1. 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

2. 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

3. 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

4. Image of sun glint area constructed from 1-km radiances at 3.7 mb
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

5. 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

6. Terra 0.84/0.64-m
Aqua 0.84/0.64-m
NOAA /0.64-m
NOAA /0.64-m

7. Terra 1.60/0.64-m
Aqua 1.60/0.64-m
NOAA /0.64-m

8. 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
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

9. Terra 0.84/0.64-m
Aqua 0.84/0.64-m
NOAA /0.64-m
NOAA /0.64-m
Terra 1.60/0.64-m
Aqua 1.60/0.64-m
NOAA /0.64-m

10. 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)

11. 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
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%

12. 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,
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,
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,
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,
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).