ATOMICS INTERNATIONAL

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NAA-SR-7408

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APPLICATIONS OF SNAP REACTOR SYSTEMS TO COMMUNICATIONS SATELLITES

AEC Research and Development Report

ATOMICS INTERNATIONAL

DISCLAIMER

DISCLAIMER

LEGAL NOTICE

T h i s report was prepared as an account of Government sponsored work Neither the U n i t e d States, nor the Commission, nor any person a c t i n g on behalf of t h e Commission

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NAA-SR-7408 SNAP REACTOR,

SNAP PROGRAM

^ J2rPAGE^

-M-3679 (26111 Ed.)

APPLICATIONS OF SNAP REACTOR SYSTEMS TO COMMUNICATIONS SATELLITES

By R. E. WIMMER

DISTRIBUTION

Category: SNAP REACTOR, SNAP PROGRAM M-3679 (26th E d . ) Advanced R e s e a r c h P r o j e c t s Agency A e r o j e t - G e n e r a l C o r p o r a t i o n (NASA) A e r o n a u t i c a l S y s t e m s Division A e r o s p a c e T e s t Wing (AFSC) A i r F o r c e Space S y s t e m s Division A i r F o r c e Special Weapons C e n t e r A i r T e c h n i c a l Intelligence C e n t e r A i r U n i v e r s i t y L i b r a r y Argonne National L a b o r a t o r y A r m y B a l l i s t i c M i s s i l e Agency Atomic E n e r g y C o m m i s s i o n , Washington Battelle M e m o r i a l Institute Brookhaven National L a b o r a t o r y B u r e a u of Naval Weapons B u r e a u of Ships B u r e a u of Y a r d s and Docks California P a t e n t Group Chicago P a t e n t Group

Defense Atomic Support Agency, Sandia D e p a r t m e n t of the A r m y

D i r e c t o r of Defense R e s e a r c h and E n g i n e e r i n g (OABCW) G e n e r a l E l e c t r i c Company, Cincinnati

J e t P r o p u l s i o n L a b o r a t o r y

Los Alamos Scientific L a b o r a t o r y M a r t i n - M a r i e t t a C o r p o r a t i o n Naval Ordnance L a b o r a t o r y

Naval Radiological Defense L a b o r a t o r y Naval R e s e a r c h L a b o r a t o r y

Naval U n d e r w a t e r Ordnance Station New York O p e r a t i o n s Office

New York O p e r a t i o n s Office, Canel Division

Office of the A s s i s t a n t G e n e r a l Council for P a t e n t s (AEC) Office of the Chief of O r d a n c e (DOFL)

Office of the Chief of Naval Operations

Office of the Chief of Naval O p e r a t i o n s (OP-03EG) Office of Naval R e s e a r c h

P h i l l i p s P e t r o l e u m Company (NRTS) P r a t t and Whitney A i r c r a f t Division RAND C o r p o r a t i o n

Rome A i r Development C e n t e r Sandia C o r p o r a t i o n

School of Aviation Medicine

Union Carbide N u c l e a r Company (ORNL) U n i v e r s i t y of California, L i v e r m o r e U n i v e r s i t y of P e n n s y l v a n i a

U n i v e r s i t y of Washington (APL)

Westinghouse E l e c t r i c C o r p o r a t i o n (NASA) Division of T e c h n i c a l Information E x t e n s i o n AI L i b r a r y (Includes 2 copies for CPAO)

CONTENTS P a g e C. Vehicle Configurations 74 D. M i s c e l l a n e o u s C o n s i d e r a t i o n s 74 E. Snap R e a c t o r S y s t e m s 76 Appendixes

A. The Use of Decibel Notation 78 B. Modulation S c h e m e s and Signal I m p r o v e m e n t F a c t o r s 80

C. S i g n a l / N o i s e Ratio R e q u i r e m e n t for V a r i o u s P r o g r a m Types . 93

D. Antennas 97 E. S p e c t r u m N o m e n c l a t u r e and F r e q u e n c y Allocations 109

F. Satellite C o m m u n i c a t i o n s P r o g r a m s and P r o p o s a l s I l l G. A b b r e v i a t i o n s , Symbols, Constants, and C o n v e r s i o n F a c t o r s . 113

