NASA-TH-111634
) , j -
INT. J. RUMOTK SHNSING, 1995, VOL. 16, NO. 11, 1931-1942 //^ / "' V /K
Inter-satellite calibration linkages for the visible and near-infared
channels of the Advanced Very High Resolution Radiometer on the
NOAA-7, -9, and -11 spacecraft
C. R. NAGARAJA RAO
NOAA/NESDIS Satellite Research Laboratory, Washington, D.C. 20233,
U.S.A.
and J. CHEN
SM Systems and Research Corporation, Bowie, Maryland 20716, U.S.A.
{Received 29 December 1994; in fatal form 16 January 1995)
Abstract. The post-launch degradation of the visible (channel I:
«0-58 0-68/jm) and near-infrared (channel 2: ^0-72 M/<m) channels of the
Advanced Very High Resolution Radiometer (AVHRR) on the NOAA-7. -9, and
-11 Polar-orbiting Operational Environmental Satellites (POES) was estimated
using the south-eastern part ol" the Libyan desert as a radiometrically stable
calibration target. The relative annual degradation rates, in per cent, for the two
channels are, respectively: 3-6 and 4-3 (NOAA-7); 5-9 and 3-5 (NOAA-9); and 1-2
and 2-0 (NOAA-11). Using the relative degradation rates thus determined, in
conjunction with absolute calibrations based on congruent path aircraft/satellite
radiance measurements over White Sands. New Mexico (U.S.A.), the variation in
time of the absolute gain or 'slope' of the AVHRR on NOAA-9 was evaluated.
Inter-satellite calibration linkages were established, using the AVHRR on
NOAA-9 as a normalization standard. Formulae for the calculation of calibrated
radiances and albedos (AVHRR usage), based on these interlinkages, are given
for the three AVHRRs.
1. Introduction
One of the main objectives of the National Oceanographic and Atmospheric
Administration (NOAA)/National Aeronautics and Space Administration (NASA)
Advanced Very High Resolution Radiometer (AVHRR) Pathfinder program is the
establishment of long-term, accurate records of environmental products such as
vegetation cover, cloud morphology, aerosols, and sea surface temperature gener-
ated from broad-band spectral measurements made by the AVHRRs on board the
NOAA Polar-orbiting Operational Environmental Satellites (POES) (Ohring and
Dodge 1992). There is evidence (e.g., Brest and Rossow 1992) that the visible
(channel 1: %0-58 0-68 jun) and near-infrared (channel 2: a 0-72 11 fim) channels,
which have no on board calibration devices, degrade in orbit, initially because of
outgassing (e.g., water vapour from filter interstices) and launch associated contami-
nation (e.g., rocket exhaust and outgassing), and subsequently because of the
continued exposure to the harsh space environment. It is therefore necessary, in
order to ensure the quality and continuity of the long-term records of AVHRR-
derived environmental products, to evaluate the in-orbit degradation of the two
channels, and develop correction algorithms; and to establish inter-satellite calib-
ration linkages. Accordingly, we wish to present here the work done at the NOAA
( 1W5 U.S. (invernnicnl
1932 C. R. Naaaraja Rao and J. Chen
Satellite Research Laboratory to characterize the post-launch performance of the
AVHRR visible and near-infrared channels as part of the AVHRR Pathfinder
Calibration activity (Rao el al. 1993). We have confined our attention to the
AVHRRs on NOAA-7, -9, and -1 1 spacecraft for programmatic reasons. It should
be mentioned that the AVHRR Pathfinder Calibration activity is an integral part of
the operational satellite calibration program presently underway at the Satellite
Research Laboratory, Office of Research and Applications, National Environmental
Satellite, Data, and Information Service, NOAA.
