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Sensors introducton, and marine survey
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Sensors
ImportantConcepts Mostremotesensinginstruments(sensors)aredesignedtomeasurephotons.
Thefundamentalprincipleunderlyingsensoroperationcentersonwhathappensinacriticalcomponent thedetector.
Thisistheconceptofthephotoelectriceffect (foundbyAlbertEinstein)
Thissaysthattherewillbeanemissionofnegativeparticles(electrons)whenanegativelychargedplateofsomeappropriatelightsensitivematerialissubjectedtoabeamofphotons.
Theelectronscanthenbemadetoflowasacurrentfromtheplate,arecollected,andthen
countedasasignal.
Akeypoint:Themagnitudeoftheelectriccurrentproduced(numberofphotoelectronsperunittime)isdirectlyproportionaltothelightintensity.
Thus,changesintheelectriccurrentcanbeusedtomeasurechangesinthephotons(numbers; intensity)thatstriketheplate(detector)duringagiventimeinterval(integrationtime).
Thekineticenergyofthereleasedphotoelectronsvarieswithfrequency(orwavelength)oftheimpingingradiation.
But,differentmaterialsundergophotoelectriceffectreleaseofelectronsoverdifferentwavelengthintervals;eachhasathresholdwavelengthatwhichthephenomenonbeginsandalongerwavelengthatwhichitceases.
Now,withthisprincipleestablishedasthebasisfortheoperationofmostremotesensors,letussummarizeseveralmainideasastosensortypes(classification)intwodiagrams:
Thisisafunctionaltreatmentofseveralclassesofsensors,plottedasatrianglediagram,inwhichthecornermembersaredeterminedbytheprincipalparametermeasured:Spectral;Spatial;Intensity.
Radiometer isageneraltermforanyinstrumentthatquantitativelymeasurestheEMradiationinsomeintervaloftheEMspectrum.
Whentheradiationislightfromthenarrowspectralbandincludingthevisible,thetermphotometer canbesubstituted.Ifthesensorincludesacomponent,suchasaprismordiffractiongrating,thatcanbreakradiationextendingoverapartofthespectrumintodiscretewavelengthsanddisperse(orseparate)thematdifferentanglestoanarrayofdetectors,itiscalledaspectrometer.
Thetermspectroradiometer isreservedforsensorsthatcollectthedispersedradiationinbands ratherthandiscretewavelengths.Mostair/spacesensorsarespectroradiometers.
Ternarydiagramoftheclassesofsensors
SirIsaacNewtondiscoveredthatwhitelightcouldbedispersedintoitsspectralcomponentsbypassingitthroughaprism.
Classificationofsensors
Thesecondcoversawiderarrayofsensortypes:
Thetwobroadestclassesofsensors:
Passive (energyleadingtoradiationreceivedcomesfromanexternalsource,e.g.,theSun);Passivesensorsoperateinthevisiblewaveband,thermalIRandmicrowavedomains.
Active (energygeneratedfromwithinthesensorsystem,beamedoutward,andthefractionreturnedismeasured).Activesensorsonsatellitealloperateinthemicrowave.
NotethatactivesensorsoperatinginthevisiblewavebandareknownasLIDAR.
Sensorscanbe:
Nonimaging (measurestheradiationreceivedfromallpointsinthesensedtarget,integratesthis,andreportstheresultasanelectricalsignalstrengthorsomeotherquantitativeattribute,suchasradiance)
Imaging (theelectronsreleasedareusedtoexciteorionizeasubstancelikesilver(Ag)infilmortodriveanimageproducingdevicelikeaTVorcomputermonitororacathoderaytubeoroscilloscopeorabatteryofelectronicdetectors);sincetheradiationisrelatedtospecificpointsinthetarget,theendresultisanimageorarasterdisplay.
Passivesensors Wavelength Information
Visible/NearInfraredwavelengthradiometers
400nm 1m Solarradiationreflectedbyocean/land
ThermalInfrared(TIR)radiometers
about10m Thermalemissionoftheocean/land
Microwaveradiometers 1.5 300mm Thermalemissionoftheocean/landinthe
microwave
Activedevices
Altimeters 3 30GHz Seasurfacetopography
Scatterometers 3 30GHz Seasurfaceroughness
Syntheticapertureradars 3 30GHz Sea/landsurfaceroughnessandmovement
Natureofspacebornesensorsandtheirapplicability
Visible/NearInfraredRemoteSensing
Theobservationmethodtoacquirevisiblelightandnearinfraredraysofsunlightreflectedbyobjectsontheground.
Byexaminingthestrengthofreflection(atdifferentwavelengths),wecanunderstandaconditionsoflandsurface,e.g.,distributionofplants,urbanareas,rivers,seas.
Duringperiodofdarkness,Thismethodcannotmakeobservation.Also,cloudsblockthereflectedsunlight,sothismethodcannotobserveareasunderclouds.
ThermalInfraredRemoteSensing
Theobservationmethodtoacquirethermalinfraredrays,whichisradiatedfromlandsurfaceheatedbysunlight.Alsoitcanobservethehightemperatureareas,suchasvolcanicactivitiesandfires.
