Lecture 3: Remote Sensing

Spectral signatures, VNIR/SWIR, MWIR/LWIR

Radiation models

VideoVideohttp://www.met.sjsu.edu/metr112-videos/MET%20112%20Video%20Library-MP4/energy%20balance/

• Solar Balance.mp4

Jin: We failed to show this one on class, you can access it from the link above

Spectral signature Much of the previous discussion centered around the

selection of the specific spectral bands for a given theme

In the solar reflective part of the spectrum (350-2500 nm), the shape of thespectral reflectance of a material of interest drives the band selection

Recall the spectral reflectance of vegetationSelect bands based on an absorbing or reflecting feature in the materialIn the TIR it will be the emissivity that is studied

The key will be that different materials have different spectral reflectances

As an example, consider the spectral reflectance curves of three different materials shown in the graph

1) Visible-Near IR (0.4 - 2.5); 2) Mid-IR (3 - 5);3) Thermal IR (8 - 14); 4) Microwave (1 - 30 centimeters)

VNIR - visible and near-infrared ~0.4 and 1.4 micrometer (µm) Near-infrared (NIR, IR-A DIN): 0.75-1.4 µm in wavelength, defined by the water absorption

Short-wavelength infrared (SWIR, IR-B DIN): 1.4-3 µm, water absorption increases significantly at 1,450 nm. The 1,530 to 1,560 nm range is the dominant spectral region for long-distance telecommunications. Mid-wavelength infrared (MWIR, IR-C DIN) also called intermediate infrared (IIR): 3-8 µm

Long-wavelength infrared (LWIR, IR-C DIN): 8–15 µm Far infrared (FIR): 15-1,000 µm

These divisions are not precise and can vary depending on the publication

Spectral SignatureSpectral signature is the idea that a given material has aspectral reflectance/emissivity which distinguishes it fromother materials

Spectral reflectance is the efficiency by which a material reflects energy as a function of wavelengthThe success of our differentiation depends heavily on the sensor we use and the materials we are distinguishingUnfortunately, the problem is not as simple as it may appear since other factors beside the sensor play a role, such as•Solar angle•View angle•Surface wetness•Background and surrounding materialAlso have to deal with the fact that often the energy measured by the sensor will be from a mixture of many different materialsThis discussion will focus on the solar reflective for the time being

Spectral Signature - geologicMinerals and rocks can have distinctive spectral shapes basedon their chemical makeup and water content

For example, chemically bound water can cause a similar feature to show up in several diverse sample types However, the specific spectral location of the features and their shape depends on the actual sample 1

Spectral signature - Vegetation Samples shown here are for a variety of vegetation types

All samples are of the leaves only That is, no effects due to the branches and stems is included

Vegetation spectral reflectance Note that many of the themes for Landsat TM were based on

the spectral reflectance of vegetation

Show a typical vegetation spectra - KNOW THIS CURVE Also show the spectral bands of TM in the VNIR and SWIR as well as some of the basic physical process in each part of the spectrum

Spectral signature - Atmosphere Recall the graph presented earlier showing the transmittance

of the atmosphere

Can see that there are absorption features in the atmosphere that could beused for atmospheric remote sensing

Also clues us in to portions of the spectrum to avoid so that the ground isvisible

A signature is not enough Have to keep in mind that a spectral signature is not always enough

Signature of a water absorption feature in vegetation may not indicate thedesired parameter

Vegetation stress and health Vegetation amount

Signatures are typically derived in the laboratory Field measurements can verify the laboratory data Laboratory measurements may not simulate what the satellite sensor

would see

Good example is the difficult nature of measuring the relationship betweenwater content and plant health

Once the plant material is removed from the plant to allow measurementit begins to dry out Using field-based measurements only is limited by the quality of thesensors

The next question then becomes how many samples are needed todetermine what signatures allow for a thematic measurement

This is a black spruce forest in the BOREAS experimental region in Canada. Left: backscattering (sun behind observer), note the bright region (hotspot) where all shadows are hidden. Right: forwardscattering (sun opposite observer), note the shadowed centers of trees and transmission of light through the edges of the canopies. Photograph by Don Deering.

http://www-modis.bu.edu/brdf/brdfexpl.html

A soybean field. Left: backscattering (sun behind observer). Right: forwardscattering (sun opposite observer), note the specular reflection of the leaves. Photograph by Don Deering. http://www-modis.bu.edu/brdf/brdfexpl.html

