Passive Microwave Remote
Sensing
Lecture 10
Nov 06, 2007
Principals

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While dominate wavelength of Earth is 9.7 um (thermal), a
continuum of energy is emitted from Earth to the atmosphere.
In fact, the Earth passively emits a steady stream of microwave
energy as well, though it is relatively weak in intensity due to
its long wavelength.
The spatial resolution usually low (kms) since the weak signal.
A suit of radiometers can record it. They measure the
brightness temperature of the terrain or the atmosphere. This
is much like the thermal infrared radiometer for temperature.
A matrix of brightness temperature values can then be used to
construct a passive microwave image.
To measure soil moisture, precipitation, ice water content, seasurface temperature, snow-ice temperature, and etc.
Rayleigh-Jeans approximation of
Planck’s law
2hc 2
L( , T )  5 hc /(kT )
 (e
 1)
Thermal infrared domain (Planck’s law):
Microwave domain (Rayleigh-Jeans approximation):
2hc 2
L ( , T ) 
 (e
5
Let
We have
hc
kT
2hc 2

 1)
h
x
, and
kT
hv  kT
 (e
5
h
kT
2hc 2
2hc 2
2hc 2 2hc 2 kT 2ckT
 5 x
 5



5
h

 (e  1)  (1  x  1) 5
 h
4
 1)
kT
Recall
We have
x x2
e  1    1 x
1! 2!
x
v
c

,...  dv 
2
c

2
d
2 2ckT 2kT 2
| L(v, T )dv || L( , T )d |,...  L(v, T )  L( , T )  
 2 v
4
c
c

c
Unit is Wm-2Hz

For a Lambertian surface, the surface
brightness radiation B(v,T),
Unit is W•m
-2•Hz•sr
2kT 2
L(v, T )  B(v, T ),...  B (v, T )  2 v
c

The really useful simplification involves
emissivity and brightness temperature:
In comparison with thermal infrared:
(TB)4 = ελ (T)4
Some important passive
microwave radiometers
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Special Sensor Mirowave/Imager (SSM/I)
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It was onboard the Defense Meterorological
Satellite Program (DMSP) since 1987
It measure the microwave brightness
temperatures of atmosphere, ocean, and terrain at
19.35, 22.23, 37, and 85.5 GHz.
TRMM microwave imager (TMI)

