I N S T I T U T D ' . A E R O N O M I E S P A T I A L E D E B E L G I Q U E 3 - A v e n u e C i r c u l a i r e
B • 1 1 8 0 B R U X E L L E S
A E R O N O M I C A A C T A
A - N° 2 8 1 - 1 9 8 4
A sample performance of the grille spectrometer aboard Spacelab
by
M . - P . LEMAITRE, J. LAURENT, J. BESSON, A. GIRARD, C. LIPPENS, C. MULLER, J. VERCHEVAL and M. ACKERMAN
B E L G I S C H I N S T I T U U T V O O R R U I M T E - A E R O N O M I E
3 - R i n g l a a n
B • 1 1 8 0 B R U S S E L
FOREWORD
This is a draft of a short article which will be part of a special issue of the "Science" journal to be dedicated to the Spacelab One mission.
AVANT-PROPOS
Ce texte sera proposé pour un numéro spécial de la revue
"Science" consacré h la première mission Spacelab.
VOORWOORD
Deze tekst zal deel uitmaken van een speciaal nummer van het tijdschrift "Science" dat zal gewijd worden aan de eerste vlucht van Spacelab.
VORWORT
Dieser Text wird vorgestellt werden für eine Spezialausgabe
der Zeitschrift "Science", geschrieben für den erten Flug Spacelabs.
A SAMPLE P E R F O R M A N C E OF THE G R I L L E S P E C T R O M E T E R A B O A R D S P A C E L A B
by
M . - P . L E M A I T R E * , J. L A U R E N T * , J. B E S S O N * , A . G I R A R D * , C. L I P P E N S * * , C. M U L L E R * * , J. V E R C H E V A L * * and M. A C K E R M A N * *
Abstract
This short article on preliminary results of the grille spectro- meter flown on the Spacelab One mission intends only to show the quality of the obtained spectral data. Among ten trace species observed in the atmosphere, methane has been choosen for the spectacufar-^spect of Q branch of the intense v3 band. Other molecules, as well as the atmospheric significance of the results, will be discussed elsewhere.
Résumé
Ce court article sur les résultats préliminaires du spectro- mètre a grille embarqué sur la première mission Spacelab se limite à montrer la qualité des données spectrales obtenues. Parmi dix constituants minoritaires observés dans l'atmosphère, le méthane a été choisi pour l'aspect spectaculaire de la branche Q de sa bande intense Vg. D'autres molécules, ainsi que la signification atmosphérique des résultats, seront discutés ailleurs.
* Office National d'Etudes et de Recherches Aérospatiales, F-9230 Châtillon, France.
** Belgium Institute for Space Aeronomy, Avenue Circulaire 3, B-1180 Brussels, Belgium.
S a m e n v a t t i n g
Dit korte a r t i k e l o v e r v o o r l o p i g e r e s u l t a t e n v a n de r a s t e r - s p e c t r o m e t e r tijdens de eerste v l u c h t v a n Spacelab wil enkel de kwaliteit v a n de bekomen s p e c t r a aantonen. O n d e r de tien m i n o r i t e i t s - c o n s t i t u e n t e n waargenomen in de atmosfeer w e r d methaan u i t g e k o z e n omwille v a n het s p e c t a c u l a i r aspect v a n de Q - t a k v a n zijn v^ b a n d . De a n d e r e molecules en de b e t e k e n i s v a n de r e s u l t a t e n v o o r de atmosfeer z u l l e n e l d e r s b e s p r o k e n w o r d e n .
Zusammenfassung
Diese k u r z e A r t i k e l ü b e r den p r o v i s o r i s c h e n Resultaten vom G r i l l e - S p e k t r o m e t e r m i t g e f ü h r t w ä h r e n d des e r s t e n F l u g e s S p a c e l a b s , b e s c h r ä n k t sich die Qualität d e r bekommen s p e k t r a l e n Daten nach z u weisen. Z w i s c h e n 10 M i n o r i t ä t s b e s t a n d t e i l e beobacht in d e r A t m o s p h ä r e , w u r d e das Methan gewählt f ü r den a u f f a l l e n d e n A s p e k t des Q - Z w e i g e s seines i n t e n s i v e n vg B a n d e s . A n d e r e Molekülen wie auch., die atmo- s p h ä r i s c h e B e d e u t u n g d e r R e s u l t a t e n , w e r d e n a n d e r s w o b e s p r o c h e n w e r d e n .
