Energy Sources and Radiation Principles

Energy and Radiation

• Forms of electromagnetic energy:– visible light– heat– ultraviolet and X-rays– radio waves

Two Components of EM Radiation

• The waves are characterized by electrical and magnetic fields.

• The vibration of these fields is perpendicular to the direction of the wave.

• Electrical field (E): varies in magnitude in a direction perpendicular to the direction of propagation

• Magnetic field (M): at right angle to the electrical field, is propagated in phase with the electrical field

Components of EM Radiation

Three Properties of EM Energy

• Wavelength () • Frequency ()• Amplitude

Wavelength ()

• The linear distance between two successive wave crests or troughs.

• It is measured in meters (m), nanometers (nm)(10-9 m) or micrometers (10-6 m)

Frequency ()

• The number of wave crests or troughs that pass a fixed point per unit time.

• Measure units: hertz (cycle per second)

Amplitude

• The height of each peak• Measured as watts per square meter (energy

level)

Speed of EM

• Electromagnetic energy is traveling at the velocity of light:

velocity of light (c) = frequency (v)* wavelength ()

• Among the three properties, wavelength is the most commonly used in the field of remote sensing

Wavelength/Frequency

Long

Short

Low

High

Electromagnetic spectrum

• In RS, electromagnetic waves are categorized by their wavelength location within the electromagnetic spectrum.

• The total range of wavelengths is commonly referred to as the Electromagnetic spectrum

Major Divisions of EM Spectrum

• Ultraviolet spectrum: 0.3 - 0.38 micrometer (m)

• Visible portion: 0.4 – 0.7 m

– Blue: 0.4 - 0.5 m – Green: 0.5 - 0.6 m – Red: 0.6 - 0.7 m

Major Divisions of EM Spectrum

• Infrared spectrum (IR):

        - near infrared: 0.72 - 1.3 m         - mid infrared:  1.30 - 3.0 m         - thermal infrared:  beyond 3 – 14 m ,

emitted from the earth

• Microwave spectrum: 1mm - 1m

Electromagnetic spectrum

The sun produces a full spectrum of electromagnetic radiationThe sun produces a full spectrum of electromagnetic radiation

Electromagnetic (EM) Spectrum

Energy and Radiation

• Common RS sensors operate in visible, IR, or microwave portions.

• Only thermal IR energy is directly related to the sensation of heat.

Energy and Radiation

• Using particle theory (instead of wave theory), the electromagnetic radiation is composed of many discrete units called photons or quanta.

• The energy of a quantum is:Q=hv

Where Q is in Joules (J), h is Planck’s constant (J sec), and v is the frequency.

Energy and Radiation

• Combining the previous two equations (c=v and Q=hv):

Q=hc/

i.e. the longer the wavelength of a quantum, the lower its energy content.

Energy and Radiation

Wavelength and frequency

Four different types of electromagnetic waves, illustrationThe inverse relationship between wavelength and frequency

Wavelength and frequency

Energy and Radiation

• All matter at temperatures above 0 K (-273 C) continuously emits electromagnetic radiation.

• The amount of energy radiated by any object is a function of the surface temperature, emissivity, and the wavelength

• There are no blackbodies is nature. Blackbody is a matter that is capable of absorbing and re-emitting all electromagnetic energy that it receives.

• All natural objects are graybodies, they emit a fraction of their

maximum possible blackbody radiation at a given temperature T. This fraction is called emissivity (ε)

ε = E/Eb

Where E is actual energy and Eb is the blackbody energy at a given temperature.

Blackbody Radiation Curves

Wien’s Displacement Law

Notice that the peak of the Blackbody curve shirts to shorter wavelengths astemperature increases

This peak represents the wavelength of maximum emittance (λmax)

Wien’s Displacement Law

• As the temperature of an object increases, the total amount of radiant energy (area under the curve, in W/m2) increases and the radiant energy peak shifts to shorter wavelengths.

• To determine this peak wavelength (λmax) for a blackbody:

λmax = A/T where A is a constant (2898 μm K) and T is the

temperature in Kelvins.

