introduction to the course & principles of radiative transfer, scattering & orbits

Michael D. King, EOS Senior Project Scientist August 25, 20021

Introduction to the Course & Principles of Radiative Transfer,

Scattering & Orbits

Introduction to the Course & Principles of Radiative Transfer,

Scattering & OrbitsMichael D. King

NASA Goddard Space Flight Center

Outline Physical principles behind the remote sensing of atmosphere,

land, and ocean properties from Terra Light scattering and emission of the Earth-atmosphere-surface

system– Spacecraft, spatial resolution, swath width, and sensor

characteristics Satellite orbits and repeat coverage required for global

observations Fundamental concepts and terminology of radiative transfer

– Atmospheric absorption and transmission characteristics– Radiance & irradiance– Scattering phase function– Optical thickness– Single and multiple scattering


Remote Sensing OverviewRemote Sensing Overview

What is “remote sensing”?– Using artificial devices, rather than our eyes, to observe or

measure things from a distance without disturbing the intervening medium» It enables us to observe & measure things on spatial,

spectral, & temporal scales that otherwise would not be possible

» It allows us to observe our environment using a consistent set of measurements throughout the globe, without prejudice associated with national boundaries and accuracy of datasets or timeliness of reporting

How is remote sensing done?– Electromagnetic spectrum

» Passive sensors from the ultraviolet to the microwave» Active sensors such as radars and lidars

– Satellite, airborne, and surface sensors– Training and validation sites


Remote Sensing Applications to be Covered in this Course

Remote Sensing Applications to be Covered in this Course

History of remote sensing & global change Remote sensing of land surface properties

– Spectral and angular reflectance, land cover & land cover change

– Fire monitoring and burn scars– Leaf area index & flux of photosynthetically active radiation– Temperature & emissivity separation of terrestrial surfaces

Remote sensing of atmospheric properties– Cloud cover, cloud optical properties, and cloud top properties– Aerosol properties– Water vapor– Atmospheric chemistry (carbon monoxide and methane)– Earth radiation budget and cloud radiative forcing

Remote sensing of the oceans from space– Chlorophyll concentration and biological productivity of the

oceans – Sea surface temperature using thermal methods

Angular directional models of the Earth-atmosphere-ocean system


Remote sensing uses the radiant energy that is reflected and emitted from Earth at various “wavelengths” of the electromagnetic spectrum

Our eyes are only sensitive to the “visible light” portion of the EM spectrum

Why do we use nonvisible wavelengths?

The Electromagnetic SpectrumThe Electromagnetic Spectrum


Visible Spectrum

Wavelength (µm)

0.4 0.5 0.6 0.7

Schematic Wave of RadiationSchematic Wave of Radiation

From Parkinson, C. L., 1997: Earth from Above


Blackbody Radiation CurvesBlackbody Radiation Curves


Spectral Characteristics of Energy Sources and Sensing Systems

Spectral Characteristics of Energy Sources and Sensing Systems


Atmospheric Absorption in the Wavelength Range from 0-15 µmAtmospheric Absorption in the

Wavelength Range from 0-15 µm


Basic Interactions between Electromagnetic Energy and the

Earth’s Surface

Basic Interactions between Electromagnetic Energy and the

Earth’s Surface


Generalized Spectral Reflectance Envelopes for Deciduous and

Coniferous Trees

Generalized Spectral Reflectance Envelopes for Deciduous and

Coniferous Trees


Typical Spectral Reflectance Curves for Vegetation, Soil, and

Water

Typical Spectral Reflectance Curves for Vegetation, Soil, and

Water


Atmospheric Effects Influencing the Measurement of Reflected

Solar Energy

Atmospheric Effects Influencing the Measurement of Reflected

Solar Energy


Atmospheric Transmission SpectraAtmospheric Transmission Spectra

Wavelength (µm)

0.2 250.0

0.2

0.6

0.8

0.4

1 10

1.0

Tra

nsm

issi

on

UV VNIR

SWIR

MWIR

LWIR


Low Earth Orbit ConceptsLow Earth Orbit Concepts

Equator

South Pole

Ground track

Ascending node

Inclination angle

Descending node

Orbit

Perigee

Apogee

Orbit


Terra Satellite in Low Earth Orbit (LEO)

Terra Satellite in Low Earth Orbit (LEO)


Satellites in Geosynchronous Orbits are used as Relay Satellites

for LEO Spacecraft

Satellites in Geosynchronous Orbits are used as Relay Satellites

for LEO SpacecraftImaging

System (e.g., Terra)

Communication relay system

Communication relay

system (e.g., TDRSS)

GEO

LEOGround station


Scattering of Sunlight by the Earth-Atmosphere-Surface System


A = radiation transmitted through the atmosphere and reflected by the surface

B = radiation scattered by the atmosphere and reflected by the surface

C = radiation scattered by the atmosphere and into the ‘radiometer’

G = radiation transmitted through the atmosphere, reflected by background objects, and subsequently reflected by the surface towards the ‘radiometer’

