SeaWinds Radiometer (SRad)

Brightness Temperature

Calibration/Validation

Mayank Rastogi

EE Masters Defense

March 29, 2005

Major Advisor: Dr. W. Linwood Jones

Presentation Outlines

• Background

• Principles of Scatterometry/Radiometry

• Revealed Tb differences - Need for Asc/Dsc Corrections

• Radiometric Calibration Procedure

• SRad/AMSR Empirical Asc/Dsc Correction

• Validation of Empirical Correction with independent data-set.

• Conclusions

Background

• SeaWinds is a satellite-borne radar scatterometer

used to remotely sense oceanic surface winds

– Launched June 1999 on the low-earth polar orbit

QuikSCAT satellite

• Issue

– When rain occurs the antenna noise increases

– Approx. 10% of wind retrievals are corrupted by rain

• Resolution

– Previously, CFRSL developed a signal processing

algorithm to measure the antenna noise and “flag” rain

contamination

• MS theses: Mehershahi, 2000 & Susanj 2000

Background cont.

• A second SeaWinds instrument launched on

Japan’s Advanced Earth Observing Satellite

in December 2002

– This thesis concerns the evaluation of the

SeaWinds Radiometer (SRad) on ADEOS-II

Thesis Objective

• Radiometric calibration of the SeaWinds Radiometer (SRad) on ADEOS-II– Evaluate previous QuikSCAT Radiometer (QRad)

transfer function

– Tb cross-calibration with the Advanced Microwave Scanning Radiometer (AMSR) also on ADEOS-II

• Validate SRad brightness temperature (Tb) measurements over ocean using independent AMSR equivalent Tb’s

Principles of

Scatterometry/Radiometry

Black Body Radiation

1

1hc2)S(

5

2

kT

ch

e

S() 2ckT

4

Rayleigh-Jean’s Approximation

(valid over microwave region):

Planck’s law:

According to thermodynamic principles, all matter at

temperatures greater than absolute zero absorb and emit non-

coherent EM energy.

Where: h = Planck’s constant

k = Boltzmann’s constant

c = Speed of light

T = Physical temperature

Region of interest

Microwave

Plank’s Radiation Law

Microwave Radiometry• Blackbody power is:

Pblackbody = k *Tphys* Bandwidthmicrowave antenna measures equivalent blackbody power

For non-blackbodies, microwave brightness temperature, Tb, of a medium is:

where:

• Brightness temperature is the equivalent blackbody temperature

is the emissivity, which varies with– EM wavelength

– Incidence angle and Polarization

– Complex index of refraction

Physb TT

Microwave Brightness Temperature over Ocean

• Measured brightness temperature:– Upwelling atmospheric emission

– Downward atmospheric emission reflected by the sea surface

– Ocean emission

Surface

Emission

Upwelling

Atmospheric

Emission

Reflected

Atmospheric

Emission

Microwave

Antenna

Ocean

A

otr dA

R

GPP

4

2

3

2

)4(

Satellite Microwave Scatterometer

• Radar Equation

• o Normalized Radar

Cross-section of the ocean

surface

Geometry of SeaWinds

Scatterometer on ADEOS-II

54°

46°

Swath = 1800 km

SRad Simplified Functional Diagram

Be

Bn

Frequency

Pow

er

Received Spectrum

Radar Echo

Echo Channel

Noise Channel

SeaWinds Received Power Spectrum

antenna noise = noise chan pwr - echo chan pwr

Early On-orbit Check-out of SRad

Revealed Anomalous Tb Differences Ascending & Descending Orbit Segments

• Compared 3-days averages of asc/desc revs

for QRad and SRad

– QRad 3-day average ascending and descending

Tb’s were equal, but

– SRad ascending Tb’s were colder by ~ 5 K

• Both polarizations exhibited this anomaly

SRad Ascending / Descending Tb

Anomaly (3-day ocean avg)

SRad Ocean Tb, H-Pol QRad Ocean Tb, H-Pol

Descending:Mean = 109

Ascending:Mean = 103 Ascending:

Mean = 106

Descending:Mean = 106

SRad Ascending / Descending Tb

Anomaly (3-day ocean avg)

SRad Ocean Tb, V-Pol QRad Ocean Tb, V-Pol

Ascending:Mean = 176

Descending:Mean = 181

Ascending:Mean = 180

Descending:Mean = 180

Ascending-Descending Tb Anomaly

ADEOS-II

(SRad)

QSCAT

(QRad)

ASDS

Radiometric Calibration

Procedure:Cross-calibration of SRad &

AMSR Brightness Temperatures

Why calibrate the instrument ?