H. Orbit P a r a m e t e r s 117

R e f e r e n c e s 123

TABLES

I. R e c e i v e r Noise F i g u r e s 37 II. P o s s i b l e Satellite T r a n s m i t t e r s ( F i n a l R F Amplifier,

F M Ratings) 38 III. S u m m a r y of E x a m p l e C a l c u l a t i o n s 70

IV. SNAP R e a c t o r S y s t e m C h a r a c t e r i s t i c s 77 B - 1 . Bandwidth R e q u i r e m e n t s for V a r i o u s P r o g r a m s 89

C - 1 . Output S/N R e q u i r e m e n t s for Audio P r o g r a m s 94

D - 1 . Typical Antenna P a r a m e t e r s 108

FIGURES

Pag

1. Typical Receiving I n s t a l l a t i o n 15 2. P r o p a g a t i o n Between I s o t r o p i c Antennas 18

3. P r o p a g a t i o n Between Gain Antennas 18 4 . C o s m i c Noise T e m p e r a t u r e vs F r e q u e n c y 23

5. Noise T e m p e r a t u r e of Undisturbed Sun 24 6. I o n o s p h e r i c A b s o r p t i o n a s F u n c t i o n s of F r e q u e n c y and

P a t h Angle with H o r i z o n t a l 26 7. T r o p o s p h e r i c A b s o r p t i o n ( I n t e r n a t i o n a l Standard Atmosphere). . . . 27

8. T e m p e r a t u r e Contribution Due to T r o p o s p h e r i c L o s s e s 28

9. P a t h L o s s Due to Rainfall (Light, 0.1 c m / h r ) 29 10. P a t h L o s s Due to Rainfall (Heavy, 1.6 c m / h r ) 30 11. C o s m i c and T r o p o s p h e r i c Noise v s F r e q u e n c y

( T a r g e t N e a r Galactic Pole) . . . 7 7 31 12. Composite Sky T e m p e r a t u r e (Includes C o s m i c ,

T r o p o s p h e r i c , I o n o s p h e r i c , R a i n Noise) 33 13. Two-Hour Orbit G e o m e t r y 48 14. S t a t i o n a r y Orbit G e o m e t r y 52 15. C o v e r a g e Zone,Stationary Satellite 54 16. 5000 k m Orbit G e o m e t r y (Example E) 64 17. M i l i t a r y C o m m u n i c a t i o n s Satellite S y s t e m (Example F) 66 18. Weight C o m p a r i s o n of Space P o w e r S o u r c e s 72 19. SNAP Specific Weight vs P o w e r Output 73

B - 1 . AM P o w e r S p e c t r a 82 B - 2 . SSB P o w e r S p e c t r a 82 B - 3 . F M P o w e r S p e c t r a 82 B - 4 . P r e d e t e c t i o n Bandwidth R e q u i r e d to P a s s Significant Modulation T e r m s of FM Signal 85 B - 5 . I m p r o v e m e n t Due to P r e e m p h a s i s 88 B - 6 . Typical P o w e r Output Envelope for U . S . Standard TV

Video Signal 91 B - 7 . P o w e r S p e c t r u m of TV T r a n s m i s s i o n 92

C - 1 . Subjective E v a l u a t i o n s of TV P i c t u r e Quality in the

P r e s e n c e of Random Noise ( F r o m IRE TASO P a n e l Studies,

FIGURES

P a g e

C - 2 . TV Subjective Quality vs S i g n a l / R a n d o m Noise Ratio 96

D - 1 . Typical Antenna P a t t e r n s 99 D - 2 . Gain and Beamwidth of Ideal {rj - 1) P a r a b o l o i d s 100

