VOR indicator flag

D

Dixiebluesky

Pre-takeoff checklist
Joined
Feb 25, 2020
Messages
166
Display Name
Dixiebluesky
Is anyone able to tell me just what a VOR receiver is using to drive its TO/FROM flag? I am as far from a genius in the inner workings of radio receivers as you can be. If the gap in the sine waves is different for each radial from 360 back around to 359 I don't see how it would work as far as the TO/FROM is concerned.
 
Google says:

How the Receiver Calculates TO/FROM
    • Phase Comparison: The ground station sends out a reference pulse and a signal that spins around like a lighthouse. Your plane's receiver looks at the timing difference between these two signals to find your exact magnetic bearing (radial) from the station. [1, 2, 3, 4, 5]
    • Course Matching: The computer inside the indicator compares your actual physical position to the course number you twisted into the knob. [1, 2]
    • The Result: If the course you dialed points toward the station relative to your position, it shows TO. If it points out behind you into open space, it shows FROM. [1, 2]

Key Rules to Remember
    • Independent of Heading: Your airplane's actual nose direction does not change the TO/FROM reading; it only cares where you are located compared to the station.
    • TO Indication: Means flying the selected number will drive you closer to the VOR tower.
    • FROM Indication: Means flying that number will lead you outward along that radial, away from the VOR tower.
 
It's far easier than you think, conceptually.

First, of course, you are only ever on ONE radial. Say you're directly north of the station. You are on the 360 radial. The receiver is able to figure that out. It's internal phase comparison results in one and only one result, the 360 radial.

So if you spin the OBS around to 360, it shows FROM, meaning that's the radial you are on. If instead, you spin the OBS to 180, that is the opposite of the radial you are on, so it shows TO.

The receiver itself doesn't know anything about which way you're going or which way you want to go. It just shows the radial you're on with a FROM if you have it dialed in, or a TO if you've set the opposite.

It may be easier to think about if you realize that, electronically, ALL radials are FROM the VOR. The whole TO thing is just a concession to humans brains to ease navigation.
 
RussR said:
It's far easier than you think, conceptually.

First, of course, you are only ever on ONE radial. Say you're directly north of the station. You are on the 360 radial. The receiver is able to figure that out. It's internal phase comparison results in one and only one result, the 360 radial.

So if you spin the OBS around to 360, it shows FROM, meaning that's the radial you are on. If instead, you spin the OBS to 180, that is the opposite of the radial you are on, so it shows TO.

The receiver itself doesn't know anything about which way you're going or which way you want to go. It just shows the radial you're on with a FROM if you have it dialed in, or a TO if you've set the opposite.

It may be easier to think about if you realize that, electronically, ALL radials are FROM the VOR. The whole TO thing is just a concession to humans brains to ease navigation.
Click to expand...

It is unfortunate that they labeled those flags with TO and FROM. The arrow pointer is far more meaningful. It points to the hemisphere of headings where the station lies.
 
sarangan said:
It is unfortunate that they labeled those flags with TO and FROM. The arrow pointer is far more meaningful. It points to the hemisphere of headings where the station lies.
Click to expand...

Using the flag and needle together isolates your aircraft's location down to a specific quadrant
 
Dixiebluesky said:
Is anyone able to tell me just what a VOR receiver is using to drive its TO/FROM flag? I am as far from a genius in the inner workings of radio receivers as you can be. If the gap in the sine waves is different for each radial from 360 back around to 359 I don't see how it would work as far as the TO/FROM is concerned.
Click to expand...
It's simple logic, not part of the VOR signal. The TO/FROM flag depends upon the OBS setting. If the OBS is set within +/- 90 degrees of the current radial, the flag shown is a FROM flag. Otherwise, the flag shown is a TO flag. (If the OBS is set exactly 90 degrees from the current radial, the flag shows OFF or blank.)
 
Clip4 said:
Using the flag and needle together isolates your aircraft's location down to a specific quadrant
Click to expand...

The CDI and flag does, but TO and FROM doesn't. Both up and down arrows should ideally say "TO", and the tail of the arrow should say "FROM". Or better yet, don't say anything. Just the pointer is enough.
 
sarangan said:
The CDI and flag does, but TO and FROM doesn't. Both up and down arrows should ideally say "TO", and the tail of the arrow should say "FROM". Or better yet, don't say anything. Just the pointer is enough.
Click to expand...
Options, TO, TO, and the RW bar is not a correct representation.
 
It's done by comparing the position of the course resolver (selected Omni-Bearing) to the received radial phase of your position.

A VOR works by using two RF carrier signals.

The reference signal is a 9960Hz sub-carrier FM modulated at 30Hz.

The rotating carrier is a unidirectional continuous wave RF carrier whose frequency is the VOR frequency.

Originally, VOR ground stations used a rotating antenna to aim the CW carrier. The antenna rotated at a speed of 1800 RPM. As the antenna pointed directly opposite the receiver, the receiver received the RF energy at its lowest amplitude. As the antenna pointed directly at the receiver, the RF is received at its greatest amplitude. At 1800 RPM, the received RF is modulated at 30Hz (1800RPM = 30 Revolutions Per Second) by the mechanical rotation of the antenna and the directional nature of the RF radiator.

The phase/time difference between the received reference signal and the modulated CW signal is the radial you are located on.

The Omni-Bearing Selector knob rotates a syncro in the nav display known as the "course resolver". The nav converter compares the position of the course resolver to the received phases of the ground station signal to drive the TO/FROM flag.

If the course selected by the course resolver is within 90 degrees of your actual position's radial phase, the FROM flag shows.

If the course selected by the course resolver is outside 90 degrees (within 90 degrees of the reciprocal), the TO flag shows.

Nowadays, VOR ground stations are solid state, using electronic doppler steering to "rotate" the CW signal at 1800 RPM.
 
Last edited:
I just wish they’d go back to having blue and yellow sides of the instrument for localizers
.
 
You must log in or register to reply here.
Back
Top