Signature and resolution

The next thing to be concerned about is the fact that we will not fully sample the entire spectrum but rather use fewer bands

In this case, all fourbands will allow us todifferentiate clay andgrass

Using bands 1, 3, and4 would also besufficient to do this

Even using just bands 3 and 4 would allow us to separate clay and grass

Signature and resolution Band selection and resolution for spectral signatures should

be chosen first based on the shapes of the spectra

That is, it is not recommended to rely on the absolute difference betweentwo reflectance spectra for discrimination

Numerous factors can alter the brightness of the sample while notimpacting the spectral shape

Shadow effects and illumination conditionsAbsolute calibrationSample purity

Bands showngive Gypsum - Low, high, lower Montmorillonite - High, high, low Quartz - high, high, not so high

Quantifying radiationIt is necessary to understand the energy quantities that are typically used in remote sensing

Radiant energy (Q in joules) is a measure of the capacity of an EM wave to do work by moving an object, heating, or changing its state.

Radiant flux (Φ in watts) is the time rate (flow) of energy passing through a certain location.

Radiant flux density (watts/m2) is the flux intercepted by a planarsurface of unit area. Irradiance (E) is flux density incident upon a surface. Exitance (M) or emittance is flux density leaving a surface.

The solid angle (Ω in steradians) subtended by an area A on a spherical surface of radius r is A/r2

Radiant intensity (I in watts/sr) is the flux per unit solid angle in a given direction. Radiance (L in watts/m2/sr) is the intensity per unit projected area.

Radiance from source to object is conserved

Radiometric Definitions/RelationshipsRadiant flux, irradiance (radiant exitance), radiance

The three major energy quantities are related toeach other logically by examining their unitsIn this course, we will deal with the special case

Object of interest is located far from thesensor (factor of five)Change in radiance from object is smallover the view of the sensor

ThenΦdetector = L object × Areacollector × ΩGIFOV

Φdetector = E object × Areacollector

E detector = L object × ΩGIFOV

ΩGIFOV= AreaGIFOV/H2

ΩGIFOV= Areadetector/f2

Electromagnetic Spectrum: Transmittance, Electromagnetic Spectrum: Transmittance, Absorptance, and ReflectanceAbsorptance, and Reflectance

Radiometric Definitions/Relationships Emissivity, absorptance, and reflectance

All three of these quantities are unitless ratios of energy quanities

Emissivity, ε, is the ratio of the amount of energy emitted by an objectto the maximum that could possibly emitted at that temperature

Absorptance, α, is the ratio of the amount of energy absorbed by anobject to the amount that is incident on it

Reflectance, ρ, is the ratio of the amount of energy reflected by anobject to the is incident on it

All three can be written in terms of the emitted, reflected, incident, andabsorbed radiance, irradiance, radiant exitance, or radiant flux (but sinceabove three quantities are unitless, numerator and denominator must beidentical units)In terms of radiant flux we would have

Radiometric Laws - Cosine LawCosine Law - Irradiance on surface is proportional to cosine of the angle

between normal to the surface and incident radiance

E = E0cosθ

In figures below, if E0 (or L0 converted to irradiance using the solid angle) isnormal to the surface, we have a maximum incident irradianceFor E0 that is tangent to surface, the incident irradiance is zero

Cosine effect exampleGraph on this page shows the downwelling total irradiance as a function of time for a single day as measured from a pyranometer

Radiometric Laws - 1/R2

Distance Squared Law or 1/R2 states that the irradiance from a point

source is inversely proportional to the square of the distance from the source

Only true for a point source, but for cases when the distance from thesource is large relative to the size of the source (factor of five givesaccuracy of 1%)

Sun can be considered a point source at the earthSatellite in terrestrial orbit does not see the earth as a point source

Can understand how this law works by remembering that irradiance has a1/area unit and looking at the cases below

In both cases, the radiant fluxthrough the entire circle is same

Area of larger sphere is 4 times that of the smallersphere and irradiance for a point on the sphere is ¼ that of the smaller sphere

Radiometric Laws - Lambertian SurfaceLambertian surface is one for which the surface-leaving radiance is constant with angle

It is the angle leaving the surface for which the radiance is invariant Lambertian surface says nothing about the dependence of the surface-

leaving radiance on the angle of incidence In fact, from the cosine law, we know that the incident irradiance will

decrease with sun angle If the incident irradiance decreases, the reflected radiance decreases

as well The radiance can decrease, as long as it does so in all directions

equally 2

Radiometric Laws - Lambertian SurfaceUsing the integral form of the relationship between radiance and irradiance we can show that

