Encyclopedia of Physical Science and Technology - Aeronautics PDF
Preview Encyclopedia of Physical Science and Technology - Aeronautics
P1:FHK Qu:00,00,00,00 EncyclopediaofPhysicalScienceandTechnology EN001H-913 May25,2001 21:11 Aircraft Avionics Robert G. Loewy GeorgiaInstituteofTechnology I. DefinitionsofAvionicsComponents(Glossary) II. AircraftAvionicsSystems,General III. TraditionalAvionics,MEP IV. AvionicsApplicationsInfluencingAircraft Design;VMS V. ImpactOf“SmartMaterials” VI. Summary I. DEFINITIONS OF AVIONICS PowerSource Mostavionicssystemcomponentsrequire COMPONENTS (GLOSSARY) powersourcesindependentofpilot/crew;i.e.,avionics systems are “active” systems. Power sources may be Actuator Anelementofacontrolsystemthatwillmove electrical(e.g.,batteries,generators,fuel-cells)orme- anotherelement,byprovidingaforce,pressureormo- chanical (e.g., hydraulic pumps and reservoirs, pneu- ment(forceactingthroughaleverarm)inresponseto matics,etc.) acommandsignal. Processor Asystemcomponentwhichmayanalyze(i.e., Effector Acontrolsystemelementthatwillprovidethe extractusefulinformationfrom),combineorstoresig- desiredchangeinanaircrafts’behavior;e.g.,aerody- nals or may model aircraft behavior for comparative namic control surface such as a “rudder,” to change purposes. Such operations may be analog or digital; heading,ora“speedbrake”toreduceflightspeed. whenthelatter,processorshavemuchincommonwith Linkage A control system component that carries use- computers, but usually having special, i.e., more lim- ful signals, forces or moments from one location to itedfunctions,ratherthanbeinggeneral-purpose. another location. These useful signals can be analog Sensors A device that responds to some physical quan- electromagneticoropticalordigital,i.e.,quantitative, titysuchaspressureortemperature(orconceivablya and such transport can be within the aircraft or from chemicalquantitysuchasacidity)byconvertingittoa andtopointsexternaltotheaircraft.Onlywhenforces usefulsignal. andmomentsaretransmittedarelinkagesmechanical. Software The capability of digital processors and the When digital signals are transmitted the linkages are complexityoftheirfunctions,definedabove,aresuch oftencalled“databuses.”Databusesaretheconduits thatthe(usually)specializedcodesthatcommandtheir through which outputs are sent or inputs are received operations are considered a separate avionics “com- by a digital system or subsystem in order to perform ponent.” In written form such computer or processor itsfunction. codes may require tens of thousands or millions of 319 P1:FHK EncyclopediaofPhysicalScienceandTechnology EN001H-913 May8,2001 14:54 320 AircraftAvionics lines of instructions and their development may in- ages (fly-by-optics, FBO) are used less often than elec- volve equal or greater expense than the “hardware” trical linkages (fly by wire, FBW) and in FBW systems elements of avionics systems whose components are there is no need for electro optical transducers. Where definedelsewhereinthisglossary. fiberopticslinkagesareuseditisusuallybecauseoftheir Transducer A device that takes a useful signal in one superiorcapabilitiesforcarryinglargequantitiesofinfor- form, say electrical, and converts it to another useful mation(highbandwidth)andinsensitivitytoElectroMag- form,perhapsoptical.(Notethat“sensors”and“actu- netic Interference (EMI), including that associated with ators” are, in a more general sense “transducers,” but lightning. commonusagerestrictsthemeaningofthetermasde- Onceaircraftwererecognizedasvehicleswithrealiz- finedhere.) ablepotentialfortransportation,theneedforanumberof Transponder Acomponentwhich,onreceivinganElec- kinds of electronics-based equipment became apparent, troMagnetic(EM)signal,oftencoded,willrespondby basedontheimportanceofincreasingaircraftutilityand sending a similar signal, usually after a known, con- safety.These functions, roughly inthe chronologicalor- trolleddelaytime. der in which the related avionics equipments were first adaptedforuseonaircraftinserviceuse,areasfollows: II. AIRCRAFT AVIONICS 1. Communication SYSTEMS, GENERAL a. Groundtoair b. Airtoground The term “avionics” results from combining “aviation” c. Airtoair with“electronics,”inrecognitionofthegrowinguseand 2. Allweather,blindflying importance of the application of devices making use of 3. Navigation electronicsinaircraftdesign,developmentandoperation. 