I = ‘adjacency effect’ of reflectance from a surface outside the field of view of the sensor into its field of view


Thermal Emission from the Earth-Atmosphere-Surface System

Thermal Emission from the Earth-Atmosphere-Surface System

D

H

E

F

D = radiation emanating directly from the target

E = radiation emanating from the atmosphere downward and subsequently reflected by the surface towards the ‘radiometer’

F = radiation self-emitted by the atmosphere

H = radiation emitted by background objects and subsequently reflected by the target into the direction of the observer


Irradiance (Flux per unit Area)Irradiance (Flux per unit Area)

E0

E = E0

cos

E0

E=irradiance = flux per unit area [Wm-2]

= ddA


Element of Solid AngleElement of Solid Angle

d = [sr]dAr2


d2dAcosd

dEdcos

N

I

Intensity (or Radiance)Intensity (or Radiance)

I = flux per unit area per unit solid angle normal to the direction of propagation [Wm-2sr-1]

= =


Projected Area Effects on Irradiance

Projected Area Effects on Irradiance

E0

A = Au/cos

Au

Au

N

E0

E0

E = E0cosAu

N

N

Irradiance crossing area A at angle of incidence is reduced from that on a normal surface due to the growth in cross sectional area

P

P

P


Solid Angle Representation on Spherical Coordinates

Solid Angle Representation on Spherical Coordinates

sin

z

d

d

d

sind

d

y

x

d

d = sindd


Angular Scattering CoefficientAngular Scattering Coefficient

Propagating beam

d

Scattering center

Unit length

Angular scattering coefficient [()]– Fractional amount of energy scattered into the direction

per unit solid angle per unit length of transit [m-1 sr-1]


Volume Scattering and Extinction Coefficient

Volume Scattering and Extinction Coefficient

Volume scattering coefficient [sca]

– Fractional amount of energy scattered in all directions per unit length of transit [m-1]

sca =

=

Volume absorption coefficient [abs]

– Fractional amount of energy absorbed per unit length of transit [m-1]

Volume extinction coefficient [ext]

– Fractional amount of energy attenuated per unit length of transit [m-1]

ext= sca + abs

Single scattering albedo [0]

– Fraction of energy scattered to that attenuated

0 = sca/(sca + abs)

€

()d∫

€

()sinddφ0

π

∫0

2π

∫


Optical depth []– Total attenuation along a path length, generally a function

of wavelength [dimensionless]

Total optical thickness of the atmosphere [t]

– Total attenuation in a vertical path from the top of the atmosphere down to the surface

Transmission of the direct solar beam

Optical ThicknessOptical Thickness

t =exp[-t()]

t =exp[-t()/µ0]

0

µ0 = cos0

€

()= extdx0

X

∫

€

t()= extdz0

∞

∫


Scattering phase function is defined as the ratio of the energy scattering per unit solid angle into a particular direction to the average energy scattered per unit solid angle into all directions

with this definition, the phase function obeys the following normalization

Rayleigh (molecular) scattering phase function

Scattering Phase FunctionScattering Phase Function

€

(cos)= ()()d∫

4π

=4π()sca

€

1= 14π

(cos)d0

1

∫0

2π

∫

€

=12

(cos)dcos−1

1

∫

€

(cos)=34

(1+cos2 )


Shapes of Scattering Phase Function


Rayleigh (molecular)Composite

180°

90°

270°

0°

45°135°

225° 315°




Nonselective scattering

Mie scattering

180°

90°

270°

0°

45°135°

225° 315°


Composition of Atmospheric Transmission

Composition of Atmospheric Transmission

Exoatmospheric solar irradiance

Exitance (300 K)

Atmospheric

transmission

Wavelength (µm)

0.2 2510

-2 1 10

Irra

dia

nce (

Wm

-2µ

m-

1)

10-1

100

101

102

103

104

0.0

0.5

1.0

Tra

nsm

ission


Absorption Properties of the Earth’s Atmosphere


0

50

100

0

50

100

0

50

100

0

50

1000 2 4 6 8 1

012

14Wavelength (µm)

0

50100

0

50100

0

50100

0

50100

H2O

Ab

sorp

tion

O3

CO

CO2




0

50

100

0

50

100

0

50

100

0

50

1000 2 4 6 8 1

012

14Wavelength (µm)

0

50100

0

50100

0

50100

0

50100

CH4

Ab

sorp

tion

N2O

O2

Total




Exoatmospheric solar irradiance F0()

Solar irradiance reaching the surface F()

0 21Wavelength (µm)

3

2000

Irra

dia

nce (

W m

-2 µ

m-1)

1500

1000

500

0


Definition of Solar Zenith, View Zenith, and Relative Azimuth

Angle

Definition of Solar Zenith, View Zenith, and Relative Azimuth

Angle

0

N

E

W

S

0


The reflection function is defined by

R(t, 0; µ, µ0, ) =where

t = total optical thickness

0 = the single scattering albedo (ratio of scattering to total extinction)

µ = absolute value of the cosine of the zenith angle |cos|µ0 = cosine of the solar zenith angle cos0

