Remot Sensing

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During WWII, infrared detectors gained great importance because of their ability to
distinguish between

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real vegetation and green plastic camouflage – led to military
development in multi-spectral remote sensing research after the war.

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After the WWII, U.S. Army scientists obtained the 1st photograph of the Earth
(White Sand, NM) from "space"

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using captured German V-2 rocket.

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The detection, recording and analysis of electromagnetic energy is the

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foundation
of aerial photographic interpretation an remote sensing.

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Detection and recording of electromagnetic energy are made possible because of a
complex series of energy matter environment interaction which combine to
produce the

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contrast which are recorded (by the camera, scanner, spectrometer, or
radiometer) between target and its background.

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Propagation of solar energy can be described in 2 ways:

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o Electromagnetic waves – occur in a "continuum" of energy frequencies
o Quanta – minimum measurable energy units of electromagnetic radiation
known as "photons".

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This "continuum" of energy is subdivided into

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different spectral regions and
wavelength bands based on the frequency range.

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In reality, none of these spectral bands have

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distinct boundaries.

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It should be noted that the limits of the electromagnetic spectrum are

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not yet fully
known.

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EMR

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– self-propagating wave in space with electric and magnetic components.
These components oscillate at right angles to each other and to the direction of
propagation

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Creation of EMR (energy):

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o Atom orbiting electron absorbs additional energy → electron (with surplus
energy) “accelerates” to a higher (faster) orbit around the atom → electron
eventually “dumps” surplus energy and “decelerate” (return) back to the
original stable orbit → EMR (energy) is released through the deceleration of
the electron

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The wavelength of EMR depends on

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the length of time over which acceleration
occurs along with the number of accelerations per second.

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Basically, the hot surface of the sun produces

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radiation of all wavelengths. (i.e., the
sun produces a “continuous” spectrum).

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The radiant power peaks within this continuous spectrum produced by the sun

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approximates a “black body” at 6,000°K.

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A “black body” is defined as

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a body which absorbs and re-radiates all energy fallen
upon it.

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Roughly 46% of the solar energy striking the earth falls between

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0.4 - 0.7um
(visible spectrum).

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Therefore, the sun is an excellent source of energy for measuring the

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reflectance of
objects or scenes in the visible spectrum.

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The Earth has an average ambient temperature of about

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300°K

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The radiant power of the Earth peaks at around

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9.6um (thermal-IR spectrum).

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The Earth then is an excellent energy source for

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“passive” imaging of the Earth
surface in the thermal-IR region.

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Examples of passive imaging system

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– aerial camera, scanner, and
radiometers.

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The pathway of EMR energy in passive imaging environment:

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The sun → EMR energy propagates through space → penetrates the
atmosphere (atmospheric interactions) → strike object (target) → be
reflected directly, or absorbed, transmitted → then be emitted (re-radiated)
or reflected or transmitted back to the atmosphere (more atmospheric
interaction) → then be recorded by the imaging system.

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Wave Theory

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• James Clerk Maxwell in his 1864 paper A Dynamical Theory of the
Electromagnetic Field – introduced a set of equations that quantify the
relationship among magnetic field, electric field in the propagation of EMR
through space.
• Maxwell states that EMR travels at the speed of light (~3 x 100,000km/sec) in a
wave pattern.
• The wave consists of 2 force fields – electric and magnetic – running
"orthogonal” (at right-angle) to each other and perpendicular to the direction of
propagation.
• The electric field of EMR is used to define polarization of the wave – often
important in determining target-signal response (horizontal vs. vertical
polarization).

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• Parameters to describe the wave:

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o Frequency (v) – # of vibrations (oscillation) per second (hertz)
o wavelength (λ) – distance between two adjacent wave crests.
o period (cycle) – time required for the passing of one complete wave.
o amplitude – the height of the wave.

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In the particle model of EMR,

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a wave consists of discrete packets of energy (like a
train), or quanta, called photons.

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The frequency of the wave is

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proportional to the magnitude of the particle's energy.

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Since photons are emitted and absorbed by charged particles, they act as

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transporters of the EMR energy.

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As a wave, EMR is characterized by a

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velocity (the speed of light), wavelength,
and frequency.

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When considered as particles, they are known as

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photons, and each has an energy
related to the frequency of the wave given by Planck's equation:

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Radiant flux

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– time rate of flow of energy (joule/sec or watts) onto, off of, or
through a surface – i.e. radiant energy per unit time.

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When a radiant flux is intercepted by a plane surface, what is the flux intercepted
per unit area of the surface?

