[{"data":1,"prerenderedAt":510},["ShallowReactive",2],{"learn-\u002Flearn\u002Fvor-dme-and-ndb-explained":3,"learn-nav-\u002Flearn\u002Fvor-dme-and-ndb-explained":484},{"id":4,"title":5,"body":6,"date":400,"dateModified":401,"description":402,"draft":403,"extension":404,"faqs":405,"howTo":415,"keyTakeaways":424,"meta":431,"metaDescription":432,"navigation":433,"path":434,"quiz":435,"seo":461,"series":401,"seriesOrder":401,"sources":462,"stem":481,"topic":482,"__hash__":483},"learn\u002Flearn\u002Fvor-dme-and-ndb-explained.md","VOR, DME and NDB explained",{"type":7,"value":8,"toc":387},"minimark",[9,13,19,24,46,50,73,98,113,144,148,167,171,203,207,217,242,246,265,272,295,299,329,333,373,377],[10,11,12],"p",{},"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.",[14,15,16],"blockquote",{},[10,17,18],{},"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.",[20,21,23],"h2",{"id":22},"one-idea-three-flavours","One idea, three flavours",[10,25,26,27,34,35,40,41,45],{},"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 ",[28,29,33],"a",{"href":30,"rel":31},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Fannex-10-aeronautical-telecommunications-volume-i-radio-navigational-aids",[32],"nofollow","ICAO Annex 10, Volume I"," and described for pilots in the ",[28,36,39],{"href":37,"rel":38},"https:\u002F\u002Fwww.faa.gov\u002Fair_traffic\u002Fpublications\u002Fatpubs\u002Faim_html\u002Fchap1_section_1.html",[32],"FAA AIM",". Because VOR and DME work at VHF and UHF, they are ",[42,43,44],"strong",{},"line of sight",": terrain and the curve of the earth block them, so usable range grows with altitude.",[20,47,49],{"id":48},"the-vor-360-spokes-of-a-wheel","The VOR: 360 spokes of a wheel",[10,51,52,53,56,57,60,61,64,65,68,69,72],{},"A ",[42,54,55],{},"VOR"," (VHF omnidirectional range) transmits between ",[42,58,59],{},"108.0 and 117.95 MHz"," and defines ",[42,62,63],{},"360 radials",", one per degree, like the spokes of a wheel. A ",[42,66,67],{},"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 ",[42,70,71],{},"position, not direction of flight",".",[10,74,75,78,79,82,83,86,87,93,94,97],{},[42,76,77],{},"The bottom of that band is not all VOR."," From ",[42,80,81],{},"108.10 to 111.95 MHz",", the channels on ",[42,84,85],{},"odd tenths"," (108.10, 108.15, 108.30, 108.35, and so on) are not VOR frequencies at all: they are reserved for ",[42,88,89],{},[28,90,92],{"href":91},"\u002Flearn\u002Fthe-ils-explained","ILS localisers",". VORs in that stretch live on the ",[42,95,96],{},"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.",[10,99,100,101,104,105,108,109,112],{},"Before using any VOR, ",[42,102,103],{},"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 ",[42,106,107],{},"four"," letters beginning with ",[42,110,111],{},"I",", so the ident also tells you immediately if you have tuned a localiser by mistake.",[10,114,115,116,119,120,123,124,127,128,131,132,135,136,139,140,143],{},"In the cockpit, the classic display is the ",[42,117,118],{},"omni bearing selector (OBS)"," and the ",[42,121,122],{},"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 ",[42,125,126],{},"10 degrees",", roughly 2 degrees per dot. A ",[42,129,130],{},"TO\u002FFROM flag"," resolves the ambiguity between a course and its reciprocal. The crucial habit is to ",[42,133,134],{},"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 ",[42,137,138],{},"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 ",[42,141,142],{},"cone of confusion",", where the needle swings and the flag flickers; hold heading, wait, and watch the flag settle on FROM as you pass.",[20,145,147],{"id":146},"flying-a-course-a-worked-example","Flying a course: a worked example",[10,149,150,151,154,155,158,159,162,163,166],{},"Suppose you are east of a VOR and want to track straight to it, westbound. Dial the OBS until the CDI centres with ",[42,152,153],{},"TO"," showing: it reads ",[42,156,157],{},"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 ",[42,160,161],{},"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 ",[42,164,165],{},"FROM",", and you are now outbound on the 270 radial west of the station without touching the OBS.",[20,168,170],{"id":169},"the-ifr-accuracy-check","The IFR accuracy check",[10,172,173,174,177,178,184,185,190,191,194,195,198,199,202],{},"Because a VOR receiver can drift out of tolerance, the FAA requires it to be ",[42,175,176],{},"checked within the preceding 30 days"," before ",[28,179,183],{"href":180,"className":181},"\u002Flearn\u002Fglossary#gt-ifr",[182],"glossary-link","IFR"," flight under ",[28,186,189],{"href":187,"rel":188},"https:\u002F\u002Fwww.ecfr.gov\u002Fcurrent\u002Ftitle-14\u002Fchapter-I\u002Fsubchapter-F\u002Fpart-91\u002Fsubpart-C\u002Fsection-91.171",[32],"14 CFR 91.171",". The permitted error is ",[42,192,193],{},"plus or minus 4 degrees"," using a dedicated VOR test facility (VOT) or a designated ground checkpoint, ",[42,196,197],{},"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 ",[42,200,201],{},"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.",[20,204,206],{"id":205},"dme-distance-by-stopwatch","DME: distance by stopwatch",[10,208,209,212,213,216],{},[42,210,211],{},"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 ",[42,214,215],{},"paired"," with VHF frequencies, so tuning the VOR or ILS automatically tunes its associated DME.",[10,218,219,220,223,224,227,228,231,232,237,238,241],{},"The number on the display is ",[42,221,222],{},"slant range",": the straight-line distance from aircraft to antenna, not distance over the ground. The discrepancy is greatest ",[42,225,226],{},"close to the station and high above it",". Cross directly overhead at 6000 ft above the site and the DME reads about ",[42,229,230],{},"1 NM",", because 6000 ft is about a ",[28,233,236],{"href":234,"className":235},"\u002Flearn\u002Fglossary#gt-nautical-mile",[182],"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 ",[42,239,240],{},"directly to or from"," the station, and it decays toward zero as you pass overhead, however fast you are moving.",[20,243,245],{"id":244},"the-ndb-and-its-needle","The NDB and its needle",[10,247,248,249,252,253,256,257,260,261,264],{},"The ",[42,250,251],{},"NDB"," (non-directional beacon) is the oldest of the three, a simple low and medium frequency transmitter, typically between ",[42,254,255],{},"190 and 535 kHz"," in the US allocation, that broadcasts equally in all directions. The aircraft's ",[42,258,259],{},"ADF"," (automatic direction finder) does the clever part: its needle simply ",[42,262,263],{},"points at the station",", like a compass that has decided the beacon is north.",[10,266,267,268,271],{},"Because the needle is referenced to the aircraft's nose, you combine it with heading to get something chartable: ",[42,269,270],{},"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.",[10,273,274,275,278,279,282,283,286,287,290,291,294],{},"The NDB demands more scepticism than the VOR. Its ground wave bends and reflects, so it suffers ",[42,276,277],{},"coastal refraction"," near shorelines, ",[42,280,281],{},"night effect"," when sky waves return after dark, ",[42,284,285],{},"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 ",[42,288,289],{},"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 ",[42,292,293],{},"identify the station and keep monitoring the ident"," the whole time you are using it.",[20,296,298],{"id":297},"where-the-beacons-fit-today","Where the beacons fit today",[10,300,301,302,305,306,310,311,315,316,320,321,324,325,72],{},"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 ",[42,303,304],{},"minimum operational network"," retained as a backup for GNSS outages. But conventional navaids still define airways on the ",[28,307,309],{"href":308},"\u002Flearn\u002Freading-an-ifr-enroute-chart","IFR enroute chart",", anchor ",[28,312,314],{"href":313},"\u002Flearn\u002Fholding-patterns-explained","holding patterns"," and ",[28,317,319],{"href":318},"\u002Flearn\u002Fnon-precision-approaches","non-precision approaches",", and back up the ",[28,322,323],{"href":91},"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 ",[28,326,328],{"href":327},"\u002Flearn\u002Frnav-rnp-and-gnss-explained","RNAV, RNP and GNSS",[20,330,332],{"id":331},"common-pitfalls","Common pitfalls",[334,335,336,343,349,355,361,367],"ul",{},[337,338,339,342],"li",{},[42,340,341],{},"Navigating an unidentified station."," No Morse ident, no navigation. A beacon under maintenance may radiate a usable-looking but false signal.",[337,344,345,348],{},[42,346,347],{},"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.",[337,350,351,354],{},[42,352,353],{},"Reverse sensing."," Set the OBS to agree with your direction of flight, or every needle deflection will tempt you the wrong way.",[337,356,357,360],{},[42,358,359],{},"Confusing radials with headings."," A radial is where you are relative to the station; your