Developments from Planck’s LawStefan-Boltzmann Law

The area under the Planck curve represents the total energy emitted by an objectat a given temperature

The Stefan-Boltzmann lawgives this energy for a blackbody

Developments from Planck’s LawStefan-Boltzmann Law

The Stefan-Boltzmann law is derived by integrating the Planck function with respect to wavelength:

M = σT4

σ is called the Stefan-Boltzmann constant.σ = 5.667 x 10-8

Energy or the radiant flux (rate of flow of EM energy)

Active and passive remote sensing

• Active systems provide their own source of energy (e.g. radar and laser)

• Active sensors emit a controlled beam of energy to the surface and measure the amount of energy reflected back to the sensor

• The main advantage of active sensor systems is that they can be operated day and night, have a controlled illuminating signal, and are typically not affected by the atmosphere.

• Passive systems depends on an external source of energy, usually the sun, and sometimes the Earth itself (e.g. photographs, multispectral scanners).

• Passive sensor systems based on reflection of the Sun’s energy can work only during daytime.

• Passive sensor systems that measure the longer wavelengths do not depend on the sun as a source of illumination and can be operated at any time.

Passive Remote Sensing

Active Remote Sensing

Energy interactions in the atmosphere

- The composition of the atmosphere influences both the incoming solar radiation and the outgoing terrestrial radiation

- The radiance (the energy reflected by the surface) received at a satellite is a result of electromagnetic radiation that undergoes several processes which are wavelength dependent

Scattering

- Scattering is the unpredictable diffusion of radiation by particles in the atmosphere

- Scattering is a function of the ratio of the particle diameter of the material doing the scattering to the wavelength of the incident radiation

- Types of scattering

1- Rayleigh 2- Mie 3- Nonselective

Rayleigh scattering- Common when radiation interacts with atmospheric molecules

and other tiny particles that are much smaller in diameter than the wavelength of the interacting radiation

- Rayleigh scattering is proportional to the inverse of the wavelength raised to the fourth power: shorter wavelengths are scattered more than longer wavelengths

- At daytime, the sun rays travel the shortest distance through the atmosphere- Blue sky

- At sunrise and sunset, the sun travel a longer distance through the Earth’s atmosphere before they reach the surface- The sky appears orange or red.

- Tends to dominate under most atmospheric conditions

Mie scattering

- It exists when atmosphere particle diameters is similar in size to the wavelength of the incoming radiation.

- Water vapor and dust are major causes of Mie scattering

- Mie scattering tends to influence longer wavelengths.

- It is restricted to the lower atmosphere where large particles are more abundant, and dominates under overcast could conditions.

Nonselective scattering

- The diameters of the particles causing scatter are much larger than the wavelengths of the energy being sensed.

- Water droplets (5-100 μm) and larger dust particles- Non-selective scattering is independent of wavelength,

with all wavelengths scattered about equally (A could appears white)

- It scatters all visible and near to mid IR wavelengths.

Absorption

- The process by which incident radiant energy is retained by a substance

- Energy converted to another form- light to heat

- Water vapor, carbon dioxide, and ozone all absorb electromagnetic energy

Transmission, reflection, scattering, and absorption

Atmospheric windows (transmission bands )

-The wavelength ranges in which the atmosphere is particularly transmissive

-The dominant windows in the atmosphere are the visible and radio frequency regions

-X-Rays and UV are very strongly absorbed and Gamma Rays and IR are somewhat less strongly absorbed.

Atmospheric windows

Basic interactions between electromagnetic energy and an earth surface feature

- The interaction of incoming radiation with surface features depends on both the spectral reflectance properties of the surface materials and the surface smoothness relative to the radiation wavelength

- The percentages of energy reflected, absorbed, and transmitted vary for different earth features, depending on their material type and condition.

- The percentages of energy reflected, absorbed, and transmitted vary at different wavelengths.

Basic interactions between electromagnetic energy and an earth surface feature

Specular versus diffuse reflectance

- Specular reflectors are flat surfaces that manifest mirrolike reflections. The angle of reflection equals the angle of incident

- Diffuse (or Lambertian) reflectors are rough surfaces that reflect uniformly in all the directions

- Diffuse contain spectral information on the color of the reflecting surface, whereas specular reflections do not.

- In remote sensing we are often interested in measuring the diffuse reflectance of objects.

