i s s n 0 0 6 5 - 3 7 1 3
I N S T I T U T D ' A E R O M 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 - 1180 B R U X E L L E S
A - N° 3 1 6 - 1 9 8 7
L a t i t u d e c o v e r a g e o f o b s e r v a t i o n s o f t h e m i d d l e a t m o s p h e r e b y s o l a r a b s o r p t i o n s p e c t r o m e t r y
f r o m a h e l i o s y n c h r o n o u s o r b i t
b y
J . V E R C H E V A l
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 0 N O . M I E
FOREWORD
This article will be published in "ESA Journal".
AVANT-PROPOS
Cet article paraîtra dans "ESA Journal".
VOORWOORD
Dit artikel zal in "ESA Journal" gepubliceerd worden.
VORWORT
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LATITUDE COVERAGE OF OBSERVATIONS OF THE MIDDLE ATMOSPHERE BY SOLAR ABSORPTION SPECTROMETRY FROM A HELIOSYNCHRONOUS ORBIT
by
J. VERCHEVAL
Abstract
The object of the present article is to analyse the conditions in which atmospheric measurements by solar absorption spectrometry could be performed from a space platform put on a heliosynchronous orbit. Those conditions, and particularly the annual coverage in latitude of the observations, essentially depend on the mean local time at the ascending node of the orbit.
Résumé
L'objet de cet article est d'analyser les conditions dans les- quelles des mesures atmosphériques par spectrométrie d'absorption solaire pourraient être effectuées à partir d'une plate forme spatiale placée sur une orbite héliosynchrone. Cés conditions, et en particulier la couver- ture annuelle en latitude des observations, dépendent essentiellement du temps local moyen au noeud ascendant de l'orbite.
Samenvatting
Het doel van dit artikel is de voorwaarden te analyseren waarin atmosferische metingen door middel van zonneabsorptiespectrometrie zouden kunnen gedaan worden vanuit een ruimteplatförm dat in een helio- synchrone baan geplaatst is. Die voorwaarden, en in het bijzonder de jaarlijkse zone van geografische breedten voor de waarnemingen, hangen hoofdzakelijk af van de gemiddelde plaatselijke tijd bij de klimmende knoop van de baan.
Zusammenfassung
Die Absicht dieses Artikels ist die Bedingungen zu analysieren worin atmosphärische Messungen durch Sonnenabsorptionspektrometrie würden getan können werden aus einer Raumplattform der in einer heliosynchronen Bahn gesetzt ist. Diese Bedingungen, und besonders das jährliche Gebiet der geographischen Breiten für den Beobachtungen hängen hauptsächlich von die mittlere lokale Zeit am aufsteigenden Knoten der Bahn ab.
02
1 . INTRODUCTION
Measurements of a t m o s p h e r i c t r a c e g a s e s in t h e middle atmosphere were performed d u r i n g t h e f i r s t S p a c e l a b m i s s i o n , on board t h e s p a c e s h u t t l e from November 28 t o December 8 , 1983. The measuring method p r i n c i p l e was a b s o r p t i o n s p e c t r o m e t r y , in the i n f r a r e d , u s i n g the Sun a s a l i g h t s o u r c e d u r i n g s u n s e t o r s u n r i s e p e r i o d s . S e v e r a l d e s c r i p t i o n s of t h e i n s t r u m e n t a l s e t - u p and t h e s c i e n t i f i c programme have a l r e a d y been p u b l i s h e d [1-M]. A few thousand s p e c t r a were o b t a i n e d d u r i n g 18 s o l a r o c c u l t a t i o n p e r i o d s and t h e f i r s t r e s u l t s were h i g h l y s i g n i f i c a n t [ 5 - 1 1 ] . The p e r f o r m a n c e of such o b s e r v a t i o n s d u r i n g o c c u l t a t i o n p e r i o d s depends on t h e c h a r a c t e r i s t i c s of t h e o r b i t and, in p a r t i c u l a r , on t h e o r i e n t a t i o n of i t s p l a n e with r e s p e c t t o t h e d i r e c t i o n of t h e s u n . Moreover, the l a t i t u d e coverage of the measurements a l s o depends on t h e s e g e o m e t r i c a l c o n d i t i o n s .
As p a r t of t h e American s p a c e s t a t i o n programme, a ' s p a c e p l a t f o r m could be s e t on a h e l i o s y n c h r o n o u s o r b i t and the o p p o r t u n i t y t o conduct a t m o s p h e r i c measurements by a b s o r p t i o n s p e c t r o m e t r y from such an o r b i t c a n n o t be n e g l e c t e d . For t h i s r e a s o n , i t i s i n t e r e s t i n g t o see what w i l l b e , f o r a h e l i o s y n c h r o n o u s o r b i t , t h e p o s s i b i l i t i e s f o r o c c u l t a t i o n and the r e s u l t i n g l a t i t u d e coverage of the o b s e r v a t i o n s . T h i s a n a l y s i s i s p r e s e n t e d in t h i s p a p e r .
