Learning hub · Radio Navigation · Lesson 1

Radio navigation: VOR, NDB, DME and GNSS

Radio aids tell you where you are by measuring a bearing, a distance or both. This lesson covers the four you will meet first: VOR, NDB, DME and satellite navigation.

About 14 minutes, plus the quiz. Part of Radio Navigation.

1. VOR: radials and the CDI

A VOR (VHF omnidirectional range) is a ground station that transmits in the VHF band, 108.0 to 117.975 MHz in ICAO Annex 10. It gives your receiver a magnetic bearing from the station. Think of 360 lines drawn out from the station like the spokes of a wheel. Each line is a radial, named by its magnetic direction away from the station. The 090 radial runs due east (magnetic) from the station. Radials are referenced to magnetic north as it is at the station.

In the cockpit you have an OBS (omni bearing selector), a knob that selects a course, and a CDI (course deviation indicator), a needle that shows whether you are on that course line. A small TO/FROM flag shows whether that course, if you flew it, takes you towards the station or away from it.

• Needle centred, FROM flag: you are on the selected radial, and flying the selected course would take you away from the station.
• Needle centred, TO flag: you are on the line that leads to the station, which is the reciprocal of the radial. Flying the selected course takes you to the station.
• Needle off centre: you are to one side of the selected line. On most indicators full scale is roughly 10° to 12° either side, and on many of them each dot is about 2° (five dots each side). Some indicators use fewer dots, so check the manual for your equipment.

The needle is a position indicator. It tells you where the line is, not which way you are pointing. It only gives sensible left and right steering when you fly roughly the same direction as the course set on the OBS. If you fly the opposite way, the needle appears to be the wrong way round (reverse sensing).

2. Intercepting a radial: a worked example

The station and the aeroplane here are invented. You want to fly inbound to the VOR on the 090 radial. You are east of the station, and not yet on the line.

1. Identify the station. Tune the frequency and listen to the Morse code. Check that it matches the identifier on your chart. Never navigate on a VOR you have not identified.
2. Set the course. To fly inbound on the 090 radial you fly the reciprocal, so set the OBS to 270. The flag should read TO.
3. Fly roughly that way. Turn to a heading near 270. Say you are heading 270 and the needle is deflected to the left. The line is to your left, to the south.
4. Turn towards the needle by a set angle. Turn left to an intercept heading about 30° off the course, here 240°. A larger angle intercepts faster, but gives you less time to react as the needle moves.
5. Hold the heading until the needle starts to move. As you close in, the needle comes in towards the centre. (If it is at full scale, it will stay there until you are within roughly 10° to 12° of the radial, measured at the station.)
6. Turn onto the course before it centres. Start the turn back to 270 as the needle nears the centre, and you will roll out on the line instead of passing through it. Stay ahead of the aeroplane: the nearer you are to the station, the faster the needle moves.
7. Correct for the wind. If the needle drifts, turn a few degrees towards it, wait, and take out a smaller correction once it centres. Small changes work better than large ones.

Over the station the needle becomes unreliable and the flag flips from TO to FROM. This area is called the cone of confusion. Hold your heading through it and the indications return after a short time.

VOR signals travel in a straight line, so range depends on your height and on terrain in the way. A VOR can also be off air for maintenance or have a fault. In training, check the notices for the station you plan to use, and use the chart, official data and the flight manual for the real flight.

3. NDB and ADF, in brief

An NDB (non-directional beacon) is a low and medium frequency transmitter, roughly 190 to 1,750 kHz in ICAO Annex 10 (US beacons are mostly 190 to 535 kHz), that sends the same signal in all directions. Your ADF (automatic direction finder) receiver has a needle that simply points at the beacon. It shows the relative bearing: the angle from your nose to the station.

Magnetic bearing to station = magnetic heading + relative bearing

An invented example: you are on a heading of 060° and the needle shows a relative bearing of 030°. The magnetic bearing to the beacon is 060 + 030 = 090°. If the sum is over 360, subtract 360.

ADF is simple but has weaknesses. Thunderstorms can pull the needle towards them. The signal bends at a coastline. At night, signals bouncing off the upper atmosphere can cause errors. Many NDBs are being withdrawn, so do not rely on one being there.

4. DME: slant range

DME (distance measuring equipment) works like a secondary radar. Your equipment sends pairs of pulses in the UHF band (960 to 1,215 MHz in Annex 10), the ground station replies, and the time for the round trip gives the distance. The result is the slant range, the straight-line distance from you to the station through the air, not the distance along the ground.

