How VOR Navigation Works: Radials, TO/FROM and the CDI

Before GPS was in every cockpit and every phone, pilots found their way across the sky using radio beacons on the ground. The most important of these is the VOR, and it is still a core topic for every student pilot. If you have seen the round instrument with a needle, a course knob and a little "TO" or "FROM" window, that is the VOR indicator. This guide explains, in plain language, what a VOR is, what a radial is, how the CDI needle and the TO/FROM flag work, and the classic mistakes beginners make, using official FAA material.

This is neutral, educational information, not flight training. For Indian student pilots, VOR sits under Radio Navigation (Radio Aids) in the DGCA CPL Air Navigation syllabus (CAR Section 7, Series B, Part IV). Always check the current syllabus and notices on dgca.gov.in.

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What is a VOR?

VOR stands for VHF Omnidirectional Range. It is a radio station on the ground that transmits in the VHF band of 108.0 to 117.95 MHz. "Omni" means all: the station sends out navigation signals in every direction at once. According to the FAA's Pilot's Handbook of Aeronautical Knowledge (PHAK), Chapter 16, a VOR provides magnetic bearing information to and from the station.

You will see three versions on charts:

• VOR: bearing information only.
• VOR/DME: a VOR with distance measuring equipment (DME) added, so the aircraft can also show distance to the station.
• VORTAC: a VOR combined with the military TACAN system; DME is always part of a VORTAC.

An important point for exams: a VOR on its own does not tell you distance. For that you need DME. And a VOR is not GPS; it is a ground-based radio aid.

What is a radial?

Picture the VOR station as the hub of a bicycle wheel, with spokes going out in every direction. Each spoke is a radial. The PHAK defines a radial as a line of magnetic bearing extending outward from the VOR station. Radials are numbered from 001 to 360, referenced to magnetic north: radial 001 is 1° east of magnetic north, and they go all the way round the circle to 360.

The key idea to remember: a radial is always measured FROM the station. If you are directly east of a VOR, you are on its 090 radial, whichever way your aircraft is pointing. To fly from there to the station, you would fly a magnetic course of 270, the reciprocal.

On aeronautical charts, a compass rose aligned to magnetic north is drawn around each VOR so pilots can read radials off the chart.

The VOR indicator: OBS, CDI needle and TO/FROM flag

In most training aircraft, the VOR is read on a simple instrument with a course deviation indicator. The PHAK describes three parts:

• OBS (omnibearing selector): also called the course selector. You turn this knob to choose the course or radial you are interested in.
• CDI needle (course deviation indicator): the left-right needle. It shows where the selected course is relative to your aircraft. When it is centred, you are on the selected course or its reciprocal.
• TO/FROM indicator: tells you whether the course set on the OBS, if flown, would take you towards the station or away from it.

The same VOR information can also be shown on a horizontal situation indicator (HSI), which combines it with a heading display, or on a radio magnetic indicator (RMI), whose needle simply points to the tuned station.

How to find which radial you are on

This is the first VOR skill every student learns:

1. Tune the VOR frequency and identify the station by its Morse code (or voice) identifier.
2. Turn the OBS until the CDI needle centres.
3. If the flag shows FROM, the number under the course index is the radial you are on.
4. If the flag shows TO, the number is the course to fly to the station, and your radial is the reciprocal (add or subtract 180).

Example: you are east of the station. Centre the needle with a FROM flag and the OBS reads 090, so you are on the 090 radial. Turn the OBS to 270 instead and the needle centres again, this time with a TO flag: 270 is the course that takes you to the station.

Tracking to and from a station

Once you know the course, flying it is about keeping the needle centred:

• Going to the station: set the inbound course on the OBS so the flag shows TO, turn to that heading, and keep the needle centred.
• Wind drift: if a crosswind pushes you off course, the needle moves to one side. The needle shows the direction of the course, so you turn towards it to get back, then hold a small wind correction so you stay on track. The PHAK warns that simply chasing the needle causes S-turns either side of the course; fly a heading that re-intercepts it and then correct for wind.
• Station passage: as you fly over the VOR, the needle fluctuates and the flag flips from TO to FROM.
• Going away from the station: if you continue on the same course, leave the OBS alone. The flag now reads FROM and the same tracking technique applies.

