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CapabilitiesandLimitations

RemoteSensors*

EARLS.LEONAHDO,

MemberTechnicalSta.fl,

FerminAnalysisPrograms,

Tl?XaSInstrumentsIncorp.

ABSTRACT:Jlfodernremotesensorimageryhasgreatlyimprovedinrecentyears.

Untilrecently,airborneimagery,

whetherformappingorintelligencepurposes, wascollectedonsunny,cloud-freedayswithinpredeterminedtimeperiods.Now passiveandactiveremotesensorscancollectdatatwenty-fourhoursadayinthe visible, infraredandmicrowaveregionsoftheelectromagneticspectrum. Noindivl:dualsensorhasreachedsuchastateofdevelopmentthatitsstoreof informationcannotbeincreasedbysupplementaluseofothersystems.Radar, forexample,collectsitsdatathroughcloudsoratnight.Infraredsystemsdetect thermalvariationsbetweenadjacentfeatures.Neithersystem,howetler, ap proachesmoderncamerasinresolution,dynamicrangeanddetail. S

INCE\\'orldWarIItheterm"remotereo

connaissance"hasinvolvedmorethan better cameras,sharperlensesorfasterfilms.

Nolongerareimageanalystsrestrictedto

aerialphotographyalone.Instead,their analysesspantheelectromagneticspectrum bymeansofinfrared,radarandothereven moresophisticatedsystems.Theyextract datafromsectionsoftheelectromagnetic spectrumseveralmilliontimeswiderthan thatavailabletoconventionalcamera systems.

Ofwhatvalueisthisadditionalcoverage?

Canitreallyprovideinformationnotobtain

ablewithgoodaerialcameras?Howdoimages generatedbyinfraredandside-lookingradar systemscomparewithconventionalphotogra phy?'vVillthey,assomeclaim,makeaerial photographyobsolete?

Theaddedcoveragehasreyealednewiden

tificationsignaturesforbothnaturalandcul turalfeaturesinspectralareasbeyondcam eras'capabilities.Undercertainconditions, infraredandradarsystemscanproduce imageryofspecificsubjectsofnearlyphoto graphicquality.Buttheywillonlysupple ment,notreplace,aerialcameras-atleast notforafewyears.Acomprehensive systemstillneedsseveralsensorstosatisfytheneeds ofmilitaryreconnaissanceorcommercialex ploration.

EAHLS.LEONARDO

Torealizethecapabilitiesandlimitations

ofthesesensorsonemustfirstunderstandthe electromagneticspectrum. All electromagneticenergyisgeneratedas waveswhoselengthschangefrommicro scopically smallatoneendofthespectrumto fantasticallylongattheother.Figure1 showsasegmentoftheelectromagneticspec trum.Thevisiblespectrum-untilrecently theonlypartavailabletotheinterpreter- *Presentedat30thAnnualMeetingoftheSociety,HotelShoreham,Washington,D.c.,March

17-20,1964.

1005

1006PHOTOGRAMMETRICENGINEERING

FIG.2.Relationofwavelengthtofrequency.

-t- -+ / PULSE [L -+- • n

FIG.3.Differencebetweenpassiveandactivesensors.

INFRAREORADIATIONINFRAREDDETECTOR

sorsareeitherpassiveoractive.Passivesys tems(someinfrareddevices,cameras)detect radiationsthatwouldbepresentwhetheror notthesensorwereoperating.

Active

sensors,likeradar,recordechoesof reflected electromagneticenergywhichthey themselvestransmit.

Aerialcamerasproducetheirbestimagery

oncloudless,hazefreedays,butwithnew techniquesandequipmenttheydoobtain reasonablygoodimageryonclearnights.Fig ure4showsthatradarandinfraredsystems canovercometheselimitations.

Infraredsystemsalsoproducegoodday

timeimagery.However,sincetheyrespond toenergyradiatedfrombeyondthevisible spectrum,nightinfraredmissionswithmiddle andfarinfraredsensitivityyieldexcellent results.Formanypurposes,farinfrareddata flightsobtaintheirbestimageryafterdark whenthereisnointerferencefromsolarinso lation.Militaryneedsfornighttimeopera tionsareobvious.

Infraredradiationmaypenetratedustand

haze,dependingonthesizeoftheaerosolpar ticles,butclouds,highsurfacewindsandrain greatlyreduceimagequality.