2. Degradation rates
2.1. General
Relative changes in the gains of the three AVHRRs, expressed in units of
Counts/(W m " 2 sr " ' /mi '), or equivalently, the relative changes in the 'slope', the
reciprocal of gain, expressed in units of W/(m 2 sr/<m count), were determined using
the south-eastern part of the Libyan desert (21 23" N latitude; 28 29 "E longitude)
as a radiometrically stable calibration target. It is assumed that the isotropic albedo,
defined as (7t/,/F, cos () () ), does not vary in time; here /, and F iu are, respectively the
in-band upwelling radiance and the extraterrestrial solar irradiance in the /' lh
channel; and o is the solar zenith angle. This is a reasonable assumption since the
highly variable atmospheric contribution to the upwelling radiance at the top of the
atmosphere is small compared to the contribution of the radiation reflected by the
bright desert surface (e.g., Brest and Rossow 1992, Kaufman and Holben 1993). The
recent work of Cosnefroy el al. (1993) on the radiometric stability of desert targets
also lends support to this assumption; based on an analysis of Meteosat images for a
100 by 100 km area in the general vicinity of the calibration site, they found the
reflectance of the region to be very uniform, with the ratio of the standard deviation
to the mean value of the reflectance of the 2-5 km resolution pixels to be of the order
of 3 per cent; temporal stability was found to be of the same order after the seasonal
trends in reflectance had been removed.
The International Satellite Cloud Climatology Project (ISCCP) B3 data (Schiffer
and Rossow 1983) for the region of the calibration target were used in the present
study; the data comprised the measured counts in the visible, near-infrared, and
thermal infrared channels of the AVHRR; the satellite and solar zenith angles, and
the azimuth angle of observation; the latitude and longitude of the ISCCP B3 pixel;
and the date of measurement. A variant of Minnaert's reflection law (Barkstrom
1972) was used to characterize the dependence of the upwelling radiance on the solar
and satellite zenith angles. Following a scheme suggested by Staylor (1990) where it
is assumed that the degradation of the radiometer in time is exponential in its nature,
the degradation rates are estimated from solutions of the following equation:
Y=AX"exp(-kd) (1)
where Y: p 2 (C w -C\ ) )cosO; C i0 : measured signal in 10-bit counts; C„: offset in
10-bit counts; p: Earth Sun distance in astronomical units for the date of measure-
ment; X: {cosOcosO n )/(cos() + cos() ), and o being the satellite and solar zenith
angles, respectively; k: daily rate of the degradation of the gain of the radiometer,
assuming it to be exponential in time; d: days after launch of the spacecraft; and A,
B: regression parameters. Standard numerical techniques were used to determine the
value of the daily degradation rate, k. We have confined our attention to the B3 data
corresponding to satellite zenith angles sg 14 to minimize the effects, if any, of the
Inter-satellite calibration linkages
1933
azimuthal dependence of the upwelling radiation at the top of the atmosphere; also,
only data corresponding to solar zenith angles ^60 have been included in the
analysis. Greater details of the method of determination of the degradation rates, of
the quality control criteria adopted to detect the presence of clouds, and of the
ISCCP B3 data used are found in Rao and Chen (1993).
2.2. The AVliRRs on NOAA-7, -9, and -11 spacecraft
The degradation rates k for the two channels of the three AVHRRs. determined
using the method outlined in the previous section, and the relevant ancillary data are
shown in table 1; the k values yield the daily rate at which the AVHRR signal
associated with an Earth scene, set equal to unity (1) on the day of launch, would
apparently decrease in time, all other conditions remaining unaltered, because of the
degradation of the radiometer. Thus, the corrected AVHRR signal can be obtained
from the uncorrected signal on day d after launch by multiplying the latter
(uncorrected signal) by exp (kd).
