what is remote sensing? “the acquisition and measurement of data/information on some property(ies)...
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What is remote sensing?“the acquisition and measurement of data/information on some property(ies) of a phenomenon, object, or material by a recording device not in physical, intimate contact with the feature(s) under surveillance; techniques involve amassing knowledge pertinent to environments by measuring force fields, electromagnetic radiation, or acoustic energy employing cameras, radiometers and scanners, lasers, radio frequency receivers, radar systems, sonar, thermal devices, seismographs, magnetometers, gravimeters, scintillometers, and other instruments”.
Source: NASA tutorial on remote sensinghttp://rst.gsfc.nasa.gov/Intro/nicktutor_I-1.html
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Shorter definition
• Remote sensing is the collection of information about an object or system without coming into direct physical contact with it
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How are measurements made?
• Ground-based
• Airborne
• Satellite
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Why do we do remote sensing?
• Unobtrusive• Automated• Useful for extreme conditions• Offers excellent spatial and temporal coverage• Provides real time or near-real time
observations• Often cost-effective• Extends our senses
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Remote Sensing Systems• Active Sensor - illuminates the subject from an artificial
energy source
• Passive Sensor - uses natural radiation from the Sun or Earth
• Imaging Sensor - creates a “picture” by scanning across a linear array of detectors while the array moves through space
• Non-imaging Sensor - measures along a transect or at a point; or uses a non-photonic approach
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REMOTE SENSING DATA TYPES
Visible, infrared, thermal, and microwave are most common
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The Remote Sensing Cycle
Sensor
Data
Processing
DataAnalysis
Interpretation
Information
Assessment
Development
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Electromagnetic Radiation• Its harmonic wave form can be described
according to the Maxwell equations:
E E0 cos(t kx)Where, E is the electric fieldEo is a constant vector = angular frequency = c/= wavelengthc = speed of light in a vacuum (300,000,000 ms-1)k = wavenumber (x = distance along the x-axist = timeThe value of E at any point depends only on x and t
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Electromagnetic Waves
Described by: • Wavelength• Frequency• Amplitude
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Frequency vs. Wavelength
= distance of separation between two successive wave peaks
= number of wave peaks passing in a given time
The product of wavelength and frequency is a constant:
c=
c = speed of light in a vacuum = 3.0 108 ms-1
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Energy vs. FrequencyWhen considering the particle form of energy,
we call it a photonThe energy of a photon, Q, is proportional to its
frequency, :Q = h = c/
Q = hch = Planck’s constant = 6.63 10-34 Jsc = speed of light = 3.0 108 ms-1
Thus, Q
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Spatial Resolution• “A measure of the smallest angular or linear
separation between two objects that can be resolved by the sensor”. (Jensen, 2000)
• Resolving power is the ability to perceive two adjacent objects as being distinct– size– distance– shape– color– contrast with background– sensor characteristics
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• Instantaneous field of view (IFOV) is the angular field of view of the sensor, independent of height
• IFOV is a relative measure because it is an angle, not a length
• It can be measured in radians or degrees
sensor
IFOV
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GIFOV
• Ground-projected instantaneous field of view (GIFOV) depends on satellite height (H) and the IFOV
GIFOV 2H tanIFOV
2
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1 meter resolution 250 meter resolution
IKONOS image of Gunnison River Basin, CO1
kilo
met
er
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Digital Number
imaging opticsdetectors
electronics
at-sensorradiance
DN
The DN that is recorded is proportional to the radiance at the sensor
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Digital Raster Imager Format
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Radiometric Resolution
• Determined by the number of bits of within which the digital information is encoded
21 = 2 levels (0,1)22 = 4 levels (0,1,2,3)28 = 256 levels (0-255)212 = 4096 levels (0-4095)
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• Surface components with very distinct spectral differences can be resolved using broad wavelength ranges
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Temporal Resolution
• The frequency of data acquisition over an area
• Temporal resolution depends on:– the orbital parameters of the satellite
– latitude of the target
– swath width of the sensor
– pointing ability of the sensor
