Introduction to Remote Sensing The Electromagnetic (EM) Spectrum.
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Transcript of Introduction to Remote Sensing The Electromagnetic (EM) Spectrum.
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Introduction to Remote Sensing
The Electromagnetic (EM) Spectrum
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Electro-magnetic radiation is a wave
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Radiation
• All objects emit electro-magnetic radiation in some form
• This radiation moves through space until it hits something
• The thing it hits may then absorb the radiation and get its energy
• Alternatively it may deflect, scatter or reflect the radiation
• It may even simply allow the radiation to pass through - transmission
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Wavelength• We can describe the radiation by:
– Wavelength• The actual length (metres) between wave
peaks.• Wavelengths for radiation vary greatly
– radio waves (100 cm to 160 metres)–Light (10-9 metres).– I will often talk in cm or mm for
microwaves–μm for IR–nm or Ǻ for visible
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Frequency
• Frequency
– The number of wave crests that pass by a point per second (Hz). Often we will talk in terms of GHz (109)
• We also sometimes refer to a wavenumber; this is 1/λ, or the number of wavelengths in a unit length (normally in cm-1)
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λ & These two quantities are inversely proportional
because
cWhere c is the speed of light (which varies with the medium, but not very much for the media we will consider)
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Radiation
The distance between wave crests is the wavelength.
Shorter waves: x-rays, UV, visible light
Longer waves: infrared, microwave, radar, TV, radio
One Wavelength
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Solar Spectrum
max = 0.55 m
© 1998 Wadsorth Publishing -- From Ahrens Essentials of Meteorology
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Passive instruments
• These are instruments that simply receive radiation.– Eg radiometer, camera, retina– Most satellite instruments are passive
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Active instruments
• These send out a signal and measure what is returned to them– Eg radars, lidars– We will pretty much ignore these– We will occasionally present examples of both
active and passive images for discussion
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Other classifications
• We can also classify instruments by their position:– Ground based
• Fixed• Mobile
– Airborne– Satellite borne
• Geostationary• Other orbits
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Ground based
• Most common is radar, but also profilers, ceilometers etc
• Advantages?
• Disadvantages?
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Satellite borne• Mostly passive visible and radiometers
• Advantages?
• Disadvantages?
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More spectrum
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Wavebands
• We often, in this field, talk of wavebands. These can be broad, e.g. the visible band (400nm - 700nm) or narrow, e.g. the radar S-band (around 10cm)
• These are related to the sensitivities of particular receivers
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Today’s quiz
• What is the speed of light?
• What is your favourite colour?
• What is its wavelength?
• What is its frequency?
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Units
• You will become familiar with many concepts, including things we measure. These will have units.
• Temperature: in Kelvin (K)
• Radiance in Wm-2
• Albedo (unitless)
• And many more
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Why do things have colour?
• Emission
• Scattering (reflection) and absorption
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Interactions with matter
• When radiation hits matter 3 things can happen– Scattering (radiation bounces off – includes
reflection which is a special kind of scattering)– Transmission (radiation passes through)– Absorption (radiation is… erm… absorbed)
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Can define more thingsAbsorptance
aλ = radiation absorbed / incident
Reflectance
ρλ = radiation scattered / incident
Transmittance
Tλ = radiation transmitted / incident
Aλ + ρλ + Tλ = 1
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What we need to do
• How do we get values of temperature and albedo (reflectance) using the instruments on the satellites?
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Albedo
• Albedo varies with wavelength
• Many substances have high albedo (reflectance) in the visible (e.g. snow), but low albedo in the microwave (e.g. snow)
• Can also have different albedo for different colours and therefore appear coloured (e.g. leaves)
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Specular and Lambertian reflection
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Reflection/scattering
• The type of reflection is determined by the smoothness of the surface– This is really the ratio of the wavelength of the
radiation to the size of uneven features
• Very few natural surfaces produce true specular reflection
• Give me some examples!!
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Solid angles
• The solid angle is the proportion of the surface area of a sphere subtended by the 2 dimensional angle. (See picture drawn on board)
• It is measured in steradians – sr.
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Measuring Reflectance (albedo)
• Measure the energy impacting the sensor in the visible waveband channel
• In Wm-2sr-1μm-1
• Energy reflected per unit time per unit area: Normalised for width of waveband and solid angle view.