GEOG 271 Flashcards
Rayleigh scatter
diameter of matter < wavelength of incident microwave radiation
ie. scattering of blue light by atmosphere (small specks of dust or nitrogen and oxygen molecules)
Mie scattering
diameter of matter is approximately equal to wavelength.
ie. scattering from dust, pollen, smoke and water vapour
Non-selective scattering
diameter of matter is several times larger than incident radiation.
ie. water droplets and large dust particles (visible light)
Blackbody
Emits a continuous spectrum of radiation across the EM spectrum proportional to its physical temperature.
The wavelength that is emitted with the most intensity (peak spectral exitance) is determined by its temperature.
Hotter objects emit peak spectral exitance at shorter wavelengths.
Blackbodies absorb and emit energy at the same rate. Good absorbers are good emitters.
ε = 1
Grey body
Object that emits a fraction of the radiation of a blackbody, governed by the object’s emissivity
emissivity (ε) is the ratio of radiant energy of an object compared to that of a blackbody at the same temperature.
Blackbody: ε= 1
Greybody: ε < 1
emissivity (ε) definition + 2 facts
the ratio of radiant energy of an object compared to that of a blackbody at the same temperature.
Emissivity has great influence in passive microwave emissions as the depth of emission of media may be well below the surface.
Shorter frequency thermal infrared is less affected by emissivity as the shorter wavelenths are emitted at depth much closer to the surface (ie. skin temperature)
Selective radiator
Emits radiation of varying intensity at different wavelengths, depending on the object’s emissivity at those wavelengths.
Absorptance
fraction of incident radiation that is absorbed
Brightness temperature
TB is a descriptive measure of radiation in terms of the temperature of a hypothetical blackbody emitting an identical amount of radiation at the same wavelength
Defined as the product of the emissivity and the physical temperature of the surface
Thermal capacity
The ability of a material to store heat
Water has highest thermal capacity- stores heat very well relative to all other materials
Thermal conductivity
Rate that heat will pass through a material.
Many rocks and soils are poor conductors of heat.
Important to know for diurnal studies, with heat increasing/ decreasing throughout the day.
Thermal inertia
Measurement of thermal response of a material to temperature changes.
How long does an object hold on to heat (and how much heat)?
How long does it take an object to heat up/ cool down?
What implication does this have for remote sensing?
- Rocks/water = high thermal inertia (long time to heat/cool)
- Sand = low thermal intertia (heat/cool quickly)
Relative complex dielectric permittivity (complex dielectric constant) ε*
describes the basic electrical properties of a material, which determine electromagnetic (EM) wave propagation, scattering, reflection, attenuation, and (for passive sensors) emission.
Two components:
Real part: ε’
Imaginary part: ε”
Real part: ε’ (component of relative complex dielectric permittivity)
Referred to as ____________.
Describes ___________.
Definition + 2 examples.
Referred to as relative dielectric constant or relative permittivity.
Describes what happens when electric field interacts with an object’s boundary (Reflection)
Sets the absolute backscatter level (ie. the degree of scattering is proportional to its dielectric constant). When microwaves are incident upon an object, if the permittivity of the first medium greatly constrast that of the second, the incident microwaves will be reflected.
ie. (1) Air and Ice (2) Ice and Water
ε’ (air) = 1, ε’ (ice) = 3.17, ε’ (water) = 80
1) ε’(air) - ε’(ice): low contrast; transmission of signal into medium
2) ε’(ice) - ε’(water): high contrast; reflection of signal
Imaginary part: ε” (component of relative complex dielectric permittivity)
Referred to as ____________.
Describes ___________.
Definition + 2 examples.
Referred to as dielectric loss.
Describes ability of material to dissipate penetrated energy (absorption). AKA how much energy is lost in the material volume once it passes across a dielectric interface.
When microwaves are incident upon an object, if the loss factor is high, the material will absorb, and none returned to the sensor
ie. (1) Ice, (2) Water
1) ε”(ice): 0.000065 (low loss)- transmission of microwaves through ice
2) ε”(water): 18.41 (high loss)- absorption of microwaves due to H20 molecule excitation
Atmospheric windows
Regions in the electromagnetic spectrum where the wavelengths of incoming solar radiation or emitted radiation from the ground are largely not absorbed by molecules in the atmosphere
Important atmospheric absorption bands
O2: 0.1- 0.3 & 0.7
H20: 1 & 5.5-7
CO2: mid-infrared (1.3-3) and thermal infrared (3-5) & 10.5-12.5)
O3: 8-14
Thermal infrared (3-14): absorption from:
CO2 (3-5 & 10.5-12.5), H2O (5-8) and O3 (8-14) - Greenhouse effect in this region
Path radiance
Paths 2,4: radiance observed at the sensors from areas other than the study area
Atmosphere Effects
Energy-matter interactions in the atmosphere, at the surface, and a the remote sensing system (sensor)
What does atmospheric correction do?
