Long before GNSS, pilots found their way by listening to the ground, and the beacons they used are still out there: the VOR, its companion the DME, and the older NDB. They remain woven into charts, holds and approaches, they are the backup plan when satellite navigation degrades, and they still appear in every instrument syllabus. This guide explains how each one actually works and how to read the needles without getting fooled.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
One idea, three flavours
All ground-based radio navigation rests on the same idea: a station at a known position broadcasts a signal, and equipment in the aircraft turns that signal into a line of position, a bearing or a distance. Cross two lines of position and you know where you are. The systems are defined internationally in ICAO Annex 10, Volume I and described for pilots in the FAA AIM. Because VOR and DME work at VHF and UHF, they are line of sight: terrain and the curve of the earth block them, so usable range grows with altitude.
The VOR: 360 spokes of a wheel
A VOR (VHF omnidirectional range) transmits between 108.0 and 117.95 MHz and defines 360 radials, one per degree, like the spokes of a wheel. A radial is a magnetic bearing outward from the station: the 090 radial extends due east of the beacon, the 180 radial due south. An aircraft sitting on the 090 radial is east of the station whatever its heading, because a radial describes position, not direction of flight.
The bottom of that band is not all VOR. From 108.10 to 111.95 MHz, the channels on odd tenths (108.10, 108.15, 108.30, 108.35, and so on) are not VOR frequencies at all: they are reserved for ILS localisers. VORs in that stretch live on the even tenths (108.00, 108.05, 108.20, 108.25). Above 112.00 MHz the whole band is VOR and the question does not arise. This matters the moment you type a frequency into the navigation radio from memory or from a misread chart: land on the wrong tenth and the box will happily tune a localiser, whose needle looks like a CDI and behaves nothing like one, because a localiser gives you a fixed course to one runway rather than 360 radials to select from. The Morse ident is what saves you, which is the next paragraph and not a coincidence.
Before using any VOR, identify it: each station transmits a three-letter Morse ident, and a station under maintenance transmits no ident or the code TEST. No ident means do not use it. An ILS localiser identifies with four letters beginning with I, so the ident also tells you immediately if you have tuned a localiser by mistake.
In the cockpit, the classic display is the omni bearing selector (OBS) and the course deviation indicator (CDI). You dial a course with the OBS; the CDI needle then shows where that course lies relative to the aircraft, with full-scale deflection at about 10 degrees, roughly 2 degrees per dot. A TO/FROM flag resolves the ambiguity between a course and its reciprocal. The crucial habit is to set the OBS to match your direction of flight. Do that and the instrument reads naturally: needle left means course to your left, so fly toward the needle. Set the reciprocal instead and every indication is backwards, the classic reverse sensing trap; an HSI avoids this by rotating the whole course arrow with the compass card. Directly overhead the station the signals converge in the cone of confusion, where the needle swings and the flag flickers; hold heading, wait, and watch the flag settle on FROM as you pass.
Flying a course: a worked example
Suppose you are east of a VOR and want to track straight to it, westbound. Dial the OBS until the CDI centres with TO showing: it reads 270. You turn to a heading of 270 and the needle sits centred. A few minutes later a wind from the north has drifted you south of course, and the needle creeps right, telling you the course is now to your right. You turn right to 280, ten degrees of correction, and hold it until the needle centres again, then split the difference and try 275 as a wind-corrected heading. The needle stays put: 5 degrees of drift correction is what this wind needs. Nearing the station the needle grows twitchy, then swings through the cone of confusion; the flag flips to FROM, and you are now outbound on the 270 radial west of the station without touching the OBS.
The IFR accuracy check
Because a VOR receiver can drift out of tolerance, the FAA requires it to be checked within the preceding 30 days before IFR flight under 14 CFR 91.171. The permitted error is plus or minus 4 degrees using a dedicated VOR test facility (VOT) or a designated ground checkpoint, 6 degrees at an airborne checkpoint, and if the aircraft has two independent VOR receivers you may instead tune both to the same station and accept a spread of no more than 4 degrees between them. The date, place, bearing error and a signature go in a log. Other authorities handle receiver checks through their own maintenance and operational rules, so check what applies to your aircraft, but the idea is universal: know your needle is telling the truth before you bet an approach on it.