Byexaminingthestrengthofradiation,wecanunderstandsurfacetemperaturesoflandandsea,andstatusofvolcanicactivitiesandforestfires.
Thismethodcanobserveatnightwhenthereisnocloud,butcannotmakeobservationundercloudyconditions.
MicrowaveRemoteSensing
Microwavesensorsreceivemicrowaves,whichislongerwavelengththanvisiblelightandinfraredrays,andobservationisnotaffectedbyday,nightorweather.
Therearetwotypesofobservationmethodsusingmicrowavesensor:activeandpassive.
ActivetypeThesensoraboardearthobservationsatelliteemitsmicrowavesandobservesmicrowavesreflectedbysea(orland)surface.Itissuitabletoobservelandandseaareas
PassivetypeThistypeobservesmicrowavesnaturallyradiatedfromlandsurface.Itissuitabletoobserveseasurfacetemperature,snowaccumulation,thicknessofice.
**Microwavealtimeterandscatterometerarenonimagingtypesofsensorsthatcomeunderactivesensors.Theirobservationsarenotaffectedbyday,nightorweather.
Schematicillustratingthedifferentremotesensingmethodsandclassesofsensorsusedinoceanremotesensing,alongwiththeirpotentialapplications
OceanSensorsandtheirapplicability
Past,presentandnearfutureoceansatellitemissions
Past,presentandnearfutureoceansatellitemissionscontd
Past,presentandnearfutureoceansatellitemissionscontd
Past,presentandnearfutureoceansatellitemissionscontd
Past,presentandnearfutureoceansatellitemissionscontd
PresentStatusSeriesofOperationalandR&Dsatellitesensorsforoceanographyhasbeenandwillbefunctioningmorethantenyears
AltimeterScatterometerSSTsensorIcesensorOceancolour Sensor
LandRemoteSensors
Bands with AVHRR
Sensor Resolution(m)SwathWidth
(km)Sensor
Channels SpectralBands(m)
LinearImagingSelfScanningSystemI(LISSI) 72 148
LISSI1LISSI2LISSI3LISSI4
0.450.52(blue)0.520.59(green)0.620.68(red)
0.770.86(nearIR)
LinearImagingSelfScanningSystemII(LISSII) 36 74
LISSII1LISSII2LISSII3LISSII4
0.450.52(blue)0.520.59(green)0.620.68(red)
0.770.86(nearIR)
LinearImagingSelfScanningSystemIII(LISSIII)
23
50
142
148
LISSIII2LISSIII3LISSIII4LISSIII5
0.520.59(green)0.620.68(red)
0.770.86(nearIR)1.551.70(midIR)
6 70 PAN 0.50.75
HighResolutionLinearImagingSelfScanningSystemIV(LISSIV) 5.8 24 70
LISSIV2LISSIV3LISSIV4
0.520.59(green)0.620.68(red)
0.770.86(nearIR)
WideFieldSensor(WiFS) 188 774WiFS1WiFS2
0.620.68(red)0.770.86(near IR)
AdvancedWideFieldSensor(AWiFS) 5670 370740
AWiFS1AWiFS2AWiFS3AWiFS4
0.520.59(green)0.620.68(red)
0.770.86(nearIR)1.551.70(midIR)
Indian Remote Sensing (IRS) Satellite Sensors
Multi-angle Imaging Spectroradiometer (MISR) Onboard Terra
Airbornehyperspectral
sensors:
1. AVIRIS2. CASI
Satellitehyperspectral
sensor:
Hyperion
Hyperspectral sensors:
The IFOV and FOV can be calculated using trigonometry
(d is the detector size, f is the focal length). The general unit of IFOV is milliradians (10-3 radians). 1= 0.01745 radians = 17.45 milliradians1 milliradian= 0.057
IFOV and FOV
Thetermscanning canbeappliedbothtomovementoftheentiresensorforlightgathering.
Twobroadcategoriesofmostscannersaredefinedbytheterms"opticalmechanical"and"opticalelectronic
(b)opticalmechanical containinganessentialmechanicalcomponent(e.g.,amovingmirror)thatparticipatesinscanningthescene
TheCrossTrackmodenormallyusesarotating(spinning)oroscillatingmirror(makingthesensoranopticalmechanicaldevice)tosweepthescenealongalinetraversingtheground.ThisissometimesreferredtoastheWhiskbroommodefromthevisionofsweepingatablesidetosidebyasmallhandheldbroom.
(c)opticalelectronic havingthesensedradiationmovedirectlythroughtheopticsontoalinearortwodimensionalarrayofdetectors(CCDs).
TheAlongTrack Scannerhasalineararrayofdetectorsorientednormaltoflightpath.TheIFOVofeachdetectorsweepsapathparallelwiththeflightdirection.Thistypeofscanningisalsoreferredtoaspushbroom scanning(fromthementalimageofcleaningafloorwithawidebroomthroughsuccessiveforwardsweeps).
Characterization of optical sensors (visible, NIR and TIR)