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4 V.h.f .omnidirect ionalange(voR)lntroductionPrior to World War II it wasrealized hat thepropagation nomaliesxperienced ith low- andmedium-frequencyavigation ids imited heirusefulnes3sstandard ystemsor a sky whichwasbecoming vermorecrowded.A system alledfour-courseow-frequencyangewaswidelyimplenrentedn the UnitedStates uring he 1930s;thisgave our courseso or from eachgroundstationand ltted n quitenicelywith a system f fixedairways.A problenrwith the four-courseystemsthateach tationonly providesor two intersectingairways; morecomplexunction requiresnorecourses. he above, oupledwith increased ltitudeof flying rnaking ineof-sight requenciessefulatlonger anges, nd he developtnentf v.h.f.comms,led o the adoptionof v.h.f.omnidirectionalange(VOR)asstandardn theUni tedStatesn 1946andinternationallyn 1949. The competition trr aninternational tandard ystemwas ierce, he eadingcontender fterVOR beingDeccaNavigator. t isdebatable hether he technically uperior vstemwaschosen, ut certainlyVOR wascheaper. ad headvantagef a arge omemarket,andhasdone hejob adequately ver ince.The VOR system peratesn the 108-l 8 MHzbandwith channels piced at sO kHiffiT[shared ith ILS localizer t. ffi ing allocatedo160of the 200 availablehannels.Of these160cfrannels 2Oareallocatedo VOR stationsntendedfor en routenavigation hile he other orty are ortClrn!ryLYQR statim!_(TVORL The output porverof anen route tat ion i l l be about200 W prov id insa service p tL49-laulieal n,iles.itTfleouencywilfbewi th in l reband l2- l l8 MHz. A TVOR wi l lhavean output powerof Ebgnl5)-t{-providing a service fup to about 5_!sg.l[sal i les, ts frequencywill bewithin hebundI9&I2_I4I&, thisbelng he partofthe otalband hared ith ILS ocalizer.The crewof an approp.iately quipped ircraftcantune nto a VOR stationwithin range nd read hebearingo the stationand he relative earing f thestation. Should he flight plan call for an approach58

to, or departurerom, a stationon a particularbearing, teeringnformationcanbe derived rom thereceived OR signals. t is this atter acil itywhichmakes OR so useful n airways lying,stations anbe placed trategically longso-called ictor airwaysand he pilot can hen,by selection f the appropriateradials,ly from station o stationeitherby obeyingsteering ommands r by feeding he same o theautopilot.To obtain a position fix liom VOR one needsbearingso two separate tationst henused n thisway VOR canbe considered theta-tl-retaystem. f aVOR station s colocatedwith a DME stationan \aircraft an obtaina fix using he pair asa rho-thetasystem.The VOR/DME system rcurrentliTilinternational hort-rangeavigation tandard. nrecent ears his systenr asbecome venmoreversatile ' ith the adventof airborne quipmentwhichcaneffectively eposition n existingVOR/DMEstat ion o givea 'phantombeacon'completei thradials hich canbe flown usingVOR-derivedsteeringnforrnation.Thisdevelopments consideredin Chapter 2.

Basic Pr inc ip lesA sinrple nalogy o VOR isgivenby imagininglig;hthousehichenritsan omnidirectional ulse flight every ime the beam s pointingdue north. Ifthe speed f rotation of the beam s known, a distantobserverould record he time ntervalbetweerrseeinghe onrnidirectionallashand seeinghe beam,and 'rence alculatehe bearing f the ighthouse.In realitya VOR station adiates .h.f.energymodulatedwith a retbrence hase ignal theomnidirectionalight - and a variable hase ignalthe rotatingbeam. The bearing f the aircraftdepends n the phase ifference etween eferenceandvariable hases- time difference etween ightandbeam.The radiation rorn a qgnventionalVOR (CVOR)station s a horizontallypolarized .h.f.wavemodulated s ollows:

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Sinpt coc

Cos qrt

Variable3O Hz a .m.

\a/

Cos trcosII

II

2r.3OHz2n.1"MHz

t .2 .

Fig.4.l Ground tation lockdiagram,.o.r,

30 Hz a.m.: the variablephase ignal. at Xo magneticbearing/roz the station he variable9960H2a.m.: th is isazubcarr ier f requency, phasewi l l /agtherefer "encepnut"uy i ; .igures+.t ,modulatedat 30 Hz with a deviationof 4.2 and4.3 illustrate h. b.ii" priniiples.1480 Hz. The 30 Hz signal s the reference The airborneequipment eceiveshe compositephase' signal adiatedby ihe station to which the receiversl02QFz a'm': identification signalkeyedto tuned. After deiection he variousmodulatingsignalsprovidemorsecode dentification at least hree- areseparated y filters. The 30 Hz referenceigrul isgjnes each39-s. Wherea voR and5ffitre-co-locatedhe dentification ransmissionsre

3 .

synchronizedassociateddentity,seeChapter ).4. Voicea.m.: the VOR system anbe usedasaground-to-air ommunicationchannelas ongasthisdoesnot interferewith its basicnavisationalfunction. The frequency angeof the vo]cemodulation s limited to SQOXZ-The 30 Hz variablephase s spacemodulated nthat the necessarymplitudevariation n the receivedsignalat the aircraft s achieved y radiatinga cardioidpattern otatingat 1800 .p.m. The frequencymodulated9960 Hz sub-carrier mplitudemodulatesthe r.f. at sourcebeforeradiation. It is arransed hatan aircraftdue north of the beacon ryill eceiievariableand referencesignals n phase, or an aircraft Flg 4.2 Frequencyspectrum:CVOR space ignals59

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REFER'{CE PHASES I G N A L ( F M )A L L R A O IA L S

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VARIAELPHASES I G N A L A M )

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Fig 4.3 Phaseelationships,VOR (courtesyKing RadioCorp.)

= ' .c