Phase signals.
            The Reference Phase is OMNIDIRECTIONAL which means that the same information is transmitted on all bearings at exactly the same time.
            The Variable Phase is DIRECTIONAL, which is achieved by rotating the transmitter through 360°, 30 times per second.
            Therefore; the modulation of 30 Hz corresponds with the antenna rotation of 30 times per second, which in turn means that the signal is modulated through 360° of phase at the same time as it passes through 360° of rotation, 30 times per second.
            This can be further interpreted as each bearing being determined by the number of degrees of phase of the Variable Phase signal.
            However, the receiver needs something to compare the Variable Phase signal to and this is where the Reference Phase signal comes in.
            On a bearing of Magnetic North from the VOR the Variable Phase antenna is aligned so that the Reference and Variable Phase signals are at the same point of phase in the 360° cycle, but on a bearing of 001°M the Variable Signal is 1° of phase behind (lags) the Reference Phase, on a bearing of 002°M it is 2° behind and so on through all 360°.
            By comparing PHASE DIFFERENCE the receiver can calculate which radial the aircraft is on.
            Of course, the receiver needs to know which signal is leading and which is lagging. If the Variable Phase is lagging the Reference Phase by 90° it stands to reason that this could be interpreted as the Reference Phase leading the Variable Phase by 270°. If the receiver could not identify which signal is which, this may be interpreted as either R-090 or R-270. Hence the reason that the signals are modulated differently; the receiver identifies the Reference Phase as Frequency Modulated and the Variable Phase as Amplitude Modulated and the amount by which the AM signal lags the FM signal identifies that radial.
            Each signal will pass through 360° of phase repeatedly on each bearing but it is the Phase Difference between the two signals that identifies the radial.

            To Be Continued...........
            Last edited by Tony Pike; 04-11-2013, 20:37.
            Best Regards,

            Tony Pike

            Give Sergei back his dignity......Simples!

            Comment


            • #7
              Re: 620102

              Continued from previous post..........

              Now, let us look at a scenario of two aircraft on the R-090 but at different distances.

              Each aircraft, at any point in time, will receive a Reference Signal and a Variable Phase signal, the latter of which will lag 90° behind the former, identifying the aircraft?s position as being on the R-090.
              It could be that Aircraft A and Aircraft B are receiving signals which are at the following points of phase:

              ?A – RP = 000° and VP = 090° (Radial = R-090)
              ?B – RP = 050° and VP = 140° (Radial = R-090)

              It could equally be as follows:

              ?A – RP = 000° and VP = 090° (Radial = R-090)
              ?B – RP = 000° and VP = 090° (Radial = R-090)

              The correct answer is completely down to interpretation of the above. The Reference Phases may be equal for both aircraft or they may be unequal for both aircraft.

              It depends on all cases on the distance of the aircraft from the transmitter as different distances correlate with different points of phase in the cycle.

              Let us look at the supposed correct answer:

              "At a certain moment of time, both the phase of the reference signals and of the variable signals are unequal for both aircraft."

              That suggests that each Reference Phase signal is different to its corresponding Variable Phase signal and that would be the case on (nearly) all radials?..provided that the distance between the aircraft did not correspond to a number of degrees of phase exactly divisible by 360. If that was the case the Reference Phase of each aircraft would be equal, as would the Variable Phase.

              Also, it still does not take into account the two aircraft being on a bearing of Magnetic North when the Reference and Variable Phases will be equal at one aircraft but POSSIBLY (not definitely) different from the other aircraft. If the distance between the aircraft corresponded to a number of degrees of phase exactly divisible by 360, the following is also correct:

              "At a certain moment of time, both the phase of the reference signals and of the variable signals are equal for both aircraft."

              ?A – RP = 000° and VP = 000° (Radial = R-000)
              ?B – RP = 070° and VP = 070° (Radial = R-000)

              Or even:

              ?A – RP = 145° and VP = 145° (Radial = R-000)
              ?B – RP = 145° and VP = 145° (Radial = R-000)

              This is very confusing and the questions are absolute nonsense - I think the examiner is only considering phase difference. I suggest you simply learn the answers to each of them.
              Best Regards,

              Tony Pike

              Give Sergei back his dignity......Simples!

              Comment


              • #8
                Re: 620102

                Wow that is confusing. Thanks though I appreciate you showing me this.

                Think I'll just remember that both are unequal for same radial different distance and hope this is the last time I see that question!!

                Cheers once again, enjoy the rest of your weekend

                Comment


                • #9
                  Hi Guys

                  I was working through this question and was also shocked that my answer (same as you guys) was shown as incorrect.

                  If you look at the question from two angles then I think the answers could be either of what we think is the right one or the one that is shown as right. As follows:

                  1. If we look at it from reference to the VOR, then; if you are on the 090 radial then the phase of the reference signal and the variable signal will be unequal, and this will be the case for both aircraft who are on the same radial. The only time they will all be equal is on the 360 radial.

                  2. If you look at it from the aircraft perspective, then; if they are both on the same radial then they will both receive the same reference phase as well as the same variable phase and thus both are equal for both aircraft.

                  There is another question floating about where the aircraft are on two different radials and the answer is that both reference signals will be the same but the variable signals will be different. This answer leads me to believe that the examiner is referencing it to the aircraft. If it is correct, then you would imagine that the answer should be the 2nd one above.

                  Has this question been resolved at all?

                  Cheers

                  Comment

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