Infrared absorption spectrometry of the atmosphere using the sun as a light source at sunrise or sunset has, for the past 15 years, proven to be a powerfull method for studying vertical distributions of trace species (1). The largest possible amount of light absorbing molecules is so observed on the optical path tangent to the earth surface at various altitudes allowing the deconvolution of very low concentrations versus altitude. Much information has already been gathered through this method from high altitude platforms such as air- craft and balloon. An orbiting platform provides access to higher altitudes and to geographic locations leading to a nearly global coverage in various seasons of great interest for geophysical purposes. While on previously used platforms the earth rotation provides the altitude scan at sunrise or at sunset it is achieved at a much higher rate from orbit by the spacecraft motion itself requiring a fast spectral scanning, thus high throughput instrument.
The choice of instruments satisfying these requirements is rather limited (2). The grille spectrometer (3) is well adapted as eventually demonstrated by the first Spacelab flight. A single grille mounting, acting as entrance and exit light ports for the spectrometer was selected. The instrumentation description and operation (4) can be summarized as follows.
- Optics : a two axis steerable frontal plane mirror tracks the sun in front of the 30 cm aperture, 6 meters focal length Cassegrain telescope imaging the sun on the grille which intercepts a square portion of the solar image (8 arc minutes). The spectrometer uses a 59 grooves per mm grating illuminated by a parabolic mirror oscillating at 436 Hz with an amplitude of ± 20 arc seconds, its average position being controlled within 5 arc seconds. The exit light flux, split in two beams,' reaches through interference filters the two detectors ( I n S b , .2.5 to 5.5 pm and HgCdTe, 2.5 to 10.5 pm). The spectral resolving power was 1.3 x 104
(instrumental line width at half peak height).
- Electronics : the electronics in the Spacelab module interfaces the pallet instrument with the Command and Data Management S u b s y s t e m ( C D M S ) and the high rate multiplexer ( H R M ) . Using data originating from the orbiter (time, attitude, orbit parameters) and from Spacelab (timeline, on board and g r o u n d commands, sun ephemeris) it manages the execution of the stored as well as in-flight updated measurement programs. The electronics on the pallet instrument provides the electro- mechanical control and the signal detection and formating functions. The main role of the crew for this flight was to check the instrument wave- length calibration, spectral resolution and sensitivity b y monitoring the display of a calibration spectrum generated inside the spectrometer b y means of a calibration lamp shining t h r o u g h a gas cell. The mission specialist in charge of this task performed a wavelength alignment 12 hours after launch.
The pallet instrumentation weighting 122.8 k g , stood 1.8 meter h i g h , occupying 0.7 square meter. The weight of the module equipment was 15 kg. The data rate in operation was 51.6 kbits per second.
Due to the multidisciplinary philosophy of Spacelab One only 25 solar occultation r u n s were allocated. Due to the "launch window time-season" combination the r u n s were scheduled in the first d a y s of the mission since the full orbit was in sunlight d u r i n g the last five d a y s . Performances took place at sunset in the northern hemisphere at latitudes ranging from 56° to 30°. The sunrise observations took place at high southern latitudes providing information pertinent to inter- hemispheric-seasonal variations of the observed atmospheric species;
thermospheric C O , for instance.
In addition to solar infrared absorption features, many of which had not been observed before, telluric spectral absorption due to C O and CO£ spectra were observed at thermospheric tangent heights ( H ) (H > 85 km); 0 - , H - 0 , C H . and N?0 added their contributions in
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the mesosphere (H > 50 km) while the strongly coupled molecules NO - N 02 and HCI - HF were simultaneously observed by pairs in the same stratospheric air parcels.
