INTRODUCTION TO REMOTE SENSING

CONCEPTS OF REMOTE SENSING

REMOTE SENSING PROCESSES
PHYSICS OF REMOTE SENSING

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Remote Sensing

 Remote Sensing” is the acquisition of information about an object or phenomena (or anything) without being in
physical contact with it
 RS-The Measurement of reflected or emitted energy from distance (in Earth Observation Science).
EXAMPLES

 Reading a book (in this case eyes act as sensors)

 Listing to a Voice

 Taking Photograph with a camera

 Airborne, satellite RS

 Ground measurement of precipitation by collecting rain in a bucket or rain gauge is “in situ”

 Temperature measurement of a body/surface with Thermometer is “in situ”

 Temperature measurement with TIR Camera is “remote sensing”

 Radar measurement of precipitation is “remote sensing”

PHYSICS OF REMOTE SENSING

 Remote sensing is possible by the interaction EMR with a target (RS make use of the information present in the
EMR that was reflected or emitted from the earth )
 EMR can be modeled by two ways
 Wave form
 Photons (discreet energy particles)
WAVE THEORY

 EMR/Light travels as Sinusoidal Waves

 Light has two oscillating components perpendicular to each other

 Electric and magnetic fields

WAVE THEORY (1970)
WAVELENGTH AND FREQUENCY ARE IMPORTANT PROPERTIES
OF EMR

 Wavelength is the distance between two consecutive trough or crest

 Frequency is the number of cycles passing a point per second(unit=Hz)

 The speed of light is constant (c is constant=3*10 8ms-1)

 Frequency is inversely proportional to wavelength

PARTICLE THEORY

 EMR consist of energy particles called Photons

 At certain wavelength, Energy carries by photon is

 Q is the Photon energy in Joules(J).

 h is plank constant= 6.626x10-34 Js

 Relationship between Q, λ and ν

Combining both Theories

 Short wavelength implies high frequency, while long wavelength mean low frequency

 Fig. Which one has the higher frequency?

 Ultraviolet, Blue, infrared, Thermal

SOURCES OF EM ENERGY
PLANCK'S LAW (BLACKBODY RADIATION)

 Objects with Temperature above Absolute Zero emit EM radiation due to molecular agitation

 Planck's Law summarizes the amount of radiation emitted per unit of solid angle (in term of wavelength and
object’s temperature)
TEMPERATURE AND BLACKBODIES
 Planck’s Law is applicable to Black Bodies

 Black bodies (perfect absorptivity) absorb 100% of incident Radiation (reflect nothing).

 BB also emit EM radiation at every wavelength (called BB radiation)

 Real world object are not perfectly BB, they can re-emit 80-98% of the received radiation.

 Emissivity is the emission ability of an object

 ε=emission from object/BB emission at object

Temperature

 ε is wavelength dependent and values ranging from 0-1

 WIEN’S DISPLACEMENT LAW

 Wien’s Displacement is about the spectral region (wavelength) where maximum emittance can occur

 If the temperature of an BB is known, one can predict the peak of BB curve.

 The peak shift to shorter wavelength as the temperature of BB rises.

 Colors tells us something about Temperature

 Blacksmith Heating Iron: color change from red to orange

 Candle Flame: Blue at the Center(show the hottest flame

 Sensor band position are application Specific.

 Forest fire (1000k), 2.9 um, Earth surface(300k), 8-14 um

EM SPECTRUM

 The total range of wavelengths of EMR is referred EM Spectrum

 Different portions are referred as Cosmic, Gamma, X-rays, UV, Visible,

IR, Microwave and Radio waves.
EM SPECTRUM

 Each spectrum is continuous without any clear cut sharp boundaries between various types, rather they represent range of
wavelengths

 Different spectrum portions have relevance for different applications of earth observations.

 Visible imaging (studying various classes)

 Thermal imaging (temperature and emissivity)

 Microwave for soil moisture and surface roughness

EM SPECTRUM

 EM Spectrum and its utility in various applications

 VISIBLE PART OF EM SPECTRUM

 Different color we see are due to Visible portion of the EM Spectrum

 Human eye is only sensitive to the Visible part of the entire spectrum

 Objects reflect light in specific wavelength which determines its color.