We have compared in table 2 the annual rates of degradation in per cent, 100 [( 1-
exp (-365/c))], based on the daily degradation rates listed in table 1 with results
reported by researchers elsewhere; this comparison is only representative and not
complete. The choice of the data for comparison was mainly governed by our desire
to compare our results with those obtained over desert calibration targets (Staylor
1990, Kaufman and Holben 1993, Wu and Zhong 1994); with the results of
statistical analysis of a large body of global reflectance data (Brest and Rossow
1992); with results based on composite calibration data (Che and Price 1992); and
with those based on measurements made over a variety of targets, and on model
simulations (R. Santer, 1993, personal communication). Staylor (1990) used the
monthly means of AVHRR channel 1 albedos extracted from the Heat Budget
Parameter (HBP) data (Gruber and Winston 1978) for a broad region (% 10" km 2 )
of the Libyan desert in his work. Kaufman and Holben (1993) used the AVHRR
GAC data (nominal spatial resolution: *4km; daily and monthly means) for several
sites located in the Libyan desert to establish the rate at which the ratio of the pre-
Table 1. Relative degradation rates for the visible and near-infrared channels of the
AVHRRs on NOAA-7, -9, and -11 spacecraft.
Launch
date
Dates of
data
availability
Number of
data
points
84
86
83
Degra
rate (k)
.dalion
per day
Spacecraft
Channel 1
0-000101
0000166
0000033
Channel 2
NOAA-7
NOAA-9
NOAA-11
23 June 1981
12 December 1984
24 September 1988
August 1981
December 1984
January 1985
November 1988
January 1989
December 1991
0000120
0000098
0-000055
Note: Each data point is the average of measurements made over approximately 35 pixels
in the 1" (longitude) x 2° (latitude) area of the calibration target in the course of the day; the
entries in the 'Number of data points* column give the number of days of appropriate data
availability.
1934 C. R. Nagarqja Rao and J. Chen
Table 2. Comparison of annual relative degradation rates (in per cent) reported by different
authors.
NOAA-7 NOAA-9 NOAA-11
Source Channel 1 Channel 2 Channel 1 Channel 2 Channel 1 Channel 2
Present work
3-6
Staylor (1990)
3-5
Kaufman and Holben
(1993)
4-2
Brest and Rossow
(1992)
Che and Price (1992)
4-4
Santer (1993. personal
communication)
4-7
Wu and Zhong (1994)
43 5-9 3-5 1-2 2-0
6-0
5-6 5-7 31 1-3 2-9
4-6
46 50 6-7 6-7 3-9
4-7 7-5 4-5
5-8 46
Note: It is assumed in Brest and Rossow (1992) that there was no degradation of channel 1
of the AVHRR on NOAA-7 over the period July 1983 January 1985.
launch value of the 'slope' to its actual value varied in time. Brest and Rossow (1992)
based their findings on statistical analysis of a large body of ISCCP global
reflectance data, assuming that 'the global aggregate of regional variations of surface
visible reflectance is not changing with time'; their results indicate that the visible
channel of the AVHRR on NOAA-7 did not exhibit any perceptible in-orbit
degradation over the period July 1983 to January 1985. Che and Price (1992) used a
composite of post-launch calibration results given by several authors, using different
techniques, and established linear regression relationships between the published
values of gains (or "slopes"), and elapsed time since launch (in months) of the
spacecraft, assuming similarity in the degradation of different AVHRRs. Santer
(1993, personal communication) has used measurements of upwelling radiances over
desert surfaces, stratus cloud decks, ocean targets, and model simulations to derive
the degradation rates. Wu and Zhong (1994) used two sets of measurements made
over desert sites in China, approximately 3 years apart, to derive the degradation
rates for the visible and near-infrared channels of the AVHRR on NOAA-9; a
bidirectional reflection model, appropriate for deserts, was used to account for the
dependence of the reflected radiation on the solar and satellite zenith angles, and the
azimuth angle of observation.
The range of values of the degradation rates is an indication of the complexity of
post-launch calibration of sensors like the AVHRR in the absence of on-board
calibration, and is also traceable to the differences in the techniques employed.
