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Define remote sensing

  • The science of acquiring information about the earth’s surface without actually being in contact with it


How is remote sensing done?

  • Sensing and recording reflected and emitted energy and
    • Processing
    • Analyzing
    • Applying that information


Information received from remote sensing comes in the form of

  • Electromagnetic radiation


Electromagnetic radiation consist of:

  • Alternating
    • Electric field and
    • Magnetic field
  • The electric field vector is perpendicular to the magnetic field vector
  • The direction of propagation is perpendicular to both


Radiation is often specified by:

  • Its wavelength
    • It is the distance between crests of the electric or magnetic field


Alternate way to describe radiation

  • Give its frequency
    • It is the rate at which the electric or magnetic field oscillates when observed at a point


What is the unit of frequency?

  • Hz or
  • One cycle per second


Give the equation of finding the frequency

  • V=c/lamda
    • C is the speed with which electromagnetic radiation travels and is known as the speed of light
    • In vacume c is 299,792 km s-1
    • In the atmosphere c is slower 299,703 km s-1 due to interaction with air molecules


Radiation is specified by

  • Wavenumber k
    • The recoprical of the wavelength
    • Inversely proportional to wavelength
    • Directly proportional to frequency


Property of electromagnetic radiation:

  • Can transport energy
    • Many of the units used to quantify EM radiation are based on energy


Unit of radiant energy

  • Joule


Radiant flux

  • Radiant energy per unit time


Radiant flux unit:

  • Watts
  • Joules per second


Radiant flux depend on

  • Area
    • It is usually normalized by surface area


Define radiant flux density

  • Radiant flux crossing a unit area


Radiant flux density unit:

  • Watts per square meter


Why is radiant flux density subdivided?

  • To indicate which way the energy is traveling


Radiant flux density is subdivided to

  • Irradiance (E)
    • Radiant flux density incident on an area
  • Radiant exitance
    • Radiant flux density emerging from an area


In nature radiation is a function of

  • Direction
    • The direction dependence is taken into account by employing the solid angle


Solid angle:

  • If one draws lines from the center of the unit sphere to every point on the surface of an object, the area of the projection on the unit sphere is the solid angle


Unit of solid angle:

  • Steradians (sr)


Equation of solid angle:

  • Ω = A/r2
    • Object with cross sectional area
    • A << r2
    • R is the radius of the sphere


In a sphere of one foot radius a steradian would correspond to a solid angle that

  • Subtended an area of one square foot on the surface of the sphere. Since the total area of a sphere is 2pi r2 there are 4 pi steradians in a sphere


Define radiance

  • Radiant flux density per unit solid angle leaving (or incident on) the surface perpendicular to the beam


Radiance represents

  • The radiation leaving (or incident on) an area perpendicular to the beam,
  • For other directions, we must
    •  Weight the radiance by cos ©(where © is the zenith angle, i.e., the angle measured from the normal to the surface).


Radiance (in certain wavelength) can be expressed as:


Monochromatic radiance (L)

  • The most fundamental radiation unit for satellite meteorology
  • The energy per unit time per unit wavelength per unit solid angle crossing a unit area perpendicular to the beam.


Radiance has the useful property that it is

  • Independent of distance from an object as long as the viewing angle and the amount of intervening matter are not changed.


Consider a satellite viewing a small object. The irradiance reaching the satellite from the object will

  • Will decrease inversely as the square of the distance of the satellite.
  • The solid angle of the object subtended at the satellite will also decrease inversely as the square of the distance of the satellite


The radiance of the object as viewed by the satellite

  • Which is simply the irradiance divided by the solid angle, is, therefore, independent of distance.