Radar,anactivesensor,providesitsown

sourceofenergy.Therefore,ittooisindepend entoftime-of-day.Itslongerwavelengths :-10 4 Me ,3eM

F:-I09MC

>.'.5I'

5000II

GAMMARAYS

r>.-I

VISIBLELIGHT

!-->.-

INFRARED

MICROWAVES

->.------1 ------

FREQUENCY(CPS)

10 20 to'S10'·10 '3

IOIt10·10'

(/) " w .........w.....w>- Q>- ..'" • :::E.."'W '" .. " • :::E '" ........J -C).. 0" ..I ...JQ !:!?:i '" "," .. C)x ::» > "- '" :::E .03A3A300A30".3CM3M300M

WAVELENGTH

FIG.t.Segmentofelectromagneticspectrum.

uses onlyaminutepartoftheavailableen ergy(wavelengthsfromabout4,000to7,600

Angstromsor.4-.76microns).

Otherpartsofthespectrumtransmit

energyatdifferentwavelengths:

Infraredradiations-lessthan3toabout

1,000microns

Microwaves(includingradar)-Iessthan

acentimetertoabout3meters

Gammarays-about.03Angstroms

Nosharpcutoffexistsbetweenbands.

Rather,asmoothtransitionoccurs,like

gradualchangesinarainbow.There,forex ample.onecannotselecttheexactpointwhere bluebecomesgreen.

Frequency(thenumberofwavespassinga

given pointpersecond)variesinverselywith wavelengthbecauseallelectromagnetic energytravelsatthespeedoflight.Figure2 showsrelationshipsbetweenwavelengthand frequencyformicrowaves,infraredradia tions,visiblelightandgammarays.This figuremerelyshowsrepresentativecompari sons.Itisnottoscale.

Eachsensorreactsonlytoenergybandsof

specific frequencyandwavelength;radarre ceiverscannotdetectvisiblelight;trans mittedmicrowavesareinvisibletoinfrared scanners.

Figure3sholl'showairborneremotesen-

CAPABILITIESANDLIMITATIONSOFREMOTESENSORS1007

CAMERAIMFRAREDRADAR

daynil.!'.l ,

GOODTOMEDIUM

WEATHER

GOODTOPOOR

WEATHER

FIG.4.Time-weathercapabilitiesofremotesensors.

FIG.5.Spectraldetectivityofmercurydopedgermaniuminfrareddetectorcomparedtoinfraredenergytransmission.

paratively)torecordnaturalandcultural featureswithalmostphotographicclarity.

Systemcomponents(suchasCathodeRay

Tubeandfilm)cannotrecordwithequaldis

criminationallsignallevelsreceivedatthe antenna.Whetherthefilmrecordsmaximum differencesbetweenhighorlowintensitysig nalsdependsonthesettingsaccordingtomis sion'smainpurpose.InFigure6,ifmostof theavailablefilmdensityrangeisusedto 25

1481012

INMICRONS

51015

WAVELENGTHINMICRONS

246

WAVELENGTH

INFRAREDENERGYTRANSMISSIONTHROUGH

ASTANDARDATMOSPHERE

SENSITIVITYOFGe'HgWINDOW

60
80
40
o > t:: > .... (/) z w (/) t-- o 20 Q. penetratefog,hazeandcloudswithminimum signalloss.Rainsattenuatethesignal,but theextentdependsonsystemwavelengthand rainfallrate.Thick,moisture-ladenclouds, however, caneffectivelyblocktransmitted waves.Towhatextentthesefactorsaffect radarimagerydependsonseveralsystem parameters.

Allthreesystemscanbe"tuned"tobe

moreselectivetospecificfrequencieswithin theiroperationalbands.Narrowbandfilm tilter combinationsenablecamerastorecord spectralresponsesofonecolor.Filtersare oftenaddedtoinfraredsystemstoeliminate effectofsolarreflectionbelowthemiddleor farinfraredrange,dependingonthesystem.

Figure5showsinfraredenergytransmis

sionthroughastandardatmosphereand spectralsensitivityofamercury-doped germaniumdetector.Theatmosphere'scom positionallowsrelativelyundisturbedtrans missionofinfraredradiationsinthe2-5and

7-15micronranges.However,thedetector,a

standardintheindustry,transmitsfromthe edgeofthevisiblespectrumtoabout15mi cronswithouttheinterruptionbetween5and

7microns.Duringdaylighthours,therefore,

itrecordssomesolareffects.