2.3. Absolute calibration of the AVHRR on NOAA-9 spacecraft
The in-orbit degradation of the visible and near-infrared channels of the
AVHRR on NOAA-9 spacecraft has been studied very extensively (e.g., Whitlock
et al. 1990) by researchers employing a variety of techniques since its effective
operational life (% 1985-1988) encompassed important, international, multi-agency,
multi-platform experiments such as the First ISCCP Regional Experiment (FIRE),
and the First International Satellite Land Surface Climatology Project (ISLSCP)
Field Experiment (FIFE). Also, the availability of the results of several aircraft/
Inter-satellite calibration linkages 1935
satellite congruent path radiance measurements over White Sands, New Mexico
(U.S.A.) during October/November 1986 (Smith el al. 1988), employing a well
calibrated spectrometer on board a U-2 aircraft, made it possible to translate the
relative degradation rates into variations in time of the gain or 'slope'. Accordingly,
using the aircraft-based absolute calibrations as anchors, and employing the relative
degradation rates (table 1), the rate of variation of the 'slope' in time over the
operational life of the spacecraft was determined. The resulting expressions for the
calculation of radiances /,, (in units of W m 2 sr ' /mi ' ) on day d after launch, are:
Channel 1:
Channel 2:
/ d = 0-5465xexp[l-66x 10 4 x (J-65)](C, -C ) (2)
/,, = 0-3832 xexp [0-98 x 10 4 x(r/-65)](C l0 -C„) (3)
C n has been given values of 370 and 39-6 in channels 1 and 2, respectively, and d is
taken to be zero on the day of launch. Also, the effective counts (C [0 — C ) have
been normalized to mean Earth Sun distance in the above formulae. The formulae
have been recommended to the user community by the NOA A/NASA AVHRR
Pathfinder Calibration Working Group (Rao et al. 1993); it was also the aim of the
Working Group that the AVHRR on NOAA-9 should be used as the reference or
normalization standard in the post-launch calibration of the AVHRRs on NOAA-7
and -11.
3. Inter-satellite calibration linkages
We have used the AVHRR on NOAA-9 as the normalization standard to
establish interrelationships among the visible and near-infrared radiances measured
by the three AVHRRs. The method is based on the use of matched data sets
consisting of the measured counts, satellite zenith and solar zenith angles, and the
dales of measurements over the south-eastern part of the Libyan desert, which was
used as the calibration target in the present study. The following selection criteria
were used to generate two sets of matched data, one for the NOAA-7/NOAA-9
combination, and the other for the NOAA-9/NOAA-1 1 combination:
(a) The solar zenith angle, () a , and the satellite zenith angle, 0, for the
measurements made by the AVHRRs on NOAA-7 and NOAA-1 1 should be
within 1 of the corresponding angles for measurements by the AVHRR on
NOAA-9;
{b) The two measurements should have been made in the same calendar month,
with the days of the month being as close to one another as was practicable,
to allow for seasonal variations, if any, in the radiometric characteristics of
the target. It is thus likely that measurements made by the AVHRR on either
NOAA-7 or -II in a given calendar month in the ith year in orbit of the
relevant spacecraft could be matched with measurements made in the same
calendar month, but in a different year (y'th) in orbit of NOAA-9.
These selection criteria resulted in 1 1 sets of matched data for the NOAA-7/
NOAA-9 combination, and 10 sets of matched data for the NOAA-ll/NOAA-9
combination.
The method of normalization of the AVHRRs on NOAA-7 and NOAA-1 1 to the
AVHRR on NOAA-9 is illustrated below, using the NOAA-7/NOAA-9 matched
data set as an example:
1936 C. R. Nauuraju Rao and J. Chen
(a) For either channel, the effective counts (C 1() -C ) were corrected for the
NOAA-7 AVHRR degradation, using the k values listed in table 1; the
elapsed time in days, d, since launch was calculated from the known dates of
launch and of the measurements in the matched data set.
(b) The matched, calibrated radiance measured by the AVHRR on NOAA-9 was
calculated, using either (2) or (3) as was appropriate.
