vor signal
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vor signal
hi everyone
going over the notes i am puzzled about the working of the VOR.
Correct me if i am wrong: VOR broadcats 360 radials; each radial signal consists of an un-modulated omnidirectional carrier wave transmitted on the VHF frequency allocated to the VOR between 108.00 and 117.95MHz and constant throughout the compass rose, and of a variable signal obtained by rotating an antenna at 1800rpm 30 revolutions per second generating a subcarrier modulated at 30Hz and the phase of which varies progressively from the carrier as the signal is rotated about the station.
The onboard receiver when tuned to the VOR frequency allocated to the carrier, measures the phase difference between the carrier and the subcarrier determining on which radial the aircraft is and then feeds the data to the VOR indicator providing a pictorial view of the position of the selected radial relative to the aircraft position.
The VOR station transmits a third signal encoded on the carrier on the 1020Hz frequency containing the station identifier and any voice messages, if applicable.
In a conventional VOR or CVOR the subcarrier is modulated in amplitude while in a Doppler VOR or DVOR the subcarrier is modulated in frequency, although from a practical standpoint that doesn’t have any effect on the way the VOR is referenced to.
on an article i read that the carrier is frequency modulated at 30MHz and the variable is amplitude modulated at 30MHz and the two signals are separated by a subcarrier at 10Khz.
My question is: am i misunderstanding the article or actually the carrier is modulated in frequency - in this case the carrier has a frequency which differs from the allocated one by 30Mhz, and the variable is modulated both in frequency at 30Mhz and in amplitude, basically stretching the signal?
Is that how a CVOR works?
Also as far as i know the doppler vors to limit the site errors - cone of confusion and line of ambiguity -, allows the frequency of the signal to increase when moving towards the navaid and to decrease when flying away from it, by transmitting a 30MHZ reference signal surrounded by 50 other signals energized at 30Hz in sequence in a counter-clockwise direction:
how is that translating in an indication that the airplane is approaching the station or moving away from it? and is the doppler reference signal replacing the frequency modulated omnidirectional reference signal of the conventional vor?
is there anyway to find out the type of vor?
pretty trivial questions, just trying to understand how it all works.
many thanks
Baobab72
going over the notes i am puzzled about the working of the VOR.
Correct me if i am wrong: VOR broadcats 360 radials; each radial signal consists of an un-modulated omnidirectional carrier wave transmitted on the VHF frequency allocated to the VOR between 108.00 and 117.95MHz and constant throughout the compass rose, and of a variable signal obtained by rotating an antenna at 1800rpm 30 revolutions per second generating a subcarrier modulated at 30Hz and the phase of which varies progressively from the carrier as the signal is rotated about the station.
The onboard receiver when tuned to the VOR frequency allocated to the carrier, measures the phase difference between the carrier and the subcarrier determining on which radial the aircraft is and then feeds the data to the VOR indicator providing a pictorial view of the position of the selected radial relative to the aircraft position.
The VOR station transmits a third signal encoded on the carrier on the 1020Hz frequency containing the station identifier and any voice messages, if applicable.
In a conventional VOR or CVOR the subcarrier is modulated in amplitude while in a Doppler VOR or DVOR the subcarrier is modulated in frequency, although from a practical standpoint that doesn’t have any effect on the way the VOR is referenced to.
on an article i read that the carrier is frequency modulated at 30MHz and the variable is amplitude modulated at 30MHz and the two signals are separated by a subcarrier at 10Khz.
My question is: am i misunderstanding the article or actually the carrier is modulated in frequency - in this case the carrier has a frequency which differs from the allocated one by 30Mhz, and the variable is modulated both in frequency at 30Mhz and in amplitude, basically stretching the signal?
Is that how a CVOR works?
Also as far as i know the doppler vors to limit the site errors - cone of confusion and line of ambiguity -, allows the frequency of the signal to increase when moving towards the navaid and to decrease when flying away from it, by transmitting a 30MHZ reference signal surrounded by 50 other signals energized at 30Hz in sequence in a counter-clockwise direction:
how is that translating in an indication that the airplane is approaching the station or moving away from it? and is the doppler reference signal replacing the frequency modulated omnidirectional reference signal of the conventional vor?
is there anyway to find out the type of vor?
pretty trivial questions, just trying to understand how it all works.
many thanks
Baobab72
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You probably know this already, but the to/from indication is only based on location of the aircraft, not on in which direction it is going.
So if radial 090 is selected, it shows ,,to'' if the ac is west of the vor.
So the indication just depends on the setting of the OBS and the radial where the ac is on.
Sorry
So if radial 090 is selected, it shows ,,to'' if the ac is west of the vor.
So the indication just depends on the setting of the OBS and the radial where the ac is on.
Sorry
Last edited by Da-20 monkey; 18th Dec 2011 at 14:03.
Erm... the "D" in the ident box means the station has DME capability.
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Sepp:
Exactly.
That "D" for doppler text disappeared quickly.