heading is separate, and wind sits between the two.",[337,362,363,366],{},[42,364,365],{},"Trusting DME as ground distance close-in."," Overhead at altitude it reads your height, not zero.",[337,368,369,372],{},[42,370,371],{},"Treating the ADF as fail-safe."," It has no flag; only the ident tells you the beacon is still alive.",[20,374,376],{"id":375},"in-pilot-efb","In Pilot EFB",[10,378,379,380,315,382,386],{},"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 ",[28,381,309],{"href":308},[28,383,385],{"href":384},"\u002Flearn\u002Freading-an-instrument-approach-chart","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.",{"title":388,"searchDepth":389,"depth":389,"links":390},"",2,[391,392,393,394,395,396,397,398,399],{"id":22,"depth":389,"text":23},{"id":48,"depth":389,"text":49},{"id":146,"depth":389,"text":147},{"id":169,"depth":389,"text":170},{"id":205,"depth":389,"text":206},{"id":244,"depth":389,"text":245},{"id":297,"depth":389,"text":298},{"id":331,"depth":389,"text":332},{"id":375,"depth":389,"text":376},"2026-07-08",null,"How the classic ground-based navaids work: VOR radials and the CDI, DME slant range, the NDB and ADF, IFR accuracy checks, and where each fits in a GNSS world.",false,"md",[406,409,412],{"q":407,"a":408},"What is a VOR radial?","A radial is a magnetic bearing outward from the VOR station. Every VOR defines 360 of them, one per degree, and each is named for its direction from the station, so the 090 radial extends due east of the beacon. An aircraft on the 090 radial is east of the station regardless of which way its nose is pointing, because a radial describes position, not heading.",{"q":410,"a":411},"Why does DME read a longer distance than my chart says?","DME measures slant range, the straight line from the aircraft to the antenna, not distance over the ground. The error is largest when you are close to the station and high above it: directly overhead at 6000 ft above the site, the DME still reads about 1 NM, because that is how far away the antenna actually is. At longer ranges the slant and ground distances converge and the difference stops mattering.",{"q":413,"a":414},"Do pilots still need to know NDBs and VORs now that GPS exists?","Yes. GNSS is the primary navigation source almost everywhere, but conventional beacons remain in charts, procedures and checkrides, and they are the fallback when GNSS is degraded or jammed. The United States is deliberately retaining a minimum operational network of VORs as a backup, while many NDBs are being decommissioned. Knowing how the needles behave is what lets you use the backup when you need it.",{"name":416,"steps":417},"How to tune and track a VOR course",[418,419,420,421,422,423],"Tune the VOR frequency on the navigation radio. The band runs from 108.0 to 117.95 MHz, but between 108.10 and 111.95 MHz the odd tenths are ILS localiser channels, not VOR ones, so a VOR down there sits on an even tenth.","Identify the station by its three-letter Morse code ident. No ident, or a TEST ident, means the signal is unusable, so do not navigate by it. A four-letter ident beginning with I means you have tuned an ILS localiser rather than a VOR.","Rotate the omni bearing selector (OBS) until the course deviation indicator (CDI) centres with a TO indication. The selected course is now your course to the station.","Turn to a heading that matches the selected course, and hold the CDI centred with small heading corrections into the wind.","Interpret the needle: it shows where the course is relative to you, so fly toward the needle to reintercept when it drifts, provided the OBS matches your direction of flight.","Approaching the station, expect the needle to become sensitive and then unreliable in the cone of confusion overhead, and the flag to swing from TO to FROM as you pass.",[425,426,427,428,429,430],"A VOR broadcasts 360 magnetic radials outward from the station; the CDI shows where your selected course is relative to you, independent of heading.","The VOR band runs 108.0 to 117.95 MHz, but between 108.10 and 111.95 MHz the odd tenths belong to ILS localisers, which ident with four letters beginning with I.","Set the OBS to match your direction of flight, or the needle's left-right sense reverses, the classic reverse sensing trap.","DME measures slant range, not ground distance: overhead a station at 6000 ft it still reads about 1 NM.","The ADF needle simply points at the NDB; magnetic bearing to the station is