The classic mistake: reverse sensing

The PHAK gives a simple rule: when flying to a station, fly the selected course with a TO indication; when flying from a station, fly it with a FROM indication. If you fly towards a station with FROM showing, or away with TO showing, the needle works backwards. It points away from the course instead of towards it, and turning "towards the needle" takes you further off course. This is called reverse sensing, and it is one of the most common beginner errors.

Common misunderstandings

• "The VOR shows which way the aircraft is heading." It does not. A simple CDI shows your position relative to the selected course, regardless of heading. Heading comes from the compass or heading indicator (an HSI combines both on one display).
• "TO means I am flying towards the station." Not exactly. TO means the course set on the OBS would take you to the station if you flew it. Your actual heading could be anything.
• "VOR tells me how far away I am." Only with DME. A plain VOR gives bearing information only.
• "VOR reaches everywhere." VHF signals are line of sight. The PHAK says reception at 1,000 ft above ground level is generally about 40 to 45 miles, and range increases with altitude. If you are too low or too far, a warning flag shows the signal is unreliable.

Checking the station is usable

Always listen to the Morse code identifier before navigating with a VOR. According to the PHAK, when a VOR is out of service for maintenance its coded identifier is removed, which tells pilots not to use it. Receivers also have a warning flag that appears when the signal is too weak. The FAA also describes accuracy checks of the aircraft's VOR receiver; in training, follow your instructor and the approved procedures for your aircraft and country.

Why VOR still matters for Indian student pilots

Satellite navigation is widely used today, but VOR remains firmly on the DGCA syllabus. The Air Navigation syllabus lists VOR principles, presentation and interpretation (including the RMI) under Radio Aids, and VOR/DME-based procedures appear in the procedures sections too. Learn it as concepts, not memorised lines: radials are FROM the station, the needle shows where the course is, and the flag tells you whether the selected course leads to or from the station.

It also ties into your radio licence: you need clear radio skills to work with ATC, which our RTR(A) guide covers. Our DGCA exams explained post lists all the CPL papers. No study method guarantees a result, but practising radial and TO/FROM questions until they feel obvious makes the navigation paper far more manageable.

Frequently asked questions

Is a VOR radial measured to or from the station?

From the station. Radials are lines of magnetic bearing going outward from the VOR, numbered 001 to 360.

Are VOR radials true or magnetic?

Magnetic. The FAA PHAK says the radials are referenced to magnetic north.

What frequencies do VORs use?

VOR ground stations transmit in the VHF band from 108.0 to 117.95 MHz.

Can a VOR tell me my distance from the station?

Not on its own. You need DME, which is installed at VOR/DME and VORTAC stations.

What happens to the TO/FROM flag when I pass over the station?

The needle fluctuates and the flag changes from TO to FROM.

Is VOR on the DGCA CPL syllabus?

Yes. VOR is listed under Radio Navigation (Radio Aids) in the Air Navigation syllabus. Check the current version with DGCA.

Wrap-up

A VOR is a ground radio station that sends out 360 magnetic radials, always measured from the station. You choose a course with the OBS, the CDI needle shows where that course is, and the TO/FROM flag tells you whether it leads to or away from the station. Keep TO when flying to it and FROM when flying away, identify the station before you use it, and remember that distance needs DME. Master these few ideas and VOR questions stop being scary.

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General educational information only, not flight training. VOR descriptions are based on the FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25, Chapter 16); Indian requirements and syllabus are set by DGCA (dgca.gov.in). Always follow your instructor's guidance and approved procedures in training. No study method guarantees exam results.