(c) A linear regression relationship was established between the NOAA-9
radiances from (b), and the corrected effective counts for the NOAA-7
AVHRR from (a).
The regression relationship established in (c) above links the AVHRR on NOAA-7
to the AVHRR on NOAA-9, and yields the 'slope', in units of W/m 2 sr/im count),
for the given channel on the day of launch of NOAA-7. The AVHRR on NOAA-1 1
was linked to the AVHRR on NOAA-9 in a similar manner. The regressions had
correlation coefficients in excess of 0-9. The resulting formulae for the calculation of
radiances are given in table 3. The reflectance factor (Rao 1987) (or albedo (AVHRR
usage) or scaled radiance (Brest and Rossow (1992)), given by (100n/,/F, ), is also
used in different applications of AVHRR data; to facilitate this activity, we have
listed in table 4 the formulae for the calculation of the albedos; the extra-terrestrial
solar spectral irradiance values given by Neckel and Labs (1984) were used in the
conversion of the radiance formulae (table 3) to the albedo representation; the
conversions are effected by multiplying the radiances by ( \00nw : /F U) ), vv, being the
equivalent width of the /th channel. We have given in table 5 the values of w, and F io
used in the conversion of the radiance formulae given in table 3 to the albedo
(AVHRR usage) representation (table 4).
It is felt that the attainable accuracies in the radiance or albedo calculated using
the formulae we have given (tables 3 or 4) are at best comparable to the estimated
Table 3. Formulae for the calculation of calibrated radiances.
Spacecraft Radiance (Wm 2 sr '/(in ')
NOAA-7
Channel I 0-5753 exp (1-01 x 10 4 ,/) (C 10 36)
Channel 2 0-3914 exp (1-20 x 10 4 d) (C ]( , -37)
NOAA-9 (Set A)
Channel 1 0-5465 exp [l-66x 10 4 (d- 65)] (C,„-37)
Channel 2 0-3832 exp [0-98 x 10 4 (J- 65)] (C,„-39-6)
NOAA-9 (Set B)
Channel 1 0-5406 exp (1-66 x 10 4 d) (C 1() --37)
Channel 2 0-3808 exp (0-98 x 10 4 d) (C',,,-39-6)
NOAA-1 1
Channel 1 0-5496 exp (0-33 x 10 4 d) (C 10 -40)
Channel 2 0-3680 exp (0-55 x 10 4 d) (C,„-40)
Note: The two sets of formulae given for NOAA-9 yield the same radiances; the quantity
exp (-65A) occurring in Set A has been incorporated into the numerical coefficient appearing
at the beginning of the formulae in Set B to render their format the same as that of the
formulae for the AVHRRs on NOAA-7 and NOAA-1 1.
Inter-satellite calibration linkages 1937
Table 4. Formulae Tor the calculation of calibrated AVHRR albedos.
Spacecraft Albedo (per cent)
NOAA-7
Channel I 01 100 exp ( 101 x 10 4 d) (C 1() -36)
Channel 2 0-1 169 exp ( 1-20 x 10 4 rf)(C 10 -37)
NOAA-9 (Set A)
Channel 1 01050 exp [1-66 x 10 4 (J-65)] (C 1() -37)
Channel 2 01 143 exp [0-98 x 10 4 (J-65)] (C l(l -39-6)
NOAA-9 (Set B)
Channel 1 01 039 exp (l-66x 10 4 d) (C 10 -37)
Channel 2 01 136 exp [0-98 x 10 4 d) (C 10 -39-6)
NOAA-11
Channel 1 0-1060 exp (0-33 x 10~ 4 d) (C,„-40)
Channel 2 01098 exp (0-556 x 10 4 d) (C lo -40)
Note: The two sets of formulae given for NOAA-9 yeild the same albedos; the quantity
exp ( — 65/c ) occurring in Set A has been incorporated into the numerical coefficient appearing
at the beginning of the formulae in Set B to render their format the same as that of the
formulae for the AVHRRs on NOAA-7 and NOAA-1 1.
accuracies of the order of a few per cent (G. Smith, 1993, personal communication)
of the Fall 1986 aircraft-based absolute calibrations of the AVHRR on NOAA-9
which we have used as the normalization standard in the present work.