magnetic heading plus relative bearing, and the ident must be monitored because there is no failure flag.","For FAA IFR flight, the VOR receiver must have been checked within the preceding 30 days: plus or minus 4 degrees on the ground, 6 degrees airborne.",{},"How VOR radials, DME slant range and the NDB\u002FADF actually work, the IFR accuracy checks, and where ground-based navaids fit in a GNSS world.",true,"\u002Flearn\u002Fvor-dme-and-ndb-explained",[436,445,453],{"q":437,"options":438,"answer":443,"explanation":444},"A VOR radial is measured in which direction?",[439,440,441,442],"Magnetic, toward the station","Magnetic, outward from the station","True, outward from the station","Relative to the aircraft's nose",1,"Radials are magnetic bearings outward from the station, so the 090 radial lies due east of the VOR. A radial describes where you are, not where you are pointing.",{"q":446,"options":447,"answer":443,"explanation":452},"You cross directly over a DME station at 6000 ft above the antenna. Roughly what does the DME read?",[448,449,450,451],"Zero","About 1 NM","About 6 NM","It depends on your groundspeed","DME measures slant range, the straight-line distance to the antenna. Overhead at 6000 ft, that straight line is about 6000 ft, which is roughly 1 NM.",{"q":454,"options":455,"answer":443,"explanation":460},"Under FAA rules, how recently must the VOR receiver have been checked for it to be used under IFR?",[456,457,458,459],"Within the preceding 12 calendar months","Within the preceding 30 days","Before every flight","There is no check requirement","14 CFR 91.171 requires the VOR equipment to have been operationally checked within the preceding 30 days, with a maximum error of plus or minus 4 degrees on a ground check or 6 degrees airborne.",{"title":5,"description":402},[463,465,467,469,472,475,478],{"label":464,"url":37},"FAA Aeronautical Information Manual, Chapter 1 Section 1 (Navigation aids)",{"label":466,"url":30},"ICAO Annex 10: Aeronautical Telecommunications, Volume I (Radio Navigation Aids)",{"label":468,"url":187},"14 CFR 91.171: VOR equipment check for IFR operations",{"label":470,"url":471},"FAA Instrument Procedures Handbook","https:\u002F\u002Fwww.faa.gov\u002Fregulations_policies\u002Fhandbooks_manuals\u002Faviation\u002Finstrument_procedures_handbook",{"label":473,"url":474},"SKYbrary: VHF Omnidirectional Radio Range (VOR)","https:\u002F\u002Fskybrary.aero\u002Farticles\u002Fvhf-omnidirectional-radio-range-vor",{"label":476,"url":477},"SKYbrary: Distance Measuring Equipment (DME)","https:\u002F\u002Fskybrary.aero\u002Farticles\u002Fdistance-measuring-equipment-dme",{"label":479,"url":480},"SKYbrary: Automatic Direction Finding (ADF)","https:\u002F\u002Fskybrary.aero\u002Farticles\u002Fautomatic-direction-finding-adf","learn\u002Fvor-dme-and-ndb-explained","Operations","SGf2n3RzexXXbXDOX3eiC-wWCmax8rg31yWFCu5mkNM",{"related":485,"newer":492,"older":503,"series":401},[486,492,497],{"path":487,"title":488,"description":489,"date":490,"topic":482,"draft":403,"minutes":491,"series":401,"seriesOrder":401},"\u002Flearn\u002Ftcas-and-airborne-collision-avoidance","TCAS and airborne collision avoidance","How TCAS watches the sky through other aircraft's transponders, what a traffic advisory and a resolution advisory demand of you, and why the RA outranks ATC.","2026-07-10",6,{"path":327,"title":493,"description":494,"date":495,"topic":482,"draft":403,"minutes":496,"series":401,"seriesOrder":401},"RNAV, RNP and GNSS explained","What performance-based navigation means: how GNSS fixes a position, what RAIM and SBAS add, RNAV versus RNP, and how to read the minima on an RNP approach.","2026-07-09",8,{"path":498,"title":499,"description":500,"date":501,"topic":482,"draft":403,"minutes":502,"series":401,"seriesOrder":401},"\u002Flearn\u002Fv-speeds-explained","V-speeds explained","What the V in V-speeds means, the difference between a design limit and an operating speed, and a tour of the speeds a pilot lives by, from stall to Vne.","2026-06-20",4,{"path":504,"title":505,"description":506,"date":507,"topic":508,"draft":403,"minutes":509,"series":401,"seriesOrder":401},"\u002Flearn\u002Ffitness-to-fly-imsafe-alcohol-and-medication","Fitness to fly: IMSAFE, alcohol and medication","The IMSAFE checklist, the FAA, EASA and UK alcohol rules and why the legal floor is not the safe ceiling, medication traps, and the scuba diving wait times.","2026-07-07","Regulations",9,1783767261352]