Radiation Framework Flashcards
What are 3 methods to collect RS data?
- Satellite
- Airborne
- Ground-based in situ
ENVISAT Mission
- Set of instruments and sensors that analyze different parts of the spectrum
- Failed mission
Sentinel-1 Mission
- Single Instrument, 1 sensor mission
- C-Band Synthetic Aperture Radar (SAR)
- Imaging Radar
Most often means?
- Utilize electormagnetic radiation (EMR) recorded by an instrument and converted to digital format
What are some exceptions to EMR?
Sound, gravity fields
What is the generalized RS process?
- Energy Source (sun or sensor)
- Atmospheric Interaction
- Target Interaction (Earth surface)
- Energy Recorded (at sensor)
- Processing (image)
- Analysis and Interpretation
- Application
What are the 3 basic models for digital remote sensing?
Passive:
- Reflected solar radiation, sun as direct energy source
- Emitted radiation, sun as original energy source, absorbed, then re-radiated
Active:
- Backscattered radiation, instrument is own source of illumination (Pulse and echo)
EMR structure, wave and particle theory
- Wave: Streams of continuous waves, classical physics, for EMR structure
- Particle: Discrete packets of particles as per modern physics, quantum theory, for EMR energy content
Wave theory
- Oscillating electric and magnetic fields
- Orthogonal to each other
- Perpendicular to the direction of travel
- Travel at speed of light
- Both electric and magnetic field travel at speed of light perpendicular to direction of travel
- For EMR structure
Quantum theory
- EMR as discrete packets of energy called quanta
- A single quantum or ‘particle’ of energy called a photon
Wavelength and frequency, eqn
velocity of light (3.8 x 10^8m/s) = wavelength (m) x frequency in cycles per second (Hertz)
- wavelength can be derived from freq and vice versa
- Freq inversely propotional to wavelength, shorter wavelength = higher frequency
Electromagnetic Radiation (EMR)
- Sun’s visible surface or photosphere radiates EMR over continuous spectrum of wavelengths
- Wavelengths from 10m plus down to micrometers (gamma)
Atmospheric Transmission
- Atm windows: EMR passes through atm w/ minimal or no absorption or scattering
- Almost complete transmission at microwave spectrum
- Therefore transmits trough cloud cover and minimal sunlight (or no sun)
What is the frequency of Microwaves often expressed as?
- GigaHertz (GHz)
- 1 GHz = 10^9 Hz
Microwave desc
- Interval of continuous EMR spectrum that includes wavelengths from 1mm or 1cm to 1m (extends into radio waves)
- Range not rigidly defined
What is the length of microwaves in comparison to optical portion of EMR spectrum?
- Microwaves approx. 100,000 times longer than optical
What are 2 big benefits of microwave sensing>
- Sunlight not required
- Clouds are transparent
What are some benefits of all weather, day/night sensing?
- Emergency response (rapid mapping of floods)
- Iceberg detection for shipping
- Maritime safety
- Management of hazards
- Oil spills
- Arctic sea ice area and long-term change
Can microwaves be attenuated by atmospheric particles and rain?
- Yes, small wavelength microwaves
Quantum theory, calculating radiant energy (Q)
- Radiant energy (Q), the energy content of a photon = planck’s constant (h, 6.6 x 10^-34Js) x frequency (Hz)
- If frequency is unknown, can sub in wavelength/freq calc (freq = speed of light (c)/wavelength
- Q = hc/wavelength
- Longer wavelength radiation has lower energy content
What is the relationship with small wavelengths to radiant energy (Q) and frequency?
- Smaller wavelength = high frequency and high Q
- Q of microwaves much less than Q of visible
Radiant energy (Q)
- Energy of electromagnetic radiation
- Its capacity to do work
- Units in Joules
Radiant energy (Q)
- Energy of electromagnetic radiation
- Capacity of radiation to do work
- Units in Joules
Directional Power
- Sunlight measured on Earth
- Sun approximates an isotropic radiator, a point source which radiates energy uniformly in all directions
Radiant flux density
- Power per unit area
- Units in Watts per sq. m (W m^-2, or J/s*m^2)
Irradiance and Exitance
- Radiant flux incident upon a surface, per unit area of that surface
- Radiant flux leaving a surface, per unit area of that surface
Why does passive radiation have low spatial resolution and large swaths?