Staylor (1990) has established linkages among the visible channels (channel 1) of
the AVHRRs on NOAA-6, -7 and -9, using the instrument on NOAA-7 as the
normalization standard. Starting with the regions of overlap in the X variable in the
plots of Von X (1), he determined adjustment factors (multipliers) for Y that would
force the regression plots for the radiometers on NOAA-6 and -9 to coalesce with
the regression plot for the AVHRR on NOAA-7. The ratio of the adjustment factors
he has given for the AVHRRs on NOAA-7 and NOAA-9 is 1 07 which is very close
to the ratio (1064) of the channel 1 slopes for the day of launch (d = 0) listed in table
3 for the two AVHRRs. Brest and Rossow (1992) have also established a linkage
between the visible channels of the AVHRRs on NOAA-7 and -9, using reflectance
data from the two satellites, obtained over a 3-week overlap period from 18 January
to 8 February 1985; it should however be noted that they have assumed that there
was no perceptible degradation in the performance of NOAA-7 AVHRR channel 1
over the period from the middle of 1983 to early 1985.
Table 5. Equivalent widths and in-band extraterrestrial solar irradiances.
Wfl/rni)
i
F i0
(Wm
2 )
Spacecraft
Channel
1
Channel 2
Channel 1
Channel 2
NOAA-7
NOAA-9
NOAA-11
0-108
0-117
0-113
0-249
0-239
0-229
177-5
191-3
184-1
261-9
251-8
241-1
1938 C. R. Nagaraja Rao and J. Chen
4. Validation
To evaluate the reasonableness of the inter-satellite calibration linkages we have
established using the AVHRR on NOAA-9 as a normalization standard we shall
first compare the variation in time of the 'slopes' of channels 1 and 2 of the AVHRR
on NOAA-11, derived from these formulae, with the absolute calibrations based on
congruent path aircraft/satellite radiance measurements over the White Sands area,
New Mexico, U.S.A. (Abel et al. 1993). The results are shown in figure 1; we have
also included results of several other investigators for purposes of comparison. It
should be mentioned that Mitchell et al. (1992) obtained their results using the split-
pass imagery technique and model simulations of the upwelling radiation over an
oceanic target in the Southern Ocean off the north-west coast of Tasmania.
The noticeable feature is that the variation in time of the 'slope', expressed in
units of W/(m 2 sr/nn count), is within one standard deviation of the mean values of
the majority of aircraft-based absolute calibrations in both channels. This behaviour
can be taken as an indicator of the viability of the method we have used to develop
interrelationships among the three radiometers, using the AVHRR on NOAA-9 as a
normalization standard. The differences, practically systematic, between the trends
in 'slope' in the present study, and those reported in Kaufman and Holben (1993)
may perhaps be due to differences in the absolute calibration anchors used.
We used about 15 per cent of the ISSCP B3 (table 1) data for the south-eastern
Libyan desert calibration site in the establishment of the matched data set; we thus
felt it may be scientifically meaningful to apply the inter-satellite calibration linkages
to the entire set of data to determine the impact of in-orbit degradation on the
calculated albedos. Accordingly, the time series (1981 1991) of albedos
\0OnlJ(F i0 cos0 o ) in channels 1 and 2 of the calibration target site obtained using
the pre- and post-launch calibrations are shown in figure 2; we have included the
albedo values calculated using the post-launch calibrations reported by Kaufman
and Holben (1993) and by Che and Price (1992) for purposes of comparison. The
noticeable feature is that the calibrated radiance formulae given here (table 3)
establish continuity of record of the isotropic albedo among the three satellites, and
also remove the spurious downward trends in the albedos obtained with the pre-
launch coefficients. The mean and standard deviation (in parentheses) of the
corrected albedos (in per cent) in AVHRR channels 1 and 2 are found to be,
respectively 37-8 (0-7) and 42-6 (1-5). It has been observed that channel 2 data are
generally noisier than those for channel 1; this may be due to the uncertainties
introduced by the variations in the atmospheric precipitable water, which exhibits
absorption in the spectral region of channel 2.
