The Meteor-3/TOMS instrument is the second in a series of Total Ozone Mapping Spectrometers (TOMS) following the 1978 launch of Nimbus-7/TOMS. TOMS instruments are designed to measure total ozone amounts over the entire earth on a daily basis, and have been the cornerstone of ozone trend monitoring. Consequently, calibration is a critical issue, and is receiving much attention on both instruments. Performance and calibration data obtained by monitoring systems aboard the Meteor-3 instrument have been analyzed through the first full year of operation, and indicate that the instrument is performing quite well. A new system for monitoring instrument sensitivity employing multiple diffusers has been used successfully and is providing encouraging results. The 3-diffuser system has monitored changes in instrument sensitivity of a few percent despite decreases in diffuser reflectivity approaching 50 percent since launch

Ahmad, Zia

Cebula, Richard P.

Jaross, Glen

Krueger, Arlin J.

English

NASA Technical Reports Server

N95- 11128
POST LAUNCH PERFORMANCE
OF THE
METEOR-3/TOMS INSTRUMENT
Glen Jaross, Zia Abroad, and Richard P. Cebula
Hughes STX Corporation
Lanham, Maryland 20706, USA
Arlin J. Krueger
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771, USA
Abstract
The Meteor-3/TOMS instrument is the sec-
ond in a series of Total Ozone Mapping Spectrome-
ters (TOMS) following the 1978 launch of Nimbus-
7/TOMS. TOMS instruments are designed to mea-
sure total ozone amounts over the entire earth on a
daily basis, and have been the cornerstone of ozone
trend monitoring. Consequently, calibration is a crit-
ical issue, and is receiving much attention on both
instruments.
Performance and calibration data obtained
by monitoring systems aboard the Meteor-3 instru-
ment have been analyzed through the first full year
of operation, and indicate that the instrument is per-
forming quite well. A new system for monitoring in-
strument sensitivity employing multiple diffusers has
been used successfully and is providing encouraging
results. The 3-diffuser system has monitored changes
in instrument sensitivity of a few percent despite de-
creases in diffuser refiectivity approaching 50 percent
since launch.
Introduction
The Meteor-3/TOMS instrument was launch-
ed aboard a Soviet Meteor-3 spacecraft on August
15, 1991. The TOMS instruments are part of a class
of ozone monitoring instruments which measure at-
mospheric albedo through the Backscatter Ultravi-
olet (BUV) technique (Klenk et al. 1982). As with
other BUV instruments, M3/TOMS is in a polar orbit
(82.5 ° inclination), though at the somewhat higher
altitude of 1200 km. Unlike other BUV instrument
platforms, the orbital plane of the Meteor-3 space-
craft precesses relative to the sun-earth vector with
a period of approximately 212 days. This rapid pre-
cession rate impacts the ability to monitor and cali-
brate the instrument, as well as having implications
for ozone retrieval.
With the exception of its solar diffusers, the
TOMS instrument optics are identical to those of its
predecessor (NT/TOMS) aboard the Nimbus-7 space-
craft (Heath et al. 1975). While some of the elec-
tronics have been updated, major features such as
the spectrometer design, wavelength channels, cross-
track scanning, and field of view remain unchanged
on the newer instrument. Three major calibration
and monitoring functions are used aboard both in-
struments. Two of these, the wavelength and elec-
tronic calibration systems are completely unchanged
on M3/TOMS. The principles of solar irradiance mea-
surement are the same as on N7/TOMS but are func-
tionally different with a change from one to three
diffuser plates. Since this change represents a sig-
nificant improvement over N7/TOMS for calibration
of instrument sensitivity, and hence in the accuracy
of ozone measurements, a more detailed discussion of
the solar irradiance measurements will follow.
Wavelength Calibrations
The wavelength calibration system is designed
to monitor the wavelength stability of the spectropho-
tometer. Entrance slits separate from those used for
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ozone retrieval are used to sample light from a Hg
lamp at 4 wavelengths near the 296.7 nm emission
line. Changes in the relative signal levels measured
at each wavelength are an indirect measure of a com-
mon shift in the band center of the ozone channels.
Calibrations are made once per day and are capa-