DME: distance by stopwatch
DME (distance measuring equipment) gives you distance rather than direction. The aircraft interrogates the ground station in the UHF band, the station replies after a fixed delay, and the box times the round trip and converts it to nautical miles. DME channels are paired with VHF frequencies, so tuning the VOR or ILS automatically tunes its associated DME.
The number on the display is slant range: the straight-line distance from aircraft to antenna, not distance over the ground. The discrepancy is greatest close to the station and high above it. Cross directly overhead at 6000 ft above the site and the DME reads about 1 NM, because 6000 ft is about a nautical mile and that is genuinely how far you are from the antenna. Far from the station the slant and the ground distance converge. The same geometry corrupts the groundspeed readout many DME units offer: it is the rate of change of slant range, so it only approximates your true groundspeed when you are flying directly to or from the station, and it decays toward zero as you pass overhead, however fast you are moving.
The NDB and its needle
The NDB (non-directional beacon) is the oldest of the three, a simple low and medium frequency transmitter, typically between 190 and 535 kHz in the US allocation, that broadcasts equally in all directions. The aircraft's ADF (automatic direction finder) does the clever part: its needle simply points at the station, like a compass that has decided the beacon is north.
Because the needle is referenced to the aircraft's nose, you combine it with heading to get something chartable: magnetic bearing to the station equals magnetic heading plus relative bearing. Heading 060 with the needle 30 degrees right of the nose puts the station on a magnetic bearing of 090. Merely keeping the needle on the nose, called homing, traces a curved path in wind; proper tracking means offsetting the needle by your drift correction so the bearing stays constant.
The NDB demands more scepticism than the VOR. Its ground wave bends and reflects, so it suffers coastal refraction near shorelines, night effect when sky waves return after dark, thunderstorm effect, where the needle happily points at a nearby cumulonimbus instead of the beacon, and dip errors in turns. And unlike a VOR, a typical ADF has no failure flag: if the signal dies, the needle can simply park where it was. That is why the discipline with an NDB is to identify the station and keep monitoring the ident the whole time you are using it.
Where the beacons fit today
GNSS has taken over as the everyday source of navigation, and the beacon network is shrinking around it: many NDBs have been switched off, and the United States is thinning its VORs to a deliberate minimum operational network retained as a backup for GNSS outages. But conventional navaids still define airways on the IFR enroute chart, anchor holding patterns and non-precision approaches, and back up the ILS you fly to minima. When GNSS is jammed or degraded, the crew that can still track a radial has options. How satellite navigation itself works, and what RNAV and RNP mean, is the subject of our companion guide to RNAV, RNP and GNSS.
Common pitfalls
- Navigating an unidentified station. No Morse ident, no navigation. A beacon under maintenance may radiate a usable-looking but false signal.
- Assuming the whole VOR band is VOR. Between 108.10 and 111.95 MHz the odd tenths are ILS localiser channels. A four-letter ident beginning with I is the giveaway that you have tuned one.
- Reverse sensing. Set the OBS to agree with your direction of flight, or every needle deflection will tempt you the wrong way.
- Confusing radials with headings. A radial is where you are relative to the station; your heading is separate, and wind sits between the two.
- Trusting DME as ground distance close-in. Overhead at altitude it reads your height, not zero.
- Treating the ADF as fail-safe. It has no flag; only the ident tells you the beacon is still alive.
In Pilot EFB
Pilot EFB is a study and planning companion, not a navigation system: it does not tune, display or navigate by VOR, DME or NDB, and nothing in the app substitutes for your aircraft's certified avionics and current charts. Where it helps is on the ground: learning how the needles work here in Learn, alongside the IFR enroute chart and approach chart guides, and keeping your own planning notes and records in one offline-first place. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and navigate by your certified equipment and official sources.