AgTech - Science: Lecture 1

Describe the remote sensing process

• Remote sensing: technique to collect data about the Earth without touching its surface; records information from visible, infrared, and radar regions; from a distance.

• Passive sensors: record natural EM radiation reflected/emitted by surfaces (e.g., sunlight).

• Active sensors: emit their own energy and measure returned signal; usable 24/7 as they don’t rely on sunlight.

• Data is collected from platforms that include ground, air (aircraft, balloons), and space (satellites).

Describe the main features of solar electromagnetic radiation (EMR)

• An E/M wave consists of propagating electric (E) and magnetic (B) fields that move at a finite speed c = 3\times 10^8\ \mathrm{m\,s^{-1}}.

• Key properties of waves include wavelength \lambda, amplitude A, frequency f, and velocity c.

• The relationship between speed, frequency, and wavelength is described by the formula c = f\lambda.

• The electromagnetic spectrum ranges from gamma rays to radio waves, with the visible range relating to color.

• When EMR encounters the atmosphere or surface, interactions include:

  • Refraction

  • Scattering

  • Absorption

  • Reflectance

• Atmospheric effects alter the speed, wavelength, intensity, and spectral distribution of EMR.

• Scattering depends on factors such as wavelength, particle abundance, and travel distance through the atmosphere.

• Absorption can lead to re-emission of energy as long-wave radiation, influencing the heating of surfaces.

• The Sun is the principal light source for remote sensing, and the color of objects arises from the scattering of solar EMR.

Understand how remote sensing works for agriculture applications

• Spectral Reflectance and Signatures:

  • Spectral reflectance varies by material (e.g., soil, vegetation, stubble) across different wavelengths.

  • Vegetation and soil exhibit characteristic spectral signatures across blue, green, red, and Near Infrared (NIR) bands.

  • The visible spectrum typically ranges from 400–750 nm, while NIR is beyond 700 nm.

• Plant Spectral Behaviour:

  • Healthy vegetation generally reflects significantly more in the NIR band than in visible bands.

  • Plant cells contribute to high NIR reflectance due to their leaf structure, with leaves reflecting 40{-}60\% of incident NIR energy.

• Soil Spectral Behaviour:

  • Soil reflectance is highly dependent on its moisture content.

  • Dry soil: typically exhibits higher reflectance.

  • Wet soil: shows more absorption and consequently lower reflectance due to increased moisture.

• Types of Remote Sensing for Agriculture:

  • Multispectral: Utilizes a few broad bands (e.g., 4–8 bands), including visible and NIR, for general assessment.

  • Hyperspectral: Employs many narrow, continuous bands across the spectrum, enabling better discrimination of biomass, vigor, nutrients, and species by highlighting the amplitude and shape of reflectance profiles for detailed material identification.

• Plant Health and Vegetation Indices:

  • Vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), use the differential reflectance between NIR and Red bands to gauge plant vigor and biomass.

  • NDVI can be derived from aerial or satellite imagery to map and monitor vegetation health over time, serving as a practical summary index derived from spectral reflectance.