Electro Magnetic Radiation

Getting information about an object without direct contact with said object

Examples of EMR: Microwaves, Gamma rays

EMR is a broad form of radiation

• Visible spectrum 400nm (blue) - 700nm (red)

• Size of waves in order (long - short)

  1. Radio
  2. Micro
  3. Infrared
  4. Visible
  5. Ultraviolet
  6. XRay
  7. Gamma Ray

• Waves are measured from crest to crest

  • Shorter wavelengths have more energy

Passive vs. Active

Passive: Sensor receives energy from the sun (e.g., Camera)

Active: Sends and receives energy (e.g., Radar, Lidar)

Resolutions

4 Different types

• Spatial: Size of pixel on the ground
• Spectral: Number and size of spectral bands
• Temporal: How often the sensor acquires data
• Radiometric: Data Precision, sensitivity of the sensor

Radiometric

How many bits to describe information

Stored in binary/bits/bytes

Atmospheric Windows

• When EMR can transmit through the atmosphere without distortion or absorption

Sensor Geometry

Nadir: Location directly under the satellite

Scanners

• Whisk Broom Sensor

  • Sweeps back and forth across the planet

• Push Broom Sensor

  • Pushes along forward

• Geostationary: Fixed on one position to monitor, 35800km away

  • Can see half of the earth
  • Lower resolution

• Polar orbitors: Move around earth, 850 km away

  • ~100 minute orbit
  • Crosses the equator at the same time each day (sun-synchronous)
  • Ascending Track (night) and descending track (morning)

Sine Wave

\[ e = \alpha\sin\left(\frac{2\pi}{\lambda}x + \varphi\right) \]

Can be characterized by three parameters

• Wavelength \(\lambda\): Differentiating property of the various types of EM radiation
• Amplitude \(\alpha\): Peak value of the wave. The larger the amplitude, the higher the energy.
• Phase \(\varphi\): Expresses the fraction the start of the wave differs from the origin of counting distance (i.e., \(x=0\)). Phase can take any value in the range from 0 to \(2\pi\).

Period: Amount of time needed by an EM wave to complete one cycle. Corresponds to wavelength.

Frequency is measured in hertz, \(1Hz = 1 \text{cycle per second}\)

Relationship between wavelength and frequency is:

\[ c = \lambda \times v \]

\(c\) is commonly used as the symbol for the speed of light.

\(v\) is used for frequency.

Short wavelength means high \(v\) while long wavelength implies low \(v\). Blue light has a higher frequency than red light.

Particle Theory

In some cases, particle theory - energy by photons - can better explain EM energy phenomena than wave theory.

Amount of energy held by a photon relating to a specific wavelength:

\[ Q = h \times \upsilon = h \times \frac{c}{\lambda} \]

\(Q\) is energy of a photon measured in joules.

\(h\) is Planck’s constant (\(6.6262 \times 10^{-34} \text{ jouleseconds}\)).

The energy carried by a single photon of light is just sufficient to excite a single molecule of a photosensitive cell of the human eye, leading to vision.

Long \(\lambda\), low energy | Short \(\lambda\), high energy.

Beyond Red Light

Infrared (IR) is light beyond red light in the spectrum.

Discrimination of vegetation types and the stress state of plants can be determined by analyzing near-infrared (NIR) and mid-infrared radiation. I.e., deciduous trees reflect more NIR energy than conifers do, so they appear brighter.

Healthy vegetation has a high reflectance in the NIR range, which decreases with increasing damage caused by a plant disease.

Mid-IR is referred to as short-wave infrared (SWIR) and is used to monitor surface features at night.

Thermal infrared (TIR): Infrared radiation with a wavelength longer than \(3 \mu m\). It causes the sensation of heat.

Absorption / Transmission

The most efficient absorbers in the atmosphere:

• Ozone (\(O_3\))
• Water vapor (\(H_2O\))
• Carbon dioxide (\(CO_2\))

Useful ranges for wavelengths are referred to as atmospheric transmission windows.

• Window from \(0.4\) to \(2 \mu m\)

  • Visible, NIR, SWIR
  • Referred to as the optical range.

• Three windows in the TIR range:

  • Two narrow windows around \(3\) and \(5 \mu m\)
  • A third broad window extending from approximately \(8\) to \(14 \mu m\)

Strong absorption occurs at longer wavelengths due to atmospheric moisture.

Hardly any transmission of energy in the range from \(22 \mu m\) to \(1\) mm.

“Transparent” range beyond 1mm is the microwave range.

Atmospheric Scattering

Occurs when particles or gaseous molecules present in the atmosphere cause EM radiation to be redirected from its original path.

• Rayleigh Scattering: Dominates where electromagnetic radiation interacts with particles smaller than the wavelengths of light, such as dust, nitrogen (\(NO_2\)), and oxygen (\(O_2\)). Light of shorter wavelengths (blue) is scattered more than longer wavelengths (red). Rayleigh scattering diminishes the crispness of photos and has a negative effect on classification using multispectral sensors.

• Mie Scattering: Occurs when the wavelength of EM radiation is similar in size to atmospheric particles. Caused by aerosols (gases, water vapor, dust). Mie scattering affects longer wavelengths more than Rayleigh scattering.

• Non-selective Scattering: Occurs when particle size is larger than the wavelength. Clouds are the most prominent cause.

Reflection

• Specular Reflection: Mirror-like reflection from smooth surfaces (e.g., water).
• Diffuse Reflection: Rough surfaces cause energy to be reflected uniformly in all directions.

Spectral Reflectance Curve

Each material type has a reflectance curve that shows the portion of incident energy reflected as a function of wavelength.

Reflectance readings are stored in spectral libraries.

• Vegetation: Reflectance depends on properties of the leaves (e.g., orientation, structure). Pigmentation, thickness, and water content affect reflectance. The highest reflection occurs in the NIR range.
• Bare Soil: Reflectance influenced by soil color, moisture, carbonates, and iron oxide.
• Water: Water has lower reflectance than vegetation and soil, reflecting energy mainly in the visible range.