Electromagnetic Radiation, Satellite Sensors, Orbits, and Image Processing

Comprehensive vocabulary flashcards covering electromagnetic radiation principles, radiometric terms, radiation laws, scattering, sensor geometry, orbital mechanics, image statistics, and atmospheric correction methods.

Passive Remote Sensing

A remote sensing technique that records reflected electromagnetic radiation from external sources, most commonly reflected sunlight.

Active Remote Sensing

A remote sensing technique that actively emits its own electromagnetic radiation and records the signal that bounces back (such as a laser-beam system).

Electromagnetic Wave

A wave characterized by electric and magnetic fluctuating orthogonal fields travelling perpendicular to the direction of travel at the speed of light.

Speed of Light ($c$)

The velocity of electromagnetic radiation in a vacuum, defined exactly as $2.99722458 \times 10^8\,\text{m/s}$ (approximately $3 \times 10^8\,\text{m/s}$).

Photon

An individual, discrete packet or quantum of electromagnetic energy whose energy is given by $q = h \times \nu = \frac{hc}{\lambda}$, where $h$ is Planck's constant ($6.62607015 \times 10^{-34}\,\text{J}\cdot\text{Hz}^{-1}$).

Radiant Flux ($\Phi$)

The rate of flow of energy per unit time ($\Phi = \frac{dQ}{dt}$), measured in watts ($\text{W}$) where $1\,\text{W} = 1\,\text{joule/s}$, without surface area information.

Irradiance ($E$)

The radiant flux received by a surface per unit time per unit area ($E = \frac{d\Phi}{dA}$), expressed in units of $\text{W}\,\text{m}^{-2}$.

Radiant Exitance ($M$)

The radiant flux emitted by a surface per unit time per unit area ($M = \frac{d\Phi}{dA}$), expressed in units of $\text{W}\,\text{m}^{-2}$.

Solid Angle ($d\Omega$)

A conic angle encompassing an area element $dA$ on the surface of a sphere of radius $r$, calculated as $d\Omega = \frac{dA}{r^2}$ and measured in steradians ($\text{sr}$).

Radiant Intensity ($I$)

The radiant flux per unit solid angle emitted from a point source into a particular direction ($I = \frac{d\Phi}{d\Omega}$), expressed in units of $\text{W}\,\text{sr}^{-1}$.

Radiance ($L$)

The energy received or emitted by a surface per unit time per unit projected area per unit solid angle ($L = \frac{d^2\Phi}{dA\cos(\theta)\,d\Omega}$), expressed in $\text{W}\,\text{m}^{-2}\,\text{sr}^{-1}$.

Blackbody

An idealized physical surface or cavity that perfectly absorbs all incident electromagnetic flux (absorptivity $= 1$, reflectivity $= 0$) and reradiates it.

Planck's Law

A law describing the spectral exitance ($M_\lambda$) of a blackbody as a function of temperature ($T$) and wavelength ($\lambda$): $M_\lambda = \frac{2\pi hc^2 \lambda^{-5}}{e^{\frac{hc}{\lambda kT}} - 1}$.

Stefan-Boltzmann Law

A law derived by integrating Planck's equation over all wavelengths, showing that total blackbody exitance varies with the fourth power of absolute temperature: $M = \sigma T^4$, where $\sigma = 5.67 \times 10^{-8}\,\text{W}\,\text{m}^{-2}\,\text{K}^{-4}$.

Wien's Displacement Law

A relationship stating that the wavelength of maximum spectral exitance is inversely proportional to temperature: $\lambda_{\max} = \frac{b}{T}$, where $b = 2897.79\,\mu\text{m}\cdot\text{K}$.

Emissivity ($\varepsilon_\lambda$)

The ratio of spectral exitance from an object at temperature $T$ to the spectral exitance from a perfect blackbody at the same temperature, ranging from $0$ to $1$.

Kirchoff's Law

A physical principle stating that for any surface in thermodynamic equilibrium, spectral emissivity is numerically equal to spectral absorptance ($\varepsilon = \alpha$).

Rayleigh Scattering

Scattering that occurs when atmospheric particles or molecules are much smaller than the wavelength of incident radiation, with scatter intensity proportional to $\frac{1}{\lambda^4}$.

Mie Scattering

Scattering that occurs when the atmospheric particles (such as aerosols, smoke, or dust) are approximately equal in size to the wavelength of incident radiation.

Non-selective Scattering

Scattering occurring when atmospheric particles (such as water droplets and large dust particles) are much larger than incident wavelengths, scattering all visible wavelengths equally.

Lambertian Surface

An ideal, perfectly diffuse surface that reflects radiant energy equally in all directions, relating radiance and exitance by the factor $L = \frac{M}{\pi}$.

Normalized Difference Vegetation Index (NDVI)

A spectral index serving as a proxy for vegetation photosynthetic greenness, calculated as $\text{NDVI} = \frac{\text{NIR} - \text{Red}}{\text{NIR} + \text{Red}}$.

