Transcript Document

Passive Microwave Remote
Sensing
Outline for 22 Feb 2008
• Passive Microwave Radiometry
• Active microwave remote sensing
Passive Microwave Radiometry
• Microwave region: 1-200 GHz (0.15-30cm)
• Uses the same principles as thermal remote
sensing
• Multi-frequency/multi-polarization sensing
• Weak energy source so need large IFOV and
wide bands
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
At long wavelengths, such
as in the microwave region,
the relationship between
spectral emittance and
wavelength can be
approximated by a straight
line.
Rayleigh-Jeans Approximation
a constant
Ll  e
2kcT
l
4
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 > 2.57mm @300 K)
Brightness Temperature
eT is also called the “brightness temperature”
typically shown as TB
TB 
l
4
2kc
Ll
Brightness temperature can be related to kinetic
temperature through emissivity
Tb  eTkin
Thus, passive microwave brightness
temperatures can be used to monitor
temperature as well as properties related to
emissivity
Microwave Radiometers
• Advanced Microwave Sounding Unit (AMSU) 1978-present
• Scanning Multichannel Microwave Radiometer (SMMR) 19811987
• Special Sensor Microwave/Imager (SSM/I) 1987-present
• Tropical Rainfall Measuring Mission (TRMM) 1997-present
• Advanced Microwave Scanning Radiometer (AMSR-E) 2002present
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
Comparative Operating Characteristics of SMMR, SSM/I, and AMSR
Param eter
Tim e Period
Frequencies
(GHz)
Sam ple
Footprint
Sizes (km ):
(Nim bus-7)
SMMR
1978 t o 1987
(DMSP-F08,F10,
F11,F13) SSM/I
1987 t o Present
6.6, 10.7, 18, 21, 37 19.3, 22.3, 36.5, 85.5
148 x 95 (6.6 GHz)
27 x 18 (37 GHz)
37 x 28 (37 GHz)
15 x 13 (85.5 GHz)
(Aqua)
AMSR-E
2002 t o Present
6.9, 10.7, 18.7,
23.8, 36.5, 89.0
74 x 43 (6.9 GHz)
14 x 8 (36.5 GHz)
6 x 4 (89.0 GHz)
Passive Microwave Applications
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Soil moisture
Snow water equivalent
Sea/lake ice extent, concentration and type
Sea surface temperature
Atmospheric water vapor
Surface wind speed
Cloud liquid water
only over the oceans
Rainfall rate
Monitoring Temperatures with
Passive Microwave
• Sea surface temperature
• Land surface temperature
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
Snow Emissivity Example
Dry
Snow
dry snow
(2)
Soil
snow water equivalent
Wet
Snow
(1)
Soil
(3)
Soil
Wet snow is a strong
absorber/emitter
SSM/I
Northern
Hemisphere
snow water
equivalent
(mm of water)
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
Atmospheric Mapping
•
•
Mapping
global water
vapor
85 GHz
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
Rainfall from
passive
microwave
sensors:
Accumulated
precipitation from
the Tropical
Rainfall
Measuring
Mission (TRMM)
Similar to SSM/I
Passive Microwave Remote Sensing from Space
Advantages
Disadvantages
• Penetration through nonprecipitating 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)
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.
Active Microwave Remote
Sensing
Radar=Radio Detection and Ranging
Radar system components
Radar: How it Works
• A directed beam of
microwave pulses are
transmitted from an antenna
• The energy interacts with the
terrain and is scattered
• The backscattered
microwave energy is
measured by the antenna
• Radar determines the
direction and distance of the
target from the instrument as
well as the backscattering
properties of the target
Radar Parameters
• Azimuth Direction
– direction of travel of aircraft or orbital track of satellite
• Range angle
– direction of radar illumination, usually perpendicular to
azimuth direction
• Depression angle
– angle between horizontal plane and microwave pulse (near
range depression angle > far range depression angle)
• Incident angle
– angle between microwave pulse and a line perpendicular to
the local surface slope
• Polarization
– linearly polarized microwave energy emitted/received by the
sensor (HH, VV, HV, VH)
Radar Nomenclature
• Nadir
• azimuth flight direction
• look direction
• range (near and far)
• depression angle ()
• incidence angle ()
• altitude above-ground-level, H
• polarization
Slant Range vs. Ground Range
Radar Pulse Length
Synthetic Aperture Radar
•
Antenna “length” is increased synthetically by building up a history of
backscattered signals from the landscape along the track of the sensor
•
Implemented by keeping track of the Doppler shift of the reflected signal
(frequency of the transmitted signal is known)
Layover
Layover
occurs when
the incidence
angle () is
smaller than
the foreslope
(a+)
i.e.,  < a+.
This
distortion
cannot be
corrected!
Radar Shadowing
Radar shadowing can be useful for interpreting
geomorphological features
Radar Backscatter
Power received
= Power per unit area at target
x
Effective scattering area of the target
Spreading loss of reradiated signal
Effective receiving area of antenna
x
x
Radar Backscatter Coefficient
The efficiency the terrain to reflect the radar pulse is termed the
“radar cross-section”, 
The radar cross-section per unit area, (A) is called the “radar
backscatter coefficient” (˚) and is computed as :
 
o

A
The radar backscatter coefficient determines the percentage of
electro- magnetic energy reflected back to the radar from within
a radar pixel
This is similar to the reflectance in optical remote sensing

Radar Backscattering
Radar Backscattering
Depends on the properties of the target:
– roughness
– dielectric constant
Depends on characteristics of the radar:
– depression angle
– frequency/wavelength
– polarization
Rayleigh Criterion for Roughness
• A surface is considered smooth at or below a height, h, if:
h
l
8sin 
[ cm ]
h = the vertical relief (average height of surface irregularities)
l = the radar wavelength (measured in cm)
 = the depression angle
Surface
Roughness in
RADAR
Imagery
Nile River
Sudan
Space
Shuttle
ColorInfrared
Photograph
SIR-C Color Composite:
• Red = C-band HV
• Green = L-band HV
• Blue = L-band HH
C-band, l= 6cm
L-band, l= 24cm
Radar and the Dielectric
Constant
• Dielectric constant depends on the type of material
as well as its moisture state
– it is analogous to the refractive index of the material
– it is primarily a function of moisture content
– also depends on chemical properties such as salinity
• Dielectric constant is the ratio of the capacitance of a
material to that of a vacuum. Also known as the
“relative permittivity”
Dielectric Constant
dielectric constant of liquid water is 80; dry soil is 2-4.
Radar frequency and backscatter
• Depth of radar
penetration
through the
vegetation
canopy varies
directly with l
Types of Active
Microwave Surface
and Volume
Scattering that Take
Place in a
Hypothetical Pine
Forest Stand
Response of A Pine Forest Stand to X-, C- and L-band Microwave Energy