GEOGG141/ GEOG3051 - PowerPoint Presentation

Slide1

GEOGG141/ GEOG3051Principles & Practice of Remote SensingEM Radiation (ii)

Dr. Mathias (Mat) DisneyUCL GeographyOffice: 113, Pearson BuildingTel: 7679 0592Email: mdisney@ucl.geog.ac.ukhttp://www2.geog.ucl.ac.uk/~mdisney/teaching/GEOGG141/GEOGG141.htmlhttp://www2.geog.ucl.ac.uk/~mdisney/teaching/3051/GEOG3051.html Slide2

2

EMR arriving at EarthWe now know how EMR spectrum is distributedRadiant energy arriving at Earth’s surfaceNOT blackbody, but close“Solar constant”solar energy irradiating surface perpendicular to solar beam~1373Wm

-2

at top of atmosphere (TOA)

Mean distance of sun ~1.5x10

8

km so total solar energy emitted = 4



r

2

x1373 = 3.88x10

26

W

Incidentally we can now calculate T

sun

(radius=6.69x10

8

m) from SB Law



T

4

sun

= 3.88

x10

26

/

4



r

2

so T = ~5800K Slide3

3

Departure from blackbody assumptionInteraction with gases in the atmosphereattenuation of solar radiationSlide4

4

Radiation Geometry: spatial relationsNow cover what happens when radiation interacts with Earth SystemAtmosphereOn the way down AND way upSurfaceMultiple interactions between surface and atmosphereAbsorption/scattering of radiation in the atmosphereSlide5

5

Radiation passing through mediaVarious interactions, with different results

From http://rst.gsfc.nasa.gov/Intro/Part2_3html.htmlSlide6

6

Radiation Geometry: spatial relationsDefinitions of radiometric quantitiesFor parallel beam, flux density defined in terms of plane perpendicular to beam. What about from a point?

Schaepman-Strub

et al. (

2006)

see

http://www2.geog.ucl.ac.uk/~

mdisney

/teaching/PPRS/papers/

schaepman_et_al.pdf

Slide7

7

Radiation Geometry: point source

d



d

ϕ

d

A

Point source

r

Consider flux

dϕ

emitted from point source into solid angle d



, where

d

F

and d



very small

Intensity

I

defined as flux per unit solid angle i.e.

I

=

dϕ

/

d



(Wsr

-1

)

Solid angle d



=

dA

/r

2

(

steradians

,

sr

)Slide8

8

Radiation Geometry: plane sourcedS cos



d

ϕ

Plane source dS

What about when we have a plane source rather than a point?

Element of surface with area

dS

emits flux

d

ϕ

in direction at angle

 to normal

Radiant

exitance

,

M =

d

ϕ

/

dS

(

Wm

-2

)

Radiance L

is

intensity in a particular direction

(

dI

=

d

ϕ

/

)

divided by the apparent area of source in that direction

i.e. flux per unit area per solid angle (

Wm

-2

sr

-1

)

Projected

area of

dS

is direction



is

dS

cos

, so…..

Radiance L = (

d

ϕ/) / dS cos  = dI/dS cos  (Wm-2sr-1)



Slide9

9

Radiation Geometry: radianceSo, radiance equivalent to:intensity of radiant flux observed in a particular direction divided by apparent area of source in same directionNote on solid angle (steradians):3D analog of ordinary angle (radians)1 steradian = angle subtended at the centre of a sphere by an area of surface equal to the square of the radius. The surface of a sphere subtends an angle of 4 steradians at its centre.Slide10

10

Radiation Geometry: solid angleCone of solid angle  = 1sr from sphere = area of surface A / radius2

From http://www.intl-light.com/handbook/ch07.html

Radiant intensitySlide11

11

Radiation Geometry: cosine lawEmission and absorptionRadiance linked to law describing spatial distn of radiation emitted by Bbody with uniform surface temp. T (total emitted flux = T4)Surface of Bbody then has same T from whatever angle viewedSo intensity of radiation from point on surface, and areal element of surface MUST be independent of , angle to surface normal

