The Nimbus 7 SBUV measures the same latitude ozone at widely different sun angle conditions at the ascent and descent part of the orbit during the summer solstice. This situation is used in a particular procedure (Ascent/Descent) to obtain the relative channel-to-channel calibration error for channels 273 nm to 306 nm. These estimated errors are combined with results from the Pair Justification procedure to correct the sun-view diffuser drift in calibration from November 1978 to February 1987 for the shorter wavelength channels that measure upper stratospheric ozone. Some preliminary re-calirated Nimbus 7 SBUV data in 1989 is compared with the first set of SBUV measurements flown on the Space Shuttle

Bhartia, P. K.

Mcpeters, R. D.

Taylor, Steven L.

English

NASA Technical Reports Server

N95-11123
PROCEDURES TO VALIDATE/CORRECT CALIBRATION ERROR
IN SOLAR BACKSCATTERED ULTRAVIOLET INSTRUMENTS
Steven L. Taylor
Hughes/STX
Lanham, Maryland
R.D. McPeters. PK. Bhartia
Goddard Space Flight Center
Greenbelt, Maryland
ABSTRACT
The Nimbus 7 SBUV measures the same latitude ozone at
widely different sun angle conditions at the ascent and decent
part of the orbit during the summer solstice. This situation is
used in a particular procedure (Ascent/Descent) to obtain the
relative channel-to-channel calibration error for channels 273
nm to 306 rim. These estimated errors are combined with
results from the Pair Justification procedure to correct the
sun-view diffuser drift in calibration from November 1978 to
February 1987 for the shorter wavelength channels that mea-
sure upper stratospheric ozone. Some preliminary re-calibrat-
ed Nimbus 7 SBUV data in 1989 is compared with the first
set of SSBUV measurements flown on the Space Shuttle.
1. INTRODUCTION
The 1988 ozone trends panel report found the Version 5
Nimbus 7 SBUV and TOMS total column ozone trends from
1979 to 1987 to be 3.5 percentage points more negative than
the Dobson measurements. The Nimbus SBUV profile shows
a negative 20 percent trend at 48 kilometers. The panel con-
cluded that the SBUV and TOMS trends were in error due to
an unaccounted degradation of the instruments' diffuser
(Fleig, 1990). Since this finding, a new procedure (designa-
tion Pair Justification) has been developed and published to
estimate and correct the diffuser calibration drift for both the
Nimbus 7 TOMS and SBUV total ozone. TOMS data have
been reprocessed and released as Version 6 data (Herman,
1991 ). This paper describes a separate new procedure to esti-
mate and correct the diffuser calibration drift for the Nimbus
7 SBUV upper stratospheric ozone (profile) channels.
The SBUV type instrument measures earth radiance and solar
irradiance at 12 channels with wavelengths from 255 nm to
340 nm. AIbedos (radiance/irradiance) from 273 nm to 306
nm are in general used to derive profile ozone. The 255 nm
channel is not used in Nimbus 7 because of nitric oxide emis-
sions. The four longest wavelength channels (313. 318, 331
and 340 nm) are paired to derive total column ozone: A-pair
(313 and 331 nm), B-pair (318 and 331 nm) and C-pair (331
and 340 nm).
The Pair Justification procedure uses a new pair of channels,
306 nm and 313 nm, designated the D-pair, for assessing total
ozone. In general, the 306 nm channel is a profile channel.
However, at equatorial latitudes with low total column ozone
and small sun angles, 306 nm "sees" the same column ozone
as the operational pairs (A, B and C). Pair Justification is
based on a particular comparison of D versus A-pair albedos.
Analytical leverage for the procedure exists because the D-
pair is significantly less sensitive to a wavelength dependent
calibration drift than the A-pair (smaller wavelength separa-
tion between the channels), plus the D-pair has a greater sen-
sitivity to ()zone. Nimbus 7 TOMS does not have a 306 nm
channel. Thus, the TOMS calibration drift is corrected on the
SBUV D-pair using coincident TOMS nadir-view albedo
measurements.
The Pair Justification procedure estimates calibration drift for
the Nimbus 7 SBUV channels 306 nm to 34(1 rim. When
combined with the Ascent/Descent Procedure using a particu-