The scope of this article being limited and the data interpretation still being a preliminary state, we will present only the first results on methane in the mesosphere, where it . had never been measured before.
Since the only atmospheric methane source is at ground level, it has been used by atmospheric modelers as a vertical transport indicator. Its increasingly efficient oxidation at higher altitudes in the stratosphere leads to a continuous reduction of its volume concentration with altitude. CH^ absorptions were observed from Spacelab in three sunset runs at two different spectral intervals in the 3.3 pm v^
fundamental band. A spectrum of most of its Q branch, recorded in 2 seconds while the line of sight to the sun center was grazing at 26 km altitude above the geoid's sea surface is shown in Figure 1a. Figure 1b shows a synthetic spectrum of the same spectral region computed using the results of the inversion of the measured absorptions.
Figure 2a shows the measured equivalent width of the absorption feature at 3017.8 cm . It is essentially due to five absorption lines which parameters (5) have been used to deduce, the vertical distribution shown on figure 2b. The verification of this result using synthetic spectra of the whole region confirms it except below about 30 km where it leads to slightly lower values, this discrepancy could originate from the saturation of these lines at very low altitudes.
Figure 2c shows the equivalent widths versus altitude of the C H .
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manifold centered at 2979 cm and figure 2d shows the corresponding inverted data and allows the comparison between the volume concentra- tion profiles deduced in two wavelength ranges.
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Figure 1 A. Spectrum of the CH^ Q branch of the 3.3 pm band recorded in 2 seconds on December 3, 1983 at 3 hours, 47 minutes and 49 seconds G . M . T . The signal amplitude is shown versus wavenumbers in cm . B. Synthetic spectrum of the same absorption feature computed using the AFGL molecular parameters (5) and the CH^ vertical distribution deduced from the spectra themselves.
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-1
Figure 2 A : Equivalent width, W, in cm of the absorption feature at 3017.8 cm versus tangent altitude (km) of the line of sight to the sun center. B. Inverted C H4 volume mixing ratio computed on the basis of the U . S . standard atmosphere (1966) for the atmospheric temperature and total number density profile versus altitude. C. Equivalent width of the 2979 cm C H4 manifold versus tangent altitude. D. Deduced C H . vertical distribution.
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These measurements taken at low latitudes agree with the previous data (6) reported for the 25° to 35° latitudes band in the stratosphere. At higher altitudes, where the chemical oxidation processes become less efficient an other destruction process must play a role in order to explain the mesospheric concentration decrease with altitude. The photodissociation of CH^, in particular' by the solar Lyman a radiation plays a dominant role in modeling this aspect as will be described elsewhere.
We wish to acknowledge the support of the Belgian Ministries of Education and of Science Policy as well as the French Centre National d'Etudes Spatiales and the French Ministry of Research and Industry.
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1. M. A C K E R M A N , in Atmospheric P h y s i c s from Spacelab, J . J . B u r g e r , A . Pedersen and B . Battrick, Ed. ( D . Reidel Publishing Company, Dordrecht-Holland, 1976), pp. 107-116.
2. A . G I R A R D and P. J A C Q U I N O T , in Advanced Optical Techniques, A . C . S . Van Heel, Ed. (North-Holland Publishing C o . , Amsterdam, 1967), pp. 73-121.
3. A . G I R A R D , A p p l . Optics, 1, 79 (1963).
4. J. L A U R E N T , M . - P . L E M A I T R E , C . L I P P E N S and C . M U L L E R , L'Aéronautique et l'Astronautique, 98, 60 (1983).
5. R O T H M A N , L . S . , G A M A C H E , R . R . , B A R B E , A . , G O L D M A N , A . , G I L L I S , J . R . , B R O W N , L . R . , T O T H , R . A . , F L A U D , J . M . , C A M Y - P E Y R E T , C . , A p p l . Optics, 22, 2247 (1982).
6. The Stratosphere 1981 T h e o r y and Measurements ( N A S A / G o d d a r d Space Flight Center, Greenbelt, Md. 20771, 1982), pp. 1-37.
