Remote Sensing Fundamentals

Remote Sensing Definition

• Measurement/observation of a target without physical contact.
• Technical definition: Using electromagnetic radiation sensors to record images and interpret them for useful information.

Key Components

• Electromagnetic radiation: detected by sensors to produce images.
• Sensors: record electromagnetic radiation.
• Images: of the environment, analyzed for useful information.

Remote Sensing: Science or Art?

• Science: Utilizes physical, biological, social sciences, and mathematics.
• Art: Image interpretation requires skill and background knowledge.

Remote Sensing Process

• Begins with problem statement and hypothesis formulation.
• Approaches: Deductive (theory to specific), inductive (specific to theory), technological (algorithm-based).
• Data Collection: In situ measurements, collateral data (soil maps, DEMs), and remote sensing data.

Types of Remote Sensing

• Passive: Uses external source of light (e.g., sun).
• Active: Sensor has its own light source (e.g., LIDAR, radar).

Energy Transfer

• Conduction: Transfer through kinetic energy from molecule to molecule.
• Convection: Transfer through the movement of hot material.
• Radiation: Energy transfer via electromagnetic waves.

Electromagnetic Radiation (EMR)

• Composed of electric and magnetic fields at 90° to each other.
• Travels at the speed of light (approximately $3 * 10^8$ m/s in vacuum).

Characteristics of EMR

• Wavelength ($\lambda$): Distance between two crests of a wave.
• Amplitude: Height of the wave from the middle point.
• Frequency: Number of cycles per second (Hertz).
  • $c = \lambda * f$ (c = speed of light)
• Frequency and wavelength are inversely proportional.

Electromagnetic Spectrum

• Range of electromagnetic radiation from gamma rays to radio waves.
• Shorter wavelengths have higher frequency and energy.
• Longer wavelengths have lower frequency and energy.

Key Regions of EM Spectrum

• Visible light: 0.4 to 0.7 μm.
• Infrared radiation: 0.7 μm to 1 mm.
• Microwaves.

Blackbody Radiation

• Theoretical concept: a perfect blackbody absorbs and radiates all incident radiation.
• Stefan-Boltzmann Law: Total emitted radiation is proportional to the fourth power of absolute temperature ($E = \sigma * T^4$).
• $\sigma = 5.6697 * 10^{-8} \frac{W}{m^2 * K^4}$

Wien's Law

• Dominant wavelength is inversely proportional to temperature ($\lambda_{max} = \frac{k}{T}$).
• $k = 2897 \mu m * K$
• Solar radiation is shortwave, Earth's emitted radiation is longwave.
• $Sun \approx 0.48 \mu m$
• $Earth \approx 9.66 \mu m$

Particle Theory (Quantum Theory)

• EMR composed of discrete units called photons.
• Energy of a photon: $E = h * f$, where h is Planck's constant.

Atmospheric Interaction

• Atmosphere affects EMR's wavelength, energy, and magnitude.
• Absorption by atmospheric gases (e.g., water vapor, carbon dioxide, ozone).
• Scattering: Unpredictable diffusion of radiation by particles.

Types of Scattering

• Rayleigh scattering: By particles smaller than the wavelength (e.g., blue sky).
• Mie scattering: By particles similar in size to the wavelength.
• Non-selective scattering: By particles much larger than the wavelength.

Atmospheric Windows

• Regions of the EM spectrum with minimal atmospheric absorption.

Interaction with Matter

• Reflection
• Absorption
• Transmission
• Reflectance: Ratio of reflected radiation to incident radiation.

Surface Characteristics

• Specular reflection (smooth surface).
• Diffuse reflection (rough surface).

Remote Sensing Platforms

• Satellites.
• Airplanes.
• Helicopters.
• Unmanned Aerial Vehicles (UAVs) / Drones.

Resolution Characteristics

• Spatial Resolution: Determined by pixel size.
• Spectral Resolution: Ability to define finer wavelength regions (number of bands).
• Radiometric Resolution: Sensitivity of the sensor to differences in energy (measured in bits).
• Temporal Resolution: Revisit time of the sensor.