lar comparison of ascent versus descent profile measurements
at or near the summer solstice, calibration drift for channels
273 nm to 302 nm are estimated and corrected to produce
Version 6 SBUV ozone data (upper level profile and total
ozone).
2. ASCENT/DESCENT PROCEDURE
During or near summer solstice at the higher latitudes, ozone
is measured twice in the same orbit, once in the ascent and
once in descent. Thus, the same latitude is measured twice at
widely different solar zenith angles. Figure l sh()ws the alli-
tude of normalized single scattered albedo sensitivity to
ozone for two measurements at the same latitude (72 degrees
north) with a solar zenith angles of 52 degrees for the ascent
and 87 degrees for descent. Also shown is the vertical distri-
923
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, , , , , , , J L
4,4
ASCENt
,T,ZA 52
015 ! 0
. #S 292
#6 297
_SCEN| #7 302 _-_
SZA 87 #8 306 \
• #9 313
m
O.S 1.0 0.8, 1.0
Fig. I.Altitude of normalized single _anered albedo sensitivity to ozone
for two measurements at 72 degrees latitude during summer solstice plus
the respective ozone density distribution (normalized).
bution of ozone. In Figure 1 the altitude of the 273 nm chan-
nel (the shortest channel) in ascent is very close to the altitude
of the 292 nm channel (a middle channel) in descent. At the
higher solar zenith angles, the altitude where a profile channel
is sensitive to ozone is displaced upward. Assuming an exact
overlap of these albedo sensitivities to ozone, a difference in
measured albedo from a reference standard (Ame a) versus
calibration error (E) can be described by two equations for the
ascent and descent channels, respectively, as follows:
Log(Amea273) = $273 x OZtrue + E273 (1)
Log(Amea292) = $292 x OZtrue + E292 (2)
where S is the logarithm of albedo sensitivity to changes in
ozone and OZtrue is the difference in "true" or actual ozone
in the region of overlap from the reference standard. With a
"close" overlap of albedo sensitivity to ozone ascent $273is
essentially equal to descent $292 and Equation 1 and 2
reduces to:
E292 - E273 = Log(Amea292) - Log(Amea273 ) (3)
where E292 - E273 is the calibration error of 292 nm relative
to 273 nm. The analytical leverage for computing a relative
channel-to-channel error from Equation (3) is the result of
different channels "seeing" the same ozone. As a practical
matter, the above formulation is not completely independent
of profile ozone. In order to characterize the altitude of the
sensitivities, an assumed reference standard ozone profile is
required. Furthermore, a particular set of ascent and descent
measurements at a particular latitude band may or may not
have a set of"very closely" overlapping sensitivities. Thus, a
matrix fl_rmulation is derived which includes an ozone profile
retrieval (OZRET) and theoretical albedo calculation (ALB)
and the following overall relationships:
Atrue(ascent ) = ALBIOZRET(Atruc(descent)) 1 {4)
Log/Amea(ascent) ] = Log[Atrue(ascent)] + E (5)
Log[Amea(descent) 1 = Log[Atrue(descent)] + E (67
where Atrue is column matrix of "true" albedos divided by
albedos for the reference standard profile, Ame a is the mea-
sured albedos divided by albedos for the reference standard
profile and E is the calibration error. Equation 4, 5 and 6 art"
combine to derive and expression for the calibration error (E).
A computer code to estimate E uses routines from the Version
5 Profile Processing Algorithm for the OZRET and ALB cal-
culations and ,sets of average ascent and corresponding decent
measurements (two week, two degree latitude averages).
To test and characterize the computer code formulation, the
theoretical albedo calculation (ALB) is u_d to compute sim-
ulated albedos for a defined ozone profile and simulated cali-
bration error. In other words, the simulated albedos are theo-
retical albedos computed from a known ozone profile and cal-
ibration error. In instead of the measured albedos, these sim-
ulated albedos are used as input to the computer code formu-
lation. The computed calibration error is compared with the