2 . DESCRIPTION OF THE ORBIT
. Due t o t h e f l a t t e n i n g of t h e E a r t h a t t h e p o l e s , a s a t e l l i t e o r b i t h a s a p r e c e s s i o n motion of i t s p l a n e c h a r a c t e r i z e d by a l i n e a r v a r i a t i o n of t h e r i g h t a s c e n s i o n of i t s a s c e n d i n g node (J2) with t i m e . The r a t e of change of t h i s o r b i t a l e l e m e n t , e x p r e s s e d in d e g r e e s per day, can be w r i t t e n a s :
AO R 3 . 5
-T-— = - 9 . 9 7 ( — ) ( 1 - e ) * cos i (1 ) At a
where Rg d e n o t e s t h e e q u a t o r i a l r a d i u s of t h e E a r t h ; a , e , and i a r e r e s p e c t i v e l y t h e s e m i - m a j o r a x i s , t h e e c c e n t r i c i t y and t h e i n c l i n a t i o n of t h e o r b i t .
A h e l i o s y n c h r o n o u s o r b i t h a s t h e p a r t i c u l a r i t y t o p r e s e n t a p r e c e s s i o n motion which c o u n t e r b a l a n c e s t h e mean a p p a r e n t m o t i o n of t h e sun on t h e c e l e s t i a l s p h e r e ( 0 . 9 8 5 6 2 d e g / d a y ) , i n s u c h a way t h a t t h e E a r t h ' s r e g i o n s a r e a l w a y s o v e r f l o w n by t h e s p a c e c r a f t a t t h e same mean l o c a l t i m e s . The o r b i t which c o u l d be a d o p t e d would be c i r c u l a r a t a n a l t i t u d e of 1000 k i l o m e t e r s w i t h , t h e r e f o r e , an i n c l i n a t i o n of 9 9 . 4 ° a s a c o n s e q u e n c e of r e l a t i o n ( 1 ) .
3 . CONDITION FOR OCCULTATION
The s p e c t r o s c o p i c o b s e r v a t i o n of t h e a t m o s p h e r e , i s o n l y p o s s i b l e when t h e r e i s o c c u l t a t i o n of t h e sun by t h e e a r t h a s s e e n f r o m t h e s p a c e p l a t f o r m . The p o s s i b i l i t i e s of o c c u l t a t i o n m a i n l y depend on t h e 3 a n g l e which i s d e f i n e d a s t h e a n g l e b e t w e e n t h e s o l a r v e c t o r (OA) and i t s p r o j e c t i o n i n t o t h e o r b i t p l a n e (OA' ) , ' t a k e n a s p o s i t i v e when t h e s o l a r v e c t o r h a s a normal component t o t h e o r b i t a l p l a n e d i r e c t e d a l o n g t h e a n g u l a r momentum v e c t o r of t h e p l a t f o r m ( f i g . 1 ) . The p o i n t s of t h e o r b i t c o r r e s p o n d i n g t o t h e b e g i n n i n g and t h e end of an o c c u l t a t i o n a r e f i x e d by two p a r t i c u l a r v a l u e s u1 and u2 of t h e a r g u m e n t of l a t i t u d e u which i s an a n g l e measured i n t h e o r b i t p l a n e f r o m t h e a s c e n d i n g node N, p o s i t i v e i n t h e d i r e c t i o n of o r b i t t r a v e l . For a c i r c u l a r o r b i t w i t h r a s r a d i u s , i t c a n be shown t h a t f o r an o b s e r v a t i o n a t a l t i t u d e z t h e s e v a l u e s a r e g i v e n by [ 1 2 ]
• _ . . 2 1 / 2
, \ , (R + z) r . f o,
c o s ( u , - u ; ) = c o s ( u0 - u ) = - „ - r - 1 ( 2 )
1 ©. 2 0 r c o s 8 (R + z)
where R i s t h e E a r t h r a d i u s and u i s t h e a r g u m e n t of l a t i t u d e of t h e © p r o j e c t i o n of t h e s o l a r v e c t o r on t h e o r b i t p l a n e ( f i g . 1 ) .