Elambertian=¶Llambertian

To obtain the irradiance we have to consider the radiance through an entire HemisphereBecause of the large range of angles, we cannot simply use E=LΩ

Radiometric Laws - Planck’s Law States that the spectral radiant exitance from a blackbody depends only on wavelength and the temperature of the blackbody

A blackbody is an object that absorbs all energy incident on it, α=1Corrollary is that a blackbody emits the maximum of energy possible for anobject a given temperature and wavelength

Radiometric Laws - Planck’s LawOnce you are given the temperature and wavelength you can develop a Planck curve

Planck curves never crossCurves of warmer bodies are above those of cooler bodies

Radiometric Laws - Wien’s LawPeaks of Planck Curves get lower and move to longer wavelengths as temperature decreases Maximum wavelength of emission is defined by Wien’s Law

λmax=2898/T [μm]

Solar Radiation

Sun is the primary source of energy in the VNIR and SWIR

Peak of solar curve at approximately 0.45 μmDistance to sun varies from 0.983 to 1.0167 AUIrradiance (not spectral irradiance) at the top of the earth’s atmosphere fornormal incidence is 1367 W/m2 at 1 AU

Terrestrial RadiationEnergy radiated by the earth peaks in the TIR

Effective temperature of the earth-atmosphere system is 255 KPlanck curves below relate to typical terrestrial temperatures

Solar-Terrestrial ComparisonWhen taking into account the earth-sun distance it can be shown that solar energy

dominates in VNIR/SWIR and emitted terrestrial dominates in the TIRSun emits moreenergy than the earthat ALL wavelengthsIt is a geometry effectthat allows us to treatthe wavelengthregions separately

Solar-Terrestrial ComparisonPlots here show the energy from the sun at the sun and at the top of the earth’s atmosphere

Also show the emitted energy from the earth

Vertical Profile of the Atmosphere

Understanding the verticalstructure of the atmosphere allows one to understand better the effects of the atmosphere

Atmosphere is divided into layersbased on the change intemperature with height in thatlayer Troposphere is nearest thesurface with temperaturedecreasing with height Stratosphere is next layer andtemperature increases with height Mesosphere has decreasingtemperatures

Atmospheric compositionAtmosphere is composed of dust and molecules which vary spatially and in concentration

Dust also referred to as aerosols Also applies to liquid water, particulate matter, airplanes, etc. Primary source of aerosols is the earth's surface

Size of most aerosols is between 0.2 and 5.0 micrometers Larger aerosols fall out due to gravity Smaller aerosols coagulate with other aerosols to make larger particles

Both aerosols and molecules scatter light more efficiently at short wavelengths Molecules scatter very strongly with wavelength (blue sky) Molecular scattering is proportional to 1/(wavelength)4 Aerosols typically scatter with 1/(wavelength) Both aerosols and molecules absorb Molecular (or gaseous absorption is more wavelength dependent Depends on concentration of material

AbsorptionMODTRAN3 output for US Standard Atmosphere, 2.54 cm column water vapor, default ozone 60-degree zenith angle and no scattering

AbsorptionSame curve as previous page but includes molecular scatter

Angular effect Changing the angle of the path through the atmosphere effectively changes the concentration More material, lower transmittance Longer path, lower transmittance

AbsorptionAt longer wavelengths, absorption plays a stronger role with some spectral regions having complete absorption

Absorption

Absorption The MWIR is dominated by water vapor and carbon dioxide absorption

Absorption In the TIR there is the “atmospheric window” from 8-12 μm with a strong ozone band to consider

Radiative Transfer Easier to consider the specific problem of the radiance at a sensor at the top of the atmosphere viewing the surface

Radiation components There will be three components of greatest interest in the

solar reflective part of the spectrum

Unscattered, surface reflected radiation Lλ

su

Down scattered, surface reflected Lλ

sd

skylight Up scattered path Lλ

sp

radiance Radiance at the sensor is the sum of these three

Radiative Transfer Radiative transfer is basis for understanding how sunlight and emitted surface radiation interact with the atmosphere

For the atmospheric scientist, radiative transfer is critical for understanding the atmosphere itself

For everyone else, it is what atmospheric scientists use to allow others to get rid of atmospheric effects

Discussion here will be to understand the effects the atmosphere will have on remote sensing data