simulated calibration error. Figure 2 shows the results using a
simulated albedos computed with a calibration error that is a
linear function of channel wavelength and ozone profiles
which are uniformly le_ (same fractional decrease at all alti-
tudes) than the standard reference. When an ozone profile is
the same as the reference standard profile used in the formu-
lation, there is no difference in the calibration error computed
by the matrix formulation and the simulated calibration error.
However, as Figure 2 shows, when the profile is different
from the standard reference, the computed calibration error is
not the same as the simulated. On the other hand, Figure 2
2 i
1 -
0 I
-2-
-3- CALIBRATION
-4 - _ -)0,0 % ERROR
-7-
-8-
--9-
-IO -
-13 -
-14 l
aT0
WITH UNIFORM CHANGE IN OZONE;
ACCURATE CHANNEL-TO-CHANNEL ESTIMATE
BUT INACCURATE ABSOLUTE VALUE
290 310 33o
WAVELENGTH (NM)
Fig. 2. Computed c:dibration errors using simulalcd albedos from a sim-
ulated calibration error which is a linear function of wavelength and from
ozone profiles lh_tl are uniformly less than the reference s_tndard prolilc.
924
showsthatherelativechannel-to-channelcalibration error is
accurately determined. Figure 3 shows the results for a simu-
lated calibration error that is a non-linear function of channel
wavelength and "bump" changes in profile ozone. A "bump"
has minus 20 percent change in ozone in the designated
Umkehr Layer and minus 10 percent change in the layer
above and the layer below. As with the uniform differences
in ozone in Figure 2, the "bump" ozone results in large differ-
ences in computed versus simulated calibration error but the
relative channel-to-channel calibration error has useful accu-
racy.
3. CALIBRATION DRIFT FOR PROFILE CHANNELS
Figure 4 shows how the relative channel-to-channel calibra-
tion error computed by the formulation can be combined with
other procedures to develop a "total calibration error" for all
channels. Total calibration error includes both a "Drift" and
an "Initial" calibration error. As discussed above, to-date
only the drift in calibration for the longer total ozone wave-
lengths has been determined by Pair Justification. Thus, the
drift in the computed relative channel-to-channel calibration
error for channels 273 to 306 nm is joined to the results of
this Pair Justification to estimate the calibration drift for all
the channels. A calibration correction for this estimated drift
is used to produce Version 6 Nimbus 7 SBUV ozone profiles.
4. NIMBUS 7 SBUV CALIBRATION DRIFT RESULTS
For Nimbus 7 SBUV, only the northern hemisphere summer
solstice data is used. In the southern hemisphere, there
appears to be a instrument temperature anomaly as the satel-
lite comes out of darkness causing some differences between
the ascent versus the descent orbit measurements. Work is
4
3
2
i
o-1.
>
_. -2.
_ -3-
_ -4.
_ -5-
-2"
_-3
_-4
#9#10
#8 ERROR
"t. CENTER LAYEROFOZONECHANGE
270 290
_ITO #2
310 330
WAVELENGTH (NM)
Fig. 3. Computed calibration errors using simulated albedos from a sim-
ulated calibration error which is a non-linear traction of wavelength and
from ozone profiles that have "bump" reductions in ozone from the reti:r-
ence slandard profile.
RELATIVE I
CHANNEL-TO-CHANNEL I
CAUBRATION
ERROR
I
°.,-,. r/ ",NmA,"I
CALIBRATION _-- _ CALIBRATION I
e.ROR Jt EnRo. 1
TOTAL I
CAUBRATION
• ERROR .
Fig. 4. Overall procedural steps to derive an estimated tolal calibration
error.
planned to further investigate this issue. In the northern
hemisphere, individual scan albedo data is averaged for two
degree latitude bands during two week periods. Figure 5
shows the computed Version 5 relative channel-to-channel
calibration error from six sets of these averages during the
1986 northern summer solstice. As with the simulation, there
are large differences between the different averages but a con-
sistent relative channel-to-channel error. The drift in calibra-
tion error is computed by subtracting the result of each year
from the average of years 1980 and 1981. Figure 6 shows the
computed drift in 1986 from the average of the same six data
set in Figure 5. Also shown is the drift for the longer wave-
