Encyclopedia of Physical Science and Technology - Aeronautics

227 Pages·2001·10.97 MB·English

by  Robert Allen Meyers (Editor)

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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
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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
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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
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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
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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
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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.)
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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.)
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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,
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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
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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.