Despiteexcellenceofnighttimeinfrared

imagery,somemineralsurveysarebestcon ductedduringthedayb.ecauseoftherateof absorptionandtransmissionofinfraredradia tion.However,theinfraredsystemmustin cludefilterssensitivetoinfraredradiations only,toeffectivelyeliminatereAectionsfrom therestofthespectrum.

Pulserepetitionrate,polarization,power,

radarsystemresolutionandsensitivityare functionsof\\'avelengthandgainsetting.For example,weatherradars(whichrequireonly moderateresolution)uselongwavelength systems;terrainreconnaissanceandmapping radarsusemuchshorterwavelengths(com-

1008PHOTOGRAMMETRICENGINEERING

FIG.6.Effectsofgainsettingonnegativedensity.

CAMERAINFRAREDRADAR

51010
3610
1l9 210
46
10l

DIFFUSEREFLECTIONSPECULARREFLECTION

GOOD=10

FIG.8.Remotesensorcomparison.

POOR=0

DAY/NIGHT

HAZE-FOGPENETRATION

CLOUDPENETRATION

TEMPERATURE

DISCRIMINATION

SUB-SURFACEDETECTION

STEREOCAPABILITY

ACCURAT£IMAGE

REPRESENTATION

LONG-RANGECAPABILITY

RESOLUTION

INTERPRETABILITYOF

IMAGERY

AVAILABILITYOFEQUIP-

MENT10

FIG.7.Specularvs.diffusereflections.

rangesoftenoccuronthesamenegative.

Manytimes,thesedensitychangesvarywith

respecttowaterdepth;othertimestothesun angle. \Vithradar,water'ssmoothsurfacereflects mosttransmittedmicrowaveenergyspecu lady;itgivesa"noreturn"image.

RadarcanI-ecordsomeofwater'sphe

nomena,however.Surfacesbrokenbybreak ers,waves,orsubmergedrocksareoftende tectable.Contrastbetweennormalno-return imagesandslightreturnsoffbrokenwater usuallyaresufficienttoassuresurfacedetec tionofsubmergedfeatures.

Infraredsystemscanextractconsiderable

informationfrombodiesofwater.Hotefflu entsdischat-gedintostreamsorlakesare detectablebecauseoftemperature differencesbetweenthem.Densitometric analysesshowhowfarandinwhatdirection V FINAL

AVAILABLE

DENSITY

RANGE , v' , , ... , ... , ",, , , ... , , '-',/ /"'-.. > !:::et CIlZ Zz Ww t-t et ...J ett-. zet (!) -0 CIl W 0:::: etw ou etw 0:0: recorddifferencesbetweenlowlevelsignals, strongintensityreturnsarecompressedintoa smalldensityrange.Muchofthesignalis,in reality,"clipped"andrecordedatonedensity level.Conversely,settingthegaintodis criminatebetweenstrongsignalsloseslow level separation.Farmland,orchardsand opencountryaretypicallow-levelreturns.

Urbanandindustrialareasgivestrongre

turns.

Anobject'ssurfacesmoothnessandorien

tationaffectsitsradarimage.Surfaces smoother,i.e.,withirregularitiessmallerthan thewavelengthoftheimpingingelectromag neticenergy,willreflectmostenergyspecu larly,ormirror-like,whileroughsurfcaes createmainlydiffusereflections.Thesmooth surfaceofFigure7(a)actslikeamirror:the angleofreflectionequalstheangleofinci dence.

Inthiscase,impingingenergyisre

flected awayfromthetransmitter-receiveL

ThemultifacetedroughsurfacesofFigure

7(b)scatterenergyunequally(diffusely)in

all directions.Someenergyeventuallyreturns toandisrecordedbythereceiver.

Becausevisiblespectrumwavelengthsare

soshort,mostsurfacesreflectlightdiffusely regardlessoforientation.Longermicrowaves createmorespecularreflectionsoffthesame surfaces.Forexample,aimaflashlightata wall,firstperpendicularly,thenatanangle.

Oneseesthewallequallyaswellregardlessof

theilluminatingangle.Aradarsystemillumi natingasimilarwallshowsmuchstrongerre turnsforhead-onorientationsthanoblique.