Specular and diffuse reflectors

Specular reflection Diffuse reflection

Energy interactions with earth features

- Albedo - Spectral reflectance R(): the average amount of incident radiation reflected by an object at some wavelength interval

R() = ER() / EI() x 100

Where

ER() = reflected radiant energy

EI() = incident radiant energy

Identification of Surface Materials Based on Spectral Reflectance

Absorption is dominant process in visibleScattering is dominant process in near infraredWater absorption is increasingly important with increasing wavelength in the infrared.

Spectra of vegetation

Spectra of soil

• What are the important properties of a soil in an RS image

-Soil texture (proportion of sand/silt/clay)

-Soil moisture content

-Organic matter content

-Mineral contents, including iron-oxide and carbonates

-Surface roughness

Dry soil spectrum

20

60

100

Perc

ent R

efle

ctan

ce

0.5 0.7 1.1 1.30

Wavelength ( m)

80

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

Silt

Sand

10

30

50

70

90

Increasing reflectance with increasing wavelength through the visible, near and mid infrared portions of the spectrum

Soil moisture and texture

• Clays hold more water more ‘tightly’ than sand.

• Thus, clay spectra display more prominent water absorption bands than sand spectra

Soil moisture and texture

20

60P

erce

nt R

efle

ctan

ce

0.5 0.7 1.1 1.30

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

22 – 32%

10

30

50

Sand

20

60

0.5 0.7 1.1 1.30

Wavelength (m)

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

35 – 40% 10

30

50 2 – 6%

0 – 4% moisture content

5 – 12%

Clay

a.

b.

Per

cent

Ref

lect

ance

SandSandSand

ClayClayClay

Soil Organic Matter

Organic matter is a strong absorber of EMR, so more organic matter leads to darker soils (lower reflectance curves).

Iron Oxide

Recall that iron oxide causes a charge transfer absorption in the UV, blue and green wavelengths, and a crystal field absorption in the NIR (850 to 900 nm). Also, scattering in the red is higher than soils without iron oxide, leading to a red color.

Surface Roughness

• Smooth surface appears black.

• Smooth soil surfaces tend to be clayey or silty, often are moist and may contain strong absorbers such as organic content and iron oxide.

• Rough surface scatters EMR and thus appears bright.

Spectra of water

Reflectance peak shifts towardReflectance peak shifts toward longer wavelengths as morelonger wavelengths as more

suspended sediment is addedsuspended sediment is added400 450 500 550 600 650 700 750 800 850 9000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Wavelength (nm)

Per

cent

Ref

lect

ance

50

100

150 200

250

clear water

400 450 500 550 600 650 700 750 800 850 9000

Wavelength (nm)

2

4

6

8

10

12

14

Per

cent

Ref

lect

ance

300

1,000 mg/l

1,000 mg/l

600

clear water

50

100

150

200 250 300 350

400

450 500 550

a.

b.

Clayey soil

Silty soil

Data acquisition and image interpretation

Data acquisition and interpretation

• Image is used for any pictorial representation of image data

• Photograph: Images that were detected as well as recorded on film

Digital Image

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72707478859397888179

71616770768290979387

727479777579798910396

4459778785848897110105

563951839195101100104104

7558435684106106938693

78797541406587867988

78828672453244698280

85858684775938457779

87918881858464415370

72808492979485787581

72707478859397888179

71616770768290979387

727479777579798910396

4459778785848897110105

563951839195101100104104

7558435684106106938693

78797541406587867988

78828672453244698280

85858684775938457779

87918881858464415370

72808492979485787581

72707478859397888179

71616770768290979387

727479777579798910396

4459778785848897110105

563951839195101100104104

7558435684106106938693

78797541406587867988

78828672453244698280

85858684775938457779

87918881858464415370

bands (z)

row

s (y

)

columns (x)Image size: The no. of rows (or lines) and no. of columns (or samples) in one scene

Digital Image data

Digital Images

72808492979485787581

72707478859397888179

71616770768290979387

727479777579798910396

4459778785848897110105

563951839195101100104104

7558435684106106938693

78797541406587867988

78828672453244698280

85858684775938457779

87918881858464415370

Digital numbers (DNs) typically range from 0 to 255; 0 to 511; 0 to 1023, etc. These ranges are binary scales: 28=256; 29=512; 210=1024.

