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ELECTROMAGNETIC RADIATION (EMR) AND REMOTE SENSINGGABRIEL PARODI & DIANA CHAVARRO-RINCON
What is remote sensing?
Remote sensing is a way of collecting and analysing data to get information about an object without the instrument used to collect the data being in direct contact with the object.
Normal photography is an example of remote sensing
EarthEmission processes
Thermal emission
Atmospheric emission
Reflection processes
Reflectedradiation
LEARNING SEQUENCE IN THIS LECTURE
TOA
Clouds Scattered radiation*
Atmospheric absorption
scatteredradiation**
transmittedradiation
SunEM radiation
Source
ReflectionAtmosphere
1
2
3
4
1. EMR AND REMOTE SENSING: PROPAGATION
https://www.youtube.com/watch?time_continue=266&v=lwfJPc-rSXwVideo courtesy of ScienceAtNASA
1. ELECTROMAGNETIC RADIATION
Wave–Particle duality
Light is Electromagnetic (EM) radiation It can be modeled in 2 ways: by waves by photons (energy bearing particles)
WAVE‐PARTICLE DUALITY
Source: https://toutestquantique.fr/en/
𝐸 = Electric vector𝑀 = Magnetic vector
𝑐 = speed of light
EM RADIATION: WAVE MODEL
EMR travels as waves Waves are characterized
by 2 fields: Electric and Magnetic
The 2 fields oscillate in time The 2 fields oscillate in
space perpendicularly to each other and to the direction of travel
Waves travel with speed of light:
WAVELENGTH AND CYCLE
Frequency 𝒇 is the number of cycles passing a fixed point per second
Frequency is inversely proportional to wavelength (c = speed of light)
units: 𝜆 in metres m𝑓 in s Hz hertz
Particle theory: EM radiation is composed of particles called photons.
Particle theory is useful for describing the amount of energy measured by the sensor
(Planck-Einstein relation)
𝑄 – amount of energy per photon Jℎ – Planck’s constant, ℎ 6.626 ⋅ 10 J s
The photon energy is proportional to the frequency
EM RADIATION: PARTICLE MODEL
Combination of models
and 𝑄 have inverse relationship (since ℎ and 𝑐 are constant). The photon energy is proportional to the frequency (inversely proportional to )
Q = [Joule = watt . sec]h = [Joule . sec]f = [sec-1]c = [meter . sec -1]λ = [meter]
THE EM SPECTRUM
THE EM SPECTRUM
EarthEmission processes
Thermal emission
Atmospheric emission
Reflection processes
Reflectedradiation
LEARNING SEQUENCE IN THIS LECTURE
TOA
Clouds Scattered radiation*
Atmospheric absorption
scatteredradiation**
transmittedradiation
Sun
Source2
SOURCES OF EMRTHE BLACK BODY CONCEPTEM FOR REAL OBJECTS
All matter above T = 0 K radiates electromagnetic radiation IN ALL WAVELENGTHS. Max Planck investigated how much…
SOURCES OF EM RADIATION
Earth’s surface ~ 27 ºC = ? K
Sun’s surface ~ 6000K = ? ºC
27ºC +273 = 300K
6000K -273 =5763ºC
0 K= -273ºC
0ºC= 273 K
PLANCK’S RADIATION LAW
A body absorbs part of the EMR that hits it. A black body (BB) is an ideal radiator that absorbs all
incoming radiation. Planck’s law for a black body
𝐿 Spectral radiance W sr m µm h Planck constant 6.62606896.10 34 J.s 𝑘 Boltzmann’s constant - c speed of light
k 1.38 ⋅ 10 J K 𝑇 Absolute temperature in Kelvin K
Stefan-Boltzmann law: Total emitted radiation M in all wavelength (area under the curve):
Wien’s displacement law:Wavelength with maximum radiation
BLACK BODY RADIATION CURVES
Stefan-Boltzmann constant:𝜎 5.67 ⋅ 10 W m K
Wien’s displacement constant:𝑏 2898 μm 𝐾]
Real objects also reflect and transmit a part of incident radiation Energy is conserved
𝛼 𝜏 𝜌 1 Applies in all wavelengths Real objects absorb less than black body In equilibrium object re-emits all absorbed radiation So, in equilibrium, what is absorbed is being emitted for both real
and blackbodies!!