What causes haze?
Removes haze/path radiance
Haze caused by:
- high level clouds
- O2
- H2O
____________ surfaces act as specular reflectors and ____________ energy ___________ from the surface
(Active Microwave)
Smooth, reflect, away
____________ surfaces act as diffuse reflectors and ______________ energy ___________ from the surface
(Active Microwave)
Rough, diffuse, back to
Scattering mechanisms of:
snow: ______ permittivity; [scattering mechanism]
urban: _________ permittivity; [scattering mechanism]
Snow: low, volume scatter
Urban: high, extremely angular double bounce
Radar remote sensing: Primary advantages
- Penetrates clouds and can be an all-weather remote sensing system
- Operates at user-defined dates and times
- Permits imaging at shallow look angles, resulting in different perspectives that cannot alwats be obtained with other sensor types.
- Senses at wavelengths outside the visible and infrared regions of the EM spectrum, providing information on surface roughness, dielectric properties, and moisture content
- Synoptic views of large areas, for mapping at 1:25,000 to 1:400,000; cloud-shrouded countries may be imaged
What is RADAR?
Radio Detection and Ranging
Form of Active Microwave
A RADAR image is a two dimensional representation of returned power/voltage (termed: backscatter) from a specific area on the ground, represented as a pixel.
Backscatter is typically reported two ways
- Linear power: values between 0-2, very small values
- Decibels (dB): Typical range between -40 (low return) and 0 (all incident power returned to sensor).
Polarized energy
The pulse of energy is filtered so that its electrical wave vibrations are only in a single plane that is perpendicular to the direction of travel.
The pulse of energy that is sent out by the antenna can be vertically or horizontally polarized.
Polarization types
Depending on the radar it may be possible to:
- Send vertically polarized energy and receive vertically polarized energy (VV)
- Send horizontally polarized energy and receive horizontally polarized energy (HH)
- Send horizontal and receive vertical polarized energy (HV)
- Send vertical and receive horizontal polarized energy (VH)
Foreshortening
Steep slope on the front side reflects a great deal of incident energy back to the sensor (appears bright), while the gradual far side of the slope has less incident energy, and appears dark.
Layover
Extreme case of foreshortening where so much relief exists that the peak backscatters energy before the pulse reaches the mountain base. Causes displacement of features from true ground position.
Shadowing
Occurs when backslope of terrain is greater than depression angle, resulting in no illumination by the sensor. Can enhance geomorphology and texture of terrain, or obscure important features in acquisition.
Vegetation: Visible to Mid-IR
Leaf pigments in the palisade mesophyll are the dominant factors controlling leaf reflectance (chlorophyll a, b, beta carotene, etc.) in the visible band (0.4-0.7).
Blue and Red are the Chlorophyll absorption bands, the primary absorption bands.
Scattering in the spongy mesophyll occurs in the near infrared (0.7-1.3).
Water absorption occurs in the MIR (1.3- 3)
Vegetation: TIR
Physical temperature of surface canopy/foliage (and ground canopy if open canopy) is the main control on brightness temperature.
Vegetation: Passive Microwave
The signal received is a combination of the emission from vegetation layer and the ground surface (includes emissivity of the ground, vegetation transmission, and reflectance of the vegetation)
The last two parameters are a function of biomass, plant moisture and shape, the wavelength/frequency, and the view angle.
Vegetation: Active Microwave
Sources of contribution to total backscattering:
Surface scattering from the top of the canopy
- volume scattering
- surface and volume scattering from the ground
Longer wavelengths have greater penetration (L-band: 23.5 cm; C-band: 5.8 cm; X-band: 3 cm)
Soil: TIR
Physical temperature of ground surface is the main control on brightness temperature.
Surface moisture/wetness conditions do influence brightness temperature.
Soil: Visible to Mid-IR
Spectral reflectance characteristics of soils are a function of several characteristics:
- soil composition (% of sand, silt and clay)
- soil moisture content (dry, moist, saturated)
- organic matter content
- iron oxide content
- surface roughness
Soil: Passive Microwave
For a relatively smooth bare soil surface, the emissivity for passive is strongly influenced by
- the moisture content of the soil (dielectric properties)
- surface roughness causes emission to increase.
While in TIR it is the temperature of a thin layer on the order of the micron that controls the emission of a surface, in the microwaves it is a thicker layer that intervenes because wavelengths are on the order of a few centimeters.
