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!