REAL OBJECTS
0t
arAbsorptivity []: absorbed radiation /
incident radiation
Transmissivity []: transmitted radiation/ incident radiation
Reflectivity []: reflected radiation/ incident radiation
𝛼 𝜌 1
Real object emits less radiation than black body with the same temperature, 𝐿 𝜆, 𝑇
How much less: described by emissivity 𝑳 𝝀, 𝑻 𝛜 𝛌 ⋅ 𝑳𝑩𝑩 𝛌, 𝐓 Emissivity of black body: 𝜖 𝜆 1 Emissivity of real objects: 𝜖 𝜆 1 Radiation measured by a sensor is a sum of radiation reflected
and emitted by the Earth Not possible to separate directly Emitted radiation: spectrum depends only on 𝑇 and 𝜖 If 𝜖 is known, 𝑇 can be derived from 𝐿 Otherwise, 𝑇 cannot be determined from 𝐿
EMISSIVITY
EarthEmission processes
Thermal emission
Atmospheric emission
Reflection processes
Reflectedradiation
LEARNING SEQUENCE IN THIS LECTURE
TOA
Clouds Scattered radiation*
Atmospheric absorption
scatteredradiation**
transmittedradiation
Sun
Reflection
3
INTERACTION OF EMR WITH SURFACE: REFLECTION
REFLECTION IN NATURE
Used for photosynthesis
(a) Specular reflection from a smooth surface
(b) Diffused reflection froma rough surface
()
REFLECTIONS FROM THE SURFACE
SPECULAR REFLECTION ‐ EXAMPLE
Energy reaching the surface: irradiance [W m‐2]
Energy reflected by the surface: radiance [W m‐2]
Reflectance curve: fraction of irradiance that is reflected as a function of wavelength
radianceirradiance
Reflectance curves are material specific: spectral signature
SPECTRAL REFLECTANCE CURVES
26
REFLECTANCE BY SENSORS: DIFFERENT AT DIFFERENT HEIGHTS?
YES!!!
BOA orSUR
Canopy
TOA𝐿 ↓
𝐿 ↓
𝐿 ↓
𝐿 ↑
𝐿 ↑
𝐿 ↑ _𝐿 ↑ _
𝜌 𝜆 _𝐿 ↑ _𝐿 ↓ 𝜌 𝜆 _
𝐿 ↑ _𝐿 ↓
𝜌 𝜆𝐿 ↑𝐿 ↓
𝜌 𝜆𝐿 ↑𝐿 ↓
27
SEQUENCE OF REFLECTANCE ESTIMATION IN THE LAB
1 2
3
65
7 ?
8 ?
Factors Contributing
to leaf reflectance
Leafpigment Water content
Scattering by leaf cells
Absorption by free water in plant tissue
UV I n f r a r e d
FIR/TIRNIR MIR
Wavelengths (m)
50
40
20
00.1 0.4 0.5 0.6 0.7 1.35 1.4 1.9 3
14
Visiblerange
Absorption for photosynthesis
SPECTRAL REFLECTANCE ‐ HEALTHY VEGETATION
SOME TYPICAL REFLECTANCE CURVES
EXAMPLE IN IMAGES
Visible Infrared
Visible IR
31
EXAMPLE
EFFECT OF SUN ILLUMINATION ANGLE
Same amount of radiation in equal solid angle, different footprint area
Smaller footprint area – larger irradiance
Affects measured radiance, must be considered for reflectance calculations
Multi temporal studies: take into account season, date and time!
1θ 2θ
EFFECT OF RELIEF
𝜃- local incidence angle
- Depends on slope- Shadows
OTHER TYPICAL SPECTRAL REFLECTANCE CURVES
EarthEmission processes
Thermal emission
Atmospheric emission
Reflection processes
Reflectedradiation
LEARNING SEQUENCE IN THIS LECTURE
TOA
Clouds Scattered radiation*
Atmospheric absorption
scatteredradiation**
transmittedradiation
Sun
Atmosphere
4
4. INTERACTION OF ERM WITH THE ATMOSPHERE
Gases mainly absorb EM radiationknown concentrations and location (cycles) enable to predict influence (per 𝜆)
Aerosols mainly scatter EM radiationvariable and difficult to model (human and natural changing influence)
Either way the satellite senses less than what reached the Earth’s atmosphere!
ATMOSPHERIC INTERACTIONS
ENERGY INTERACTIONS WITH THE ATMOSPHERE
Visible
THE SOLAR SPECTRUM (ABSORPTION)
ATMOSPHERE (ABSORPTION)
SELECTIVE (RAYLEIGH) SCATTERING
SCATTERING TYPE: REFERENCE GRAPH
Source: many authors
The same particle hit by a different wavelengths produces different kind of scattering in the wavelength.
SUMMARY
RS is based on detecting EMR EMR is described as waves & photons EM spectrum Blackbody radiance & emissivity Interaction with the atmosphere Absorption and scattering Atmospheric windows Interaction at the surface Spectral reflectance curves (‘spectral signatures’)
THANKS
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