Most remote sensing instruments are designed to detect solar radiation and terrestrial radiation

Solar radiation

• emr emitted from sun which passes through the atmosphere and is reflected in varying degrees by Earth's surface and atmosphere
• detectable only during daylight

• Sun's visible surface (photosphere) has temperature - 6000K
• energy radiated from gamma to radio waves
• 99% of sun's radiation fall between 0.2 - 5.6um; 80% - 0.4 - 1.5um (visible and reflected infrared, atmosphere quite transparent to incoming solar radiation
• maximum radiation occurs 0.48um (visible)

Problem Set (#2)

• about 1/2 of solar radiation passes through the atmosphere and absorbed in varying degrees by surface

Terrestrial radiation

• energy emitted from the Earth and atmosphere
• detectable both day and night

• Earth's ambient temperature - 300K
• Earth radiates 160,000 times less than the sun
• essentially all energy is radiated at (invisible) thermal infrared wavelengths between 4-25um
• maximum emission occurs at 9.7um

Problem Set (#3)

Wavelengths covering most of Earth's energy output are several times longer than those covering most of the solar output.  Therefore, refer to following as:

terrestrial radiation - longwave radiation

solar radiation - shortwave radiation

Radiation-Matter Interactions

Incident radiation - energy impinges upon matter

• strongest source of incident radiation for earth is sun
• incoming solar radiation called Insolation
• full moon is 2nd strongest source

When EMR strikes matter 3 interactions may occurs:

• transmission
• reflection
• absorption

Proportion of energy that is transmitted, reflected or absorbed depends upon:

• composition and physical properties of medium
• wavelength or frequency of incident radiation
• angle at which incident radiation strikes a surface

Transmission - process by which incident radiation passes through matter w/o measurable attenuation (transparent to radiation); cause change in velocity and wavelength but not frequency

Specular reflection - process whereby incident radiation "bounces off" the surface of substance in a single, predictable direction; caused by surfaces smooth relative to wavelengths of incident radiation; no change in velocity or wavelength

Scattering (diffuse reflection) - occurs when incident radiation is dispersed or spread out unpredictably in many different directions; occurs when surfaces rough relative to wavelengths of incident radiation; no change in velocity or wavelength

Absorption - process by which incident radiation is taken in by the medium (e.g., surface, atmospheric particulates, atmospheric layer); medium opaque to incident radiation

Interrelationships between energy interactions expressed as:

   Equation (1)

Problem Set (#7)

Opaque materials transmit no incident radiation

Transparent material have little or no absorption and scattering

E.g.,

clear glass - high transmission, low reflection and absorption

fresh snow - high reflectance, low transmission and absorption

fresh asphalt - high absorption, minimum transmission and reflection

EMR-Atmosphere Interactions

EMR travels through space w/o modification

Diversion and depletions occurs as solar and terrestrial radiation interact with earth's atmosphere

Interference is wavelength selective - meaning at certain wavelengths emr passes freely through atmosphere, whereas restricted at other wavelengths

atmospheric windows (transmision bands) - areas of ems where specific wavelengths pass relatively unimpeded through atmosphere

absorption bands (atmospheric blinds) - areas where specific wavelengths are totally or partially blocked

Objective to study earth's surface - different remote sensing instruments designed to operate w/i windows where cloudless atmosphere will transmit sufficient radiation for detection

Objective to study atmosphere constituents - operate in atmospheric windows and absorption bands

EMR interacts w/ atmosphere in # of ways:

absorbed and reradiated at longer wavelengths (causes air temperature to rise)

reflected or scattered w/o change to either its velocity or wavelength

transmitted in straight-line path directly through the atmosphere

Radiation Balance

Incoming solar radiation = Outgoing longwave radiation

100 = 35 (reflected - albedo) + 65 (terrestrial emitted)

Problem Set #5

Atmospheric Absorption and Transmission

Most significant absorbers of EMR:

ozone
carbon dioxide
water vapor
oxygen
nitrogen

 

Absorption-transmission characteristics of cloud-free atmosphere shows gases responsible for EMR absorption as function of wavelength

16% of shortwave solar radiation absorbed directly by atmospheric gases

2% by clouds

Atmospheric gases - selective absorbers w/ reference to wavelength

Gamma and X-ray - completely absorbed in the upper atmosphere by Oxygen and Nitrogen

Ultraviolet (<0.2um) - absorbed by molecules of oxygen (O and O2 combine form ozone); ozone absorbs UV w/ wavelengths -0.2-0.3um in stratosphere

0.9-2.7um - water vapor and carbon dioxide absorb in narrow bands

thermal infrared

• strong absorption by water vapor between 5-8um and 20um-1,000um (1cm)
• carbon dioxide absorbs 14-20um
• ozone 9-10um

absorbed radiation heats the lower atmosphere

microwave region - 3 relatively narrow absorption bands occur between 0.1 - 0.6cm (oxygen and water vapor)

beyond 0.6cm , atmospheric gases generally do not impede passage of microwave radiation