4. Limitedvisibilitylanding Aircraftavionicssystems,however,makeuseofcompo- 5. Badweatheravoidance nentswhichmaynotallbeelectronic,andanunderstand- 6. Flightpathstabilityaugmentation ing of their functions usually requires consideration of 7. Improvedflighthandlingqualities the whole system. Figure 1 illustrates a hypothetical sys- 8. Flightdatarecording tem for control of an aircraft about its pitch axis (i.e., 9. Collisionavoidance pointing the “nose” of the aircraft up or down), which 10. Formationflying(military) would“boost”thepilot’sforceoutputinmovinganaero- 11. Targetacquisition(military) dynamic control surface by a variable and appropriate 12. Secureidentification(military) amount, depending on the aircraft’s flight speed. In this 13. Crew/passengerscomfortimprovement case “the pilot’s longitudinal sidearm controller motion 14. Structuralloadalleviation isconvertedintoanelectricalsignalbyamotion1 sensor 15. Terrainavoidance (Loewy, 2000). That electrical signal is converted to an 16. Noisereduction opticalsignalbyanelectro-opticaltransducer.Fiberoptic a. Internal linkagescarrytheopticalsignaltoaprocessor.Afterbe- b. External ingtransducedbackintoanelectricsignal,itisamplified 17. Suppressingservo-aeroelasticinstabilities orattenuatedthereaccordingtoasecondsignaloriginat- 18. Performanceimprovement ing from an airspeed sensor ( this may be simple gain changes),sothattheaircraft’spitchresponsewillbethe It will be noted that this list is long, some items involve same at all airspeeds (assuming this is a desirable char- furtherbreakdown(Items1and16),andsincesuchadapta- acteristic).Thesignalfromtheprocessorthenregulatesa tionscontinueapace,anyattemptatcompletenessislikely valveonahydraulicactuator,whichdrivestheaircraft’s tosoonbethwartedbynewdevelopments.Forexample, elevator,i.e.,pitchattitudecontrolsurface.Severalcom- in-flight entertainment systems for commercial airliners mentsmaybepertinentforthisillustrativeexample.The arenotlisted,buttheycouldbeconsideredavionicssys- electromechanical input valve on the hydraulic actuator tems.Theiruseisalreadycommonplaceandtheservices might be considered a transducer, but for our purposes theyprovidearegrowingbyleapsandbounds. it is viewed as part of the actuator. Such an assembly is As implied by the order of functions in the list, oftencalledanintegratedservoactuator.Fiberopticlink- the application of electronic devices to aircraft can be thoughtofasbeginningwithradiosforcommunications 1Forillustrativepurposes:side-armcontrollersusuallyhaveforce, (Items 1(a) and (b)), between aircraft crew members— ratherthanmotionsensors. pilots, copilots,navigators, flight engineers, etc.—and P1:FHK EncyclopediaofPhysicalScienceandTechnology EN001H-913 May8,2001 14:54 AircraftAvionics 321 FIGURE1 SchematicofavionicscomponentsinaFly-By-Optics(FBO)flightcontrolsystemforthepitchaxis.(From Loewy,R.G.(2000).“Avionics:A‘New’seniorpartnerinaeronautics.”AIAAJ.37(11),1337–1354.) ground crew members; among dispersed crew members With the advent of devices (so-called “control actua- oflargeaircraft—althoughthisismorelikelytousetele- tors”) capable of moving aircraft control effectors (e.g., phone rather than wireless technology—e.g., from the aerodynamic surfaces) reliably and as quickly or more cockpit of military aircraft such as bombers, on the one quickly than a human pilot, avionics systems could be hand,tothetailgunnerviaan“intercom,”forexample,on used, not only to help the pilot and crew perform their theother;and(Item1(c))betweenflightcrewsofdifferent missionsbutalsotoflytheaircraftsafely.Theseautomatic aircraft. systemsareoftenreferredtoasVehicleManagementSys- Later, what can be considered radio technology, i.e., tems (VMS). To emphasize the highly integrated nature transmittersandreceiversofwirelessEMsignals,wasap- of VMS into the aircraft for which they are part of the pliedtonavigation(i.e.,helpingthepilotknowwhereto control system—on an equal, flight-safety footing along go)andlandingaides(i.e.,helpingthepilottolandsafely, with airframe structure, aerodynamic shape and propul- particularlyunderreducedvisibilityconditions).Inmili- sion systems—it is useful to think of VMS avionics as taryapplicationsofaircraft,such“assistance”byavionics part of the host vehicle. As might be expected, then, for the pilot and/or other crew members was extended VMS avionics are not “added on” but are usually con- to acquiring and identifying targets; pointing, firing or sideredduringthedesignordevelopmentalstagesofthe launching weapons; countering—i.e., thwarting through introductionintoserviceofaneworsubstantiallymodi- so-called“electroniccountermeasures”—similarsystems fiedflightvehicle. used by the enemy; and identifying himself/herself as In a “gray area” between MEP and VMS are what, at friendlytomembersofthesameforces.Thelastisknown their introduction as early as 1917, were called “auto- asIFF,for“Identification,FriendorFoe.”Allsuchfunc- maticpilots”or“autopilots.”Thesesystemsbeganbyus- tionscanbeperformedmoreorlessautomaticallytosuch ing roll attitude sensors, heading sensors (e.g., magnetic anextentthat,takentogether,suchsystemsareoftenre- compass), altitude sensors (e.g., barometric altimeters), ferredtoas“thepilot’sassociate.”Thesekindsofavionics airspeedsensors(e.g.,“pitot-static”pressuretubes),etcto systems are also referred to in the military as the “Mis- automaticallyadjust(i.e.,“hold”constant)wingsinalevel sionEquipmentPackage”(MEP).Itisusefultothinkof position, aircraft direction, flight altitude and speed, re- all avionics systems which assist pilots and crew in per- spectively,bysendingappropriatecorrectivecommands, forming their “mission,” even if it is a civilian transport for example, to rudder and ailerons (yaw and bank ef- moving passengers or cargo from one place to another, fectors)andlongitudinalcontrolstickandenginethrottle