T h e r e f o r e , o c c u l t a t i o n occurs i f the c o n d i t i o n
2
1/ 2R + Z r - 1 < 1 2
r cos 0 (R + z)
i s s a t i s f i e d . This c o n d i t i o n becomes :
| B| < 90° - D
where D denotes t h e s o l a r d e p r e s s i o n angle below the l o c a l h o r i z o n t a l plane given by
n
R+z
cos D = r
For a heliosynchronous o r b i t a t 1000 km, the d e p r e s s i o n angle a t the beginning and the end of an o c c u l t a t i o n (z=0) i s 30.2 d e g r e e s . Hence, t h e s p e c t r o s c o p i c experiment w i l l be f e a s i b l e i f
| B| < 59.8° (3) I f | B| g e t s above t h i s v a l u e , t h e r e w i l l be no t e r m i n a t o r s ( f u l l sun
o r b i t ) .
Considering the s p h e r i c a l t r i a n g l e K North A in f i g u r e 1, t h e 3-angle can be expressed in the form :
s i n B = cos i s i n 6 + s i n i cos 6 s i n (fi - a ) (4)
© © ©where a and 6 denote t h e e q u a t o r i a l c o o r d i n a t e s of the sun. The © © i n c l i n a t i o n i being f i x e d , the B-angle depends f i r s t l y on the d e c l i n a t i o n
of the sun 6 , and secondly on the d i f f e r e n c e (fi-a ) which i s a f u n c t i o n © ©
of launch time.
NORTH
0 CENTER OF THE EARTH A SUN
S SPACELAB N ASCENDING NODE Y VERNAL POINT
ORBIT
EQUATOR
Fig. 1.- Geometric aspect of the occultation problem.
06
For a hel Asynchronous orbit, this difference (fi-oi ) varies
© during the year as the equation of time since
Q - aQ = (MLT)n - 12h + E ( 5 )
where (MLT)^ denotes the mean local time at the ascending node of the orbit (a constant fixed by the launch time).
The extreme values of the B-angle during one year are shown as a function of (MLT)^ at the lower part of figure 2 . I t can be seen that for (MLT)^ values between 4h and 8h or between I6h and 20h, the 3-angle takes values which do not always satisfy condition ( 3 ) including, therefore, periods of time without occultation.
COVERAGE IN LATITUDE
By using formulas given in a previous work [ 1 2 ] , the annual coverage in latitude of the measurements at the ground l e v e l , during sunrise and sunset, has been determined for different mean local times at the ascending node. The results are reported at the upper part of figure 2 . It can be noted that the observations are mainly performed at high latitude in both hemispheres. The equatorial region is investigated only for (MLT)N values close to 1 , 8 , 16, and 20 hours, that is to say where the geometrical conditions give B-angles close to the limit of 5 9 . 8 degrees.
On the other hand, the coverages in latitude for sunrise (SR) and sunset (SS) are very similar i f one considers the corresponding
(MLT)., values linked by the following relation : N
( m l t )N , S R = 1 2 h - ( m l t )N , S S ( 6 )
In particular, for (MLT)^ values of 6 and 18 hours,, the coverages in latitude at sunrise and sunset are overlapping and especially concern high latitudeis.
t
ANNUAL COVERAGE IN LATITUDE FROM A HELIOSYNCHRONOUS ORBIT AT 1000 Km ALTITUDE
4 6 8 10 12 14 16 18 20 22 24 MEAN LOCAL TIME AT THE ASCENDING NOOE (Hour)
Fig. 2.- At the upper part : annual coverage in latitude from a helio- synchronous orbit at 1000 km in terms of the mean local time at the ascending node.
At the lower part : range of values of the g-angle during one year; occultations will occur for | B| < 59.8 degrees.
08
Figures 3 and 4 show the variation with time of the latitude of the observations during sunset and for different values of the mean local time at the ascending node. Figures 5 and 6 are referring to sunrise conditions. These four figures very well display the periods without occultation; the latter can rise up to 1 0 , 5 months for (MLT) values of 6 and 18 hours. A comparison between sunset and sunrise conditions reveals that for a particular day of the year, relation ( 6 ) is not always reliable when the (MLT)^ values are close to 8 , 16 and 20 hours. Indeed, for these particular values of (MLT) , combination of formulas ( 1 ) and (5) shows that g-angle can take.values which give or not a possibility for occultation according to the values of the equation of time E. Figure 7 gives an illustration of this for the f i r s t three months of the year with (MLT)^ taking I6h and 20h values. I f the equation of time E is neglected, g-angle takes the same values for (MLT)^ equal to 16 and 20 hours (curve 1 ) . Taking into account the variation of E, g-angle values are either greater (curve 2) or lower (curve 3) than the limit value ( 5 9 . 8 degrees) between "occultation" and "no occultation"
conditions.