The information on soils that we obtain by analyzing their microwave emissions therefore comes from a layer for which the thickness is proportional to the size of the wavelength used.
For terrestrial observations, the frequency ranges of interest are the 1-20GHz band and the 37GHz atmospheric transmission window.
Snow and Ice: Visible to Mid-IR
The spectral signature of snow is influenced by :
the size and shape of crystals at/near the surface,
the water content near the surface,
the presence of impurities (ie. soot), the
depth (thin layer), and
roughness of the snow cover.
Penetration near surface only ~ 1/2 mm in blue; a few mm in NIR and MIR.
Snow and Ice: TIR
Physical temperature of snow surface is the main control on brightness temperature.
Dry Snow: Microwave Region
Scattering has an increasing effect with an increase in snow thickness (depth) and grain size.
This results in generally lower emission (brightness temperature for passive microwave and larger backscatter for active microwave)
Backscatter
The portion of the transmitted signal that is returned from a target towards a receiving antenna.
Returned power, voltage.
Factors affecting the brightness temperature (passive) and backscatter (active) of freshwater and sea ice:
Freshwater ice:
- ice thickness
- snow cover
- temperature
- moisture
- air bubbles
- surface melt
Sea Ice:
- ice thickness
- ice age
- snow cover
- temperature
- moisture
- surface salinity
- air bubbles
- brine pockets/ channels and
- surface melt/ pooling
Wet Snow: Passive Microwave
If liquid water is introduced in the snowpack by snow melt or rain, a sharp increase in brightness temperature is observed
Wet Snow: Active Microwave
Backscatter decreases sharply with an increase in liquid water content in snowpack (source of backscatter becomes mostly from the surface, not the volume of snow).
Liquid Water: Visible to Mid-IR
Total radiance, (Lₜ) recorded by a remote sensing system over water is a function of the electromagnetic energy received from:
Lₚ = atmospheric path radiance
Lₛ = free-surface layer reflectance
Lᵥ = subsurface volumetric reflectance
Lᵇ = bottom reflectance
Presence of organic and inorganic constituents have an effect on signature
Liquid Water: Thermal IR
Physical temperature of water temperature near the surface is the main control on brightness temperature on an hourly time scale. Emissivity is close to 1 in this part of the EM spectrum.
On longer time scales (daily to annual), the effect of heat capacity/storage and water mixing also has an impact on observed brightness temperatures.
Liquid Water: Passive Microwave
Emissivity is low in passive microwave (about 0.4 to 0.5) therefore resulting in low brightness temperature for open water.
Liquid Water: Active Microwave
Surface roughness increases with wind speed. This has the effect of increasing radar backscatter.
Important disadvantages of passive microwave remote sensing:
- Larger instantaneous fields of view (10 - 50+ km per pixel) compared to visible or active microwave sensors.
- Emissivity is subject to change without knowledge of in-situ conditions (emissivity is not consistent for targets on the ground within a single pixel).
- Polar orbiting satellites provide discontinuous temporal coverage of equatorial regions (is an issue for weather observation, need to create weekly composites).
Microwave remote sensing has the following important advantages over other portions of the spectrum:
- Microwave radiation penetrates clouds and, therefore, forms the basis for an all-weather observation tool.
- Low frequency microwaves partially penetrate vegetation and, therefore, allows soil moisture estimation from vegetated areas.
- Low frequency microwaves partially penetrate the soil surface and, therefore, emitted signatures contain information over the soil penetrated depth.
- High frequency microwaves are partially absorbed by vegetation and, therefore, emitted signatures contain information on vegetation properties.
- Microwave radiation is independent of solar radiation and can therefore be used during both night-time and day-time hours.
Microwave absorption bands
The microwave absorption spectrum of the atmosphere includes absorption bands around 22 and 183 GHz due to water vapour and around 60 & 120 GHz due to oxygen.
Radiometric observations at and around these absorption bands are used to estimate the water vapour and temperature profiles of the atmosphere and the water content of clouds and rain, when present.
Soil: active microwave
While in TIR it is the temperature of a thin layer on the order of the micron that controls the emission of a surface, in the microwaves it is a thicker layer that intervenes because wavelengths are on the order of a few centimeters.
The information on soils that we obtain by analyzing their backscatter (radar) therefore comes from a layer for which the thickness is proportional to the size of the wavelength used.
For a relatively smooth bare soil surface, the backscatter (radar) is strongly influenced by
- the moisture content of the soil (dielectric properties)
- surface roughness. Greater roughness causes backscatter to increase.
