VOR question
 

How does the VOR receiver know that you are on the "To" or the "From" radial. I suspect the answer is simple and I just have a vapor lock here...

Oh my God it's so long ago:*, but I'll give it a try.

With the CDI/HSI course selector one out of 360 radials is selected.

If the relative bearing of your compass is < 90° the TO pointer appears, so if RB > 90° the FROM pointer shows up. So it all depends on your HSI /CDI, I guess:ouch:

Hope this helps.

To or From
 

It knows what radial you are on by the time delay from the "central" pulse until you receive the individual radial pulse. The "To/From" indication is derived from your heading and the radial you are on. When you select a heading/radial, they it knows whether that heading will take you to or away from the station.

It simply knows which radial you are on. If you turn the OBS or HSI to within 90 degrees of the radial you are on it will tell you you are FROM the station. If you set the reciprocal of your present radial or any setting with 90 degrees of the reciprocal, it will say TO.

The airplane's heading does not matter.

Ok, I think I have it. As the beam sweeps around and hits the VOR antenna. if the receiver detects that the OBS setting and the beam heading are the same, it activates the "From" indicator. If it detects a 180 degree difference, it activates the "To" indicator. I understand that there's a little more to it to get the "To"-"From" indicators to handle the whole half-circle of headings, but now I see the basic mechanism.

It's all a pretty brilliant system given it's age and that it could be implemented with a box of vacuum tubes.

Thanks.

@MarkerInbound & grumpyoldgeek

You are right, my mistake.

If you turn the OBS or HSI to within 90 degrees of the radial you are on it will tell you you are FROM the station. If you set the reciprocal of your present radial or any setting with 90 degrees of the reciprocal, it will say TO.

a little correction here, if you are +/-80* within the selected OBS radial then "from" is displayed and if +/-80* of the reciprocal of the OBS radial then "to" is displayed.
this leaves a gap of 40*. i.e.10* of either side of +/- 90* of obs radial. if the aircraft is in this zone then you get both or none flags display, depending on the instrument.
hope this helps.

As MI says, IF you are ON the 180 radial (as read by the VOR receiver), then in broadbrush terms the box knows that an OBS/Course setting of 360 will be 'TO' and 180 'FROM' regardless of which way you are pointing at the time. It is that simple.

If you are a pilot. Unfortunately I'm an engineer and with something like this I have to literally simulate it's operation in my mind to fully understand it. In any case, I've got it now. I could even build one. Thanks all.

- I don't see that has anything to do with it. Let's say your company HQ is to the south of San Francisco - is North TO or FROM the City for you? It matters not which way you face ('Heading') - this is purely POSITION.

The way I tough it to my students:

"If you flew the same track as the course you selected in the OBS, would you be flying TOwards the VOR or away FROM it? That is the answer the OBI gives to you in the form of a TO or FM flag."

For instance if you are in a radial south east of the VOR and select the 360º course, the OBI will show a TO flag, because if you fly with a north heading, you will ge closer and closer to the VOR. Until reaching the 090 radial, at which point the flag would change to FM, because then you would be flying away from the VOR.

The same logic can be used for the CDI.

I recall that the TO /FROM meter is really an "ambiguity meter".

all radials are "from" the station.

center the needle with the obs with a ''from''indication and you have the radial you are on.

selecting a course to the station would demand the reciprocal.

locate with the from...go to the station with a to...and that is to preclude reverse sensing.

Hope this helps:

The rotating signal is fed into a calibrated phase shifter which is controlled by the OBS on the front of the instrument in the cockpit. It is turned until the two signals are in phase and the CDI is in the centre.

The TO-FROM indicator is driven by another phase shifter and phase detector operating in parallel. Because of the nature of VOR transmissions and the way they are used for direction finding, there is a 180° ambiguity, so the CDI is equally sensitive to signals coming from either of two opposite directions (i.e. two radials, 180° apart, from the same VOR). To resolve this an additional circuit indicates TO or FROM with a flag. The reference signal is shifted by another 90° and compared again to the rotating one, to tell whether it is leading or lagging the rotating signal, to make the indicator show the relevant direction.

Phil