asMEP.Emphasizingthe“added-on”natureofMEP,the (speedandclimb/rateofdescenteffectors).Suchavionics aircraft in which it is installed is often referred to as the systemswereinitiallythoughtofasrelievingpilotfatigue “hostvehicle.” on long flights (particularly in an era of low cruising P1:FHK EncyclopediaofPhysicalScienceandTechnology EN001H-913 May8,2001 14:54 322 AircraftAvionics speeds). With increasing avionics component capabili- tively. The architectures of such systems should then be ties, as discussed in Section IV, autopilots have devel- such as to include at least triple redundancy and a con- opedintothemuchmoresophisticated,AutomaticFlight tinuouscomparisonofperformanceamongthemin“real” ControlSystems(AFCS).Perhapsthemostimportantof time, so that a failure can be identified, in what is often theenablingnewcapabilitiesisthegreaterresponsiveness called “voting,” and an automatic shut off of the system to commands on the part of actuators, i.e., their higher which has failed—or has even been subject to degraded “bandwidth” or high frequency capabilities. Among the performance—willtakeplace. AFCS functions making use of these newer capabilities are included Items 4, 6, 7, and 13 through 18. The last ofthesecouldincludesuchmeasuresasautomaticpump- III. TRADITIONAL AVIONICS, MEP ingoffuelfromonetanktoanothertokeeptheaircraft’s center of gravity in its most favorable position on long A. CommunicationandNavigation flights. SystemsinGeneral AlthoughavionicssystemsintheMEPcategorycan,in Radiotechnologyisbased,fundamentally,onthefactthat manyinstancesbe“addedon”wellafterthefundamental an alternating electrical current (ac) in a wire will radi- aircraftdesigniscompleted,thoseresponsiblefortheirin- ateEMenergyintospace.Iftherelationshipbetweenthe tegrationintothehostaircraftstillmustprovide(a)space lengthofthewireandthefrequencyoftheac, f,issuch withintheairframe,(b)stress-freemountingpointswhich that the wire length is half a wavelength, λ, almost all limit the shock and vibration transmitted to this equip- the power not turned into heat in the wire will be radi- ment,and(c)anenvironmentoflimitedmaximumtemper- ated.Thisbehaviorofhalfwavelengthwiresisthebasis aturesandEMIandacousticfields.Suchmust,ofcourse, ofEM“transmittingantenna”design.Ahalfwavelength alsobeprovidedforVMSavionics.Further,whentrans- wirewhichinterceptstheEMradiationwillalsoconvert mitting/receivingantennae(theinternal/externallinkages) itsenergyintoaccurrentmostefficientlyandisthebasis areinvolved,theirlocationsshouldminimizeinterference for“receivingantenna”design.Notethattherelationship with the signals to be sent or received and “cross talk” betweenfrequencyandwavelengthisgivenby to/from other EM sources. This is particularly challeng- ingforaircraftdesignedtohavelowradarcrosssections, c (indistancepersecond) f(inH )= , so-called“stealth”configurations. z λ Another way to categorize avionics systems in broad where c is the velocity of propagation of EM radiation, terms is to note that there are those for which the con- whichisthatoflightinavacuum(about300×106m/s). sequences of failures are such that a pilot can correct or Sendingandreceivingantennaecanbebased(1)onthe compensateforthemwithreasonableeffort,orthosefor ground(terrestrial),(2)inaircraftor(3)inspacecraft.In whichitisnotreasonabletoexpectapilottodoso.Forour general,thelargertheantennaintermsofwavelengthsof purposes,inthisconsideration,itdoesnotmatterwhether theradiationtransmitted,thenarrowerwillbethepattern thefailureisofthetypeinwhichthesystemsimplystops ofradiation.Thiscanleadtosomelargeairborneantennae workingorifitcausesthesystemtodrivetoafullauthority (see Fig. 2, for example). Some antennae are designed to positionunbidden—aso-called“hard-over”failure. be omni-directional or nondirectional, i.e., they transmit Systemsforcommunication,navigation,orbadweather EMradiationinasphericalpattern.Insuchacasetheratio avoidancemaywellbeimportantforsafetyofflightand ofreceivedtotransmittedenergyisequalto hencebeduplicatedorprovidedinmultipleinstallations ReceiverAntennaArea ofhigherredundancy;buttheconsequencesoftheirfail- , ure can reasonably be expected to be compensated for 4πR2 byapilotifmeansexisttoidentifytheirimproperoper- where R is the distance, or range, between transmitting ation. As a consequence, the duplicated systems do not andreceivingantennas. usuallyhavetooperatesimultaneously,butcanbeleftin Although specific portions of the frequency spectrum a “stand-by” mode until needed. Other systems, having (i.e., all the values of f to be used) must be allocated to to have frequency response characteristics well beyond preventdifferentsystemsfrominterferingwitheachother, whatapilotcandosimplytooperateeffectively,include formanyyearsandbygeneralagreement,radiotransmis- thoseformaintainingproperaircraftattitudeinallweather sion frequencies have been designated in the following flying,performinglimitedvisibilitylandings,augmenting “bands” (Skolnik, 1962) (Table I). flightpathstability,andsuppressingaeroelasticinstabili- There is a marked tendency to use higher and higher ties.Theconsequencesofsuchsystemsfailing,therefore, frequencies, and some of the categories in Table II have willunfoldmuchtooquicklyforapilottorespondeffec- also been widely used for about 50 years, but with some P1:FHK EncyclopediaofPhysicalScienceandTechnology EN001H-913 May8,2001 14:54 AircraftAvionics 323 FIGURE2 EarlywarningE-2Caircraft.