It may rightfully be argued that the time seris of the albedos of the calibration
target site should not be used to validate our results. However, the calibrated
radiance/albedo formulae we have given here have been implemented in the
reprocessing of the Normalized Difference Vegetation Index (NDVI) global climat-
ology at NOAA (Gutman et al. 1995); in the NOAA experimental clouds from
AVHRR (CLAVR) product (L. Stowe and P. Davis, 1994, personal communica-
tion); in the validation of the NOAA aerosol product using shipboard aerosol
optical thickness measurements (Ignatov et al. 1994); and in the generation of the
NOAA/NASA Pathfinder AVHRR Land Data Set (Agbu and James 1994, James et
al. 1994). Results to date of the above application are very encouraging (personal
communications from different users) in terms of establishment of continuity
Inter-satellite calibration linkages
1939
0.68
c 0.64
Channel 1
0.6
S
•g 0.56
a. 0.52
o
0.48 4--
200 400
Days after launch
600
800
c
a
o
a.
I**
"a
a.
o
0.44
0.4
0.36
0.32
Channel 2
200 400
Days after launch
600
800
Figure 1. Comparison of the predicted variation in time of the 'slopes' of channels 1 (lop)
and 2 (bottom) of the AVHRR on NOAA-1 1 (launch date: 24 September 1988) with
aircraft-based absolute calibrations. • Abel el al. (1993), — present work,
Kaufman and Holben (1992), Che and Price (1992), ■ Mitchell el al. (1992).
1940
C. R. Nagaraja Rao and J. Chen
August
1981
March
1984
December
1986
Date
September
1989
June
1992
August
March
December
September
June
1981
1984
1986
1989
1992
Date
Figure 2. Time series of the isotropic albedo of the Libvan desert calibration target.
present work, Kaufman and Holben (1993). • Che and Price (1992),
Pre-launch.
Inter-satellite calibration linkages 1941
amongst the three AVHRRs and in terms of removal of spurious trends in the
different products.
5. Conclusion
The validity and usefulness of the inter-satellite calibration linkages we have
established, and of the radiance and albedo formulae we have given can be evaluated
only after they have been applied to the reprocessing of long-term records of
different AVHRR-derived geophysical products by a large community of users.
Several activities along these lines, some of which have been mentioned earlier, are
presently underway both within NOAA, and at other institutions elsewhere. It
should also be recognized that the improvements in the products brought about by
the use of the radiance and albedo formulae we have given are also determined by
the nature of the data (e.g.. skewness or bias, composite nature) to which they are
applied, and the approximations and assumptions made in the product-generation
algorithms. Also, in continuation of the present investigation, a sensitivity analysis
of the post-launch calibration of the AVHRR visible and near-infrared channels
using desert calibration targets is presently underway to evaluate the impact, if any,
of uncertainties or changes in the various physical parameters used in the derivation
of the degradation rates (§2.1); results of this investigation will be published in a
forthcoming paper.
Acknowledgments
The work reported here was supported by the Information Management
component (NOAA Pathfinder Program Manager: Dr Arthur Booth) of the Climate
and Global Change Program, NOAA Office of Global Programs. Useful discussions
with Dr Sullivan and Dr Gutman, NOAA Satellite Research Laboratory, Wash-
ington, D.C., and with Dr Staylor, NASA Langley Research Center, Hampton, are
herewith acknowledged.
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