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Remote Sensing: A VERY Brief Overview
Some basic concepts:
All natural surfaces emit photons (energy) when they are
above absolute zero.
This emission follows Stefan-Boltzmann Law:
WB= T4
The spectral distribution is given by Planck’s Law
WB= f( T), such that higher emitting T, smaller 
The Electromagnetic Wave
Electric field
Magnetic field
The Electromagnetic spectrum for a black body
A gray body emits a fixed proportion of the blackbody radiation at that
temperature: WB= T4 , where  is the emissivity of the surface.
Object T
WB
Sun
6000OK 105 ly/min
Earth
287OK 0.56 ly/min
Snow
273OK 0.46 ly/min
Reflection, scattering, absorption
As photons enter an object, some are reflected from the surface, some
pass through the object, and some are absorbed.
Those photons that are reflected from the surface or refracted through a
particle are said to be scattered.
Photons may also originate from a surface, a process called emission.
Emitted photons are subject to the same physical laws of reflection,
refraction, and absorption to which incident photons are bound.
Photons are absorbed in objects by several processes. The variety of
absorption processes and their wavelength dependence allows us to
derive information about the chemistry of an object from its reflected or
emitted energy.
The principles of a Radiometer
A radiometer can in general be described with 4 parameters:
Spectral range, which is important to cover enough diagnostic spectral
absorptions to solve a desired problem. (There is some confusion in literature
about the names of the spectral ranges. Depending on the field of interest
different ranges are used for the terms near infrared (NIR), shortwave infrared
(SWIR), middle infrared (MIR) and thermal infrared.
Spectral bandwidth, which is the width of an individual spectral channel in the
spectrometer.
Spectral sampling. The shape of the bandpass profile. Ideally each radiometer
channel rejects all energy except that from within a given narrow wavelength
range. The most common bandpass in radiometers is a Gaussian profile. The
width of the bandpass is usually defined as the width in wavelength at the 50%
response level of the function.
Signal-to-noise ratio (S/N). A radiometer must measure the spectrum with
enough precision to record details in the spectrum.
Other sensors
LIDAR
LIDAR stands for LIght Detection And Ranging. It is a radiometer
system that uses a light beam to obtain measurements of speed, altitude,
direction and range of a target.
Other sensors/systems
RADAR
It is a radiometer system that emits an energy beam (in the microwave
frequency) to obtain measurements of speed, altitude, direction and
range of a target.
SOUNDERS
A sounder is a special kind of radiometer which measures changes of
atmospheric temperature with height, and changes in
water vapor content of the air at various levels.
Where do we observe from space?
In view of the properties of the atmosphere, traditionally two windows are used:
1.optical window, wavelengths from 300 - 14000 nm
2.microwave window, wavelengths from 1 mm - 1 m.
The optical window is often subdivided into a reflective optical part (3003000 nm) and a thermal infrared part (3000 – 14000 nm)
Water Vapor Window
Orbits
Altitude and inclination
Orbital altitude, the height above the surface of the earth; inclination is
defined as the angle between the earth's equatorial plane and the satellite's
orbital plane.
Repeat Cycle
A repeat cycle is defined as the number of days before the satellite passes
over exactly the same orbital path relative to the earth's surface.
Descending Node Crossing (DNC)
The point in the orbit path of a satellite, where the orbit crosses the earth
equator and the spacecraft moves from North to South.
Low Earth Orbit (LEO)
A satellite orbit with a typical altitude range of several hundred kilometres is
called LEO.
Orbits (cont.)
Geosynchronous Orbits
An orbit is geosynchronous if it has a fixed position relative to the surface of
the earth
Geostationary Orbits
A geostationary satellite is a geosynchronous satellite without an inclination
angle.
Sun-synchronous Orbits
For near-earth satellite orbits (lower than 1000 km) sun-synchronous orbits
are polar, and the inclination and altitude is such that a satellite passes over a
given site always at the same local time.
Nadir Mode
In Nadir Mode, the line of sight of a sensor points from the orbit downward
to the earth’s surface or atmosphere. An alternative measurement mode of
spaceborne remote sensing instruments is the Limb Mode.
DMSP Special Sensor
Microwave/Imager (SSM/I)
Soil Moisture Algorithm Results Oklahoma
H Polarization
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VolumetricSoilMoisture(%)
• Shown here are 1.4 GHz results
obtained using an aircraft
sensor and 19 GHz satellite data
• The difference between the
sensitivity of the two
instruments is quite apparent
• Standard error of estimates:1.4
GHz = 2.6% and 19 GHz =
5.3%.
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E m issi
Aqua AMSR Three Day Composite Brightness Temperature
July 4-6
Aqua AMSR Three Day Composite Brightness Temperature
July 4-6