D - 3 . Gain D e g r a d a t i o n by I m p e r f e c t Reflector (After

Rechtin, R e f e r e n c e 6) 102 D - 4 . M a x i m u m F r e q u e n c y for High-Quality P a r a b o l o i d a l R e f l e c t o r s (After P o t t e r , R e f e r e n c e 19) 103 D - 5 . P a r a b o l o i d a l H o r n Antenna 104 D - 6 . H e l i c a l B e a m Antenna 105 D - 7 . Cost of P a r a b o l o i d a l Antenna v s D i a m e t e r . 107 H - 1 . O r b i t P a r a m e t e r s 118 H - 2 . G e n e r a l O r b i t G e o m e t r y 119

ERRATA

July 30, 1962

NAA-SR-7408, "APPLICATIONS O F SNAP REACTOR SYSTEMS TO COMMUNICATIONS SATELLITES'

P a g e 18, F i g u r e 2: Equation should r e a d : 2

(P^ = 2~2 "^^^^^^ l6Tr r

F i g u r e 3: Equation should read: 2 P^G^G^X'

(Pj. = ^ ^ watts) I67r r

P a g e 62, 7th line in table u n d e r " V a r i a b l e " should r e a d :

^ t

8th line in table u n d e r " V a r i a b l e " should r e a d :

G r

P a g e 114, 14th line u n d e r "Definition" should read:

B o l t z m a n n ' s constant ( J o u l e s / ° K )

19th line u n d e r "Symbol" should read:

m^ = Af/f^

A t o m i c s I n t e r n a t i o n a l

I. INTRODUCTION AND SCOPE

The field of s a t e l l i t e c o m m u n i c a t i o n s i s one of i n t e n s e and w i d e s p r e a d i n t e r e s t , since potential m i l i t a r y c o m m u n i c a t i o n s u s e s a r e n u m e r o u s and it also a p p e a r s to be the only field in space technology with r e a s o n a b l y i m m e d i a t e p r o s p e c t s for c o m m e r c i a l exploitation. This r e p o r t d e a l s p r i m a r i l y with the t e c h -nological c o n s i d e r a t i o n s of s a t e l l i t e c o m m u n i c a t i o n s y s t e m s , and d e t e r m i n a t i o n of the s a t e l l i t e power supply r e q u i r e m e n t s for such s y s t e m s .

C o n s i d e r a t i o n i s r e s t r i c t e d to active s a t e l l i t e s ( s a t e l l i t e s with p o w e r e d t r a n s -m i t t e r s ) a s opposed to p a s s i v e s a t e l l i t e s which -m e r e l y reflect signals r a d i a t e d from the ground. Although p a s s i v e s a t e l l i t e s have d e m o n s t r a t e d c o m m u n i c a t i o n s capability, their i n h e r e n t r e q u i r e m e n t of e x t r e m e l y high-power ground stations c r e a t e s a p r o b l e m of i n t e r f e r e n c e with o t h e r s e r v i c e s , which is expected to p r e -clude any a p p r e c i a b l e extension of this type of s a t e l l i t e r e l a y s e r v i c e into the s p e c t r u m - c r o w d e d c o m m u n i c a t i o n s future.

N u c l e a r r e a c t o r A P U ' s , such as the c u r r e n t s e r i e s of SNAP ( S y s t e m s for N u c l e a r Auxiliary P o w e r ) r e a c t o r s y s t e m s , a p p e a r p a r t i c u l a r l y a t t r a c t i v e for a wide range of s a t e l l i t e conamunications a p p l i c a t i o n s . The d e g r e e of a t t r a c t i v e n e s s of SNAP, or any o t h e r type of APU, is s t r o n g l y dependent upon the many d e -t a i l s of each specific c o m m u n i c a -t i o n s m i s s i o n , and may also depend upon m i s s i o n f a c t o r s not d i r e c t l y r e l a t e d to the c o m m u n i c a t i o n technology of the m i s s i o n . An e x a m p l e of the l a t t e r is the use of the e l e c t r i c p r o p u l s i o n capability of l a r g e n u c l e a r r e a c t o r A P U ' s to r e d u c e the cost of b o o s t e r r o c k e t s , or to enhance t h e i r pay load c a p a b i l i t i e s .