(FromSkolnik,M.I.(1962).“IntroductiontoRadarSystems,”McGrawHill,NewYork.) different applications overseas (Reference Data for Engi- tion of the electrical characteristics of the earth (which neers (1985) (Table II). influence “ground waves,” those propagating along the Many factors affect the transmission of EM radiation. Earth’s surface), atmospheric noise (such as caused by Someareafunctionofradiationfrequency,othersafunc- lightning), ionospheric properties (which influence “sky waves,” those reflected by characteristics of the Earth’s atmosphere). The influence of the Earth is, as might be TABLEI DesignatedFrequencyBandsforEMRadiation expected, important for transmissions from ground sta- Name Abbreviation Frequency Wavelength tions, and is sometimes referred to as causing “site sen- sitivity.” Very high frequencies (i.e., above 30 MHz) are Verylow VLF 3to30kHz 100to10km frequency mostlyline-of-sightwaves,andabove3GHz,atmospheric Lowfrequency LF 30to200kHz 10to1km and precipitation scattering and absorption become Medium MF 300to3000kHz 1kmto100m significant. frequency Highfrequency HF 3to30MHz 100to10m Veryhigh VHF 30to300MHz 10to1m B. TerrestrialBasedNavigationSystems frequency Ultrahigh UHF 300to3000MHz 1mto10cm The term “avionics” usually implies equipment carried frequency and/orfunctionscarriedoutaboardaircraft.Tounderstand Superhigh SHF 3to30GHz 10to1cm someoftheircomplexities,however,itisusefultoknow frequency somethingoftheground-basedsystemswithwhichtheair- Extremelyhigh EHF 30to300GHz 10to1mm borneavionicscomponentsinteract.Thereare,ingeneral, frequency twokindsofground-basednavigationsystems;so-called P1:FHK EncyclopediaofPhysicalScienceandTechnology EN001H-913 May8,2001 14:54 324 AircraftAvionics TABLE II Letter Designation of High-Frequency EM Bearing,asprovidedbydirectionfinders,anddistance, Radiation asprovidedbyaDME,fromthesamegroundstation,al- lowsthecalculationofpositionrelativetothatgroundsta- Letter Frequency Letter Frequency designation range designation range tion.Ingeometricterms,byestablishingrangeandbear- ing,ground“pointsources”allowanaircrafttoplaceitself L 0.39to1.55GHz Xb 6.25to6.90GHz onthespacecurveintersectionofasphere(fromrangein- Ls 0.90to0.95GHz Ka 10.90to36.00GHz formation) and a semi-infinite vertical plane which has S 1.55to5.20GHz Ku 15.35to17.25GHz oneedgeatthefixedgroundstation(frombearinginfor- C 3.90to6.20GHz Ka 33.00to36.00GHz mation).Ifbarometricallydeterminedaltitudeinformation X 5.20to10.90GHz Q 36.00to46.00GHz isadded,thepositionoftheaircraftwillbeknown. aIncludesKeband,whichiscenteredat13.3GHz. Anotherkindofground-basedpointsourceisintended toprovideaircraftoccasional,positiveandabsoluteloca- tioninformation,oftenknownasa“fix.”TheseEMradia- “pointsources”andthosethatestablishanEMradiation tiontransmittersareknownas“markerbeacons”andthey gridinspace.Amongpointsourcegroundsystemsareom- send a narrow, fan-like pattern vertically at fixed points nidirectional (nondirectional) beacons, which allow air- along the nation’s airways, with the pattern’s maximum bornedirectionfinderstoestablishaheadingdirectionor widthalignedwiththecenter-lineoftheairwayonwhich “bearing”totheknownpositionofthebeacon.Direction theyarelocated.Receiversintheaircraftprovidethepilot finders consist essentially of a rectangular loop antenna withinformationastowhichbeaconhasbeenorisbeing wired so as to send the difference in signals in the op- traversed. posite,verticalsidesofthelooptoa“receiver.”Thisdif- The VHF (Very High Frequency) band listed in Table I ference is zero when the sides of the loop are the same is used for voice communications to, from, and among distancefromthebeacon,atwhichpointtheplaneofthe aircraft. By combining communications and navigation loop is perpendicular to the line joining the aircraft and functions in the VHF band, some avionics components thebeacon.Theantennaloopis,therefore,rotatedabout canbemadetododoubleduty.Thesuccessofthisscheme an axis parallel to and equidistant from the two vertical has resulted in what is known as VOR (for VHF Omni- sidessensingthebeacon’ssignals,anditsorientationmust directional Range) and its adoption as an international benotedwhenthe“receiver”indicatesanullreading.To standard. In this system, the ground station radiates two minimizetheaerodynamicdragonanaircraftinwhicha signals: one is omni-directional radiation whose carrier direction-findingantennaistobemounted,twofixedan- VHF frequency is modulated at 30 Hz; the second is a tennaloopscanbemountedsothattheirplanesareat90◦ cardioid(heart-shaped)patterninthehorizontalplanethat toeachotherandthephaseoftheirsignalsarecompared rotatesat30rps.Theairbornereceiverexperiencesboth electricallyfromonetotheothertoachievethesameeffect transmitted waves as 30 Hz signals and the phase angle