1. Passive Microwave Remote Sensing

2. Outline
Passive Microwave Radiometry

3. Passive Microwave Radiometry
Microwave region: GHz ( cm)
Uses the same principles as thermal remote sensing
Multi-frequency/multi-polarization sensing
Weak energy source so need large IFOV and wide bands

4. Microwave Brightness Temperature
Microwave radiometers can measure the emitted spectral radiance received (Ll)
This is called the brightness temperature and is linearly related to the kinetic temperature of the surface
The Rayleigh-Jeans approximation provides a simple linear relationship between measured spectral radiance temperature and emissivity

5. At long wavelengths, such as in the microwave region, the relationship between spectral emittance and wavelength can be approximated by a straight line.

6. Rayleigh-Jeans Approximation
a constant
spectral radiance is a linear function of kinetic temperature
k is Planck’s constant, c is the speed of light, e is emissivity, T is kinetic temperature
This approximation only holds for l >> lmax
(e.g. l > K)

7. Brightness Temperature
eT is also called the “brightness temperature” typically shown as TB

8. Brightness temperature can be related to kinetic temperature through emissivity
Thus, passive microwave brightness temperatures can be used to monitor temperature as well as properties related to emissivity

9. Microwave Radiometers
Advanced Microwave Sounding Unit (AMSU) 1978-present
Scanning Multichannel Microwave Radiometer (SMMR)
Special Sensor Microwave/Imager (SSM/I) 1987-present
Tropical Rainfall Measuring Mission (TRMM) 1997-present
Advanced Microwave Scanning Radiometer (AMSR-E) 2002-present

10. Passive Microwave Radiometry
Passive microwave sensors use an antenna (“horn”) to detect photons at microwave frequencies which are then converted to voltages in a circuit
Scanning microwave radiometers
mechanical rotation of mirror focuses microwave energy onto horns

12. Passive Microwave Applications
Soil moisture
Snow water equivalent
Sea/lake ice extent, concentration and type
Sea surface temperature
Atmospheric water vapor
Surface wind speed
Cloud liquid water
Rainfall rate
only over the oceans

13. Monitoring Temperatures with Passive Microwave
Sea surface temperature
Land surface temperature

14. Passive Microwave Sensing of Land Surface Emissivity Differences
Microwave emissivity is a function of the “dielectric constant”
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
Thus, moisture content affects brightness temperature
Surface roughness also influences emissivity

15. Snow Emissivity Example
Soil
Dry
Snow
brightness temperature
(2)
dry snow
snow water equivalent
Wet
Snow
Soil
Soil
(1)
(3)
Wet snow is a strong
absorber/emitter

16. SSM/I Northern Hemisphere snow water equivalent
(mm of water)

17. Atmospheric Effects
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

18. Atmospheric Mapping
Mapping global water vapor
85 GHz

19. Passive Microwave Sensing of Rain
Over the ocean:
Microwave emissivity of rain (liquid water) is about 0.9
Emissivity of the ocean is much lower (0.5)
Changes in emissivity (as seen by the measured brightness temperature) provide and estimate of surface rain rate
Over the land surface:
Microwave scattering by frozen hydrometeors is used as a measure of rain rate
Physical or empirical models relate the scattering signature to surface rain rates

20. Rainfall from passive microwave sensors:
Accumulated precipitation from the Tropical Rainfall Measuring Mission (TRMM)
Similar to SSM/I

21. Passive Microwave Remote Sensing from Space
Advantages
Disadvantages
Penetration through non-precipitating clouds
Radiance is linearly related to temperature (i.e. the retrieval is nearly linear)
Highly stable instrument calibration
Global coverage and wide swath
Larger field of views (10-50 km) compared to VIS/IR sensors
Variable emissivity over land
Polar orbiting satellites provide discontinuous temporal coverage at low latitudes (need to create weekly composites)

22. Passive and Active Systems
Passive remote sensing systems record electromagnetic energy that is reflected or emitted from the Earth’s surface and atmosphere
Active sensors create their own electromagnetic energy that 1) is transmitted from the sensor toward the terrain, 2) interacts with the terrain producing a backscatter of energy, and 3) is recorded by the remote sensor’s receiver.