• SeaWinds not intended for radiometric

measurements - does not have usual “hot”

and “cold” internal brightness temperature

calibration sources.

Calibration Approach

• Use Advanced Microwave Scanning

Radiometer (AMSR) and compare

simultaneous Tb.

Advanced Microwave Scanning Radiometer (AMSR)

• Multi- frequency microwave radiometer

- 6.9, 10.65, 18.7, 23.8, 36.5, and 89.0 GHz

- Vertical and Horizontal polarizations

- Incidence angle = 55º

SeaWinds Scatterometer

• Specialized active microwave sensor

- Ku-band, 13.4 GHz

- Vertical and Horizontal polarizations

- Incidence angle » H-Pol = 46º

» V-Pol = 54º

Get AMSR Tb at 10.65 GHz & 18.7 GHz

Generate Modeled AMSR & SRad Tb using RadTb

& SSMI-F-15 Environmental Parameters

Check Biases of AMSR Tb Vs Modeled AMSR Tb

Does it Need

Correction

Yes No

Procedure Algorithm

Yes No

Find Asc/Dsc Correction

Create Spectral Ratios

Get SRad Tb

Generate SRad Equiv

AMSR Tb using Spectral

Ratios

Check Biases of SRad Vs AMSR

Find Empirical Asc/Dsc Corrections

Validation of Empirical Corrections with independent

data-set

Model AMSR/SRad Tb

RadTb was run with:

• SRad incidence angles (H-Pol=46deg V-

Pol=54deg) – get SRad Tb at 13.4 GHz

• AMSR incidence angle of 55deg with 2 freq

- get AMSR Tb at 10.65GHz & 18.7GHz

• Environmental pars from NCEP and SSMI

- SST, Water Vapor, Cloud Liquid & Wind Speed

13.4 10.65

18.7 10.65

( )( )

( )

T Tspecratio

T T

• Spectral Ratios were found to be a function of

predominantly Water Vapor (WV)

H-Pol Spectral Ratio Vs Water Vapor

AMSR Asc/Dsc Correction

0 500 1000 1500-4

-3

-2

-1

0

1

2

3Average Correction ASC/DSC H-Pol 0429 0624 0922

AMSR Asc/Dsc Correction H-Pol

0 500 1000 1500-1

0

1

2

3

4

5Average Correction ASC/DSC V-Pol 0429 0624 0922

AMSR Asc/Dsc Correction V-Pol

Generate SRad Equiv AMSR Tb using

Spectral Ratios

• SRad Equiv AMSR Tb (at 13.4 GHz) was

generated from AMSR 10.65 & 18.7 GHz

Tb’s using Spectral Ratio

13.4 10.65 18.7 10.65*( )Tb Tb specratio Tb Tb

SRAD/AMSR Tb Differences

• Calculate Tb differences (biases) between

SRad Equivalent AMSR Tb and SRad Tb for

both polarizations and ascending/descending

orbits

• Model Tb biases as a three-term Fourier

series of orbit latitude

– Latitude range for one orbit includes ascending

and descending segments = 2 x 180°

SRad/AMSR Biases Along Longitude

• There were variations in Tb biases with longitude because of the temperature differences along land/ocean boundaries

– Caused by antenna pattern differences between SRad and AMSR

– Land is radiometrically “hot” compared to ocean

• New “conservative” land mask created to eliminate of land effects

SRad/AMSR Biases along Land/Ocean Boundaries

(9-Day Average)

SRad-AMSR Bias Sep 9-Day Avg. H-Pol Asc

200 400 600 800 1000 1200 1400

100

200

300

400

500

600

700-50

-40

-30

-20

-10

0

10

20

SRad-AMSR Bias 9-Day Avg H-Pol Asc using conservative LandMask

200 400 600 800 1000 1200 1400

100

200

300

400

500

600

700-50

-40

-30

-20

-10

0

10

20

SRad/AMSR Biases after applying Conservative

Land Mask

-90° +90° -90°Orbit Latitude:

Del

ta-T

b (

SR

ad -

AM

SR

), K orbit avg

day-2orbit avg

day-3

orbit avg

day-1

SRad Asc/Dsc Tb (H-Pol) Correction

Orbital Pattern, Average for 3-days

Fourier Series

-120 -100 -80 -60 -40 -20 0 20 40 600

500

1000

1500

2000

2500

3000

3500

4000

4500 SRAD/AMSR Bias H-Pol ASC Without Correction

MEAN = - 5.7346

-100 -80 -60 -40 -20 0 20 40 600

500

1000

1500

2000

2500

3000

3500

4000

4500 SRAD/AMSR Bias H-Pol ASC With Correction

MEAN = 0.2118

SRad/AMSR Bias H-Pol Ascending

Without Correction

Mean = -5.7346

With Correction

Mean = 0.2118

-120 -100 -80 -60 -40 -20 0 20 40 600

500

1000

1500

2000

2500

3000

3500

4000

4500 SRAD/AMSR Bias H DSC Without Correction

MEAN = -3.1126

-100 -80 -60 -40 -20 0 20 40 60 800

500

1000

1500

2000

2500

3000

3500

4000

4500 SRAD/AMSR Bias H DSC With Correction

MEAN = 0.1792

SRad/AMSR Bias H-Pol Descending

Without Correction

Mean = -3.1126

With Correction

Mean = 0.1792

SRad/AMSR Asc/Dsc Tb Biases

Mean (K) STD (K) # pts

H-pol Asc -5.7 3.0 52,253

V-pol Asc -6.4 2.7 66,584

H-pol Dsc -3.1 2.3 52,031

V-pol Dsc -1.6 2.3 67,545

Mean (K) STD (K) # pts

H-pol Asc 0.21 3.0 52,253

V-pol Asc 0.41 2.1 66,584

H-pol Dsc 0.18 2.3 52,031

V-pol Dsc 0.70 2.3 67,545

(a) Original algo - before correction

(b) Revised algo - after correction

• Validation of refined SRad Tb algorithm

through independent comparisons between

SRad and AMSR

• After empirical correction, ocean delta Tb

bias is reduced to approx ± 2 K over latitude

compared to a peak value of -10 K before

correction

SRad Tb Algorithm Validation

100 150 200 250 300 350 400 450 500 550 600-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

Latitude Index

Delta

Tb(K)

H-Pol Asc Without SRad Correction 090103 L2A Data

100 150 200 250 300 350 400 450 500 550 600-10

-8

-6

-4

-2

0

2

4

6

8

Latitude Index

Delta

Tb(K)

H-Pol Asc With SRad Correction 090103 L2A Data

Without empirical correction

With empirical correction

-55° 0° +55°

Latitude

Del

ta-T

b, K

Del

ta-T

b, K

(SRad - AMSR) H-pol delta-Tb Asc

100 150 200 250 300 350 400 450 500 550 600-10

-8

-6

-4

-2

0

2

4

Latitude Index

Delta

Tb(K)

H-Pol Dsc Without SRad Correction 090103 L2A Data

100 150 200 250 300 350 400 450 500 550 600-8

-6

-4

-2

0

2

4

6

Latitude Index

Delta

Tb(K)

H-Pol Dsc With SRad Correction 090103 L2A Data

Without empirical correction

With empirical correction

+55° 0° -55°

Latitude

(SRad - AMSR) H-pol delta-Tb DscD

elta

-Tb, K

Del

ta-T

b, K

0 100 200 300 400 500 600 700 800-15

-10

-5

0

5

10

15SRad-AMSR Tb Vs Latitude H-Pol Ascending

Latitude Index

Delta

Tb (k

)

0 100 200 300 400 500 600 700 800-15

-10

-5

0

5

10

15

Latitude Index

Delta

Tb (K

)

SRad-AMSR Tb Vs Latitude Descending

SRad Tb Algorithm Validation,

Zonal Average Ocean delta Tb

South Pole North Pole

North PoleSouth Pole

3-Day Ascending orbit avg

3-Day Descending orbit avg

80 100 120 140 160 180 200

80

100

120

140

160

180

200

AMSR Brightness Temp(K)

SRad Tb Algorithm Validation,

Sept. 1, 2003 - Asc/Dsc Segments

Slope Offset

Asc 1.02 -3.98

Dsc 1.01 -0.93

"Blue" = Ascending

"Red" = Descending

AMSR Brightness Temp, K

SR

ad B

rightn

ess

Tem

p,

K

V-pol

H-pol

Conclusions

• Empirical Tb corrections are developed for SRad using AMSR simultaneous ocean Tb comparisons

– Function of orbit position (latitude, Asc/Dsc)

• After applying empirical correction, results demonstrate that good quality SRad brightness temperatures are obtained over the oceans.

SRad/AMSR Biases along Land/Ocean Boundaries

(9-Day Average)

SRad/AMSR Biases after applying Conservative

Land MaskSRad-AMSR Bias 9-Day Avg H-Pol Asc using conservative LandMask

200 400 600 800 1000 1200 1400

100

200

300

400

500

600

700-50

-40

-30

-20

-10

0

10

20