ble of resolving day to day variations of 2- ]0 -4 rim.
Overall variations as much as .04 nm, caused by ther-
mally driven changes in the instrument housing or
photomultiplier sensitivity, have been observed. Left
uncorrected, the observed variations could cause as
much as 1.5% error in derived ozone.
Diffuser Calibrations
The BUV class instruments determine ozone
amounts by measuring the earth albedo at several
wavelengths. The six TOMS wavelengths Ai are nom-
inally 312.5 nm, 317.5 nm, 331.2 nm, 339.8 nm, 360.0
nm, and 380.0 nm. The albedo at each wavelength is
determined by the ratio I/F, where the earth radi-
ances I()t i) are measured during earth viewing mode
and solar irradiances F(Ai) are measured during so-
lar viewing mode. An important advantage of albedo
measurement is that changes in instrument sensitiv-
ity and solar flux cancel when F is measured with
sufficient frequency.
TOMS instruments utilize aluminum diffusers
to fill the instrument's field of view during solar irra-
diance measurements. The diffuser is the only optical
element which is not common to both earth and solar
measurements so the albedo determination depends
on accurate knowledge of the diffuser reflectivity. Ex-
perience has shown that reflectivity degrades with
diffuser exposure, either to sunlight or to the space
environmeni (Herman et al. 1990). While the N7/
TOMS instrument has no mechanism for measuring
diffuser refiectivity, subsequent SBUV/2 instruments
have had measurement systems added based on an
onboard Hg lamp. The dilemma faced with the N7/
TOMS instrument is that of obtaining frequent so-
lar measurements while minimizing the exposure of
its single diffuser. The task of separating the diffuser
from other instrument changes is difficult as well.
The method employed on the M3/TOMS in-
strument is that of the 3-diffuser system, similar tn a
system on the Solar Mesosphere Explorer (Rottman
et al. 1982) but unique among the BUV instruments.
The diffusers, referred to as Cover, Working, and Ref-
erence, are arranged as the sides of an equilateral tri-
angle and mounted on a carousel so that they can
be rotated into the instrument field of view on com-
mand. A given diffuser must be exposed during solar
measurements, but is protected while another is be-
ing used. Thus the amount of exposure that each
diffuser receives is varied such that the most infre-
quently measured undergoes little degradation while
the other two can be measured as frequently as de-
sired. As its name implies, the Cover diffuser is ex-
posed constantly and is viewed roughly 13 times per
day. The Working diffuser is viewed once per week
and the Reference diffuser twice per 212 days. The
reflectivity of the more frequently measured, and thus
more degraded, diffusers are calibrated relative to the
Reference by nearly simultaneous measurements of
the solar flux. Thus at a given time t, F(t) _x
Ru,(t,,)
- S,,.(O.R,,( 0
where S represents the signal measured by the instru-
ment with a given diffuser and R is its time dependent
reflectivity. Integrated exposure amounts of the three
diffusers are given in the table below.
Diffuse Exposure at One Year
total exp. equivalent solar
(hours) exp. (hours 1
Cover 8800 _ 750
Working 2.2 1.72
Reference 0.35 0.34
The ratio of the solar signals S to S, on day
129 are shown in Figure 1 as a function of time since
launch for each of the three diffusers. While only the
shortest wavelength channel is shown, signals in the
other channels are qualitatively the same. The rela-
943
DaySinceLaunch
1.02
1,00
0.98
0.96
0.94
.... , .... i .... i .... i .... f .... i ....
+ (B}
o ........
++4"
..................... 0 ..................
+
.................................. +4.+ ...................
I + Working I +
.... I, °....Ref*r*"ctI....... ++ ....... ÷...,,°
++
,++.p+
0.9:' ''''' .... ' .... * .... ' .... ' .... ' ....
0 SO 100 150 200 250 300 350
Day Since Launch
Figure 1 Diffuser signals measured during the first year relative to the Reference measurement
of day 129; a) all three diffusers, b) Working and Reference diffusers alone.
tire frequency with which each diffuser is used is evi-
dent in the plot. Large gaps in the data occur during
periods when the instrument cannot view the sun due
to the orbital precession. The continuous exposure of
the Cover diffuser has lead to a significant drop in
its associated signal level, while Working and Refer-
ence signals have remained relatively stable during
