Satellite Data Analysis

1. Electro-Magnetic Radiation

Remote sensing

It means to:

• observe matter characteristics and physical quantities
• use faraway sensors (satellite-based)

EM Radiation

The radiation emitted from the sun is reflected in a particular way based on what it hits.

EM radiation is made of an electric (E) and magnetic (M) fields.

wavelength: λ = _c/_nf    c = 3.10⁸ km/h    n = refraction index

The atmosphere is transparent only at some ranges of λ:

• near the visible light (OPT - optical sensing);
• radio waves (MW - microwave sensing).

Spectral reflectance: an object creates a particular signature of light reflected.

EM spectrum:

• ultraviolet λ < 0,4 µm
• visible 0,4 µm < λ < 0,7 µm
• infrared λ > 0,7 µm up to a few µm
• microwaves λ > than mm

The most important colors to keep in mind are:

• BLUE → 0,45 µm
• GREEN → 0,55 µm
• RED → 0,65 µm

Peaks of water absorption:

• 1,5 µm
• 1,9 µm

Electro-Magnetic Radiation

Remote sensing

It means to:

• observe matter characteristics and physical quantities
• use faraway sensors (satellite-based)

EM Radiation

The radiation emitted from the sun is reflected in a particular way based on what it hits.

EM radiation is made of an electric (E) and magnetic (M) fields.

wavelength: λ = c / n f c = 3.10⁵ km/h n = refraction index

The atmosphere is transparent only at some ranges of λ:

• near the visible light (OPT - optical sensing);
• radio waves (MW - microwave sensing).

Spectral reflectance: an object creates a particular signature of light reflected.

EM spectrum:

• ultraviolet λ ≤ 0,4 μm
• visible 0,4 μm < λ < 0,7 μm
• infrared λ > 0,7 μm up to a few μm
• microwaves λ ≥ than mm

The most important colors to keep in mind are:

• BLUE → 0,45 μm
• GREEN → 0,55 μm
• RED → 0,65 μm

Peaks of water absorption:

• 1,5 μm
• 1,9 μm

Measuring the EM radiation

Energy = capability to perform work (Joules J).

Radiant energy = energy of an EM wave.

Flux of radiant energy = energy carried by an EM wave in 1 time unit.

Power of the EM wave (Watts W).

Density of radiant flux = radiant flux emitted or absorbed per surface unit ^(W/m2).

Spectral reflectance: ρ(λ) = ^{power leaving}_{power landing}

The reflectance is computed at a particular wavelength.

Irradiance = RF of an incoming EM wave.

Emittance = RF of an outgoing EM wave.

Directionality

The measurement happens at a specific angle, so it doesn't really intercept the whole emittance. The spreading of the radiation could be uneven, so the direction of measuring should be taken into account.

Radiance = density of radiant flux emitted by a surface unit and seen through a solid angle.

Diffuse reflection — the radiation is reflected evenly (ideal scenario).

Specular reflection — the reflection happens only at a specific angle.

The real-world case is a mix of the two reflections.

The less rough it is, the more the reflection is concentrated in a narrow cone.

Rayleigh Criterion

The reflection type depends on the roughness of the surface compared to the wavelength.

Critical height: h_c = ^λ_{8 cos θ}

Reflection Type:h => rough => diffuse

h smooth => mirror

In the case of visible light data acquisition, the material should be smooth at μm scale to have specular reflection (it is the case for water).

Emittance estimation

Existence = emittance associated to the EM radiation emitted by bodies [W/m²]

Planck law

Considering a black body (no reflection) (the Sun is well approximated by the BB)

Spectral emittance: Mλ(λ, T) = 2hc2/λ5 1/ehc/λkT – 1

The general shape has a peak and then goes down.If the peak goes into the visible range, the object starts to glow.

At around ~3 μm it's difficult to differentiate the radiation produced by the Sun (which is then reflected) and the radiation emitted by the Earth (it's a fuzzy boundary).Below 0.4 μm the ultraviolet radiation is blocked by the atmosphere.

• Reflection range: 0.4 μm ≤ λ ≤ ~3 μm (usually 2.4 μm)
• Emission range: λ ≫ 3 μm

Thermal remote sensing

To estimate temperature it would be sufficient to l