Using Sun Glint and Antarctic Ice Sheets to Calibrate MODIS and AVHRR Observations of Reflected Sunlight William R. Tahnk and James A. Coakley, Jr Cooperative Institute for Oceanographic Satellite Studies College of Oceanic and Atmospheric Sciences Oregon State University

Problem: Maintaining the calibration of aircraft and satellite-borne radiometers used to measure reflected sunlight Verifying the calibration while in flight and subject to the stresses of the flight environment Objective: Use reflected radiances in glint areas to assess relative accuracies of radiances at different wavelengths If the accuracy at one wavelength is known, observations in glint areas can be used to check the accuracies of radiances at other wavelengths

True color image for Terra on Dec 8 2002 over the Indian Ocean at 0700 UTC Method: Select sub-regions of reflected radiances to analyze within sun glint areas for various satellites Selection criteria: Narrow region, free of clouds Region spans a major fraction of the sun glint (edge to center) Good dynamic range in reflectance Derive estimates of the slopes and intercepts for the reflectances

Image of sun glint area constructed from 1-km radiances at 3.7 m b 1-km reflectances at 0.64, 1.6, and 2.1 m, and radiances at 3.7 m for the boxed region in the figure above

A Monte Carlo radiative transfer model (MOCARAT) was used to assess the sensitivity of the reflectance relationships at various wavelengths MOCARAT results: Values of the slopes and intercepts derived for simulations of the reflectances for atmospheres with marine aerosols are rather insensitive to surface wind speed and direction aerosol burden (for  < 0.2 and large aerosol particles) solar zenith angle (for  < 35º) The slopes and intercepts of the linear relationships among the reflectances can then be used to assess, in a relative sense, the calibration of the radiometer

Terra 0.84/0.64-m Aqua 0.84/0.64-m NOAA16 0.84/0.64-m NOAA17 0.84/0.64-m

Terra 1.60/0.64-m Aqua 1.60/0.64-m NOAA17 1.60/0.64-m

Slopes for the 0.84/0.64 and 1.60/0.64-µm linear relationships for Terra and Aqua MODIS and NOAA-16 and -17 AVHRR passes for the period December 2001-2004 Satellite Month/Year Pass Count 0.84/0.64 Slope 1.60/0.64 Terra Dec 2001 37 1.098 ± 0.008 1.084 ± 0.013 Dec 2003 47 1.114± 0.008 1.081 ± 0.009 Dec 2004 44 1.105 ± 0.005 1.082 ± 0.009 Aqua Dec 2002 42 1.107 ± 0.011 1.082 ± 0.015 61 1.108 ± 0.006 1.074 ± 0.006 32 1.110 ± 0.005 1.071 ± 0.007 NOAA-16 63 1.050 ± 0.037a -- 50 1.052 ± 0.041a 57 1.163 ± 0.018b 39 1.137 ± 0.033b NOAA-17 77 0.943 ± 0.029a 1.031 ± 0.021a 54 0.936 ± 0.026a 1.014 ± 0.012a 43 1.146 ± 0.033b 1.203 ± 0.014b aReflectances obtained using pre-launch calibration coefficients imbedded in the Level 1(B) data stream bReflectances obtained using updated calibration coefficients imbedded in the Level 1(B) data stream

Terra 0.84/0.64-m Aqua 0.84/0.64-m NOAA16 0.84/0.64-m NOAA17 0.84/0.64-m Terra 1.60/0.64-m Aqua 1.60/0.64-m NOAA17 1.60/0.64-m

NOAA16 NOAA17 Channel 1 Channel 2 Channel 1 Channel 2 Reflectances obtained using the operational calibration coefficients imbedded in the Level 1(B) data stream Reflectances obtained using calibration coefficients derived with the Antarctic ice sheet method Note: The solid line represents the NOAA9 calibration reference for Antarctica, from Loeb (1997)

Summary and Results: Antarctic ice sheets and ocean glint areas were used to check the calibration of solar reflectance channels on Terra and Aqua MODIS and NOAA16 and NOAA17 AVHRR for the period December 2002-2004 Terra and Aqua MODIS observations at 0.64, 0.84, and 1.6 m are consistent with each other and internally consistent through the period analyzed With the inclusion of updated calibrated coefficients in the Level 1(B) data stream for NOAA16 and NOAA17, the AVHRR solar reflectance channels are more consistent with MODIS The 0.64-m reflectances for both NOAA16 and NOAA17 fall short of the Antarctic ice sheet calibration by about 4% The 0.84-m reflectances are accurate within 1% for both NOAA satellites when compared with the Antarctic ice sheet calibration The 1.6/0.64 slopes derived for ocean glint regions indicate that the 1.6-m reflectances for NOAA17 (in high gain mode) and calibrated using the updated Level 1(B) calibration coefficients are too high by about 6%

References: Loeb, N.G., 1997: In-flight calibration of NOAA AVHRR visible and near-IR bands over Greenland and Antarctica. Int. J. Remote Sens., 18, 477-490. Tahnk, W. R. and J. A. Coakley, Jr., 2001: Improved calibration coefficients for NOAA-14 AVHRR visible and near-IR channels. International Journal of Remote Sensing, 22, 1269-1283. Tahnk, W. R. and J. A. Coakley, Jr., 2001: Updated calibration coefficients for NOAA-14 AVHRR channels 1 and 2. International Journal of Remote Sensing, 22, 3053-3057. Tahnk, W.R. and J.A. Coakley, Jr., 2002: Improved calibration coefficients for NOAA-12 and NOAA-15 AVHRR visible and near-IR channels. J. Atmos. Ocean. Tech. 19, 1826-1833. Luderer, G., J.A. Coakley, Jr., and W.R. Tahnk, 2005: Using sun glint to check the relative calibration of reflected spectral radiances. J. Atmos. Ocean. Technol. (in press). Tahnk, W.R. and J.A. Coakley, Jr., 2005: Calibration of visible and near-IR channels of MODIS and AVHRR through 2004 using Antarctic ice sheets and ocean glint regions. (in preparation).