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o If the surface is at a right-angle (90°) to the radiant flux intercepted, the flux
divided by the area of the plane equals to the average angles to the radiant
flux intercepted, the radiant flux density at the plane.
o Radiant flux density (or flux incidence) upon a surface is called irradiance
– i.e. power incident on a surface.
o If the irradiance is constant from point to point on the plane of interception,
then the intercepted flux is simply:
Φ = EA

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Hemispherical Reflectance
• The flux density or radiant flux leave the planar surface is called

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exitance (M

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Hemispherical reflectance ( ρ ) is defined as

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a dimensionless ratio of the exitance
from a plane of material to the irradiance on the plane.
ρ = M reflected / E

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Reflection means

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radiation “bounce-off” a surface (not being transmitted or
absorbed by the surface).

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Reflectivity is the

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reflectance (the ratio of reflected power to incident power,
generally expressed in decibels or percentage) at the surface of a material

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Basic characteristics of surface reflectance:

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o The direction of incident radiation, the reflected radiation, and a vertical axis
to the reflecting surface all lie in the same plane.2
o The angle of incidence and angle of reflection as measured from this
vertical are always equal.

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Reflectivity of a surface depends on 3 factors:

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o Angle of incident energy
o Refractive index of the reflecting material
o Extinction coefficient of the reflecting material

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refractive index:

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measure of the amount of refraction (a product of a non conducting substance)

o It is a ratio of the λ or phase velocity of EMR in a vacuum to that in the
substance.
o The ability of an object to refract EMR is a function of λ, temperature, and
pressure.

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Extinction Coefficient – a result of the interaction between a small portion of
EMR photons and particles of the surface material

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Extinction Coefficient is proportional to the absorption coefficient of the
surface material.
o The lower the absorption, the higher the reflectivity of the material. (e.g.
metals – low absorber, therefore, high reflector)

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• Specular (mirror-like) reflection:

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o The EMR incident angle is equal to its angle of reflection from the surface.
o The reflecting surface must be smooth (in the eyes of the EMR)
o For example – ice sheets, calm water (lake) surface, flat metal surface,
airport runways, large solar panels…etc

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Diffuse reflection:

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o Spreading out or scattering of radiation to all directions depending on
surface roughness and orientation.
o For example – rough surfaces, forest canopies, fresh snow, sand (dunes),
clouds, urban buildup, grassland, golf courses.
• Most man-made objects and water surface have reflectance in-between these two
extremes.

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Atmospheric Effects

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• In general, as the distance between the sensor and target↑, atmospheric
attenuation↑.
• About 50% of the Earth’s atmosphere lies within 17,500ft (~533km) of the Earth
surface.
o ~75% of the atmosphere within 35,000ft (10.67km)
o ~99% of the atmosphere within 25miles (40.2km)

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Three major types of atmospheric attenuation on EMR in remote sensing:

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o Atmospheric scattering
o Atmospheric absorption
o Atmospheric refraction

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Scattering Effect

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o Scattering – reflection by molecules or particles in the air at unpredictable
directions
o Scattering occurs mainly below 30,000ft altitude.

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o 3 kinds of scattering:

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 Rayleigh scattering
 Mie scattering
 Non-selective scattering

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Rayleigh Scattering:

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 Scatter types – air molecules and very small particles suspended in the
air.
 Scatter’s diameter – much smaller than the λ of the EMR being scattered.

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 Scattering process:

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 Involves “re-radiation” by atoms – i.e. through absorption and
re-emission of radiation by atoms and molecules.
 It is impossible to predict the direction in which a specific atom
will emit a photon.
 Scattering effect is inversely proportional to the 4th power of the
λ – i.e. when λ of EMR↓, Rayleigh scattering↑.
 For example, UV is scattering 16 times as much as that of the
red light and 4 times as much as that of the blue light.
 Rayleigh scattering is most obvious in a clear day with few
water vapor or dust in the air.
 The sky appears blue because scattering of blue light in the
visible spectrum is the greatest

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o Mie Scattering:

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 Scatter types – mainly spherical particles in the atmosphere
 Scatter diameter – about the same as the λ of visible spectrum of EMR
(0.4 - 0.7um)
 For example – water vapor, dust, smoke, or particles with diameter
ranges from a few micron to a few 10th of a micron.
 While Rayleigh scattering occurs up to 30,000ft (~9,100m) elevation,
Mie scattering largely restricts to the lower atmosphere – below 15,000ft
(~4,500m).
 In a polluted sky with abundance of dust and smoke in the air, Mie
scattering may exceed Rayleigh scattering – resulting to a reddish
looking sky

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Non-selective Scattering:

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 Scatter types – mainly large particle in the air
 Scatter diameter – several times larger than the λ of the EMR being
scattered
 For example – water droplets (in clouds) scatter all λ spectrum evenly,
which gives clouds and fog the white appearance.