It is based on SSM/I, and added one more
frequency of 10.7 GHz.
AMSR-E
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Advanced Microwave Scanning Radiometer – EOS
It observes atmospheric, land, oceanic, and cryospheric parameters,
including precipitation, sea surface temperatures, ice concentrations,
snow water equivalent, surface wetness, wind speed, atmospheric
cloud water, and water vapor.
At the AMSR-E low-frequency channels, the atmosphere is relatively
transparent, and the polarization and spectral characteristics of the
received microwave radiation are dominated by emission and
scattering at the Earth surface.
Over land, the emission and scattering depend primarily on the water
content of the soil, the surface roughness and topography, the surface
temperature, and the vegetation cover.
The surface brightness T (TB ) tend to increase with frequency due to
the absorptive effects of water in soil and vegetation that also
increase with frequency. However, as the frequency increase,
scattering effects from the surface and vegetation also increase,
acting as a factor to reduce the TB
AMSR-E
Najoku et al. 2005
Example1: Snow depth or snow
water equivalent (SWE)
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The microwave brightness
temperature emitted from a
snow cover is related to the
snow mass which can be
represented by the
combined snow density
and depth, or the SWE (a
hydrological quantity that
is obtained from the
product of snow depth and
density).
∆Tb = Tb19V-Tb37V
Kelly et al. 2003
3. Study Area (1)
Impact of snow density (4)-mean SD
AMSR-E vs ground mean snow depth
AMSER-E vs ground mean snow depth
30
30
y = 0.81x + 0.25
R2 = 0.74
RMSD=4.6 cm
EB= -17%
20
25
AMSR-E (cm)
AMSR-E (cm)
25
15
20
15
10
10
5
5
y = 0.97x + 1.45
R2 = 0.90
RMSD=3.0 cm
EB =11%
0
0
0
5
10
15
20
25
Ground snow depth (cm)
Snow density = 0.4 g/cm3
30
0
5
10
15
20
25
Ground snow depth (cm)
Multi-snow density
Xianwei, Xie, and Liang 2006
30
Results: AMSR-E vs ground- SD at
individual stations (snow density = 0.4 g/cm3)
50
50
Zhaoshu
Caijiahu
y = 0.82x + 1.46
R2 = 0.65
40
40
30
30
20
20
10
y = 1.28x - 3.20
R2 = 0.52
10
0
0
0
10
50
20
30
40
50
Qinhe
0
10
20
25
40
20
30
15
20
10
10
30
50
jinhe
y = 0.78x + 1.65
R2 = 0.40
5
y = 0.69x + 4.06
R2 = 0.40
40
0
0
0
10
20
30
40
50
0
5
10
15
20
25
Results: AMSR-E vs ground- SD at
individual stations (snow density = 0.4 g/cm3)
50
40
30
Baitashan
y = 0.55x + 2.58
R2 = 0.74
Tuoli
y = 0.42x + 3.15
R2 = 0.56
25
20
30
15
20
10
5
10
0
0
0
0
10
20
50
30
40
40
40
30
30
20
10
10
0
0
20
15
20
25
30
30
40
y = 0.94x - 0.75
R2 = 0.50
Fuhai
20
y = 1.64x - 6.84
R2 = 0.65
10
10
50
Qitai
0
5
50
50
0
10
20
30
40
50
Results: Annual change of SWE in
YWR
Annual Change of SWE (cm) in YRW
60
Mean SWE (cm)
50
40
30
20
10
0
6
8
10
12
2
02-03
4
6
8
10
12
2
4
6
8
10
03-04
12
04-05
Hydrologic Year
2
4
6
8
10
12
05-06
2
4
Antarctic sea ice
Coverage Area (10
6
km 2)
Snow Area over Sea Ice
20
18
16
14
12
10
8
6
4
2
0
2002
2003
2004
2005
0
50
100
150
200
250
300
350
Julian Day
Mike and Xie, 2006
Snow Depth Over Sea Ice
120
02Max
02Mean
Snow Depth (cm)
100
03Max
03Mean
80
04Max
60
04Mean
05Max
40
05Mean
20
0
0
50
100
150
200
Julian Day
250
300
350
Maximum SD values exceed 50-60 cm in most data sets, (outside
range of retrievable snow depth for 37GHz) and are likely noise
Mike and Xie, 2006
Mean Snow Depth vs. Total Area
2002W
2003W
40
2004W
Mean Snow Depth (cm)
35
2005W
Summer
2002SP
30
Spring
2003SP
25
2004SP
20
2005SP
2002SU
15
2003SU
10
2004SU
Winter
Fall
2005SU
5
2003F
0
2004F
0
5
10
15
20
2005F
Coverage Area (106 km 2)
Mike and Xie, 2006
3
Snow Volume (km )
Snow Volume over Sea Ice
4000
2002
3500
2003
2004
3000
2005
2500
2000
1500
1000
500
0
0
50
100
150
200
Julian Day
250
300
350
Seasonal Comparison of Locations of Max SD Areas, 2002
Max Areas = +2σ
7/20/02
10/20/02
8/20/02
11/18/02
9/24/02
12/20/02
Seasonal Comparison of Locations of Max SD Areas, 2003
1/20/03
4/20/03
7/20/03
10/20/03
2/20/03
5/20/03
8/20/03
11/18/03
Oct 1, 2005
Oct 1, 2004
3/20/03
6/20/03
9/20/03
12/20/03
Seasonal Comparison of Locations of Max SD Areas, 2004
1/20/04
4/20/04
7/22/04
10/20/04
2/20/04
5/20/04
8/20/04
11/17/04
3/20/04
6/19/04
9/17/04
12/20/04
Seasonal Comparison of Locations of Max SD Areas, 2005
1/20/05
4/20/05
7/20/05
10/20/05
2/20/05
5/20/05
8/20/05
11/16/05
3/20/05
6/20/05
9/20/05
12/20/05
Example2: Radio-frequency
interference contaminate the 6.9
and 10.7 GHz channels
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Radio-frequency interference (RFI): the cable television relay,
auxiliary broadcasting, mobile. RFI is several orders of magnitude
higher than natural thermal emissions and is often directional and can
be either continuous or intermittent.
Radio-frequency interference (RFI) is an increasingly serious
problem for passive and active microwave sensing of the Earth.
The 6.9 GHz contamination is mostly in USA, Japan, and the Middle
East.
The 10.7 GHz contamination is mostly in England, Italy, and Japan
RFI contamination compromise the science objectives of sensors
that use 6.9 and 10.7 GHz (corresponding to the C-band and X-band
in active microwave sensing) over land.
radio-frequency interference (RFI)
index (RI)
Li et al. 2004
6.9 GHz contamination
Najoku et al. 2005