In the snippet below El Toro and Paradise have DME, Homeland does not.
Erm... the "D" in the ident box means the station has DME capability.
That "D" for doppler text disappeared quickly.
In the snippet below El Toro and Paradise have DME, Homeland does not.
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You have the basic carrier frequency.
This is amplitude modulated with 30 Hz (not MHz) to provide one reference.
Then you AM a 9960 Hz subcarrier onto the carrier frequency. This 9960 Hz subcarrier is in turn frequency modulated with a 30 Hz signal to provide a second reference.
The phase difference between these two 30 Hz reference signals is your received radial.
In a Doppler VOR, the phase of the FM signal depends on your bearing to the station while the AM signal is omnidirectional. In a CVOR, the opposite applies with an omnidirectional FM reference and the phase of the AM modulation changing with bearing to the station. The directional signal rotates different ways though, meaning the receiver doesn't know the difference.
The type of VOR may be stated in the ENR section of the AIP, under radio navigation aids. It is in my neck of the woods, but I know it's not universal.
You need pretty sophisticated gear to detect a difference in the signals. Your best indication would be to guesstimate based on how steady the radial received is, but there are both bad DVORs and good CVORs.
Cheers,
Fred
This is amplitude modulated with 30 Hz (not MHz) to provide one reference.
Then you AM a 9960 Hz subcarrier onto the carrier frequency. This 9960 Hz subcarrier is in turn frequency modulated with a 30 Hz signal to provide a second reference.
The phase difference between these two 30 Hz reference signals is your received radial.
In a Doppler VOR, the phase of the FM signal depends on your bearing to the station while the AM signal is omnidirectional. In a CVOR, the opposite applies with an omnidirectional FM reference and the phase of the AM modulation changing with bearing to the station. The directional signal rotates different ways though, meaning the receiver doesn't know the difference.
The type of VOR may be stated in the ENR section of the AIP, under radio navigation aids. It is in my neck of the woods, but I know it's not universal.
You need pretty sophisticated gear to detect a difference in the signals. Your best indication would be to guesstimate based on how steady the radial received is, but there are both bad DVORs and good CVORs.
Cheers,
Fred
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Nordian does a nice job explaining the theoretical differences between the classic rotating antenna VOR and the up-to-date and more common Doppler VOR:
http://www.nordian.no/demo_files/Rad...ation_demo.pdf
http://www.nordian.no/demo_files/Rad...ation_demo.pdf
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many thanks everyone for your time
let's see if i have gotten it right this time
the CVOR transmist a carrier and the allocated frequency.
The carrier is FM at 30Hz. Why does it need to be FM?
FM refers to how often the signal repeats itself, complete a full cycle, for lack of better word, within one second 30Hz, 30 times per second.
This carrier carries a subcarrier amplitude modulated at 9960Hz and frequency modulated at 30Hz consisting of the variable signal.
AM means that there is another wave having an amplitude, vertical displacement from the position of rest, different than the carrier and carried by the carrier.
The receiver measures the phase difference between the reference signal and the 9960 hz variable signal and detremines the position of the aircraft related to the navaid.
now why do we need to FM the carrier and the subcarrier, wouldnt inserting the 9960 hz signal be enough? and i do beg your pardon for the sillyness of this question, but for the mortals like me having a mental picture of this stuff is pretty hard!!
i am still working on understaanding the DVOR but i will come back to you as soon as i have a clue of what i am talking about!!!
baobab72
let's see if i have gotten it right this time
the CVOR transmist a carrier and the allocated frequency.
The carrier is FM at 30Hz. Why does it need to be FM?
FM refers to how often the signal repeats itself, complete a full cycle, for lack of better word, within one second 30Hz, 30 times per second.
This carrier carries a subcarrier amplitude modulated at 9960Hz and frequency modulated at 30Hz consisting of the variable signal.
AM means that there is another wave having an amplitude, vertical displacement from the position of rest, different than the carrier and carried by the carrier.
The receiver measures the phase difference between the reference signal and the 9960 hz variable signal and detremines the position of the aircraft related to the navaid.
now why do we need to FM the carrier and the subcarrier, wouldnt inserting the 9960 hz signal be enough? and i do beg your pardon for the sillyness of this question, but for the mortals like me having a mental picture of this stuff is pretty hard!!
i am still working on understaanding the DVOR but i will come back to you as soon as i have a clue of what i am talking about!!!
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Found this in my 747-400 course notes - hope it helps!
The receiver circuits process the RF signals from the VOR antenna,
which is routed through the VOR/Localizer rf power divider. The
synthesizer tunes the receiver circuits to the selected frequency.
The theory of VOR navigation includes a measurement of the phase
difference between two transmitted signals. One signal is called the
reference (REF) signal; the other signal is the variable (VAR) signal.
The reference signal is an rf carrier amplitude modulated with a 9960
Hz subcarrier. The subcarrier is frequency-modulated +/-480 Hz at a
30Hz rate.