asmechanicalrotation.Directionfindingsystemscan,al- betweenthem,asrelatedtotherotationangleofthecar- ternatively,havethebeaconplacedinthevehicleandthe dioidpattern,determinesthebearingoftheVORbeacon rotatingloopantennaandreceiveratthegroundstation. ThenearlyconstantspeedofEMradiationhasledtoits use to measure distance. Although other means of radio ranging (i.e., means to measure distance) exist, perhaps the simplest in concept is known as DME, for Distance MeasuringEquipment.Thissystemisinternationallystan- dardized. Its operation is depicted in Fig. 3 (from Kayton and Fried, 1997). A transmitter-receiver on board the air- craft,knownasan“interrogator,”sendsapairofveryshort EMpulses(3.5µslongand12µsapart),repeatedfrom5 to150timespersecond.Atransponderatafixed,known groundstation,onreceivingthesepulses,retransmitsthem aftera50µsdelay.Theavionicscomponentontheaircraft automaticallydeterminesthedifferencebetweensending andreceivingtimes(veryshortcomparedtotheperiodof the highest repetitive rate) subtracts the transponder de- lay and shows the distance from the ground station on a FIGURE 3 DME operation. (From Kayton, M., and Fried, W. R. controlpaneldisplay. (1997). “Avionics Navigation Systems,” Wiley, 2nd ed., New York.) P1:FHK EncyclopediaofPhysicalScienceandTechnology EN001H-913 May8,2001 14:54 AircraftAvionics 325 fromtheaircraft.ThereisalsoaDopplerversionofVOR (seeSectionIII.E)inwhicha9960-Hzcarrierfrequencyis frequencymodulatedbythe(simulated)rotationofalarge diameter(480wavelengthsor44-ft-diameter)antenna2so astobevariedby±480Hzat30Hz.Thesameairborne equipmentcanbeusedtosensephase,hencebearingtothe DopplerVORbeacon,aswithordinaryVOR,butwithless sitesensitivityandgreateraccuracy.Maximumbearinger- rorsat20-miledistancewithstandardVORareabout3◦ andwithDopplerVOR,about0.5◦. Themilitaryuses“pointsource”groundstationswhich combine systems for determining both bearing and dis- FIGURE4 HyperboliclinesofconstantTDforatypicalmaster- tance measurement. These systems are known as Tacan secondary pair. (From Kayton, M., and Fried, W. R. (1997). (Tactical Air Navigation). The distance measuring func- “AvionicsNavigationSystems,”2nded.,Wiley,NewYork.) tion, i.e., range determination, is accomplished using the same pulse and frequency configurations as stan- dardDME.TheTacanomni-bearingoperation,however, sition in space, as shown in Fig. 5 (Kayton and Fried, (a)usesfrequenciesfrom960to1215MHz(almost10× 1997). higher than VOR) so that smaller antennas can be used; Since LORAN-C is affected by sky waves and uses (b) employs a multi-lobe radiation principle which im- groundwaves,sophisticatedcorrectionsmustbemadeto proves bearing accuracy; and (c) enjoys equipment eco- achievemaximumaccuracy,andtheusualranges,afunc- nomicsasaresultofusingthesameradiofrequenciesfor tion of transmitter power, are measured in hundreds of rangeandbearingdetermination. miles.Accuracyofabout1/2kmisachievedroughly95% The so-called “hyperbolic” systems, such as Loran, ofthetime,whendifferentialtechniques(see,forexample, Omega and Decca, provide an alternative means of po- SectionIII.C)areused,addingredundantstationpairs. sition determination. These systems, rather than using IntheOmegasystem(whichhasbeenshutdownforsev- “pointsources,”consistofgroupsoftransmittingstations eralyears),transmittersemittedcontinuouswaves,rather thought of as forming “chains.” A chain consists of at thanpulses,andhyperboliclinesofpositionwereestab- least three stations, of which one is a master transmitter lishedbyphasedifferencesinsignalsreceivedfromamas- and the other two are secondary transmitters. Each sta- ter/secondary station pair. Because one phase difference tion in a chain transmits EM pulses which are grouped betweentwocontinuouswavesignalsdefinedaseriesof closely in time and repeated at a certain rate. The inter- hyperbolas, multiple frequencies were used to eliminate valbetweentherepeatedtransmissionsofthesegroupsof theambiguityofwhichhyperbolawasthepertinentone. pulses is known as the Group Repetition Interval (GRI), anditidentifiesaparticularchain.Thenumberofpulses inagroup,theintervalbetweenthem,theenvelopewhich definespulseshape,aswellasGRI,establishthetransmit- tedsignalformat,anditidentifieseachstationinachain. Since the positions of the stations are known, as are the timingofsignalstransmittedfrommasterandsecondary stations, the difference of the Time Of their signals’ Ar- rival(TOA)atanaircraftinformstheaircraftthatitmust be somewhere on a space curve, which happens to be a hyperbola,inahorizontalplanedeterminedbybaromet- ric altitude. A series of these TOA’s, then, establishes a series of hyperbolas, as shown in Fig. 4 (from Kayton and Fried, 1997). If follows that TOA’s from that master and another secondary station establishes a second series of hyperbola. The two specific TOA’s informs the aircraft astowhichtwohyperbolasitmustbeon;theirintersec- tions(plusbarometricaltitude)establishtheaircraft’spo- FIGURE 5 Hyperbolic lines of constant TD for a typical triad. (From Kayton, and Fried, W. R. (1997). “Avionics Navigation Sys- 2ActuallyaringofindividualEMtransmittingelements. tems,”2nded.,Wiley,NewYork.) P1:FHK