OTOH flux per unit solid angle divided by

true

area of surface must be proportional to cos

Slide12

12

Radiation Geometry: cosine lawCase 1: radiometer ‘sees’ dA, flux proportional to dACase 2: radiometer ‘

sees

’

dA

/cos



(larger) BUT T same, so

emittance

of surface same and hence radiometer measures same

So flux emitted per unit area at angle





to cos



so that product of

emittance

(



cos



) and area emitting (



1/ cos



) is same for all



This is basis of

Lambert’s Cosine Law

Radiometer

X

Y

X

Y

Radiometer

dA



dA/cos



Adapted from

Monteith

and

Unsworth

, Principles of Environmental PhysicsSlide13

13

Radiation Geometry: Lambert’s cosine lawRadiant intensity observed from a ideal diffusely reflecting surface (Lambertian surface) surface directly proportional to cosine of angle between view angle and surface normal

http://

en.wikipedia.org

/wiki/

Lambert's_cosine_law

Emission

rate (photons/s) in a normal and off-normal direction. The number of photons/sec directed into any wedge is proportional to the area of the wedge.

Observed

intensity

(W/cm2

·sr)) for a normal and off-normal observer; dA0 is the area of the observing aperture and

dΩ

is the solid angle subtended by the aperture from the viewpoint of the emitting area element.Slide14

14

Radiation Geometry: Lambert’s Cosine LawWhen radiation emitted from Bbody at angle  to normal, then flux per unit solid angle emitted by surface is  cos 

Corollary of this:

if Bbody exposed to beam of radiant energy at an angle



to normal, the flux density of absorbed radiation is



cos



In remote sensing we generally need to consider directions of both incident AND reflected radiation, then reflectivity is described as bi-directional

Adapted from Monteith and Unsworth, Principles of Environmental PhysicsSlide15

15

Recap: radianceRadiance, Lpower emitted (dϕ) per unit of solid angle (d) and per unit of the projected surface (dS cos



) of an extended widespread source in a given direction,



(



= zenith angle,



= azimuth angle)

L =

d

2

ϕ

/ (d



dS

cos



) (in Wm

-2

sr

-1

)

If radiance is not dependent on



i.e. if same in all directions, the source is said to be Lambertian. Ordinary surfaces rarely found to be Lambertian.

Ad. From http://ceos.cnes.fr:8100/cdrom-97/ceos1/science/baphygb/chap2/chap2.htm

d



Projected surface dS cos



Slide16

16

Recap: emittanceEmittance, M (exitance)emittance (M) is the power emitted (dW) per surface unit of an extended widespread source, throughout an hemisphere. Radiance is therefore integrated over an hemisphere. If radiance independent of 

i.e. if same in all directions, the source is said to be Lambertian.

For Lambertian surface

Remember L =

d

2

ϕ

/ (d



dS

cos



) = constant, so M =

d

ϕ

/

dS

=

M =



L

Ad. From http://ceos.cnes.fr:8100/cdrom-97/ceos1/science/baphygb/chap2/chap2.htmSlide17

17

Recap: irradianceRadiance, L, defined as directional (function of angle)from source dS along viewing angle of sensor ( in this 2D case, but more generally (, ) in 3D case)Emittance, M, hemispheric Why??

Solar radiation scattered by atmosphere

So we have direct AND diffuse components

Ad. From http://ceos.cnes.fr:8100/cdrom-97/ceos1/science/baphygb/chap2/chap2.htm

Direct

DiffuseSlide18

18

ReflectanceSpectral reflectance, (), defined as ratio of incident flux to reflected flux at same wavelength () = L()/I()Extreme cases:Perfectly specular: radiation incident at angle  reflected away from surface at angle -Perfectly diffuse (Lambertian): radiation incident at angle  reflected equally in all anglesSlide19

19

Interactions with the atmosphereFrom http://rst.gsfc.nasa.gov/Intro/Part2_4.htmlSlide20

20

Interactions with the atmosphereNotice that target reflectance is a function ofAtmospheric irradiancereflectance outside target scattered into pathdiffuse atmospheric irradiancemultiple-scattered surface-atmosphere interactions