 Green is due to the predominate reflectance in “Green”

 Why sun looks Yellow?

EMR INTERACTION WITH ATMOSPHERE

 Before sun light reaches earth surfaces, 3 type of interactions occur at the
atmosphere
 Absorption, Transmission and Scattering

 The Transmitted Energy is either absorbed by the surface or reflected

 Before reaching the RS sensor the reflected EMR is either absorbed or

scattered by the Atmosphere.
ABSORPTION AND TRANSMITTANCE

 All wavelengths are not equally useful for RS simply because the radiation at certain wavelengths cannot penetrate
through the atmosphere.
 As EMR moves through atmosphere , some of it is absorbed by various molecules in the atmosphere.

 The efficient absorber in the atmosphere are H2O vapors, O3, CO2 , CH4

 The portions (wavelength range) of EM spectrum where energy transmit unabsorbed is called “Atmospheric
Transmission Windows”
 ABSORPTION AND TRANSMITTANCE

 The atmospheric windows at various spectral domains, the grey areas highlight the spectral domains suitable for remote
sensing applications.
ATMOSPHERIC SCATTERING

 When the EMR interacts with particle/gaseous molecules in the atmosphere and redirect it from its original path
is called scattering
 There are three type of scattering primarily depends upon particle size in the atmosphere

1. Rayleigh scattering
2. Mie scattering
3. Non-Selective Scattering
ATMOSPHERIC SCATTERING

 Suppose ‘d’ is the diameter of atmospheric molecules and other tiny particles

 Rayleigh scattering : ( dominates where the particle size are smaller then the wavelength of light)

 Occurs when d << λ

 Rayleigh scatter is inversely proportional to the fourth power of λ:

Rayleigh α 1/λ4
 In Rayleigh scattering, shorter wavelengths scatter more than longer wave length

 Causes: Tiny Specks of dust, NO2, O2

 ATMOSPHERIC SCATTERING

 Why Sky appear Blue and orange or red at the sun rise and sun set?
MIE SCATTERING

 Occur when the particle size (d ~ λ ) are equal to the incident wavelength of EMR

 Major causes: Water vapor and dust

 Mie scattering is mostly restricted to the lower part of the atmosphere where larger particle are more abundant

 Mostly occur at overcast/ cloudy condition

 Affect the RS signals in longer wavelengths

NON-SELECTIVE SCATTERING

 Non-selective scattering: occur when the particle sizes are much larger (d >> λ) then the radiation wavelengths (d >> λ)

 Major causes: Water droplets,

 Non-selective scattering are independent of wavelengths in the optical range

 Example: cloud appear white as water droplet scatter light equally in all wavelengths
NON-SELECTIVE SCATTERING

 Remote sensors cannot “see through” clouds in the optical domain

 Causing shadow is another limiting effect of Clouds on optical RS

 RESOLUTION

•Spatial resolution
•Spectral resolution
•Radiometric resolution
•Temporal resolution

Radiometric resolution
 TYPES OF REMOTE SENSING
• Passive RS: External source of electromagnetic radiation (mostly sun)

• Passive RS examples:

• Visible

• Reflected Infrared

• Emitted Infrared

• Passive microwave / Microwave radiometry

• Active: RS sensors has its own source of electromagnetic radiation

• Active RS examples:

• Radar (Active microwave)

• Lidar (Laser remote sensing)

THE MANY ASPECTS OF REMOTE SENSING

 Physical Science & Technology Development  Data processing

 Study of physical interactions  Information retrieval

 Sensor design & development  Error analysis

 Testing & calibration  Geoscience & Earth Observation

 Observational Campaigns & Missions  Data interpretation

 System deployment  Data analysis techniques

 Data collection  Algorithm development

 Signal/Data Processing  Space Remote Sensing

 Astrophysics & Astronomy

 Spectroscopy