Different kinds of image

• Panchromatic image• True-color image• False-color image

Panchromatic image

• If airborne cameras use black/white film or satellite sensors use a single band, it produces panchromatic image (gray scale image)

Color composite

• Color primaries: RGB (Red, Green, Blue)

• Many colors are formed by combining color primaries in various proportions

• Same principles apply to producing color images taken from airborne cameras or satellite sensors

Greyscale vs. RGB

• Greyscale is typically used to display a single band

• RGB (“Red”, “Green”, “Blue”) images can display 3 bands, corresponding to the red, green and blue phosphors on a monitor.

True color and false color images

• True color image- the color of the image is the same as the color of the object imaged.

• A false color image is one in which the R,G, and B values do not correspond to the true colors of red, green and blue.

• The most commonly seen false-color images display the very-near infrared as red, red as green, and green as blue.

• For instance, different types of vegetation might appear as blue, red, green or yellow. Intuitively, vegetation would appear green.

Vegetation appear red in this color composite

True color and false color images

Describing Sensors- Spatial resolution

- Spectral resolution

- Temporal resolution

- Radiometric resolution

Spatial Resolution

Spatial Resolution

• Spatial resolution is a measure of the smallest object that can be resolved by the sensor

• In aerial photography, it is the minimum separation between two objects for which the images appear separate.

• Airborne and some spaceborne systems: cm – m resolution

• Most spaceborne systems: 10’s of m to kilometers

Spatial resolution• Spatial: The size of the smallest possible feature that can be

detected.• Pixel size is the area covered by one pixel on the ground • In a digital image, the resolution is limited by the pixel size,

i.e. the smallest resolvable object cannot be smaller than the pixel size.

• Fine or high resolution image refers to one with a small resolution size. Fine details can be seen in a high resolution image.

• Coarse or low resolution image is one with a large resolution size, i.e. only coarse features can be observed in the image.

• Aerial photo has higher resolution • The image resolution and pixel size are not equivalent.

Spatial Resolution

Spatial resolution

A low resolution MODIS scene (1km) A very high resolution image acquired

by the IKONOS satellite (1m)

Spectral Resolution

• The number, wavelength position and width of spectral bands a sensor has

• A band is a region of the EMR to which a set of detectors are sensitive.

• Multi-spectral sensors have a few, wide bands (several spectral bands)

• Hyper-spectral sensors have a lot of narrow bands (hundreds of spectral bands)

Spectral ResolutionMulti-spectral

hyper-spectral

What is Radiometric Resolution?

• The number of brightness levels the sensor can record

Radiometric Resolution

• Radiometric resolution refers to the number of possible brightness values (or light levels) in each band of data, and is determined by the number of bits into which the recorded energy is divided.

• It described the sensitivity of the sensor to variations in brightness.

• Typically, 8,10, or 12 are used for representing the radiometric levels

• In 8-bit data, the brightness values can range from 0 to 255 for each pixel (256 total possible values). In 7-bit data, the values range from 0 to 127, or half as many possible values.

Radiometric Resolution

8-bit radiometric resolution 28 = 256 levels

3-bit radiometric resolution 23 = 8 levels

Radiometric Resolution

2-bit = 4 radiance levels 8-bit = 256 radiance levels

The finer the radiometric resolution of a sensor, the more sensitive it is to detecting small differences in reflected or emitted energy .

Radiometric Resolution

2-bit 4 gray levels

8-bit 256 gray levels

Temporal resolution

• Temporal resolution is a description of how often a sensor can obtain imagery of a particular area of interest, determined by the repeat cycle of its orbit.

• For example, the Landsat satellite revisits an area every 16 days as it orbits the Earth, while the SPOT satellite can image an area every 1 to 4 days due to off-nadir viewing.

Temporal resolution

• How frequent a given location on the earth surface can be imaged by imaging system.– For satellite image, it can be regular (satellites

are orbiting the earth in regular time interval)

Geosynchronous Orbit

• A satellite in geosynchronous orbit circles the earth once each day.

• The time it takes for a satellite to orbit the earth is called its period.