- Sensor ‘hoovers up’ energy available when passing overhead
- More energy needed to acquire high spatial resolution, therefore more time over spot
- However, time is short, so a wide swath is used to build up enough signal for the sensor
Why does active sensing usually have higher spatial resolution?
- Controls radiant flux
- Therefore can control spatial res and ‘crank it up’
Black-body radiator
- Theoretical perfect absorber and re-emitter of energy at all wavelengths
- Nothing is lost to reflection and transmission (conservation of energy)
- Water is close to black-body (0.95 of 1)
Black-body radiator
- Theoretical perfect absorber and re-emitter of energy at all wavelengths
- Nothing is lost to reflection and transmission (conservation of energy)
- Real-world objects are not black-body radiators
- Most objects approach black-body
- Water is close to black-body (0.95 of 1)
Emissivity
- Ratio of radiant existence of an object to that of a black-body at the same physical temperature
- ex. snow = 0.8, soil = 0.9, water = 0.95
What is exitance from a black-body proportional too?
- Proportional to the 4th power of its temperature
- black-body total emitted radiation, M (Watts/m^2) = Stefan-Boltzman constant, sigma (5.67 x 10^-8) x Temp ^4 (Kelvin)
- Warmer objects radiate more energy
What is exitance from a black-body proportional too?
- Proportional to the 4th power of its temperature
- black-body total emitted radiation, M (Watts/m^2) = Stefan-Boltzman constant, sigma (5.67 x 10^-8) x Temp ^4 (Kelvin)
- Warmer objects radiate more energy (ex. IR camera where warmer = brighter)
Wien’s Displacement Law
- Dominant wavelength of exitance from black-body is temp dependent
- Max wavelength = Wiens constant, k (2898 um K)/ Temp (Kelvin)
- Increase temp and dominant wavelength decreases
Planck’s Radiation Law
- Combination of Stefan-Boltzman and Wiens laws
- Describes temp and wavelength-dependent exitance of black-body
What is the dominant wavelength of the sun?
- In the visible spectrum
- Fire is in IR and TIR
- Earth is in microwaves
Beyond 10um, where is the energy available at the Earth’s surface from?
- From the Earth itself, not the sun
- Due to solar energy reduced by inverse-square law dispersion of energy w/ distance from source
- Earth is closer than sun at those wavelengths
What is the energy like at microwave spectrum?
- Very low
- Not optimal for Earth surface observing but still useful b/c it can capture info regardless of time or cloud cover
What are the 3 ways the energy is transferred?
- Conduction (pot on stove)
- Convection (warm ground heats air near surface)
- Radiation (EMR, sun to Earth)
Conservation of Energy
Energy interacting w/ matter (surface or object) is subject to conservation of energy laws such that it is:
- Absorbed
- Reflected
- Transmitted
- Fractions of each all add to 1
What are the proportions based on with the factors of conservation of energy (Abs, refl, trans)?
- Vary based on properties of material, wavelength of the energy, angle of illumination
- Increase angle of illumination, and more is reflected, less absorbed, but total fractions will still = 1 (conservation of energy)
Energy-Matter interactions: Refraction
- Bending of EMR relative to surface normal as it encounters a medium of different density (ex. from air to water)
Energy-Matter interactions: Reflection - 3 types
Scattering:
- Specular reflection
- Diffuse scattering
- Isotropic scattering
Energy-Matter interactions: Specular reflection
- Perfect reflection
- Reflection angle predictable
- Calm water body
Energy-Matter interactions: Diffuse scattering
- Partially diffuse
- Reflection angle not predicable
- Slightly windy water surface
Energy-Matter interactions: Isotropic Scattering
- Lambertian
- Perfectly diffuse, energy scattered equally in all directions
- Very windy water surface
Energy-Matter interactions: What is the wavelength dependency of absorption, transmission, and scattering?
- Complex function of size of wavelength relative to material and physical/chemical properties of material
Energy-Matter interactions: Size of wavelength relative to material
- Rule of thumb: shorter wavelengths more likely to be scattered and/or absorbed, longer wavelengths transmitted
Physical and chemical properties of the material
- Complex, subject of research