EncyclopediaofPhysicalScienceandTechnology EN001H-913 May8,2001 14:54 326 AircraftAvionics Fivefrequencieswereusedateachstation;fourcommon less.” Decentralized systems clearly require the airborne andonestation-unique.ThefrequencybandsbeingVLF componentstobebothreceiversandtransmitters. (seeTableI),thesesignalswerepropagatedwithlowatten- uationbetweentheEarth’ssurfaceandaparticularlayer C. Satellite-BasedNavigationSystems oftheionosphere,togreatranges.Infact,only8transmit- tingstationsworldwideconstitutedanOmegasystemwith Relativelysoonafterthesuccessfulorbitingofman-made accuracy within about 4 nm, 95% of the time. When 30 satellitesabouttheEarth,attemptsbeganwiththeobjec- suchstationswereemployed,usingdifferentialtechniques tiveofreplacingorsupplementingterrestrialradionaviga- among redundant pairs, position errors were diminished tionsystemsthroughtheuseofearthsatellites.Themajor to about 2 km, 95% of the time, within 1000 km of the advantagesprovidedarethoseof(1)coverage,sincethe monitorstation. lines of sight, with enough satellites in the proper orbits Deccaisalsoasystembasedonphasedifferences,but can be made to reach all points on earth; and (2) very onethatuseslowfrequencycarrierwavesbetween70and stableoperation.Theirtransmissionfrequencies(Lband 130kHz.Twostationpairsaretypically110kmapartand navigationsignalsfromthesatellitesandSbandtelemetry therangeofcoverageistypically320km. downlinktoandup-linkfromthegroundstation)arealso Withtheadventofdigitaltechnology,boththeaccuracy such as to make them all-weather systems. Two major andflexibilityoftheearliersystems’informationprocess- satellite radio navigation systems are included in an in- inghaveincreased.Further,thecomponents’,particularly ternationallyrecognizedGlobalNavigationSatelliteSys- theairbornecomponents’,sizeandweighthavebeenre- tem(GNSS);theyaretheU.S.DepartmentofDefense’s duced.Sincemanyaircraftradiocommunicationandnav- NAVSTAR Global Positioning System (GPS) and the igation systems have shared both the same parts of the Russian Federation’s Global Orbiting Navigation Satel- EMfrequencyspectrumandacommontechnology,both lite System (GLONASS). Both systems have three ele- ground-basedandairbornecomponentsofterrestrialsys- ments:(1)aconstellationofearth-orbitingsatellites(each temswhichprovidedigitalcommunicationandnavigation has 24, as of this writing), (2) ground stations; and (3) functionshavebeendevelopedasintegratedsystems.That receiver/processorunitsintheuseraircraft.Thesatellites is,thesameEMcarrierwaveformsareusedtocarryboth transmitEMsignalswhichthegroundstationusestotrack functions. themandfromwhichtheuseraircraftdeterminesitsposi- Two basic types of terrestrial integrated communi- tionrelativetothesatellites.Veryaccurateatomicclocks cation-navigationsystems,centralizedanddecentralized, aboardthesatellitesaretheheartofGPS.TOAprocess- are in widespread use by the military. Operation of the inginthegroundstationsallowsimultaneousrangingfrom formerisdependentonacentralsite,fromwhichallusers multiplelocations,andsincethegroundstations’positions determine their positions on an absolute basis. Such an areknown—alsoallowsdeterminationofthesatellite’slo- arrangement facilitates the control of many users, more cation,velocity,andpredictedorbitalpositions.Thesatel- or less simultaneously, although users ordinarily receive lite’sorbitalpositioninformationissentfromtheground informationontheirpositionsautomaticallyonthebasis stationstothesatellites,whichtransmitittotheuserair- ofperiodic“requests”fromtheground-basedcentralsite craft’s receiver processor, together with timing signals. ornode.Usersinthese“PositionLocationandReporting The user aircraft’s processor uses TOA data to establish Systems” (PLRS) are “cooperating” users; i.e., they are itspositionrelativeto(atleastthree)oftheGPSsatellites equipped with Radio Sets” (RS) equipped with accurate which,combinedwiththetransmitteddataontheirposi- “clocks” and send a signal, individually identifiable, to tions,allowstheuseraircraft’spositiontobeknown.An threeormoregroundstations(MS’sforMasterStations). obvious advantage from the avionics viewpoint of GPS TheMS’salsohaveveryaccurate“clocks”andcompar- typenavigationsystemsisthattheequipmentintheuser isons of their timing signals are made with those of the aircraft can be “passive” to the extent that user aircraft RS.Knowing“clock”signaldifferencesandsignalTOA neednottransmitEMsignals. informationattwoMS’s,placestheuser(theRS)onthe A variety of corrections are required in GPS or spacecurveintersectionoftwo(imaginary)spheres.Alti- GLONASStoachievethepositionaccuracydesired;such tudeinformation,basedonbarometerdata,establishesthe includethosecompensatingforclockerrors,therotation positionoftheRSatoneoftwopointsonthisspacecurve, oftheearth,ionosphericandtroposphericrefraction,etc. andTOAdatafromathirdMSeliminatesthisuncertainty. “Differential” principles can be used to eliminate errors Operationofthesecond,decentralizedtypeofsystem commonbothtotheuserandareferencegroundstation. issuchthateachuserdeterminesitsowncoordinatesrel- In “Differential Global Positioning