From: http://www.geog.ucl.ac.uk/~mdisney/phd.bak/final_version/final_pdf/chapter2a.pdf

R

1

target

R

2

target

R

3

target

R

4

targetSlide21

21

Interactions with the atmosphere: refractionCaused by atmosphere at different T having different density, hence refractionpath of radiation alters (different velocity)Towards normal moving from lower to higher density

Away from normal moving from higher to lower density

index

of refraction (n) is ratio of speed of light in a vacuum (c) to speed

c

n

in another medium (e.g. Air) i.e. n = c/

c

n

note that n always >= 1 i.e.

c

n

<= c

Examples

n

air

= 1.0002926

n

water

= 1.33Slide22

22

Refraction: Snell’s LawRefraction described by Snell’s LawFor given freq. f, n1 sin 1 = n2

sin



2

where



1

and



2

are the angles from the normal of the incident and refracted waves respectively

(non-turbulent) atmosphere can be considered as layers of gases, each with a different density (hence n)

Displacement of path - BUT knowing Snell

’

s Law can be removed

After: Jensen, J. (2000) Remote sensing of the environment: an Earth Resources Perspective.

n

1

n

3

n

2

Optically less dense

Optically more dense

Optically less dense

Incident radiation



2



3



1

Path affected by atmosphere

Path unaffected by atmosphereSlide23

23

Interactions with the atmosphere: scatteringCaused by presence of particles (soot, salt, etc.) and/or large gas molecules present in the atmosphere Interact with EMR anc cause to be redirected from original path. Scattering amount depends on: of radiationabundance of particles or gasesdistance the radiation travels through the atmosphere (path length)

After: http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter1/chapter1_4_e.htmlSlide24

24

Atmospheric scattering 1: Rayleigh Particle size <<  of radiatione.g. very fine soot and dust or N2, O2 molecules Rayleigh scattering dominates shorter 

and in upper atmos.

i.e. Longer



scattered less (visible red



scattered less than blue



)

Hence during day, visible blue



tend to dominate (shorter path length)

Longer path length at sunrise/sunset so proportionally more visible blue



scattered out of path so sky tends to look more red

Even more so if dust in upper atmosphere

http://www.spc.noaa.gov/publications/corfidi/sunset/

http://www.nws.noaa.gov/om/educ/activit/bluesky.htm

After: http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter1/chapter1_4_e.htmlSlide25

25

Atmospheric scattering 1: Rayleigh From http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html

So, scattering





-4

so scattering of blue light (400nm) > scattering of red light (700nm) by (700/400)

4

or ~ 9.4Slide26

26

Atmospheric scattering 2: Mie Particle size   of radiatione.g. dust, pollen, smoke and water vapour Affects longer  than Rayleigh, BUT weak dependence on



Mostly in the lower portions of the atmosphere

larger particles are more abundant

dominates when cloud conditions are overcast

i.e. large amount of water vapour (mist, cloud, fog) results in almost totally diffuse illumination

After: http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter1/chapter1_4_e.htmlSlide27

27

Atmospheric scattering 3: Non-selective Particle size >>  of radiatione.g. Water droplets and larger dust particles, All  affected about equally (hence name!)

Hence results in fog, mist, clouds etc. appearing white

white = equal scattering of red

,

green and blue



s

After: http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter1/chapter1_4_e.htmlSlide28

28

Atmospheric absorption Other major interaction with signalGaseous molecules in atmosphere can absorb photons at various depends on vibrational modes of molecules

Very dependent on



Main components are:

CO

2

, water vapour and ozone (O

3

)

Also CH

4

....

O

3

absorbs shorter



i.e. protects us from UV radiationSlide29

29

Atmospheric absorption CO2 as a “greenhouse” gas

strong absorber in longer (thermal) part of EM spectrum

i.e. 10-12



m where Earth radiates

Remember peak of Planck function for T = 300K

So shortwave solar energy (UV,

vis

, SW and NIR) is absorbed at surface and re-radiates in thermal

CO

2

absorbs re-radiated energy and keeps warm

$64M question!