Start with some definitions Zenith Angle Elevation Angle Nadir Angle Airmass is 1/cos(zenith) Azimuth angle describes the angle about the vertical similar to cardinal directions

Optical Depth Optical depth describes the attenuation along a path in the atmosphere

Depends on the amount of material in the atmosphere and the type ofmaterial and wavelength of interest

Soot is a stronger absorber (higher optical depth) than salt Molecules scatter better (higher optical depth) at shorter wavelengths Aerosol optical depth is typically higher in Los Angeles than Tucson Total optical depth is less on Mt. Lemmon than Tucson due to fewer

molecules and lower aerosol loading Optical depth can be divided into absorption and scattering components

which sum together to give the total optical depth

δtotal = δ scatter + δabsorption Scattering optical depth can be broken into molecular and aerosol

δscatter = δmolec + δaerosol Absorption can be written as sum of individual gaseous components

δabsorption = δ H2 O + δO3 + δCO2 + .........

Optical Depth and Beer’s Law Beer’s Law relates optical depth to transmittance

Increase in optical depth means decrease in transmittance Assuming that optical depth does not vary horizontally in the atmosphere allows us to write Beer’s Law in terms of the vertical optical depth 1/cosθ=m for airmass is valid up to about θ=60 (at larger values must include refractive corrections) Recalling that optical depth is the sum of component optical depths

Beer’s Law also relates an incident energy to the transmitted energy

Directly-transmitted solar term

First consider the directly transmitted solar beam, reflected from the ground, and transmitted to the sensor -

the unscattered surface-reflected radiation, Lλsu

Solar irradiance at the ground

Can also write the transmittance as an exponential in terms of optical dept

Beer’s law Need to account for the path length of the sun due to solar zenith

angle of the sun in computing transmittance Account for the cosine incident term to get the irradiance on the surface

Recall m=1/cosθsolar

Eλground, solar is

the solar irradiance atthe bottom of the atmosphere normalto the ground surface (shown here tobe horizontal)Requires a 1/r2 to account for earth-sun distance

Incident solar irradiance The surface topography will play a critical role in

determining the incident irradiance Two effects to consider Slope of the surface Lower optical depth because of higher elevation Good example of the usefulness of a digital elevation model (DEM) and

assumption of a vertical atmospheric model

Example: Shaded Relief

Surface elevationmodel can be used to predictenergy at sensor

Given Solar elevation angle local topography

(slope, aspect) from DEM

Simulate incident angleeffect on irradiance Calculate incidentangle for every pixelDetermine cos[θ(x,y)]

Creates a “shaded-relief” image

TM: Landsat thematic mapper

Directly-transmitted solar term Reflect the transmitted solar energy from the surface

within the field of view of the sensor

Once the solar irradiance is determined at the ground in the directionnormal to the surface it is reflected by the surface

The irradiance is converted to a radiance Conversion from irradiance to radiance is needed because we want to

use the nice features of radiance Recall the relationship between irradiance and radiance derived earlier for a lambertian surface - E=¶L There is a similar relationship between incident irradiance and reflected radiance from a Lambertian surface

Directly-transmitted solar term Last step is to transmit the radiance from the surface to

the sensor along the view path Simply Beer’s law again, except now we use the view path instead of

the solar path

Reflected downwelling atmospheric Atmosphere scatters light towards the surface and this scattered light is reflected at the surface to the sensor

Compute an incident irradiance from the incident radiance due to atmospheric scattering

This incident irradiance is reflected from our lambertian surface to give

Still need to transmit this through the atmosphere to get the at-sensor radiance

In the shadowsImage below is three-band mix of ETM+ bands 1, 4, and 7

Note that there is still energy coming from the shadows Scattered skylight - which will have a blue dominance to it

Path Radiance Term Path radiance describes the amount of energy scattered by the atmosphere into the sensor’s view

Basically, any photon for which the last photon scattering event occurred in the atmosphere is a path radiance termCan include or exclude an interaction with the ground

If it includes a surface interaction then this can be affected byatmospheric adjacency effects

The intrinsic path radiance is the radiance at the sensor that would be measured if there were zero surface reflectance Contribution only from the atmosphere Depends only on atmospheric parameters No simple formulation Requires radiative transfer code Use Lλ

sp

Over water A similar effect can be seen over waterImages here are also bands 3, 4, and 7 of ETM+ (LANDSAT)Water is highly absorbing at these wavelengths thus almost all of the signal is due to atmospheric scattering

At-sensor radiance in solar reflective Summing the previous three at-sensor radiances will give

the total radiance at the sensor

There is a huge amount of buried information in the above This is a simplified way of looking at the problem Phase function effects from scattering and single scatter albedo are contained in Edown and the path radiance Optical depths due to scattering and absorption are combined in the transmittance termsAlso assumes lambertian surface!!!