Systems” (DGPS), a ativetootherusers’positions.Sinceitisindependentof “reference ground station” receives the same navigation centralsites,decentralizedsystemsareoftencalled“node- signalsastheuseraircraft,butsinceitspositionisknown, P1:FHK EncyclopediaofPhysicalScienceandTechnology EN001H-913 May8,2001 14:54 AircraftAvionics 327 alltheerrorsinitscalculatedpositioncanbedetermined. worththeiradditionalcomplexity.INSiswidelyusedin Thesebecomeerrorcorrectionswhenthereferenceground themilitaryandonlargecivilpassengeraircraft. stationtransmitsthemtotheuseraircraft.Throughtheuse As principal components of inertial systems, ac- of DGPS, position errors can be reduced to within 1 to celerometersandgyroscopeshavebeensubjecttointense 10m,dependingontheuser’sdistancefromthereference developmenteffortstoimprovetheiraccuracyandelimi- station. nateresponsivenesstoinfluenceswhichcontaminatetheir GPSreceiverscanbequitesmall;onemilitaryversion outputs.Designofaccelerometersforinertialnavigation isknownastheMAGR(forMiniatureAirborneGPSRe- systems are most often based on one of three concepts. ceiver)andtheentirecomponent,exceptforitsantenna, Thefirstisthatofapendulumon“flexures”—beamswith iscontainedinsideanotheravionicsassembly;e.g.,anin- very low stiffness in one direction, but stiff in the other ertial navigation system (See Section III.D, below). The two,perpendiculardirections—andelectricallyrestrained antenna itself, in a U.S. Navy system, is contained in a toazerodeflectionatzeroorreferenceacceleration.This circularhousingwithadiameteroflessthan125mm,has providesfor“rebalancing”toensurethatresponsetoone aheight(thickness)ofabout40mm,andweighsabout2n acceleration will not change the direction of sensing for (massofabout0.1kg). thenext.Thesecondmakesuseofverysmall,microma- chinedsiliconmassesmountedonspringsthatare“soft” inonedirection,stiffintheothertwo,alsoelectrostatically D. InertialNavigationSystems nulled;andthethirdemploysvibratingbeamswhosestiff- Thebasisofinertialnavigationis“deadreckoning”(see nessissolowinthedirectionofvibrationthattensileforce Section III.E, below), using accelerometers mounted on variationsalongthebeamlengthcausechangesinthefre- theaircrafttomeasureaccelerationsandintegratingtheir quencyofvibration,thusindicatingaccelerationalongthe signaloutputsovertime,firsttoobtainvelocitiesandthen beamlength. a second time to determine position. Inertial Navigation Manytypesofgyroscopesareusedinaircraftapplica- Systems(INS)areself-contained,requiringnocooperat- tionsforeitherindicatingorprovidingsignalsinautomatic ing ground stations or satellites sending EM signals to systemswhichprovidecontrolofaircraftattitudeangleor the user aircraft; thus, they are not subject to interfer- angular rates. The earlier forms used a spinning wheel encesbyanenemyortheweather.Sincetheprocessingof mounted in a gimbal (so as to be free to rotate about an thefundamentalsensoroutputisanintegrationovertime, axisperpendiculartoitsspinaxis)and“floated”atneutral however,errorsgrowwithtime,and,iftheorientationof buoyancy. Angular motion of the gimbal axis in a plane the accelerometers is not accurately known, aircraft at- containingthespinaxiswouldthencauseprecessionabout titude changes—as a result of atmospheric disturbances the gimbal axis, which would indicate the gimbal axis’ or deliberate maneuvers—will contaminate acceleration angular rate. If this response were to be available again signals with changing gravity components. Corrections forlatermotions,thisprecessionanglewouldhavetobe for these effects are accomplished in so-called “strap- “reset,” and such would be done by magnetic torquers, down”inertialsystemsinwhichgyroscopesareaddedto according to a “rebalance algorithm.” These and other sense angular motions. These “strap-down” inertial sys- gyroscopes were developed further, including such re- temshavebecomepracticalwiththeadventofRingLaser finementsastwoperpendiculargimbals,electrostaticsus- Gyros (RLG’s) and Fiber Optic Gyros (FOG’s). These pension,etc.Thelessexpensive,lessmaintenanceprone devicescorrectforchangesinaccelerationdirectionelec- versionswithdriftratesofabout0.1deg/harestilluseful, trically, so that the linear, horizontal velocity and posi- forexample,intacticalmissiles,butaretooinaccuratefor tionpredictionsareastheyshouldbeforthepurposesof long-rangenavigation. navigation. Although modern optical angular motion sensors are When inertial systems are activated, they must be stillcalled“gyroscopes,”theyfunctiononotherthanNew- “aligned,”tosettheaircraft’sinitialpositionandvelocity tonianmechanical,i.e.,“inertial,”principles.Becauseof properlyandtoorientitsaxesrelativetotheEarth;thispro- theiraccuracy,dynamicrange,linearity,maintenance-free cessisknownas“gyrocompassing.”TheEarth’srotation, nature,andreliability,RLG’sandFOG’sarenowusedin ofcourse,imposesacentripedalaccelerationwhosemag- INS’sforalmostallcommercialandmilitaryaircraft.One nitudeanddirection(withrespecttothe“vertical,”i.e.,the oftwoopticalprinciplesareusedinthesedevices,butin localnormaltotheearth’smeansurface)varieswithgeo- either application, two laser beams propagate