Does increasing CO

2



increasing T??

Anthropogenic global warming??

Aside....Slide30

30

Atmospheric CO2 trendsKeeling et al.Annual variation + trendSmoking gun for anthropogenic change, or natural variation??

Antarctic ice core recordsSlide31

31

Atmospheric “windows”As a result of strong  dependence of absorptionSome



totally unsuitable for remote sensing as most radiation absorbed

Atmospheric windowsSlide32

32

Atmospheric “windows”If you want to look at surfaceLook in atmospheric windows where transmissions highIf you want to look at atmosphere however....pick gapsVery important when selecting instrument channelsNote atmosphere nearly transparent in wave i.e. can see through clouds!V. Important consideration....Slide33

33

Atmospheric “windows”Vivisble + NIR part of the spectrumwindows, roughly: 400-750, 800-1000, 1150-1300, 1500-1600, 2100-2250nmSlide34

34

SummaryMeasured signal is a function of target reflectanceplus atmospheric component (scattering, absorption)Need to choose appropriate regions (atmospheric windows)μ-wave region largely transparent i.e. can see through clouds in this regionone of THE major advantages of μ-wave remote sensingTop-of-atmosphere (TOA) signal is NOT target signal To isolate target signal need to...Remove/correct for effects of atmosphereA major part component of RS pre-processing chainAtmospheric models, ground observations, multiple views of surface through different path lengths and/or combinations of aboveSlide35

35

SummaryGenerally, solar radiation reaching the surface composed of<= 75% direct and >=25 % diffuseattentuation even in clearest possible conditionsminimum loss of 25% due to molecular scattering and absorption about equallyNormally, aerosols responsible for significant increase in attenuation over 25%HENCE ratio of diffuse to total also changesAND spectral composition changesSlide36

36

ReflectanceWhen EMR hits target (surface)Range of surface reflectance behaviourperfect specular (mirror-like) - incidence angle = exitance angleperfectly diffuse (Lambertian) - same reflectance in all directions independent of illumination angle)

From http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter1/chapter1_5_e.html

Natural surfaces somewhere in betweenSlide37

37

Surface energy budgetTotal amount of radiant flux per wavelength incident on surface, () Wm-1 is summation of:reflected r



, transmitted t



, and absorbed, a



i.e.



(



) = r



+ t



+ a



So need to know about surface reflectance, transmittance and absorptance

Measured RS signal is combination of all 3 components

After: Jensen, J. (2000) Remote sensing of the environment: an Earth Resources Perspective.Slide38

38

Reflectance: angular distributionReal surfaces usually display some degree of reflectance ANISOTROPYLambertian surface is isotropic by definitionMost surfaces have some level of anisotropy

From: http://www.geog.ucl.ac.uk/~mdisney/phd.bak/final_version/final_pdf/chapter2a.pdf

Figure 2.1

Four examples of surface reflectance: (a) Lambertian reflectance (b) non-Lambertian (directional) reflectance (c) specular (mirror-like) reflectance (d) retro-reflection peak (hotspot).

(a)

(b)

(c)

(d)Slide39

39

Directional reflectance: BRDFReflectance of most real surfaces is a function of not only λ, but viewing and illumination anglesDescribed by the Bi-Directional Reflectance Distribution F

unction

(BRDF)

BRDF of area



A defined as: ratio of incremental radiance, dL

e

, leaving surface through an infinitesimal solid angle in direction



(



v

,



v

), to incremental irradiance, dE

i

, from illumination direction



’

(



i

,



i

) i.e.