Onaerialphotographs,imagesofbodiesof

waterfrequentlyvaryintone.vVidedensity

CAPABILITIESANDLIMITATIONSOFREMOTESENSORS

1009

FIG.9.Photoindex,ArhuckleMountains,Oklahoma.

FIG.10.Radarmosaics,ArbuckleMountains,annotatedandunannotated. readilyapparent.Mostimportant,theradar imagecouldhavebeentakendayornightin virtuallyany"'eatherthatallowedtheair crafttofly,Thedatacollectionphaseofthis geologicmappingprogramtooklessthan6 hours.

Figure11ISanunclassifiedhighaltitude

,!STREAMSi ,-."";._.-.-- <---------'1

ICONTACTSI

.... effluentstravelbeforethestreamorlakeab sorbsthem.Chemicalwastesorotherpolluted dischargesshouldbeasreadilydetectablebe causeofthetemperatureanomaliesthey create.

Figure8summarizestheadvantagesand

disadvantagesofremotesensors.Nosingle sensorpossessesallrequirementsforanopti mumdevice.Butneitherdoesanysensor havesomanydisadvantagesthatitisvalue less.Cameraspossessthebestresolutionand givegeometricallyaccuratereproductions; infraredsystemsrecordminutetemperature differences;side-lookingairborneradarsoper ateindependentlyoftheclockinalmostall weatherandmaintainconstantimagequality (upto3rdorderplanimetricaccuracy)over extremelylongranges.

Butsomeremotesensorproblemsrequire

theinterpretertobe\'erycautious.Oneof infrared'smostconfusingsituationsoccurs whenanobject'stemperatureisthesameas itsbackground.Thenthetwocannotbe separated.Properlypositionedmetalcorner reflectorsproduceradal-reflectionsasstrong aslargeindustrialcomplexes.Ho\\'ever,to hidealargeobject-likeafactory-fromra dardetectionpresentsmoreseriousproblems.

Optimumsystemsthen,consistofmore

thanonesensor,eachcontributingitsspecial information.Forexample,Figure9isaphoto indexmosaicoftheArbuckle:i\'Iountains,

Oklahoma.Detailissharp,butatthisscaleit

showsonlyrelativelygrossobjects,(Of course, stereoanalysisofthecontactprints willrevealmuchdata.)Figure10showsun classifiedduplicateradarimagesoftheAr bucklescollectedbywhatisnowaprimitive radarsystem.Manyfeaturesofgeologicin terest(annotatedonthelowerphoto)are

1010PHOTOGRAMMETRICENGINEERlG

FIG.11.TnfraredimageofDallas,Texas.

1011

VERTICALINCIDENCE

WINGTIPMOUNTED

LONGWAVELENGTH

RADARS

345

VERTICALINCIDENCE

RADIOMETERS

(3SENSITIVITYBANDS) /L-,----,---J' I

FIG.12.Multi-sensorequippedB-25aircraft.

nighttimeinfraredimageofDallas,Texas.

Onecanseeacompleteroadpattern,stream,

openareas,parksandmanyindividualbuild ingsacrosstheentireformat.

Manyofthestreamsandroadsarebelow

treecanopiesthat\\"ouldprecludetheirdetec tioninthevisualspectrum.

Toexploittheadvantagesofmultisensorre-

FIG.13.TexasInstruments'multi-sensorsystemconcept. connaissance,TexasInstrumentsconverteda B-25 twin-engineaircraftintoamultisensor platform.(Figure12)Wehaveequippedit withaside-lookingradar,afar-infrared scanner,twoaerialcamerasandtwohyper sensitiveradiometers.Withthisaircraftwe haveacquired,undergovernmentandcom mercialcontracts,multisensorimageryand performedinterpretationsofvariouscultural andnaturalfeaturesintenstates.Current planningcallsforoutfittingamuchlargerair craftwithanevenmoresophisticated,inte gratedmultisensorsystem.Figure13isa block diagramofthissystem.Itwillhave fourradarsystems(side-looking,360 0 scan, verticalincidenceandterrainavoidance)a far-i nfrareddetector,severalaerialcameras. threeradiometersandamagnetometerarray.

Theverticalincidenceradarsystemisbased

uponTexasInstrumentsterrainanalysisre searchperformedfortheArmyEngineersat

Vicksburg.Withthesesensorswewillbeable

tostudymanydecadesoftheelectromagnetic spectrumandbroadenitsapplicationtore motereconnaissanceorexplorationproblems.

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