the same time period. The degradation of the Cover
diffuser relative to the Workin9 diffuser, determined
from the ratio S,./S,,., is almost 50% by the end of
the period shown in Figure l(a). A plot of the ra-
tios in each channel, shown in Figure 2, illustrates
the strong wavelength dependence of Cover diffuser
degradation.
A similar technique has been applied to de-
termine Workin9 diffuser degradation. Close agree-
meut (< 0.25% change in S,L,/Sr) between the three
sets of coincident Workin9 and Reference measure-
ments seen in Figure lindicates little relative change
between the two. This evidence supports the hypoth-
esis that neither diffuser has undergone significant
degradation. Nevertheless Working diffuser signals
have dropped by about 8% since their first measure-
ment in November, 1991. WorkiTtg diffuser exposure
is not sufficient to cause significant degradation, nor
is the observed change wavelength dependent as with
the Cover diffuser. These facts suggest the downward
slope seen in Figure 1 is a result of changes in instru-
ment sensitivity. A drop in sensitivity affects solar
irradiance and earth radiance measurements equally
and so will not change the measured albedo I/F.
An area of concern is that the instrument
lacks a calibration system which detects diffuser degra-
dation unrelated to exposure• At present such degra-
dation is indistinguishable from a decrease in instru-
ment sensitivity. Designs for future TOMS instru-
ments supplement the multi-diffuser system with an
onboard Hg lamp to eliminate this ambiguity. If
the 8% drop seen in Figure l(b) is actually caused
by such a degradation rather than a drop in instru-
ment sensitivity, then the value of F will be incor-
rect. While such an error would not directly affect
ozone retrieval because of the wavelength indepen-
dence of the observed change, earth reflectivity de-
rived from M3/TOMS measurements might be high
by 10% over some areas of the globe. This error can
indirectly influence the derived ,_znne amnnnts by n_
much as 2%. Independent studies of M3/TOMS ra-
diances measured over Greenland and Antarctic ice
indicate the instrument sensitivity has decreased by
at least 8% during the first year of operation, thus
reaffirming the relation between diffuser exposure and
optical degradation.
944
o
L)
1.00
0.90
0.80
0.70
0.60
0.50
0.40
50
.... I .... I .... I .... | .... I ....
: 000 .
x sl=.s., %+_, i _A,,.._
r_ 1317 5 nr_ t3--_£ ÷+ i i aA
o ;--"- : _°"_"°"°' i _++.
+ _339.8 nq_ i X_ _ i 0
........• i
0 ]380.0 nm i ................................................
: E
• ,..I,.,,i,.:, J,:,,i:,,:l,, t
100 150 200 250 300 350
Day Since Launch
Figure 2 Cover diffuser reflectivity relative to
Working diffuser, at each of the 6 TOMS channels.
Summary
Monitoring and calibration systems aboard
Meteor-3/TOMS indicate that the instrument is op-
erating normally and yielding science data compara-
ble to those of the Nimbus-7/TOMS. The new dif-
fuser calibration system is performing as expected
and has shown itself to be robust against diffuser
degradation. Separation of diffuser related changes
from other inst/ument changes should be possible
with greater accuracy than on Nimbus-7/TOMS. Pre-
liminary analysis of the solar irradiance data indi-
cates that the most frequently expc, sed diff, ser has
degraded by as much as 50% and instrument sensi-
tivity has dropped by at least 8%.
Acknowledsements
This work was performed under NASA con-
tracts NAS5-29386, NAS5-31380, and NAS5-31755.
References
Heath, D.F., B.D. Henderson, A.J. Krueger, and H.A.
Roeder, 1975: The Solar Backscatter Ultravi-
olet and Total Ozone Mapping Spectrometer
(SBUV/TOMS) for Nimbus G. Opt. En_., 14,
323-331.
Herman, J.R., R.D. Hudson, and G. Serafino, 1990:
Analysis of the Eight-Year Trend in Ozone De-
pletion from Empirical Models of Solar Backscat-
tered Ultraviolet Instrument Degradation.
J. Geophys. Res., 95, 7403-7416.
Klenk, K.F., P.K. Bhartia, A.J. Fleig, V.G. Kaveesh-
war, R.D. McPeters, and P.M. Smith, 1982: To-
tal Ozone Determination from The Backscat-
tered Ultraviolet (BUV) Experiment.
J. of Appl. Meteor., 21, 1672-1684.
Rottman, G.J., C.A. Barth, R.J. Thomas, C.H.
Mount, G.M. Lawrence, D.W. Rusch, R.W.
Saunders, G.E. Thomas, and J. London, 1982:
Solar Spectral Irradiance, 120 to 190 nm, Oc-
tober 13, 1981 - January 3, 1982.
Geophys. Res. Letters, 9, 587-593.
945


Post launch performance of the Meteor-3/TOMS instrument

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