Path radianceModel output shows the spectral dependence of the at-

sensor radiance for path radiance and reflected radiance

TOA radiance, VNIR/SWIR MISR data showing the effect of view angle on TOA radiance

with brightening and blue dominance at large views

Model versus measured Comparison between measured spectra of RRV Playa using AVIRIS and predicted radiance based on ground

measurements

The airborne visible/infrared imaging spectrometer (AVIRIS)

Model versus measured Results below model the at-sensor radiance compared to

the sensor output

A raw AVIRIS spectrum (measured in digital numbers or. DN's)

TIR paths There will also be three components of greatest interest in

the emissive part of the spectrum (or TIR)

Unattenuated, surface emitted radiation Lλeu

Downward emitted, surface reflected skylight L λed

Upward emitted path radiance Lλep

Radiance at the sensor is the sum of these threeLλ

e = Lλeu + Lλ

ed + Lλep

Thermal infrared problem In the TIR, the problem is similar in philosophy as the

reflective

Still have a path radiance, and reflected downwelling Direct reflected term in reflective is analogous to the surface emitted

term in the TIR Difference is that we are now dealing primarily with emission and

absorption rather than scattering Reflective we are most concerned with how much stuff is in the

atmosphere and what it is Aerosol loading (Gives aerosol optical depth) Atmospheric pressure (Gives molecular optical depth) Types of aerosols (Phase function and absorption properties) Amount of gaseous absorbers (Water vapor, ozone, carbon dioxide) In the TIR we must also worry about where these things are vertically Temperature depends on altitude Emission depends on temperature Need vertical profile of termperature, pressure, and amounts of

absorbers

Surface-emitted term Surface emitted term will depend upon the emissivity and temperature of the surface attenuated along the view path

Easiest assumption is to assume that the surface is a blackbody butthen the temperature obtained will not correspond to the actualtemperature

Better assumption is to assume the emissivity and temperature areknown and use Planck’s law to obtain the emitted radiance

Transmitting this through the atmosphere gives

Reflected downwelling and path radiance Here, the equations are identical to the reflective case

The downwelling radiance depends on atmospheric temperature andcomposition

Equations are the same

Path radiance term is same as in reflective Must be computed from radiative transfer Depends heavily on atmospheric , Use Lλ

sp

Sum is same approach as reflective

TOA Radiance, TIRConcepts work in the other direction as well

Radiance at the sensor will depend mostly upon where the layer is thatis emitting the energy seen by the sensor

Location of the layer affects the temperature The warmer the layer, the higher

the radiance that is emitted

TIR Imagery examplesETM+ Band 6 of Tucson showing temperature effects

This image is from July Note the hot roads and cool vegetation

Bright and dark water

Water is dark inreflective bandsbut can be brightin LWIR Warm water relative to surround Water is also high emissivity (nearly unity)

Bright and dark land

Example ofNew Orleansshown herepoints out theHigh temperaturesof the urban area Water in thiscase is much colder than the land Little contrast inthe reflective

the LANSSAT TM consists of 7 bands that have these characteristics:

Band No.WavelengthInterval (µm)

SpectralResponse

Resolution (m)

1 0.45 - 0.52 Blue-Green 30

2 0.52 - 0.60 Green 30

3 0.63 - 0.69 Red 30

4 0.76 - 0.90 Near IR 30

5 1.55 - 1.75 Mid-IR 30

6 10.40 - 12.50 Thermal IR 120

7 2.08 - 2.35 Mid-IR 30

TIR ImageryClouds seen in the TIR (band 6 left) and visible (band 3

right) of ETM+ from July

CLASS Part.: WHY?

TIR Imagery TIR “Shadows” seen in the ETM+ band 6 image left are of far different nature than those of the band 3 shadows

TIR Imagery Canyons act as blackbody as well as have higher

temperatures due to lower elevations

GOES image hereshows low radianceas bright

Note the GrandCanyon is plainlyVisible

Also evident are land-water boundaries (andnot just because ofthe lines drawn toshow them)