in what is graphicalposition.Thisandothersuchsmallerrorsgrow essentially the same closed, planar path; one clockwise, sufficientlywithtimeastomake“hybrid”systems,such the other counterclockwise. If the device containing the asthosewhich“update”inertialsystemsperiodicallyus- paths rotates about an axis perpendicular to the plane of ingGPSdata,forexample,inaprocessknownas“aiding” thosepaths,theSagnacEffect(1967),whichresultsfrom P1:FHK EncyclopediaofPhysicalScienceandTechnology EN001H-913 May8,2001 14:54 328 AircraftAvionics thefactthatlightwavesareunmovedbymotionalongthe transmitter powerrequirementsaresmall;andtheywork light path of the medium in which they’re transmitted— verywellforlowvehiclevelocities. makes the light’s path in the direction of device rotation AstotheprincipleonwhichDopplerradarsarebased, appear to be longer and the light’s path in the opposite considerthatwavemotionemanatingfromasourcemov- direction appear to be shorter. The RLG makes use of a ing with respect to the receiver is sensed as having a “resonator” principle, the FOG can use that principle or changedfrequency;themagnitudeofthechangedepend- interferometry. ing on the relative velocity, higher if the source and re- Becausethefrontpartofalaserlightbeamiscoherent ceiveraremovingcloser,loweriftheyaremovingfarther (i.e., all components are in phase) the interference be- apart. This so-called “Doppler effect” is experienced al- tweentwobeamspropagatinginoppositedirectionsinan mosteverydayacoustically,forexample,ifafast-moving opticalresonatorforcesastandingwavewithintheopti- auto or train passes with its horn or whistle blowing. In calcavity.WhenthistypeofRLG’shousingrotatesabout adirectlyanalogousway,thefrequencyofaradarsignal thecircularpath’scenterline,then,thenodesand/orantin- returnshiftsifthetransmitterandreflectingsurfacehave odesofthestandingwave,whicharefixedinspace,canbe relativevelocityalongthelineofEMtransmission.This “counted”andinterpretedasanglesintheazimuthaldirec- provides a means, using reflection returns, to determine tionaroundthecircularpath.ThelightsensorofanFOG the speed of an aircraft relative to the ground or water usingtheinterferometryprincipleexperiencesphasedif- over which it is flying. Doppler radars, mounted on an ferenceswherethetwo,counter-rotatinglaserlightbeams aircraft,usemicrowavefrequenciesinaninternationally emittedsimultaneouslyarerecombined,sinceone’spath authorizedband,between13.25and13.4GHz.Thispro- islongerandtheother’sshorter,dependingonthesense videsnarrowbeamsofEMradiation,whichcanbepointed andmagnitudeoftheangularrotationofthedeviceabout at the ground at relatively steep angles. The last has the the path’s centerline. The positions of the lines of inter- additionalbenefitofreducingtheprobabilityofdetection ference can then be interpreted as a measure of rotation inmilitaryapplications. angle. Most optical gyros used in INS’s, as of this writ- ForDopplernavigation,atleastthreeradarbeamsare ing,areoftheinterferometertype;employinglightpaths neededtodeterminethreecomponentsofvelocityrelative of between 10 to 40 cm in length; weighing between 5 totheearth’ssurface,andthreeaircraftattitudemeasure- and20n(massbetween0.5–2.0Kg)peraxis;andhaving mentsinthreeperpendicularplanesareneededtoresolve root-mean-squareaccuraciesofabout0.05◦. the Doppler radar measurements into components in an The typical INS, using these components, then (Kayton earth-related, geodetic coordinate system, as needed for and Fried, 1997), requires about 8000–16,000 cc in vol- dead reckoning navigation (Fig. 6). If the three Doppler ume,30–150Wofpower,weighsapproximately85–130n radar beams are arranged as shown in Fig. 7, and a differ- (mass between 9–14 Kg) and has a velocity accuracy encetakenofthereturnsfromsignalsAandB,theDoppler of about 0.75 m/s (rms) and navigational accuracy of shiftsofthelateralcomponentswillcancel,whereasthe 1.5 km/h. These modern airborne systems are relatively longitudinalcomponents,beingofoppositesign,willbe expensive($50,000to$120,000). added. This arrangement, known as a “Janus” system (aftertheRomanGodwhocouldseebothbackwardand forward),increasessystemaccuracy.Fortheusualbeam E. DopplerRadarandDead anglestothehorizontalofabout70◦,aJanussystemwill ReckoningSystems “Deadreckoning”isanoldmaritimetermusedtodescribe navigating(itselfamaritimeterm)byusingknowninitial position, the vehicle’s velocity vector (speed and direc- tion),andhowlongthatvelocityhasbeenmaintained,to determinethevehicle’snewposition.Ifvelocityismea- sured, say, relative to the surface of a body of water, it isclearthatpositionsdeterminedbydeadreckoningwill beinerrorbytheexistenceofcurrentsinthatwater.For shipswhosespeedisnotgreatrelativetocurrents,thisis important; for fast flying aircraft it is much less so. Use of Doppler radars to measure relative speed in modern dead reckoning systems, however, has some significant advantages;forexample,likeINS,theyareselfcontained, FIGURE 6 Resolution of aircraft velocity into navigable needing no terrestrial or satellite cooperative station; their components.
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