On the other hand, the asymmetry of some of the curves between the two half-years is also due to the effect of the variation of the equation of time. More generally, this last effect has no practical influence when 0-angle is far from its limit value.
5 . CONCLUSION
Absorption spectrometry measurements of the Earth's atmosphere from a heliosynchronous orbit have an annual coverage in latitude which depends on the mean local time at the ascending node of the orbit. The high latitude regions in both hemispheres are mainly investigated. The.
coverage is extended to the equatorial region just for mean local times at the ascending node close to ^ , 8 , 1 6 and 20 hours. It is also for these particular casés that the equation of time plays a major role.
Fig. 3.~ Coverage in latitude for various mean local times at the ascending node between 0 and 12 hours and for sunset condi- tions.
Fig. 4.- Coverage in latitude for various mean local times at the ascending node between 13 and 23 hours and for sunset
Fig. 5.- Coverage in latitude for various mean local times at the ascending node between 0 and 12 hours and for sunrise condi- tions.
Fig- 6- ~ Coverage in latitude for various mean local times at the ascending node between 13 and 23 hours and for sunrise condi-
F i g . 7.~ E f f e c t of the equation of time on t h e B-angle f o r mean l o c a l
times a t the ascending node of 16 and 20 hours and f o r the
f i r s t t h r e e months of the y e a r .
REFERENCES
[1] BESSON, J., GIRARD, A., ACKERMAN, M. and FRIMOUT, D., Spectromètre pour la première mission Spacelab, ONERA T.P. 1978-89F, 1978.
[2] MULLER, C. and LAURENT, J., Scientific programs for the Spacelab ES013 grille spectrometer, Bull. CI. Sci. Acad. Roy. Belg.., 68, 431-442, 1982
[3] LIPPENS, C., Controlling the 1ES013 spectrometer for Spacelab and its data retrieval, Bull. CI. Sci., Acad. Roy. Belg., 68, 454-469, 1982.
[4] LAURENT, J., LEMAITRE, M.P., LIPPENS, C. and MULLER, C., Expérience de spèctrométrie infrarouge pour la première mission Spacelab, 1'Aéronautique et 1'Astronautique, n° 98, 60-70, 1983.
[5] LEMAITRE, M.P., LAURENT, J., BESSON, J., GIRARD, A., LIPPENS, C., MULLER, C. and VERCHEVAL, J., A sample performance of the grille spectrometer, Science, 225, 171-172, 1984.
[6] LIPPENS, C., MULLER, C., ACKERMAN, M., VERCHEVAL, J., LAURENT, J., LEMAITRE, M.P., BESSON, J. and GIRARD, A., Trace constituents measurements deduced from spectrometric observations on board Spacelab, Adv. Sp. Res., 4, 75~79, 1984.
[7] LAURENT, J., LEMAITRE, M.P., BESSON, J., GIRARD, A., LIPPENS, C. , MULLER, C. , ACKERMAN, M. and VERCHEVAL, J., Middle atmosphere NO and NO^ observed by means of the Spacelab one grille spectrometer, Nature, 31_5, 126-127, 1985.
[8] MULLER, C., VERCHEVAL J., ACKERMAN, M. , LIPPENS, C. , LAURENT, J., LEMAITRE, M.P., BESSON, J. and GIRARD, A., Observations of middle atmosphere CH^ and N20 vertical distributions by the Spacelab 1 grille spectrometer, Geophys. Res. Letters, 12, 667-670, 1985.
[9] VERCHEVAL, J., MULLER, C. , ACKERMAN, M., LIPPENS, C. , LAURENT, J., LEMAITRE, M.P., BESSON, J. and GIRARD, .A., •'CO and CO vertical distribution with middle atmosphere and lower thermosphere deduced from infrared spectra, to be published in Ann.
Geophysicae, 1986.
[10]. MULLER, C., LIPPENS, C., VERCHEVAL, J., ACKERMAN, M. , LAURENT, J., LEMAITRE, M.P., BESSON, J. and. GIRARD, A.-, Expérience
"Spectromètre à grille" à bord de la première charge utile de Spacelab, J. Optics, 16, 155-168, 1985.
[11] LAURENT, J. LEMAITRE, M.P., BESSON, J., GIRARD, A., LIPPENS, C., MULLER, C., VERCHEVAL, J. and ACKERMAN, M., Middle atmospheric water vapor observed by the Spacelab one grille spectrometer, submitted to Planetary and Space Science, 1986.
[12] VERCHEVAL, J., Détermination des conditions de lancement de Spacelab en vue de satisfaire les exigences d'un projet d'expérience par spectrométrie d'absorption, ESA Journal, 2, 19-26, 1978.