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Absorption Effect

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o Absorption – a process by which radiant energy that is neither transmitted
nor reflected is converted into other forms of energy (e.g. heat).
o Therefore, absorption is a process of energy retention and transformation.
o Certain λ are affected far more by absorption than by scattering – this is
particularly true of thermal-IR and UV spectrums.
o Major atmospheric absorption agents:
 O2
 O3
 CO2
 H2O (in particular)

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Refraction Effect

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o Refraction – the bending of light when it passes through from one medium
to another.
o It occurs when EMR passes through media which are differing densities,
modifying the speed of propagation of the EMR.
o Refraction index (n):
n = C / Cn
 C = speed of light in vacuum
 Cn = speed of light in the medium.

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A stable atmosphere can be viewed as

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a series of gas layers of different
densities.

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Clear Sky vs. Normal Sky

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• Clear sky situation:
o 80% of EMR –reaches the ground surface
o 6% of EMR – lost in atmospheric scattering and diffused reflection
o 14% of EMR – lost in atmospheric absorption
o Atmospheric refraction is minimal
• Normal sky situation:
o 50% of EMR – reaches the ground surface
o 32% of EMR – reflected (by the Earth surface)
o 5% of EMR – Rayleigh scattered
o 21% of EMR – non-selective and Mei scattered (Clouds, haze, and fog)

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The 1st generation of satellite system that remotely sensed the Earth is the

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meteorological satellite system in the 60's.

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4 classes of meteorological satellite system:

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1. TIROS family weather satellites
o TIROS
o ESSA
o ITOS / NOAA
o TIROS-N
2. Nimbus and ATS technology Satellite
3. DMSP (Defence Meteorological Satellite Program)
4. SMS / GOES

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TIROS (Television and Infrared Observation Satellite)

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The granddaddy of the current global operational meteorological
satellite system for the U.S. in the past 25 years.
o TIROS-1 – launched in 1960.
o 10 TIROS were launched from 1960 – 65.
o Payload:
a) TV camera:
 narrow angle – 12°
 medium angle – 78°
 wide angle – 104°
b) IR scanner (radiometer)
 for later missions.
c) Earth radiation budget instrument
 for later missions.

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ESSA (Environmental Science Service Administration) Satellites

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1966 – NASA introduced the TIROS Operational System (TOS) as a
commitment to provide routine daily worldwide observations without
interruption in data flow.
o The system employed 2 ESSA satellite (TIROS satellites in a different
name) – one on each side of the hemisphere.
o Through their on-board data storage system, the odd-numbered satellite
(ESSA-1, 3, 5, 7, 9) provided global weather data.
o The even number satellites (ESSA-2, 4, 6, 8) provided direct real-time
readout of their APT (automatic Picture Transmission) TV pictures
around the world.
o 9 ESSA were launched between 1966-69.
o Sun-synchronious orbit – provide constant sun illumination of target at
all time.
o Use “side-looking” camera (instead of “downward looking” camera as
of the early TIROS) – increase ground coverage.
o 1970 – the ESSA became part of the National Oceanic and
Atmospheric Administration (NOAA).

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TIROS-N (NOAA) series

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3rd generation TIROS.
o Operational since 1978.
o 1978-86 – 8 TIROS-N were launched.

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o TIROS-N sensor instruments:

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a) Advanced Very High Resolution Radiometer (AVHRR)
 4-channel scanning radiometer sensitive from visible / near-IR
and mid-IR radiation.
 Visible / near-IR spectrum – to map cloud cover, land-water
boundaries, and snow caps.
 Mid-IR range – measure temperature differences between
clouds and land surface.
 1 km spatial resolution.
b) TIROS Operational Vertical Sounder (TOVS)
 3 sensors:
 High Resolution IR Radiation Sounder (HIRS/2)
 Stratosphere Sounding Unit (SSU)
 Microwave Sounding Unit (MSU)

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Data Collection and Platform Location System (DCS)


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Objective – to obtain environmental data (such as
temperature, altitude, pressure), and earth location from fixed
or moving platforms (e.g., ships, planes, satellites, etc.).

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NIMBUS Satellites (1964 – 78)
•
• Total of 7 NIMBUS was launched (1964-78).

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NIMBUS satellite program was launched by NASA's "Space Observation
Program" in the early 60's.
• Objectives:
o To participate in the global observation program (World Weather
Watch) by expending daily global weather monitoring capability from
space.
o Serve as a test bed for advanced instruments for future operational
TIROS satellites.

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Pay load consisted of 8 major instruments:

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next

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a) Scanning Multi-channel Microwave Radiometer (SMMR)

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o Measures radiance in 5 wavelengths and 10 channels.
o Measures sea roughness, wind current, sea surface temperature, water
content in soil and the atmosphere.

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b) Stratospheric and Mesospheric Sounder (SAMS)

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o Measures vertical concentration (profiles) of H2O, N2O, CO, NO form
the stratosphere (~ 15 km above sea level) to the mesosphere (~ 90 km).