• To stay over the same spot on earth, a geostationary satellite also has to be directly above the equator.

• Otherwise, from the earth the satellite would appear to move in a north-south line every day.

Sun-Synchronous Orbit

• Because the valid comparison of images of a given location acquired on different dates depends on the similarity of the illumination conditions, the orbital plane must also form a constant angle relative to the sun direction.

• This is achieved by ensuring that the satellite overflies any given point at the same local time, which in turn requires that the orbit be sun-synchronous

• The satellite crossed the equator at approximately the same local sun time (9:42) every day

Earth Resource Satellites Operating in the Optical Spectrum

• Landsat • SPOT • Meteorological Satellites

– NOAA satellites – GOES satellites

• Ocean Monitoring Satellites – Radar Satellites – Seasat – ERS-1 – JERS-1 – Radarsat

Introduction

• Satellite systems operating within the optical spectrum (0.3-14 m): UV, visible, near-, mid-, and thermal IR wavelengths

• Landsat and SPOT

• Higher resolution, contemporary programs (IKONOS, QuickBird)

Earth history of space imaging• Cameras on rockets (Germany 1891, 1907)

• 1946-1950: beginning of space RS with small cameras aboard V-2 rockets (NM, USA)

• Meteorological satellites (initial application) TIROS-1 (1960)

• Corona, a military space imaging reconnaissance program (1960 - 1972).

Earth history of space imaging• Manned space programs:

– Mercury, Gemini, Apollo– Alan Shepard (1961): Made a 15-min suborbital Mercury

flight on which 150 excellent photographs were taken– John Glenn (1962): Made 3 historic orbits around the earth

and took 48 color photographs during Mercury mission MA-6

• Gemini GT-4: geological application

• Other geographic & oceanographic phenomena

Earth history of space imaging• Apollo 9: multispectral orbital photography

for earth resource studies

• 1973: Skylab: Earth Resources Experiment Package (EREP)

• 1975: US-USSR Apollo-Soyuz Test Project (ASTP) hand-held cameras, disappointing results

Landsat Satellite Program Overview

• Earth Resources Technology Satellites (ERTS) program (1967): a planned sequence of six satellites

• In 1975, ERTS was renamed by NASA “Landsat”

Landsat Program

• During the experimental Landsat phase, imagery was disseminated by Earth Resources Observation System (EROS) Data Center at Sioux Falls, SD.

• Satellites were operated by NASA and USGS was handling the data distribution.

Landsat Program• Gradually, NOAA took over and the Landsat

program operation was transferred to a commercial firm (Earth Observation Satellite Company – EOSAT).

• Landsat-7 operation reverted to the government; EROS Data Center is the primary receiving station, processing and distributing the data.

Landsat Program• Landsat-1,-2,-3 images are catalogued

according to their location within the Worldwide Reference System (WRS), by specifying:

– a path (each orbit within a cycle)– a row (individual nominal sensor frame centers)– a date

Landsat Program

Landsat Program

• E.g. the WRS has 251 paths for Landsat –1, -2,-3 (number of orbits to cover the Earth in 18 days).

• Paths are numbered from 001 to 251, E to W, row 60 coincides with the equator.

ERTS-1 (Landsat-1)• Launched by a rocket on 7/23/1972

• Operated until 1/6/1978

• The first unmanned satellite specifically designed to acquire data about earth resources on a systematic, repetitive, medium resolution, multispectral basis

• 43 US states & 36 countries

Landsat Satellite Program Overview

• Landsats carried combinations of 5 types of sensors:

– Return Beam Vidicon (RBV) camera systems– Multispectral Scanner (MSS) systems– Thematic Mapper (TM)– Enhanced Thematic Mapper (ETM)– Enhanced Thematic Mapper Plus (ETM+)

Landsat 1-3 orbital characteristics

• Sun-synchronous orbits: The satellite crossed the equator at approximately the same local sun time (9:42) every day

• Lunched into circular orbits at 900 km • Near-polar orbits travels northwards on one side of

the earth and then toward the southern pole on the second half of its orbit, 14 times a day

• Passing same point every (coverage repetition) • Sensors aboard imaged only 185 km swath• Globe coverage every 18 days (20 times/year)