Post on 21-Dec-2015
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
72808492979485787581
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)