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