Passive Microwave Remote Sensing
Lecture 10
Nov 06, 2007
Principals
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, sea-surface temperature, snow-ice temperature, and etc.
Rayleigh-Jeans approximation of Planck’s law
)1(
2),(
)/(5
2
kThce
hcTL
45
2
5
2
5
2
5
2
5
2
5
2 222
)11(
2
)1(
2
)1(
2
)1(
2),(
ckT
h
kThc
kT
hhc
x
hc
e
hc
e
hc
e
hcTL
x
kT
h
kT
hc
Thermal infrared domain (Planck’s law):
Microwave domain (Rayleigh-Jeans approximation):
xxx
ex 1!2!1
12
kThv
andkT
hx
,
Let Recall
We haveWe have
d
cdv
cv
2,...
224
22 22),(),(|,...),(||),(| v
c
kTckT
cTL
cTvLdTLdvTvL
Unit is Wm-2Hz
For a Lambertian surface, the surface brightness radiation B(v,T),
The really useful simplification involves emissivity and brightness temperature:
Unit is W•m-2•Hz•sr
22
2),(),...,(),( v
c
kTTvBTvBTvL
In comparison with thermal infrared: (TB)4 = ελ (T)4
Some important passive microwave radiometers
Special Sensor Mirowave/Imager (SSM/I) 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 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)
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
AMSER-E vs ground mean snow depth
y = 0.81x + 0.25
R2 = 0.74 RMSD=4.6 cm
EB= -17%
0
5
10
15
20
25
30
0 5 10 15 20 25 30
Ground snow depth (cm)
AM
SR
-E (
cm)
Snow density = 0.4 g/cm3 Multi-snow density
AMSR-E vs ground mean snow depth
y = 0.97x + 1.45
R2 = 0.90 RMSD=3.0 cm
EB =11%0
5
10
15
20
25
30
0 5 10 15 20 25 30
Ground snow depth (cm)
AM
SR
-E (
cm)
Xianwei, Xie, and Liang 2006
Results: AMSR-E vs ground- SD at individual stations (snow density = 0.4 g/cm3)
Zhaoshu
y = 0.82x + 1.46
R2 = 0.65
0
10
20
30
40
50
0 10 20 30 40 50
Caijiahu
y = 1.28x - 3.20
R2 = 0.52
0
10
20
30
40
50
0 10 20 30 40 50
Qinhe
y = 0.69x + 4.06
R2 = 0.40
0
10
20
30
40
50
0 10 20 30 40 50
jinhe
y = 0.78x + 1.65
R2 = 0.40
0
5
10
15
20
25
0 5 10 15 20 25
Baitashany = 0.55x + 2.58
R2 = 0.74
0
10
20
30
40
50
0 10 20 30 40 50
Tuoliy = 0.42x + 3.15
R2 = 0.56
0
5
10
15
20
25
30
0 5 10 15 20 25 30
Qitai
y = 1.64x - 6.84
R2 = 0.65
0
10
20
30
40
50
0 10 20 30 40 50
Fuhaiy = 0.94x - 0.75
R2 = 0.50
0
10
20
30
40
50
0 10 20 30 40 50
Results: AMSR-E vs ground- SD at individual stations (snow density = 0.4 g/cm3)
Results: Annual change of SWE in YWR
Annual Change of SWE (cm) in YRW
0
10
20
30
40
50
60
6 8 10 12 2 4 6 8 10 12 2 4 6 8 10 12 2 4 6 8 10 12 2 4
Me
an
SW
E (
cm
)
02-03 03-04 04-05 05-06Hydrologic Year
Antarctic sea ice
Snow Area over Sea Ice
0
2
46
8
10
12
1416
18
20
0 50 100 150 200 250 300 350
Julian Day
Co
vera
ge
Are
a (1
06 k
m2)
2002
2003
2004
2005
Mike and Xie, 2006
Snow Depth Over Sea Ice
0
20
40
60
80
100
120
0 50 100 150 200 250 300 350
Julian Day
Sn
ow
Dep
th (
cm)
02Max
02Mean
03Max
03Mean
04Max
04Mean
05Max
05Mean
Mike and Xie, 2006
Maximum SD values exceed 50-60 cm in most data sets, (outside range of retrievable snow depth for 37GHz) and are likely noise
Mean Snow Depth vs. Total Area
0
5
10
15
20
25
30
35
40
0 5 10 15 20Coverage Area (106 km 2)
Mea
n S
no
w D
epth
(cm
)
2002W
2003W
2004W
2005W
2002SP
2003SP
2004SP
2005SP
2002SU
2003SU
2004SU
2005SU
2003F
2004F
2005F
Summer
FallWinter
Spring
Mike and Xie, 2006
Snow Volume over Sea Ice
0
500
1000
1500
2000
2500
3000
3500
4000
0 50 100 150 200 250 300 350
Julian Day
Sn
ow
Vo
lum
e (
km
3 )
2002
2003
2004
2005
11/18/028/20/02
Max Areas = +2σ
7/20/02
9/24/02
10/20/02
12/20/02
Seasonal Comparison of Locations of Max SD Areas, 2002
Oct 1, 2004 Oct 1, 2005
Seasonal Comparison of Locations of Max SD Areas, 2003
5/20/032/20/03 8/20/03 11/18/03
1/20/03
3/20/03
4/20/03
6/20/03
7/20/03
9/20/03
10/20/03
12/20/03
5/20/042/20/04 8/20/04 11/17/04
1/20/04
3/20/04
4/20/04
6/19/04
7/22/04
9/17/04
10/20/04
12/20/04
Seasonal Comparison of Locations of Max SD Areas, 2004
5/20/052/20/05 8/20/05 11/16/05
3/20/05
1/20/05 4/20/05
6/20/05
7/20/05
9/20/05
10/20/05
12/20/05
Seasonal Comparison of Locations of Max SD Areas, 2005
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.
Example2: Radio-frequency interference contaminate the 6.9
and 10.7 GHz channels
radio-frequency interference (RFI) index (RI)
Li et al. 2004
6.9 GHz contamination
Najoku et al. 2005