Lecture 13 Flashcards
Microwave region
1-200 GHz (0.15-30cm)
Characteristics of microwave radiometry
- All weather (cloud penetration–longer wavelength)
- Daily or better coverage
- Multi-frequency/channel
- Long record (since late 60s)
- Large footprint
- Hole at the pole
- Does not rely on sun as source of illumination
- Weak energy source so need large IFOV (bigger pixel size). Special resolution because signal is weak, so need bigger area.
Is radiant temperature equal to kinematic temperature?
NO. except for a blackbody. Almost all natural surface are gray bodies (absorb EMR).
Emissivity is a ratio of what?
Ratio of radiant emittance relative to that of a blackbody
Emissivity is what kind of quantity?
Emissivity is a spectral quantity (it varies with wavelength)
What is a selective radiator?
Materials that have variable emissivity are “selective radiators”–all natural surfaces
What does Planck’s Blackbody Equation do?
Describes the spectral distribution (wavelength) of emitted energy as a function of temperature
amount of energy at each wavelength at a given temp
What does Stefan-Boltzmann Law do?
the TOTAL power (energy) emitted from a blackbodygraybody is proportional to the 4th power of temperature (area under the Plank Law curve)
What does Wien’s Law do?
there is a maximum wavelength at which a blackbody radiates and this is determined by temperature
What does Rayleigh-Jeans approximation do?
- Approximation of Planck’s Law
- Works because part of Panck’s approaches zero at long wavelengths
- valid for most microwave applications
- MW radiometers measure brightness temperature, which when we know emissivity tells us kinematic (physical) temp
(Brightness temp is linearly related to the kinematic temperature of the surface)
Dielectric constant
related to conductivity of material
- measurement of how well radar energy will be moved to a depth. It therefore it measures velocity of propagating radar energy
- Most earth materials have a dielectric constant in the range of 1 to 4 (air=1, veg=3, ice=3.2)
- Dielectric constant of liquid water is 80
Why does moisture content affect brightness temperature
Dielectric constant of liquid water is very high, and wavelength and velocity reduce (speed penetrating surface)
Snow emissivity
soil vs dry snow vs wet snow–wet snow is a strong absrober/emitter
Atmospheric Effects and Passive Microwave
- Atmospheric attenuation increases with frequency from 1 to 1000 GHz
- At frequencies less than 50 GHz, there’s little effect of clouds and fog on brightness temperature (it “sees through” clouds)
- Thus, PM can be used to monitor the land surface under cloudy conditions
- In atmospheric absorption bands, PM is used to map water vapor, rain rates, clouds
Passive Microwave Radiometry – how scan/sensors work
- sensors use an antenna to detect photons * then converted to voltages in a circuit
- mechanical rotation of antenna, or platform, or fixed antenna and oscillating mirror
