Watson W. Gregg,
Frederick S. Patt,
and Wayne E. Esaias
W. W. Gregg and W. E. Esaias are with the Laboratory for Hydrospheric Processes, NASA Goddard Space Flight Center, Code 971, Greenbelt, Maryland 20771. USA
F. S. Patt is with General Sciences Corporation, 6100 Chevy Chase Drive, Suite 200, Laurel, Maryland 20707. USA
Watson W. Gregg, Frederick S. Patt, and Wayne E. Esaias, "Initial analysis of ocean color data from the ocean color and temperature scanner. II. Geometric and radiometric analysis," Appl. Opt. 38, 5692-5702 (1999)
We assessed the geometric and radiometric performance of the ocean
color and temperature scanner (OCTS) using data acquired over the
United States. Initial results indicated a geometric offset in the
along-track direction of 4–5 pixels that was attributed to a tilt
bias. OCTS radiometric data appeared to suffer from near-field and
possibly far-field scatter effects. Analysis of radiometric
stability was inconclusive because of daily variability and the absence
of a full seasonal cycle. Comparison of OCTS-computed water-leaving
radiances with colocated in situ measurements showed that
the prelaunch calibration required adjustment from -2% to
+13%. Minor modification of OCTS data processing based on these
results and avoidance of near-field scatter effects can enable improved
and more-reliable OCTS data for quantitative scientific
analyses.
Watson W. Gregg, Margarita E. Conkright, John E. O’Reilly, Frederick S. Patt, Menghua H. Wang, James A. Yoder, and Nancy W. Casey Appl. Opt. 41(9) 1615-1628 (2002)
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Each band has 20-nm bandwidth, except bands 7 and 8, which have
40-nm bandwidth. Fo is the mean
extraterrestrial irradiance for the OCTS bands (units of mW
cm-2 µm-1 sr-1),
τr is the Rayleigh optical thickness,
τoz is the ozone optical thickness (all from
NASDA3), and Lsat is the radiance
at which OCTS saturates (with the prelaunch calibration values and
detector 1 only3).
Table 2
Information about Measured in situ
Water-Leaving Radiances Used to Evaluate the OCTS Calibrationa
All sampling events included data for
OCTS bands 1, 2, 3, and 6. Only the 18 November 1996 date contained
data from OCTS bands 4 and 5. The other events acquired data at 510
and 555 nm, which are SeaWiFS bands 4 and 5.
Coastal Services Center.
Woods Hole Oceanographic Institution.
Table 3
Statistics for Geometric Adjustments for OCTS by use of
Scan Edges (±300 pixels from usable edge) to Identify a Source of
Geometric Biasa
Error Source
Median
IQR
Percent
Second tilted segment (18 March 1997 to mission end), 44 overpasses
Pitch (deg)
-0.265
0.06
16
Time (s)
0.340
0.09
7
Tilt (deg)
-0.240
0.05
75
Nadir segment (15 December 1996 to 17 March 1997), 45 overpasses
Pitch (deg)
-0.200
0.03
22
Time (s)
0.490
0.09
40
Tilt (deg)
-0.200
0.02
38
The median represents the
central value of the error source adjustment that produced the fewest
misnavigated pixels. Percent refers to the relative number of
scenes for which the geometric adjustment produced the overall best
results of the three. A negative tilt or pitch adjustment is toward
the north (aft of the velocity vector). A negative time
adjustment indicates moving the spacecraft position toward the north
(aft of the velocity vector).
Table 4
Median and IQR Values for the Calibration Adjustment
Factora
Derived from in situ
measurements of radiance and clear-water radiances and compared with
other observations. No IQR is available for bands 4 and 5 because
only a single site was available at these wavelengths. Clear-water
values denote the median and IQR of the daily medians provided in Fig. 9.
Tables (4)
Table 1
OCTS Ocean Color Spectral Characteristicsa
Band
Center Wavelength (nm)
Fo
τr
τoz
Lsat
1
412
170.96
0.3162
0.00077
18.4
2
443
188.17
0.2346
0.0039
18.0
3
490
194.59
0.1556
0.02488
14.3
4
520
185.74
0.1263
0.04724
12.8
5
565
184.49
0.0871
0.1127
9.9
6
670
153.12
0.04436
0.04923
6.4
7
765
122.61
0.0256
0.008146
4.3
8
865
98.55
0.0158
0.00341
2.1
Each band has 20-nm bandwidth, except bands 7 and 8, which have
40-nm bandwidth. Fo is the mean
extraterrestrial irradiance for the OCTS bands (units of mW
cm-2 µm-1 sr-1),
τr is the Rayleigh optical thickness,
τoz is the ozone optical thickness (all from
NASDA3), and Lsat is the radiance
at which OCTS saturates (with the prelaunch calibration values and
detector 1 only3).
Table 2
Information about Measured in situ
Water-Leaving Radiances Used to Evaluate the OCTS Calibrationa
All sampling events included data for
OCTS bands 1, 2, 3, and 6. Only the 18 November 1996 date contained
data from OCTS bands 4 and 5. The other events acquired data at 510
and 555 nm, which are SeaWiFS bands 4 and 5.
Coastal Services Center.
Woods Hole Oceanographic Institution.
Table 3
Statistics for Geometric Adjustments for OCTS by use of
Scan Edges (±300 pixels from usable edge) to Identify a Source of
Geometric Biasa
Error Source
Median
IQR
Percent
Second tilted segment (18 March 1997 to mission end), 44 overpasses
Pitch (deg)
-0.265
0.06
16
Time (s)
0.340
0.09
7
Tilt (deg)
-0.240
0.05
75
Nadir segment (15 December 1996 to 17 March 1997), 45 overpasses
Pitch (deg)
-0.200
0.03
22
Time (s)
0.490
0.09
40
Tilt (deg)
-0.200
0.02
38
The median represents the
central value of the error source adjustment that produced the fewest
misnavigated pixels. Percent refers to the relative number of
scenes for which the geometric adjustment produced the overall best
results of the three. A negative tilt or pitch adjustment is toward
the north (aft of the velocity vector). A negative time
adjustment indicates moving the spacecraft position toward the north
(aft of the velocity vector).
Table 4
Median and IQR Values for the Calibration Adjustment
Factora
Derived from in situ
measurements of radiance and clear-water radiances and compared with
other observations. No IQR is available for bands 4 and 5 because
only a single site was available at these wavelengths. Clear-water
values denote the median and IQR of the daily medians provided in Fig. 9.
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