After: Jensen, J. (2000) Remote sensing of the environment: an Earth Resources Perspective.



is viewing vector (

v

,



v

) are view zenith and azimuth angles;

’

is illum. vector (

i

,

i

) are illum. zenith and azimuth anglesSo in sun-sensor example,

 is position of sensor and ’ is position of sunSlide40

40

Directional reflectance: BRDFNote that BRDF defined over infinitesimally small solid angles , ’ and  interval, so cannot measure directlyIn practice measure over some finite angle and  and assume valid

From: http://www.geog.ucl.ac.uk/~mdisney/phd.bak/final_version/final_pdf/chapter2a.pdf

surface area



A

surface tangent vector



i

2



-



v



v



i

incident solid angle



incident diffuse radiation

direct irradiance (



E

i

) vector



exitant solid angle



viewer

Configuration of viewing and illumination vectors in the viewing hemisphere, with respect to an element of surface area,



A.Slide41

41

Directional reflectance: BRDFSpectral behaviour depends on illuminated/viewed amounts of materialChange view/illum. angles, change these proportions so change reflectance Information contained in angular signal related to size, shape and distribution of objects on surface (structure of surface)Typically CANNOT assume surfaces are Lambertian (isotropic)

From: http://www.geog.ucl.ac.uk/~mdisney/phd.bak/final_version/final_pdf/chapter2a.pdf

Modelled barley reflectance,



v

from –50

o

to 0

o

(left to right, top to bottom).Slide42

42

Directional InformationSlide43

43

Directional InformationSlide44

44

Features of BRDF

Bowl shape

increased scattering due to increased path length through canopySlide45

45

Features of BRDFBowl shapeincreased scattering due to increased path length through canopySlide46

46Slide47

47

Features of BRDFHot Spotmainly shadowing minimumso reflectance higherSlide48

48

The “hotspot”See http://www.ncaveo.ac.uk/test_sites/harwood_forest/Slide49

49Slide50

50

Directional reflectance: BRDFGood explanation of BRDF:http://geography.bu.edu/brdf/brdfexpl.htmlSlide51

51

Hotspot effect from MODIS image over Brazil Slide52

52

Measuring BRDF via RSNeed multi-angle observations. Can do three ways:multiple cameras on same platform (e.g. MISR, POLDER, POLDER 2). BUT quite complex technically.Broad swath with large overlap so multiple orbits build up multiple view angles e.g. MODIS, SPOT-VGT, AVHRR. BUT surface can change from day to day.Pointing capability e.g. CHRIS-PROBA, SPOT-HRV. BUT again technically difficultSlide53

53

AlbedoTotal irradiant energy (both direct and diffuse) reflected in all directions from the surface i.e. ratio of total outgoing to total incomingDefines lower boundary condition of surface energy budget hence v. imp. for climate studies - determines how much incident solar radiation is absorbed

Albedo is BRDF integrated over whole viewing/illumination hemisphere

Define directional hemispherical refl (DHR) - reflectance integrated over whole viewing hemisphere resulting from directional illumination

and bi-hemispherical reflectance (BHR) - integral of DHR with respect to hemispherical (diffuse) illumination

DHR =

BHR =Slide54

54

AlbedoActual albedo lies somewhere between DHR and BHR

Broadband albedo,



, can be approximated as

where p(



) is proportion of solar irradiance at



; and



(



) is spectral albedo

so p(



) is function of direct and diffuse components of solar radiation and so is dependent on atmospheric state

Hence albedo NOT intrinsic surface property (although BRDF is)Slide55

55

Typical albedo valuesSlide56

56

Surface spectral informationCauses of spectral variation in reflectance?(bio)chemical & structural properties e.g. In vegetation, phytoplankton: chlorophyll concentration soil - minerals/ water/ organic matterCan consider spectral properties as continuous e.g. mapping leaf area index or canopy cover or discrete variable e.g. spectrum representative of cover type (classification) Slide57

57

Surface spectral information: vegetation

vegetationSlide58

58

Surface spectral information: vegetationvegetationSlide59

59

Surface spectral information: soil

soilSlide60

60

Surface spectral information: canopySlide61

61

SummaryLast weekIntroduction to EM radiation, the EM spectrum, properties of wave / particle model of EMR Blackbody radiation, Stefan-Boltmann Law, Wien’s Law and Planck functionThis week radiation geometry interaction of EMR with atmosphere

atmospheric windows

interaction of EMR with surface (BRDF, albedo)

angular and spectral reflectance properties