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• Multi-temporal imagery is important for– infrequent observational opportunities (e.g.,
when clouds often obscure the surface)– short-lived phenomenon (floods, oil spills,
etc.)– rapid-response (fires, hurricanes)– detecting changing properties of a feature to
distinguish it from otherwise similar features
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Atmospheric Effects
• EMR is attenuated by its passage through the atmosphere
Attenuation = scattering + absorption– Scattering is the redirection of radiation
by reflection and refraction
– Attenuation is wavelength dependent
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– scattering by molecules and particles whose diameters are <<
– primarily due to oxygen and nitrogen molecules
– scattering intensity is proportional to -4
– responsible for blue sky
Rayleigh Scattering
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Mie Scattering
– Spherical particles that have a mean diameter 0.1 to 10 times the incident wavelength
– Examples for visible light: water vapor, smoke particles, fine dust
– Scattering intensity is proportional to -4 to 0
(depending on particle diameter)
• Clear atmosphere has both Rayleigh and Mie scattering. Their combined influence is between -0.7 to
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• Non-selective Scattering– aerosol particles much larger than the
wavelength (> 10x)– examples: water droplets, ice crystals, volcanic
ash, smog
– independent of wavelength: 0
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Atmospheric Absorption
Absorption is the process whereby radiant energy is absorbed by atmospheric constituent and converted to thermal energy
Atmospheric absorbers are primarily:• H2O water vapor, water droplets• CO2 carbon dioxide• O2 oxygen• O3 ozone• Dust and soot
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Absorption Bands
• An absorption band is a portion of the EM EM spectrum within which radiant energy is spectrum within which radiant energy is absorbed by substances such as water absorbed by substances such as water (H(H22O), carbon dioxide (COO), carbon dioxide (CO22), oxygen (O), oxygen (O22), ),
ozone (Oozone (O33), and nitrous oxide (N), and nitrous oxide (N22O), dust, O), dust,
soot, etc.soot, etc.
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spectral absorptance
Spectral Reflectance
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Microwave Brightness Temperature
• Microwave radiometers can measure the emitted spectral radiance received (L
• This is called the brightness temperature and is linearly related to the kinetic temperature of the surface
• The Rayleigh-Jeans approximation provides a simple linear relationship between measured spectral radiance temperature and emissivity
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At long wavelengths, such as in the microwave region, the relationship between spectral emittance and wavelength can be approximated by a straight line.
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Rayleigh-Jeans Approximation
• k is Planck’s constant, c is the speed of light, is emissivity, T is kinetic temperature
• This approximation only holds for >> max
• (e.g. > 2.57mm @300 K)
L 2kcT
4
spectral radiance is a linear function of kinetic temperature
a constant
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Brightness Temperature
T is also called the “brightness temperature” typically shown as TB
TB 4
2kcL
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Brightness temperature can be related to kinetic temperature through emissivity
Thus, passive microwave brightness temperatures can be used to monitor temperature as well as properties related to emissivity
Tb Tkin
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Microwave Radiometers
• Advanced Microwave Sounding Unit (AMSU) 1978-present
• Scanning Multichannel Microwave Radiometer (SMMR) 1981- 1987
• Special Sensor Microwave/Imager (SSM/I) 1987-present
• Tropical Rainfall Measuring Mission (TRMM) 1997-present
• Advanced Microwave Scanning Radiometer (AMSR-E) 2002-present
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Comparative Operating Characteristics of SMMR, SSM/I, and AMSR Parameter (Nimbus-7)
SMMR (DMSP-F08,F10, F11,F13) SSM/I
(Aqua) AMSR-E
Time Period
1978 to 1987 1987 to Present 2002 to Present
Frequencies (GHz)
6.6, 10.7, 18, 21, 37 19.3, 22.3, 36.5, 85.5 6.9, 10.7, 18.7, 23.8, 36.5, 89.0
Sample Footprint Sizes (km):
148 x 95 (6.6 GHz) 27 x 18 (37 GHz)
37 x 28 (37 GHz) 15 x 13 (85.5 GHz)
74 x 43 (6.9 GHz) 14 x 8 (36.5 GHz) 6 x 4 (89.0 GHz)
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Radar=Radio Detection and Ranging
Radar system components
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Radar: How it Works• A directed beam of
microwave pulses are transmitted from an antenna
• The energy interacts with the terrain and is scattered
• The backscattered microwave energy is measured by the antenna
• Radar determines the direction and distance of the target from the instrument as well as the backscattering properties of the target
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History of the Landsat series
Currently, Landsat 5 and Landsat 7 (ETM+) are in orbit
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