# Presentation: 287000

## 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 Li******4***
**1****NASA Goddard Space Flight Center**
**2****University of Maryland Baltimore County**
**3****Science Systems and Applications, Inc. **
**4****SM&A 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 ****R****0.47(0.67)**** as a function of ****R****2.20****  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 0–65°**
**Repeat for different solar zenith angles—forward scattering direction**
**Select high turbidity case and repeat**
**Do atmospheric correction using 6S for different aerosol optical thickness**
**Explore  ****R****0.47(0.67)****/****R****2.20  ****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 @ 16 bit**
**395 pixels in scan line**
**4% reflectance calibration accuracy**

## Cloud Absorption Radiometer

**Horizontal mode**

**Vertical mode**

***Cloud Absorption Radiometer***

## Scatter Plots of Reflectance at 0.47 and 0.67 μm as a Function of Reflectance at 2.2 μm

****** ****= 0°**

***Scatter Plots of Reflectance at 0.47 and 0.67 μm as a Function of Reflectance at 2.2 μm***

**Reflectance (2.20 μm)**

**Reflectance (2.20 μm)**

**Reflectance (0.47 μm & 0.67 μm)**

**Brasilia**

## Reflectance Ratios R0.47/R2.20 and R0.67/R2.20 as a Function of View Zenith Angle

***R******0.47******/R******2.20***

**Brasilia**
**Aircraft height above ground = 2.4 km**
******0**** = 45.4°**
****** ****= 132.1°**

***Reflectance Ratios R******0.47******/R******2.20****** and R******0.67******/R******2.20****** as a Function of View Zenith Angle***

**Zenith Angle**

***R******0.47(0.67)******/R******2.20***

***R******0.67******/R******2.20***

## Reflectance Ratios R0.47/R2.20 and R0.67/R2.20 as a Function of View Zenith Angle

**Reflectance Ratios R****0.47****/R****2.20**** and R****0.67****/R****2.20**** as a Function of View Zenith Angle**

***R******0.47******/R******2.20***

**Cuiabá**
**Aircraft height above ground = 2.0 km**
******0**** = 54.1°**
****** ****= 109.3°**

***Reflectance Ratios R******0.47******/R******2.20****** and R******0.67******/R******2.20****** as a Function of View Zenith Angle***

**Zenith Angle**

***R******0.47(0.67)******/R******2.20***

***R******0.67******/R******2.20***

## Reflectance Ratios R0.47/R2.20 as a Function of Zenith Angle with Atmospheric Correction

**Brasilia**
**Aircraft height above ground = 2.4 km**
******0**** = 45.4°**
****** ****= 132.1°**

**Zenith Angle**

***R******0.47******/R******2.20***

## Reflectance Ratios R0.67/R2.20 as a Function of Zenith Angle with Atmospheric Correction

***Reflectance Ratios R******0.67******/R******2.20****** as a Function of Zenith Angle with Atmospheric Correction***

**Brasilia**
**Aircraft height above ground = 2.4 km**
******0**** = 45.4°**
****** ****= 132.1°**

**Zenith Angle**

***R******0.67******/R******2.20***

## Reflectance Ratios R0.47/R2.20 as a Function of Zenith Angle with Atmospheric Correction

***Reflectance Ratios R******0.47******/R******2.20****** as a Function of Zenith Angle with Atmospheric Correction***

**Cuiabá**
**Aircraft height above ground = 2.0 km**
******0**** = 54.1°**
****** ****= 109.3°**

**Zenith Angle**

***R******0.47******/R******2.20***

## Reflectance Ratios R0.67/R2.20 as a Function of Zenith Angle with Atmospheric Correction

***Reflectance Ratios R******0.67******/R******2.20****** as a Function of Zenith Angle with Atmospheric Correction***

**Cuiabá**
**Aircraft height above ground = 2.0 km**
******0**** = 54.1°**
****** ****= 109.3°**

**Zenith Angle**

***R******0.67******/R******2.20***

## Bidirectional Reflectance Measurements

**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)**

***Bidirectional Reflectance Measurements***

## 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**
******0**** = 60.4°**

**Zenith Angle**

***R******047(0..67)******/R******2.20***

***R******0.47******/R******2.20***

***R******0.67******/R******2.20***

## Spectral Measurements of Surface-Atmosphere Reflectance Ratios

**0.0              0.2             0.4              0.6              0.8             1.0**

***R******0.47******/R******2.20***

***R******0.47******/R******0.67***

***R******0.67******/R******2.20***

**Brasilia**
**Cerrado**

## Spectral Measurements of Surface-Atmosphere Reflectance Ratios

**0.0              0.2             0.4              0.6              0.8             1.0**

***R******0.47******/R******2.20***

***R******0.47******/R******0.67***

***R******0.67******/R******2.20***

**Cuiabá**
**Dense Forest**

## Summary and Conclusions

**For the pristine case the slopes of reflectance *****R******0.47******/R******2.20***** **** vary from 0.08–0.27 with a correlation coefficient *****r***** > 0.80 for zenith angles from 0°-65°**
**For measurements taken in the forward scattering direction the slopes < 0.1 and correlation *****r***** ~ 0.19**
**For the more turbid atmosphere, ********a**** = 0.68, the slopes *****R******0.47******/R******2.20***** **** ~ 0.1 for view zenith angles 0°–40°**
**Slopes of reflectance *****R******0.67******/R******2.20***** **** for the backscattering direction vary from 0.312–0.554 for zenith angles 0°–65°**
**For measurements made in the forward direction the slopes lie between 0.233–0.473 and *****r***** < 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 *****R******0.47***** = 0.25 *****R******2.20***** does not seem to hold for the cases tested here, but the relationship *****R******0.67***** = 0.5 *****R******2.2*****  seems to hold for the cases tested from 0°-45°**