The variable signal is an unmodulated rf carrier directional signal. It
is rotated through 360 degrees at 30 rotations per second. This signal
fluctuation that is received creates the same effect in the receiver’s
detector as a 30 Hz amplitude-modulated transmitted carrier will.
Phase Comparison
The reference 30 Hz signal and the variable 30 Hz signal are
in-phase when the variable signal is aligned with the magnetic north
(000) radial. An airplane on the radial receives both signals in-phase.
On any other radial, the phase difference is equal to the angle of the
radial from magnetic north.
The receiver circuits process the RF signals from the VOR antenna,
which is routed through the VOR/Localizer rf power divider. The
synthesizer tunes the receiver circuits to the selected frequency.
The theory of VOR navigation includes a measurement of the phase
difference between two transmitted signals. One signal is called the
reference (REF) signal; the other signal is the variable (VAR) signal.
The reference signal is an rf carrier amplitude modulated with a 9960
Hz subcarrier. The subcarrier is frequency-modulated +/-480 Hz at a
30Hz rate.
The variable signal is an unmodulated rf carrier directional signal. It
is rotated through 360 degrees at 30 rotations per second. This signal
fluctuation that is received creates the same effect in the receiver’s
detector as a 30 Hz amplitude-modulated transmitted carrier will.
Phase Comparison
The reference 30 Hz signal and the variable 30 Hz signal are
in-phase when the variable signal is aligned with the magnetic north
(000) radial. An airplane on the radial receives both signals in-phase.
On any other radial, the phase difference is equal to the angle of the
radial from magnetic north.
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This is engineering stuff, not pilot stuff.
It is far more important to understand how to hold correctly as per ATC instructions.
This is a bit like asking the time and having the clock built for you.
It is far more important to understand how to hold correctly as per ATC instructions.
This is a bit like asking the time and having the clock built for you.
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baobab,
I think you need to start with reading up on what amplitude modulation and frequency modulation is. I think it will make many things fall into place!
aterpster,
this is more like somebody asking to have a clock built and someone chiming in (pardon the pun) in the resulting discussion with the opinion that it's watchmaker stuff, when all you really need is to be able to determine at what time tea is served.
Cheers,
Fred
I think you need to start with reading up on what amplitude modulation and frequency modulation is. I think it will make many things fall into place!
aterpster,
this is more like somebody asking to have a clock built and someone chiming in (pardon the pun) in the resulting discussion with the opinion that it's watchmaker stuff, when all you really need is to be able to determine at what time tea is served.
Cheers,
Fred
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Doppler VOR is actually a beautiful piece of physics.
The beacon consists of a ring of antennae and a central one. They are capacitance loaded dipoles on a ground plane reflector to give an optimum radiation pattern.
The central antenna radiates a signal at the frequency of the VOR with a 30 Hz amplitude modulated signal, the beacon ident and any colocated ATIS etc. This forms the reference phase.
The ring of antennae are commutated in such a way that opposing antennae radiate a signal modulated at 9660 Hz, and they switch sequentially so that the pairs rotate around the beacon at 30 rpm.
The ring is 13.6m in diameter, and if you do the math, a distant observer will observe the signals from both antennae, with one approaching and one receding with such velocity that each signal is doppler shifted. The summation at the receiver is that of a frequency modulated 30 Hz variable signal.
The receiver calculates the phase difference between the FM variable and the AM reference 30 Hz signals to extract the bearing information.
The advantage over 'conventional' VOR is that siting errors are very much reduced eg scalping of radials and false bearings. This is because the 'aperture' is much larger for a DVOR than the smaller antenna on a CVOR and so spatial resolution is enhanced.
Think about the objective lens on a big telescope vs small telescope.
As Brian Cox would say - amaaaazing. What a shame we just use Tom Toms these days
The beacon consists of a ring of antennae and a central one. They are capacitance loaded dipoles on a ground plane reflector to give an optimum radiation pattern.
The central antenna radiates a signal at the frequency of the VOR with a 30 Hz amplitude modulated signal, the beacon ident and any colocated ATIS etc. This forms the reference phase.
The ring of antennae are commutated in such a way that opposing antennae radiate a signal modulated at 9660 Hz, and they switch sequentially so that the pairs rotate around the beacon at 30 rpm.
The ring is 13.6m in diameter, and if you do the math, a distant observer will observe the signals from both antennae, with one approaching and one receding with such velocity that each signal is doppler shifted. The summation at the receiver is that of a frequency modulated 30 Hz variable signal.
The receiver calculates the phase difference between the FM variable and the AM reference 30 Hz signals to extract the bearing information.
The advantage over 'conventional' VOR is that siting errors are very much reduced eg scalping of radials and false bearings. This is because the 'aperture' is much larger for a DVOR than the smaller antenna on a CVOR and so spatial resolution is enhanced.
Think about the objective lens on a big telescope vs small telescope.
As Brian Cox would say - amaaaazing. What a shame we just use Tom Toms these days
