VOR&NDB navigation (about dual needle RMI)

[sticky_beaver]

Hope this is the right subforum, as there is no NAVIGATIONAL topic on this forum.

NDB transmitter transits none directional signal and onboard equipment determine the direction to it so it has directional antennas that is mechanicaly or electronicaly rotated and where the signal is most powerfull there is the direction towards NDB (it really measures weakest signal and add 180°)

And VOR transmitter has two antennas one thas in none directional and second directional that is electronical rotated for 360° - meaning that there is 360 radials each representing one degree of a circle. None directional antenna transmits the VHF signal only when the radial is passing 360 or 0°. And on board VOR equipment count the time between directional and nondirectional signal thus determine on which radial you are.

Back to the thread...
I played FSX where you have planes with dual needle RMI (I know that FSX is simulation and not a reall thing), but beside ADF needle - showing the direct direction to engaged NDB, there are also a second needle showing direct direction to VOR transmitter not second NDB.

So how can RMI's VOR needle shows direction to VOR? Does it work the same as ADF, and you need extra antenna?

[hetfield]

Die NDB Nadel zeigt immer zur Station, egal was die Kompassrose (RMI) macht!
D.h., relative bearing stimmt immer!

Die VOR Nadel ist dagegen auf der Kompassrose (RMI) "zementiert". D.h. egal was die Rose macht das angezeigte Radial stimmt immer.

[CHfour]

I think the needle is positioned to reflect the relative bearing in the display which will show the QDM if the needle head is read against the compass card. Aa far as I know the relative brg for the VOR needle is just processed info. The QDM/QDR is known from the VOR receiver info so it's not difficult to calculate the relative brg if the hdg is known. Now, how the 737 IRS computes the true hdg without any magnetic input is IMHO the really impressive bit!

[sticky_beaver]

by CHfour:
"The QDM/QDR is known from the VOR receiver info so it's not difficult to calculate the relative brg if the hdg is known."

I don't see what VOR has with magnetic bearing (qdm/qdr)? VOR just receives two signals - one electronicaly directed to cover 360 radials and other none directed - VOR receiver just calculate the time diference betwen two signals - this gives you the info on whic radial you are - then a VOR gauge shows the deviation from the preselected radial. mining that if your VOR receiver calculates that you are on 90° radial and you have selected 100° radial, than you are 10° from the radial, and by DME you can determine right-angled lenght to the selected radial. This is how I understand the VOR.

"Now, how the 737 IRS computes the true hdg without any magnetic input is IMHO the really impressive bit!"

IRS(it is more known as INS) gets true nort by a gyrocompas or directional gyro. you can reed how it works on the web.

by hetfield:
"Die VOR Nadel ist dagegen auf der Kompassrose (RMI) "zementiert"."

HSI - combined VOR and DG and RMI in combined ADF and DG.
VOR or ADF needle on both swings independantly of the rose.

and jet none did answer the question. it was how dual needle RMI - VOR needle show direction towards VOR station. I reayl don't know it dual needle RMI has one VOR needle, because on the web I can only find data about dual ADF needle RMI - one needle for one ADF freq.

how I understand the vorking of VOR receiver it doesn't have directional antenna like ADF, so how can one determine direction towards VOR station. (don't mix radial bearing with relative bearing!)

[CHfour]

by CHfour:
"The QDM/QDR is known from the VOR receiver info so it's not difficult to calculate the relative brg if the hdg is known."

I don't see what VOR has with magnetic bearing (qdm/qdr)? VOR just receives two signals - one electronicaly directed to cover 360 radials and other none directed - VOR receiver just calculate the time diference betwen two signals - this gives you the info on whic radial you are - then a VOR gauge shows the deviation from the preselected radial. mining that if your VOR receiver calculates that you are on 90° radial and you have selected 100° radial, than you are 10° from the radial, and by DME you can determine right-angled lenght to the selected radial. This is how I understand the VOR.

I thought your question was about the mechanics of the VOR needle on the RMI. If your aircraft was on (say) the 270 radial from the selected VOR then the tail of the associated needle would be placed over the 270 mark on the compass card. No extra info or antenna is reqired as the radial (QDR) is known and the heading so the two are combined for display on the RMI.

[eckhard]

Gruss Gott, Sticky Beaver!

The VOR receiver in the aircraft is able to determine which radial it is on, in the manner you describe.

This information can then be displayed to the pilot in several ways:

for example, in some radio tuning displays, the actual radial you are on is always displayed as a numerical value.

In other displays, the angular displacement from a selected radial is possible (classic OBS/CDI needle).

In the RMI, the radial you are on is displayed by the tail of the VOR needle against the RMI card, as hetfield and CHfour already explained. The head of the needle therefore points to the station. It is not actually a 'direction finder' like the ADF needle, but is displaying the numerical value of the sensed radial in an analogue format.

As hetfield says, the VOR needle is 'cemented' to the RMI card (assuming the aircraft' position is constant), in other words the needle will point to the same numerical value on the card, no matter what heading you are on, and no matter if the card is incorrectly synchronised with the magnetic heading. This is because it is displaying the radial that the VOR receiver is sensing, NOT any kind of relative bearing.

The ADF needle on the other hand directly senses the direction of the NDB, again as you described, and so displays the correct relative bearing, no matter what figure the RMI card says underneath it. (I'm ignoring ADF errors to keep it simple.)

In most modern electronic displays, the pilot can select two pointers to overlay on the main Nav Display. There is normally a 'single' pointer and a 'double' pointer and either of them can be selected to display ADF or VOR signals.

CHfour, as far as the IRS sensing North without any magnetic input is concerned, my understanding is as follows:

1. You initialise the IRS by telling it your lat and long.
2. The laser gyros and accelerometers sense the movement of the IRU caused by the earth's rotation.
3. The IRS knows that the earth rotates from true west to true east.
4. It therefore knows which direction is east, relative to a 'fore and aft' longitudinal datum set by the manufacturer on installation.
5. It therefore calculates the true heading of the aircraft.
6. It has an internal database of magnetic variation and it applies a correction to the true heading, appropriate to its geographical position (which it knows from step 1.)
6. It displays this magnetic heading to the pilot.

Hope this helps!

:8