Summary

absorption of atmospheric gases has maximum influence in wavelengths <0.3um and minimum impact on wavelengths greater than 0.6cm

important atmospheric windows exploited in remote sensing -

0.3 - 1.1um         UV, visible, near infrared

1.5 - 1.8um        Mid infrared

2.0 - 2.4um        Mid infrared

3.0 - 5.0um        Mid infrared

8.0 - 14.0um       Thermal Infrared

   (below ozone layer)

10.5 - 12.5          Thermal Infrared

                                (above ozone layer)

                          >0.6cm                Microwave

Atmospheric windows become less transparent when air is moist (high humidity)

Clouds absorb most of longwave radiation emitted from Earth's surface

Microwave radiation (>0.9cm) capable of penetrating clouds

Atmospheric Scattering

Scattering process disperses radiation in all directions

Important scattering agents include:

• gaseous molecules
• suspended particulates (aerosols)
• clouds

3 types of atmospheric scattering are important in remote sensing

Rayleigh (molecular) scattering

- primarily caused by oxygen and nitrogen molecules (diameters at least 0.1 times smaller than affected wavelengths)

- most influential at altitudes above 4.5km

- amount of scattering inversely proportional to fourth power of wavelength

Equation (2)

E.g. UV at 0.3um scattered 16x as readily as red 0.6um

Blue 0.4um scattered about 5x as readily as red

- blue sky - preferential scattering of blue wavelengths, clear sky appears blue in daylight; blue wavelengths reach our eyes

- brilliant colors of sunrise/sunset - solar beam starts out as white light passes though long atmosphere path causing shorter wavelengths of sunlight to be scattered away leaving only longer red wavelengths that reach our eyes

Mie (nonmolecular) scattering

- occurs when there are sufficient particles in atmosphere w/ mean diameter 0.1 to 10 times larger than wavelength under consideration

- caused by water vapor, tiny particles of smoke, dust, volcanic ejecta, salt crystals released from evaporation of sea spray

- most pronounced in lower 4.5km of atmosphere

- wavelength dependence varies 1/wavelength

dependent on size distribution and concentration of mie particles

Clear atmosphere is a medium for both Rayleigh and Mie scattering

Nonselective scattering

- occurs when lower atmosphere contains sufficient # of suspended aerosols (diameters 10 times larger than wavelengths under consideration)

- important agents include larger equivalents of Mie particles, water droplets and ice crystals that compose clouds and fogs

- scattering is independent of wavelength (near UV, visible, near infrared)

clouds appear brilliant white - colorless water droplet and ice crystals scatter all wavelengths equally well w/i visible

Skylight and Haze

Clear sky is source of illumination because its gases preferentially scatter shorter wavelengths of sunlight

diffuse radiation (skylight, sky radiation)

Natural and man-made (cultural) features of Earth's surface interact with solar radiation differently

On average, 50% of incident shortwave radiation on TOA reaches and interacts with Earth's surface features

50% incident @ surface = 4% reflected directly + 46% absorbed

Absorbed

- proportion of absorbed shortwave radiation is reradiated or emitted back to atmosphere as longwave terrestrial radiation (5%)

- most heat emitted at wavelengths falling within thermal infrared atmospheric windows; contains information about different temperature properties of Earth's surface features

Albedo - average amount of incident radiation reflected by an object/feature

Equation (3)

Albedo of Earth's - atmosphere system (50% cloud cover) - 30%

- meaning 30% of insolation is reflected, 70% absorbed

- earth made visible from space only by its albedo

Earth's brightest features - clouds, snow and ice surfaces; darkest - water bodies

Percent reflected energy from Earth's surface objects/features

Albedo also helps explain how warm an object becomes when exposed to sunlight

-objects w/ high albedo are good reflectors but poor absorbers (dictates slow and small temperature increases)

- objects w/ low albedo are poor reflectors but good absorbers (dictates rapid and large jumps in temperature when exposed to sunlight)

E.g., walking barefoot on black asphalt versus grass

wearing light or dark clothing on summer day

 

Every natural and synthetic object reflects and emits emr over a range of wavelengths in its own characteristic way according to is chemical composition and physical state

spectral signatures - distinctive reflectance and emittance properties of objects/features and their conditions

- w/i some limited wavelength region, particular object/feature or condition exhibit a diagnostic spectral response pattern that differs from other objects

- remote sensing depends upon operation in wavelength regions where detectable differences in reflected and emitted radiation occur; features and their different conditions show enough variation to allow for individual identification
 

Typical spectral signature of vegetation, soil and water

 

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Last revised on September 23, 1999 by Tracy DeLiberty.