= relative azimuth angle between the direction of propagation of the emerging radiation and the incident solar direction

I = reflected intensity (radiance) in the outward (–µ) directionF0 = incident solar flux (irradiance) in W m-2 µm-1

Note: R, t, 0, F0 and I are all functions of wavelength

Definition of Reflection FunctionDefinition of Reflection Function

πI(0, –µ, )µ0F0


The transmitted flux (irradiance) at the Earth’s surface can be calculated as:

where the transmission function is defined in an analogous manner to reflection function

Flux (Irradiance) on a Horizontal Surface at the Surface of the EarthFlux (Irradiance) on a Horizontal

Surface at the Surface of the Earth

€

E(t ,0 ;μ0 ,μ,φ)= I(t ,0 ;μ0 ,μ,φ)μdμdφ+μ0F0 (exp−0

1

∫0

2π

∫ t / μ0 )

€

=μ0F01π

T(t ,0 ;μ0 ,μ,φ)μdμdφ+ (exp−0

1

∫0

2π

∫ t / μ0 )⎡

⎣⎢⎢

⎤

⎦⎥⎥

€

€

T(t ,0 ;μ0 ,μ,φ)=πI(t ,μ,φ)μ0F0


Reflectance Properties of Idealized Surfaces

Reflectance Properties of Idealized Surfaces

Specular reflector Diffuse

Nearly diffuse

Less idealized surface

Nearly specular

‘Lambertian’


Bidirectional Reflectance ConceptBidirectional Reflectance Concept

0

0


University of Washington CV-580University of Washington CV-580

Solar Spectral Flux

Radiometer (SSFR)

Ames Airborne Tracking

Sunphotometer (AATS)

Cloud Absorption Radiometer

(CAR)


Goddard Space Flight Center– developed in 1982-1983

University of Washington– integrated & flown in 1984 (B-23)– principal data from 1987-97 (C-

131A)– flights after 1998 (CV-580)

Sensor Characteristics– 14 spectral bands ranging from

0.34 to 2.29 µm– scan ±95° from horizon on right-

hand side of aircraft– field of view 17.5 mrad (1°)– scan rate 1.67 Hz (100 rpm)– data system 9 channels @ 16 bit– 395 pixels in scan line– 4% reflectance calibration

accuracy

Cloud Absorption RadiometerCloud Absorption Radiometer


Roll: ~20° Time: ~2 min Speed: ~80 m s-1

Height: ~600 m Diameter: ~3 km Resolution

– 10 m (nadir)– 270 m ( = 80°)

Channels– 8 continuously

sampled (0.34-1.25 µm)

– 2 filter wheel channels used for BRDF measurements (1.64 & 2.20 µm)

Bidirectional Reflectance Measurements

Bidirectional Reflectance Measurements


= 1.64 µm = 0.67 µm

0.0

Bidirectional Reflectance - Tundra Melt Season (0 = 81°)


0.2

0.4

0.6

0.8

1.0




Snow Free Tundra


= 1.64 µm = 0.67 µm

Bidirectional Reflectance - Atlantic Ocean

Sunglint (0 = 19°)


Sunglint (0 = 19°)

0.0

0.2

0.4

0.6

0.8

1.0


Atlantic Ocean (sun glint)


Sunglint (0 = 19°)


Sunglint (0 = 19°)

0.0

0.1

0.2

0.3

0.4

0.5

0.472 µm0.675 µm0.869 µm1.038 µm1.219 µm1.271 µm1.643 µm2.207 µm

Opposition0

90 60 030 30 60 90

Backward ScatteringAngle Forward Scattering

SunGlint


TRMM11/27/97

Terra12/18/9

9

QuikScat

6/19/99

Landsat 7

4/15/99

NASA Earth Science Spacecraft in Orbit



EO-111/21/00

SAGE III

12/10/01

Jason-1

12/7/01



Aqua5/4/02


Aura1/04

SORCE12/02

ICESat

12/02

EOS Spacecraft Under Development

EOS Spacecraft Under Development


NASA ER-2 High Altitude Research Aircraft

NASA ER-2 High Altitude Research Aircraft


ER-2 Pilot in SpacesuitER-2 Pilot in Spacesuit


Skukuza, South Africa Maun, Botswana

Flux TowerFlux Tower

Photograph courtesy of Michael KingPhotograph courtesy of NASA Dryden


Surface Instrumentation for Measuring Shortwave Radiation

Surface Instrumentation for Measuring Shortwave Radiation

Photograph courtesy of NASA


New EOS science results published in lay terms on NASA’s award-winning Web site:

earthobservatory.nasa.gov


Natural Hazards Section– Dust & Smoke– Fires– Floods– Severe Storms– Volcanoes– Unique Imagery

Timely, newsworthy imagery posted as thumbs, medium sized, and full-resolution

Weekly updates of maps showing locations & severity of hazards around the globe


Severe Storm ExampleSuper Typhoon Fengshen

introduction to the course & principles of radiative transfer, scattering & orbits

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