length channels from Pair Justification. The relative channel-
to-channel calibration error from 273 nm to 306 nm is con-
nected to the Pair Justification results. Figure 7 shows the
resulting estimated drift for all channels for the month of June
from 1985 to 1989.
:=
Fig. 5. Computed relative ch_mnel-to-ch_mnel calihralian error from
two-week, two-degree-latitude average ,'dbedos.
925
-2 -_ a- ---x''a'-'-----_
-4 -_ Q 0NFTIN_NIE 1_C, AU_IAl_ONERROFI(306r0340_)
-9 + _E_ENCE W¢_JNE I mB RELAT1VEC_tNCN_-TO-C_ANN_ CAIJa_T1C_
-_0 _ FROM JUNE _W0 P_ID JUNE lie1MF.AN (273 TO 3_11rim)
-+1
_ -12 _
-_4 -t
-16 i i
2;'0 29o 31o _3o
WAVELENGTH (NM)
Fig 6. Drifl m relative channel-lo-ch,'mnel c:dibralion error versus drift
in longer "o.,avelcngthsfrom pair juslificalion
The SSBUV instrument flown on the Space Shuttle is
designed to validate the calibration of SBUV type instru-
ments. Its first flight was in October 1989. Figure 9 shows
the average differences in SSBUV versus Nimbus 7 Version 5
(Uncorrected) SBUV coincident albedos. Also shown is a
preliminary estimated calibration drift from June 1979 to
October 1989. However, the comparison of SSBUV and the
estimated drift are not fully complimentary. As shown in
Figure 4, the total calibration error also needs an estimate of
the "Initial Error". In the case of Nimbus 7, the first summer
solstice is eight months after launch. Thus. an estimate of the
"Initial Error" using June 1979 data includes both an estimat-
ed "'at launch" calibration error and the drift from launch to
June 1979. The procedure to estimate "'Initial Error" is
pre_ntly under development.
Starting in March 1987, the Nimbus 7 SBUV measurements
developed a large increase in noi_ due to non-synchroniza-
tion of the chopper wheel. The evidence suggest there are
greater inaccuracies in estimating the calibration error for this
data. A procedure to correct the non-synchronization is
presently under development. Thus. the data shown in Figure
7 for 1987 through 1989 is preliminary and has larger uncer-
tainties than the pre-March 1987 data.
For the pre-March 1987 data, the largest uncertainty in the
estimated calibration drift for 273 nm is the uncertainty in
306 nm from Pair Justification. The second-largest uncertain-
ty is the uncertainty in solar zenith angle determination. At
mid-latitudes, the combined uncertaintiesgives an estimated
uncertainty in ozone derived from the 273 nm channel of +6.4
percent ozone (95 percent confidence). For ozone derived
from 306 nm channel, the uncertainty is slightly less at +5.0
_ercent ozone.
2a-
2s-4 FORJUNE
-_ 2+- FROMD-PAIRUNEAR
FR A _T/_ _'V ITH wAVELENGTH
22 - OM SCENT/
=- SSUMPTION
_ 2o- - 1989 DESCENT
o+ '
_=z _,,. lg88z _ CATION
o:_ lo-
> a-
z
o I i , i
a?0 29o 310 33O
WAVELENGTH (NM)
Fig 7+ Estimated c:dibralion drift for'all channels used for Version 6
calibralion correclion l_r the month of tune years 19X5 through log0;
l)relimmary rcsull for 1087 Ihrough It}X0.
926
30
t o ONLYDRIFTIN OCT 11NI9
2S
¢_
o _
-s i
WAVELENGTH (f_M)
Fig 8. Comparison of total calibration error in October 1089 estimated
by SSBUV versus estimated calibration drift by Ascent/Descent Procedure.
5. CONCLUSION
The drift in Calibration error for Version 5 Nimbus 7 SBUV
has been corrected for data from November 1979 to February
1987. The uncertainty in mid-latitude profile ozone derived
by the shortest wavelength channel is +6.4 percent (95 per-
cent confidence). Preliminary data for total calibration error
for October 1989 shows a relatively close comparison with
the first SSBUV flight
REFERENCES
Fleig, A.,J., etal.. An Assessment of Long-Term Drift in
SBUV Total Ozone Data, Based on Comparison with the
Dobson Network, Geophvs. Res. Lelt., 13. 1359, 1986.
Herman, J.R., etal., A New Self-Calibration Method Applied
to TOMS and SBUV Backscattered Ultraviolet Data to
Determine Long-Term Global ()zone Change. J. Geophys.
Res., Wol 96. No D4, pg 7531-7545, April 2(I. 1991.


Procedures to validate/correct calibration error in solar backscattered ultraviolet instruments

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