sensors,marine survey
DESCRIPTION
Sensors introducton, and marine surveyTRANSCRIPT
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Sensors
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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:
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Thisisafunctionaltreatmentofseveralclassesofsensors,plottedasatrianglediagram,inwhichthecornermembersaredeterminedbytheprincipalparametermeasured:Spectral;Spatial;Intensity.
Radiometer isageneraltermforanyinstrumentthatquantitativelymeasurestheEMradiationinsomeintervaloftheEMspectrum.
Whentheradiationislightfromthenarrowspectralbandincludingthevisible,thetermphotometer canbesubstituted.Ifthesensorincludesacomponent,suchasaprismordiffractiongrating,thatcanbreakradiationextendingoverapartofthespectrumintodiscretewavelengthsanddisperse(orseparate)thematdifferentanglestoanarrayofdetectors,itiscalledaspectrometer.
Thetermspectroradiometer isreservedforsensorsthatcollectthedispersedradiationinbands ratherthandiscretewavelengths.Mostair/spacesensorsarespectroradiometers.
Ternarydiagramoftheclassesofsensors
SirIsaacNewtondiscoveredthatwhitelightcouldbedispersedintoitsspectralcomponentsbypassingitthroughaprism.
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Classificationofsensors
Thesecondcoversawiderarrayofsensortypes:
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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.
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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
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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.
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MicrowaveRemoteSensing
Microwavesensorsreceivemicrowaves,whichislongerwavelengththanvisiblelightandinfraredrays,andobservationisnotaffectedbyday,nightorweather.
Therearetwotypesofobservationmethodsusingmicrowavesensor:activeandpassive.
ActivetypeThesensoraboardearthobservationsatelliteemitsmicrowavesandobservesmicrowavesreflectedbysea(orland)surface.Itissuitabletoobservelandandseaareas
PassivetypeThistypeobservesmicrowavesnaturallyradiatedfromlandsurface.Itissuitabletoobserveseasurfacetemperature,snowaccumulation,thicknessofice.
**Microwavealtimeterandscatterometerarenonimagingtypesofsensorsthatcomeunderactivesensors.Theirobservationsarenotaffectedbyday,nightorweather.
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Schematicillustratingthedifferentremotesensingmethodsandclassesofsensorsusedinoceanremotesensing,alongwiththeirpotentialapplications
OceanSensorsandtheirapplicability
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Past,presentandnearfutureoceansatellitemissions
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Past,presentandnearfutureoceansatellitemissionscontd
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Past,presentandnearfutureoceansatellitemissionscontd
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Past,presentandnearfutureoceansatellitemissionscontd
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Past,presentandnearfutureoceansatellitemissionscontd
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PresentStatusSeriesofOperationalandR&Dsatellitesensorsforoceanographyhasbeenandwillbefunctioningmorethantenyears
AltimeterScatterometerSSTsensorIcesensorOceancolour Sensor
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LandRemoteSensors
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Bands with AVHRR
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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
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Multi-angle Imaging Spectroradiometer (MISR) Onboard Terra
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Airbornehyperspectral
sensors:
1. AVIRIS2. CASI
Satellitehyperspectral
sensor:
Hyperion
Hyperspectral sensors:
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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
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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).
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Characterization of optical sensors (visible, NIR and TIR)