Using Sun Glint and Antarctic Ice Sheets to Calibrate MODIS and AVHRR Observations of Reflected Sunlight

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

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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 ?m
c
a
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
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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 ? lt 0.2 and large aerosol
  particles)
• solar zenith angle (for ? lt 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
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Terra 0.84/0.64-?m
Aqua 0.84/0.64-?m
NOAA16 0.84/0.64-?m
NOAA17 0.84/0.64-?m
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Terra 1.60/0.64-?m
Aqua 1.60/0.64-?m
NOAA17 1.60/0.64-?m
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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 Slope
Terra Dec 2001 37 1.098 0.008 1.084 0.013
Terra Dec 2003 47 1.114 0.008 1.081 0.009
Terra Dec 2004 44 1.105 0.005 1.082 0.009

Aqua Dec 2002 42 1.107 0.011 1.082 0.015
Aqua Dec 2003 61 1.108 0.006 1.074 0.006
Aqua Dec 2004 32 1.110 0.005 1.071 0.007

NOAA-16 Dec 2001 63 1.050 0.037a --
NOAA-16 Dec 2002 50 1.052 0.041a --
NOAA-16 Dec 2003 57 1.163 0.018b --
NOAA-16 Dec 2004 39 1.137 0.033b --

NOAA-17 Dec 2002 77 0.943 0.029a 1.031 0.021a
NOAA-17 Dec 2003 54 0.936 0.026a 1.014 0.012a
NOAA-17 Dec 2004 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
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Terra 0.84/0.64-?m
NOAA17 0.84/0.64-?m
NOAA16 0.84/0.64-?m
Aqua 0.84/0.64-?m
Terra 1.60/0.64-?m
Aqua 1.60/0.64-?m
NOAA17 1.60/0.64-?m
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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)
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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

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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).