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c) Solar Backscattered Ultraviolet / Total Ozone Mapping System (SBUV /
TOMS)

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o Measures direct and backscattered solar UV to monitor solar irradiance
and ozone layer.

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d) Earth Radiation Budget (ERB)

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o Measures long and short wave radiance flux and direct solar irradiance
to monitor solar constant, earth albedo and atmospheric energy budget.

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e) Coastal Zone Color Scanner (CZCS)

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o Measures chlorophyll concentration, water turbidity and salinity of
coastal waters.

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Stratospheric Aerosol Measurement II Experiment (SAM-II)

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o Measures the concentration and optical properties of stratospheric
aerosols in vertical and lateral profile.

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Temperature Humidity IR Radiometer Experiment (THIR)

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o Measure the mid-IR radiation from the earth at 1.1 – 6.7 um day and
night.
o Mapping cloud cover.
o Mapping temperature of clouds, land surface, and ocean surface

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Limb IR Monitoring of the stratosphere Experiment (LIMS)

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o Global distributional profile of selected gases in the stratosphere.
o Mapping atmospheric temperature.

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Applications Technology Satellites (ATS)

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“Geo-stationary” (~ 600 km above the earth) – equatorial orbits
• Objectives – continues observations on a single platform of almost 1/3 of the
earth's surface.
• 1st generation ATS program was short-lived (1966-67) – only 2 ATS were
launched (ATS-1 and ATS-3). 2nd generation ATS program (ATS-4, ATS-5,
ATS-6) was launch in 1968.
• Mission – to demonstrate the capability of providing 1 picture of the western
hemisphere in every 20 minutes using a spin-scan camera.
• Coverage of photo – between 55°N & S.
• Enabled “sequential” photo coverage of the same area through time – valuable
in typhoon monitoring, e.g..

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The Defense Meteorological Satellite Program (DMSP)

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A military satellite program operated by the Space and Missile Systems
Center, Los Angeles Air Force Base, Calif..
• Since 1966, > 30 DMSP satellites were built by Lockheed Martin and
launched by the US Air Force.Prof. Woo 8
• Satellites provided both tactical (direct readout) and strategic (stored readout)
data transmitted routinely to the land bases throughout the world.

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a) Block 4 (A and B)

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o Earliest version (launched between 1966-69).
o Employed 2 vidicon cameras to get TV pictures showing cloud cover
and terrain features of the earth – at the visible spectrum (.4 - .7 um).
o Spatial resolution at nadir (~ 1.5 nautical mile), but resolution degraded
rapidly toward the edges of the picture.
 1 nautical mile = 1.1 mile (or 6080')rof. Woo 9
o A “C” System (of 16 thermopile sensors) was later added on the pay
load to detect earth's radiation emission at:
 .4 - 4 um (mainly mid-IR) for monitoring reflected solar
radiation;
 8.0 - 12 um (thermal-IR) for monitoring earth's radiation
emission.

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b) Block 5 (A, B, and C)

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o 2nd generation DMSP (1970-76)
o Vidicon camera was replaced by Sensor Aerospace Vehicle
Electronics (AVE) Package (SAP) – to get visible and IR information
at finer spatial resolution (~ 1/3 nautical mile for visible band; ~ 2
nautical mile for IR data).
o Functions of the SAP:
 Measure atmospheric temperature profile.
 Electronic flux at aircraft altitude.
 Atmospheric density data.

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c) Block 5D (1, 2 & 3)

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o In operation since 1976, latest DMSP satellite 5D F-17 was launch in
2006. Satellite 5D F-18 is scheduled to be launched in late 2009.

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Primary sensor − “Operational Linescan System” (OLS)

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 It is a two channel radiometer which consists of a
Cassegranian telescope with elements for visible and infrared
viewing.
 Improved spatial resolution – 0.3 nautical mile for both visible
and IR data.
 “Smoothed” data resolution – 1.5 nautical mile.

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Secondary sensor − Special Sensor Microwave Imager Sounder

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 Provides all-weather capability for worldwide tactical
operations and is particularly useful in typing and forecasting
severe storm activity.

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Characteristics of the “D5” series:

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 Orbit – near polar, sun-synchronous.
 Altitude – 833 kmProf. Woo 10
 Period – 101 minutes
 Manufacturer – RCA.

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Special sensors on board:

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 SSB – Gamma Detector
 SSC – Snow / cloud discriminator
 SSD – Atmospheric density scanner
 SSH – Humidity, temperature, ozone sounder
 SSI/E – Ionosphere plasma monitor
 SSI/P – Passive ionospheric monitor
 SSM/I – Microwave environmental sensor system
 SSJ – Space radiation dosimeter