Slant range is larger than ground distance, and the difference matters when you are high and close. One nautical mile is about 6,076 ft. If you fly directly overhead a DME station at 6,000 ft above it, the display shows about 1 NM, although your ground distance is zero. At 10,000 ft overhead it shows about 1.6 NM. Well away from the station, at a distance several times your height, the error is small.

A DME is usually paired with a VOR, and their frequencies are paired, so one frequency setting selects both. The pair gives you a bearing and a distance from one station. That is enough to fix your position.

5. GNSS and RAIM

GNSS (global navigation satellite system) is the general name for GPS (USA), Galileo (Europe), GLONASS and BeiDou. Each satellite sends its position and an accurate time signal. Your receiver compares the arrival times and works out how far it is from each satellite. Three distances would be enough to fix a point in space, but the receiver’s own clock is not accurate enough, so it needs a fourth satellite to solve for the clock error too.

Accuracy depends on how many satellites you can see and how they are spread across the sky. Augmentation systems, such as the European SBAS called EGNOS, improve it further. A normal receiver is often accurate to a few metres, but that does not mean you can assume this on any flight.

RAIM (receiver autonomous integrity monitoring) is a check built into approved aviation receivers. It compares the signals from more satellites than the minimum and looks for one that does not agree with the rest. In ICAO terms, RAIM is a form of ABAS (airborne-based augmentation system), which checks integrity but, unlike EGNOS, does not improve accuracy. In principle, detecting a bad satellite needs at least five in view, and removing it from the solution needs at least six. If RAIM is not available, or raises an alert, you cannot trust the position as a primary source.

GNSS signals are weak and can be jammed or spoofed, and in some regions interference is common. Receivers can also fail or lose the signal in a turn or behind the airframe.

GNSS is an aid, not a replacement for airmanship. In VFR flying you must keep your map-reading skills: know your position on the chart, keep the track and the time, and be able to carry on if the screen goes blank. Real flights use approved equipment, current official data and your aeroplane’s flight manual.

6. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

1. You fly directly over a DME station at 12,000 ft above it. What does the DME show, using 1 NM = 6,076 ft? Later you are 5 NM from the station along the ground, still 6,000 ft above it. What does the DME show, and is the error large?
   Show the answer

   Overhead, the ground distance is zero and the slant range equals the height: 12,000 ÷ 6,076 ≈ 2.0 NM.

   The height of 6,000 ft is about 1 NM. At 5 NM along the ground the slant range is the diagonal of a right-angled triangle with sides 5 NM and 1 NM: the square root of (25 + 1) ≈ 5.1 NM.

   The error of about 0.1 NM is small, because you are several times further away than your height. The error matters when you are high and close.

2. Your ADF shows a relative bearing of 100° and your magnetic heading is 215°. What is the magnetic bearing to the beacon? Then repeat for a heading of 300° and a relative bearing of 120°.
   Show the answer

   Magnetic bearing to station = magnetic heading + relative bearing.

   First case: 215 + 100 = 315°. Second case: 300 + 120 = 420, which is more than 360, so subtract 360: 060°.

   The relative bearing is the angle from your nose to the station, so the beacon is on your right in both cases. Remember that thunderstorms, coastlines and night effects can make the needle unreliable.

3. The OBS is set to 090 and the CDI is centred with a TO flag. Which radial are you on, and on which side of the station? Later you fly heading 180° with the OBS set to 180°, a TO flag, and the needle deflected to the right. Which side of you is the line, and what intercept heading do you use if you want to turn 30° towards it?
   Show the answer

   With a TO flag and the needle centred, flying the selected course (090) would take you to the station, so you are on the reciprocal radial: the 270 radial, which means west of the station.

   Second case: the needle is deflected to the right, so the line is to your right. Turn towards the needle by about 30°: turn right to an intercept heading of 180 + 30 = 210°.

   Hold it until the needle starts to move, start the turn back to 180 as it nears the centre, and then make small corrections for the wind. Always identify the station by its Morse code before you rely on it.

7. Watch

A clear walk-through of radials, the OBS and the CDI, which you can follow with the first section above. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

Another explanation of how to use a VOR, with practical tips. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB.

8. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

Next: the wind triangle, to see how heading and track relate. Back to the learning hub.