Bidirectional Reflectance Distribution Function (BRDF)

A mathematical function ($f_r$) describing the scattering of a parallel beam of incident light from one direction into another direction in the hemisphere, expressed in units of $\text{sr}^{-1}$.

Bidirectional Reflectance Factor (BRF)

The ratio of flux reflected by a surface in a specific direction to that reflected by an ideal non-absorbing Lambertian surface under the same geometry and single directional illumination; interchangeable with surface reflectance.

Directional-Hemispherical Reflectance (DHR)

The ratio of flux reflected into the entire hemisphere to incident flux when target illumination is a collimated parallel beam; also known as black-sky albedo (BSA).

Bi-Hemispherical Reflectance (BHR)

The ratio of flux reflected into a hemisphere to incident flux when illuminated by an arbitrary hemispherical radiation field; known as white-sky albedo (WSA) when incident radiance is isotropic.

Astronomical Unit (AU)

A unit of length roughly equal to the distance from Earth to the Sun, defined since 2012 as exactly $1.495978707 \times 10^{11}\,\text{m}$.

Julian Date (JD)

A time measurement recording the continuous interval of time in mean solar days and fractions of a day elapsed since 12:00 UT on January 1, 4713 B.C.

Subsolar Point

The single location on the globe where the Sun is directly overhead at a given instant, with its latitude representing the solar declination.

Analemma

The figure-eight pattern traced by the Sun's position in the sky when observed from the same terrestrial location at the same clock time every day over one year.

Instantaneous Field of View (IFOV)

The angular extent through which a detector element (pixel sensor) is sensitive to electromagnetic radiation at any one time.

Ground Instantaneous Field of View (GIFOV)

The geometric projection of the IFOV onto the ground surface, defining the physical ground resolution area covered by a single pixel.

Whiskbroom Scanner

An across-track spotlight scanner that uses moving optical components to direct incoming radiation across a prism into detectors, typical of older sensor designs like MSS and MODIS.

Pushbroom Scanner

An along-track scanner using a linear array of detectors aligned perpendicular to the flight path to capture data simultaneously along a swath, minimizing pixel distortion (e.g., Landsat-8 OLI).

Radiometric Resolution

The sensitivity of a sensor's detectors to small differences in reflected or emitted electromagnetic radiation, typically expressed by bit depth (e.g., 8-bit or 10-bit).

Swath

The cross-track width of the Earth's surface imaged by a satellite sensor during a single orbital pass.

Orbital Inclination

The angle formed between a satellite's orbital plane and the plane of the Earth's equator.

Geosynchronous Orbit

A high Earth orbit located at an altitude of approximately $36,000\,\text{km}$ ($42,164\,\text{km}$ from Earth's center) where the orbital period matches the rotation period of the Earth.

Geostationary Orbit

A circular geosynchronous orbit with an inclination of $0^\circ$ parked directly above Earth's equator, keeping the satellite stationary over a fixed longitude and latitude.

Molniya Orbit

A highly eccentric 12-hour orbit with an inclination of $63.4^\circ$ and an eccentricity of $0.722$, designed to maximize observation and dwell time over high-latitude polar regions.

Sun-Synchronous Orbit

A near-polar low Earth orbit (inclination $\approx 98^\circ$) that passes over any given geographic point at approximately the same local solar time, ensuring consistent surface illumination angles.

Nadir

The point on the ground located vertically directly beneath the perspective center of an imaging camera lens or scanner detector.

A-Train (Afternoon Constellation)

A constellation of Earth observation satellites (including Aqua) in a sun-synchronous orbit crossing the equator northbound at approximately 1:30 PM local solar time.

Lagrange Points

Specific positions in space where the gravitational pulls of two large celestial bodies equal the centripetal force required for a small body to orbit along with them, such as L1 and L2 located $1.5 \times 10^6\,\text{km}$ from Earth.

Skewness

A measure of the asymmetry of a frequency distribution or histogram, categorized as positive (right-skewed, mean $>$ median) or negative (left-skewed, mean $<$ median).

Kurtosis

A statistical measure of the peakedness or sharpness of a distribution curve relative to a perfectly normal distribution (which has a kurtosis of zero).

Spatial Autocorrelation

A statistical relationship describing the spatial similarity among measured values across an image as a function of the distance and direction between them.

Spatial Semivariance

A metric that quantifies the degree of spatial dissimilarity between pixel pairs separated by a given lag distance, where higher semivariance indicates lower similarity.

H-Resolution Scene Model

A remote sensing scene condition where the spatial resolution is sufficiently high such that objects of interest are larger than the individual pixel size.

L-Resolution Scene Model

A remote sensing scene condition where objects of interest are smaller than the sensor pixel size, resulting in sub-pixel mixing.

Chavez (1996) Dark Target Method

An image-based atmospheric correction approach that assumes at least one pixel in the scene has near-zero reflectance, meaning its measured DN is entirely path radiance that can be subtracted from the whole image.

FLAASH

A physics-based atmospheric correction algorithm using MODTRAN4 to correct for water vapor, ozone, and molecular/aerosol scattering using image metadata and atmospheric models.