Remote Sensing Principles

• This lesson covers the basic principles of remote sensing and its application in agriculture.

What is Remote Sensing?

• Remote sensing involves observing an object from a distance without physical contact.
• It's the art, science, and technology of gathering information using instruments at a distance.
• The term was first used in 1950 in the USA by Evelyn Pruitt of the U.S. Office of Naval Research.
• It is the process of obtaining information.

Components of a Remote Sensing System

• Energy Source(A): Provides illumination, commonly the sun.
• Atmosphere(B): Interaction occurs between the energy and the atmosphere.
• Target(C): Interaction happens once more, between the energy and the target.
• Sensor(D): Records energy reflected or emitted from the earth’s surface.
• Data Retrieval/Storage/Distribution(E): Processes for data retrieval and storage.
• Interpretation/Correction/Analysis(F): Required for gathered information.
• Applications(G): Using processed data for practical purposes.

Remote Sensing Platforms

• Remote sensing platforms act as carriers for remote sensors, enabling data collection from various locations.
• Ground-level platforms include cranes and towers.
• Aerial platforms include helicopters, high-altitude aircraft, and low-altitude aircraft.
• Spaceborne platforms include space shuttles, geostationary satellites, and polar-orbiting satellites.
• Sputnik 1 was launched in 1957 by the Soviet Union.
• Landsat 1 was launched in 1972 by the USA and decommissioned in 2021.
• Sentinel 1 launched in 2014 by the European Space Agency (ESA); its last satellite was launched in 2020.

Principles of Remote Sensing

• Remote sensing measures the radiations of different wavelengths that are reflected or emitted from distant surfaces.
• This measurement aids in identifying and categorizing these surfaces.
• Four basic components are Energy source, Transmission path, Target, Satellite sensor.

Understanding Light

• In 1704 Isaac Newton determined light behaves like a particle.
• In 1803 Thomas Young proved travels as a wave.
• Light possesses electrical and magnetic energy components.
• Thus, light is also called electromagnetic energy, or EM energy.

EM Waves

• Wavelength (λ) is the length of one oscillation cycle, measured in meters (m), millimeters (mm), nanometers (nm), or micrometers (µm).
• Period (t) measures the time in seconds (s) required to complete one cycle.
• Frequency (f) is the number of waves passing a specified point per unit time and measured in Hertz (Hz), where Hz = s⁻¹.
• Amplitude (a) is the peak value of the wave.
• Wavelength and frequency are inversely proportional: f ∝ 1/λ.
• EM waves travel at the speed of light (c).
• The formula: c = fλ.
• Speed of light: approximately c ≈ 3 × 108 m/s.
• Energy is inversely proportional to wavelength but proportional to frequency.
• Planck's constant (h) is approximately 6.6262 × 10⁻³⁴.

EM Spectrum

• EM spectrum is the range of all types of EM radiation.
• Types of waves are Gamma rays, X rays, Ultraviolet, Visible light, Infrared, Microwaves and Radio waves.

Sources of EM Energy

• The sun is primary source of EM Energy.
• The Earth itself is a source of EM Energy.
• Remote sensors measure reflected energy.
• Remote sensors measure emitted EM energy.

EM Energy Interaction with Matter

• Incoming Energy: Sunlight i.e. visible light, near-infrared, and thermal infrared.
• Absorption: The leaf absorbs visible light for photosynthesis, raising its temperature.
• Reflection: Chlorophyll absorbs red and blue wavelengths but reflects the green wavelengths.
• Transmission: Light passes through the leaf for photosynthesis.
• Emission: Leaf emits thermal infrared radiation as heat. For visible and near-infrared radiation: α + τ + ρ = 1.
• Thermal radiation: ε + τ + ρ = 1.

Measuring Reflected EM Energy

• Spectral Radiance (Lλ(θ, φ)) refers to the radiant energy reflected by a surface in a specific direction (θ, φ) area, angle, wavelength, and time.
• Φλ(θ, φ): denotes the total power of radiation at a specific wavelength.
• Solid angle Ω: Indicates the source's angular extent seen by the sensor.
• Projected area A: Represents the surface area perpendicular to incoming or outgoing radiation.
• Term cos(θ): Factors in the angle of incidence relative to the surface.
• Radiance (L) relates to energy emitted/reflected from a source per unit area, angle, and time (Wm⁻²sr⁻¹).
• Spectral radiance (Lλ) refers to radiance per unit wavelength (Wm⁻²sr⁻¹µm⁻¹).
• Irradiance (I) indicates the amount of incident energy on a surface per unit area per unit time (Wm⁻²).
• Actual spectral properties of the object is needed, that doesnt rely on the incoming light.
• Reflectance is the proportion of radiation reflected by a surface.

Radiation Interaction with Atmosphere

• Atmospheric absorption by molecules such as O₂, H₂O, CO₂, and O₃ affects radiation transmission.
• Graph displays the transmittance levels of atmospheric windows across the electromagnetic spectrum.

Atmospheric Window

• The atmospheric window refers to the portion of the electromagnetic spectrum which transmits through the atmosphere.

Atmospheric Scattering

• Atmospheric scattering involves particles in the atmosphere deflecting and dispersing sunlight.
• Rayleigh scattering is caused by very small particles like O₂ and NO₂ that scatter shorter wavelengths more.
• Mie scattering is triggered by larger particles and affects all wavelengths, resulting in white or gray skies.
• Non-selective scattering occurs due to very large particles and is independent of the wavelength, e.g., clouds.

Passive and Active Remote Sensing

• Passive sensing measures natural radiation emitted or reflected, depending on sunlight as an energy source.
• Active sensing emits its own energy and measures the reflected or backscattered signal.