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PPT – Sensitivity of Off-Nadir Zenith Angles to the Surface Reflectance Ratio Technique PowerPoint presentation | free to download - id: 1c743e-ZDc1Z

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Sensitivity of Off-Nadir Zenith Angles to the

Surface Reflectance Ratio Technique

- Michael D. King,1 Charles Gatebe,1,2 Si-Chee

Tsay,1 - Qiang Ji,3 Tom Arnold,4 and Jason Li4
- 1NASA Goddard Space Flight Center
- 2University of Maryland Baltimore County
- 3Science Systems and Applications, Inc.
- 4SMA Corporation
- Outline
- Spectral variation of surface reflectance over

land - Important for the retrieval of aerosol optical

thickness over land

Goals Objectives

- Data Set Used
- Cloud Absorption Radiometer (CAR) data
- Smoke, Clouds, and Radiation-Brazil (SCAR-B),

August-September 1995 - Strategy Used
- Select a pristine case
- Plot reflectance R0.47(0.67) as a function of

R2.20 for view angles from nadir to 65 and then

compute the slope, correlation coefficient r, and

intercept from the best line of fit - Plot the slope of reflectance as a function of

zenith view angle from 065 - Repeat for different solar zenith anglesforward

scattering direction - Select high turbidity case and repeat
- Do atmospheric correction using 6S for different

aerosol optical thickness - Explore R0.47(0.67)/R2.20 for all azimuthal

angles from BRF measurements

University of Washington CV-580

Cloud Absorption Radiometer

- Goddard Space Flight Center
- developed in 1982-1983
- University of Washington
- integrated flown in 1984 (B-23)
- principal data from 1987-97 (C-131A)
- flights after 1998 (CV-580)
- Sensor Characteristics
- 14 spectral bands ranging from 0.34 to 2.29 µm
- scan 95 from horizon on right-hand side of

aircraft - field of view 17.5 mrad (1)
- scan rate 1.67 Hz (100 rpm)
- data system 9 channels _at_ 16 bit
- 395 pixels in scan line
- 4 reflectance calibration accuracy

Cloud Absorption Radiometer

Horizontal mode

Vertical mode

Scatter Plots of Reflectance at 0.47 and 0.67 µm

as a Function of Reflectance at 2.2 µm

Brasilia

q 30 f 132.1

q 0

Reflectance (0.47 µm 0.67 µm)

Reflectance (2.20 µm)

Reflectance (2.20 µm)

Reflectance Ratios R0.47/R2.20 and R0.67/R2.20 as

a Function of View Zenith Angle

Brasilia Aircraft height above ground 2.4 km q0

45.4 f 132.1

R0.47(0.67)/R2.20

R0.67/R2.20

R0.47/R2.20

Zenith Angle

Reflectance Ratios R0.47/R2.20 and R0.67/R2.20 as

a Function of View Zenith Angle

Reflectance Ratios R0.47/R2.20 and R0.67/R2.20 as

a Function of View Zenith Angle

Cuiabá Aircraft height above ground 2.0 km q0

54.1 f 109.3

R0.67/R2.20

R0.47(0.67)/R2.20

R0.47/R2.20

Zenith Angle

Reflectance Ratios R0.47/R2.20 as a Function of

Zenith Angle with Atmospheric Correction

Brasilia Aircraft height above ground 2.4 km q0

45.4 f 132.1

R0.47/R2.20

Zenith Angle

Reflectance Ratios R0.67/R2.20 as a Function of

Zenith Angle with Atmospheric Correction

Brasilia Aircraft height above ground 2.4 km q0

45.4 f 132.1

R0.67/R2.20

Zenith Angle

Reflectance Ratios R0.47/R2.20 as a Function of

Zenith Angle with Atmospheric Correction

Cuiabá Aircraft height above ground 2.0 km q0

54.1 f 109.3

R0.47/R2.20

Zenith Angle

Reflectance Ratios R0.67/R2.20 as a Function of

Zenith Angle with Atmospheric Correction

Cuiabá Aircraft height above ground 2.0 km q0

54.1 f 109.3

R0.67/R2.20

Zenith Angle

Bidirectional Reflectance Measurements

- Roll 20
- Time 2 min
- Speed 80 m s-1
- Height 600 m
- Diameter 3 km
- Resolution
- 10 m (nadir)
- 270 m (? 80)
- Channels
- 8 continuously sampled (0.34-1.25 µm)
- 2 filter wheel channels used for BRDF

measurements (1.64 2.20 µm)

Azimuthally Averaged Reflectance Ratios

R0.47/R2.20 and R0.67/R2.20 as a Function of q

Cuiabá - BRF Aircraft height above ground 1.7

km q0 60.4

R0.67/R2.20

R047(0..67)/R2.20

R0.47/R2.20

Zenith Angle

Spectral Measurements of Surface-Atmosphere

Reflectance Ratios

Brasilia Cerrado

R0.47/R2.20

R0.47/R0.67

R0.67/R2.20

0.0 0.2 0.4

0.6 0.8 1.0

Spectral Measurements of Surface-Atmosphere

Reflectance Ratios

Cuiabá Dense Forest

R0.47/R2.20

R0.47/R0.67

R0.67/R2.20

0.0 0.2 0.4

0.6 0.8 1.0

Summary and Conclusions

- For the pristine case the slopes of reflectance

R0.47/R2.20 vary from 0.080.27 with a

correlation coefficient r gt 0.80 for zenith

angles from 0-65 - For measurements taken in the forward scattering

direction the slopes lt 0.1 and correlation r

0.19 - For the more turbid atmosphere, ta 0.68, the

slopes R0.47/R2.20 0.1 for view zenith angles

040 - Slopes of reflectance R0.67/R2.20 for the

backscattering direction vary from 0.3120.554

for zenith angles 065 - For measurements made in the forward direction

the slopes lie between 0.2330.473 and r lt 0.80 - After the atmospheric correction is applied the

slopes of reflectance increase, with the greatest

increase occurring for the largest aerosol

optical thickness - In the case of the azimuthally averaged BRF data,

the slopes increase as a function of viewing

zenith angle - The slopes show little variation with azimuth

angle, being especially well behaved in the

backward scattering directions over dark targets

Summary and Conclusions

- From these results a relationship between visible

reflectance and shortwave infrared reflectance

exists for zenith angles from nadir to about 40 - However, the relationship R0.47 0.25 R2.20 does

not seem to hold for the cases tested here, but

the relationship R0.67 0.5 R2.2 seems to hold

for the cases tested from 0-45