Special attention h a s been given to the p r e s e n t SNAP r e a c t o r s y s t e m s , since they r e p r e s e n t a range of power output c a p a b i l i t i e s suitable to a v a r i e t y of c o m -m u n i c a t i o n s -m i s s i o n s , and will be available e a r l y in the schedule of i -m -m e d i a t e l y a t t r a c t i v e s a t e l l i t e c o m m u n i c a t i o n p r o g r a m s .

II. THE SATELLITE AS A COMMUNICATIONS NODE

A. BASIC ADVANTAGES

T h e r e a r e two b a s i c facts which m a k e a s a t e l l i t e a t t r a c t i v e for c o m m u n i c a -tions a p p l i c a t i o n s . F i r s t , a s a t e l l i t e at even a low altitude " s e e s " a l a r g e a r e a of the e a r t h s u r f a c e , and can be " s e e n " s i m u l t a n e o u s l y f r o m widely s e p a r a t e d p o i n t s . Second, a l a r g e p o r t i o n of the useful e l e c t r o m a g n e t i c s p e c t r u m p r o p a -gates r e l i a b l y only over s t r a i g h t , " l i n e - o f - s i g h t " p a t h s . The useof this s p e c t r u m region in d i r e c t s u r f a c e links is limited, by the c u r v a t u r e of the e a r t h , to a m a x -i m u m s u r f a c e r a n g e of about 30 m -i l e s .

The b a s i c value of the c o m m u n i c a t i o n s s a t e l l i t e thus l i e s in the fact that it p e r m i t s the use of a l a r g e new r e g i o n of the e l e c t r o m a g n e t i c s p e c t r u m for d i r e c t l o n g - r a n g e c o m m u n i c a t i o n s . This new s p e c t r u m region is frequently r e f e r r e d to as the " s p a c e band," and s t r e t c h e s f r o m about 100 m e g a c y c l e s to about 20,000 m e g a c y c l e s . (The " s p a c e band" i s of c o u r s e used in m a n y s u r f a c e a p p l i c a t i o n s , and will r e m a i n useful for many types of s h o r t r a n g e p o i n t - t o - p o i n t links.)

The new s p e c t r u m s p a c e , made available through u s e of c o m m u n i c a t i o n s a t e l l i t e s , p r o m i s e s to a l l e v i a t e the p r o b l e m of r a p i d l y i n c r e a s i n g crowding in existing global c o m m u n i c a t i o n s c a p a b i l i t y .

In addition to providing additional traffic capacity, t h e r e a r e s e v e r a l o t h e r advantages which will be d e r i v e d as c o r o l l a r y b e n e f i t s . One such advantage i s the p o s s i b i l i t y of handling p r o g r a m m a t e r i a l w^hich is beyond the capability of existing or f o r s e e a b l e surface l i n k s . Wide-bandwidth s i g n a l s such a s t e l e v i s i o n (about 5 Mc bandwidth) a r e p r e s e n t l y l i m i t e d to m i c r o w a v e s u r f a c e links and coaxial c a b l e , both s y s t e m s r e q u i r i n g closely spaced r e l a y or b o o s t e r a m p l i f i e r s . Although it would be p o s s i b l e in p r i n c i p l e to design an oceanic cable for t e l e v i s i o n (TV) bandwidths, such a p r o j e c t does not a p p e a r e c o n o m i c a l l y a t t r a c t i v e in the f o r s e e a b l e future. A r e l a y s a t e l l i t e , on the other hand, does a p p e a r p r a c t i c a l in the i m m e d i a t e future and can r e a d i l y be designed for bandwidths capable of s i m u l t a n e o u s l y a c c o m m o d a t i n g s e v e r a l TV p r o g r a n n s .

1. D i r e c t i v i t y and F r e q u e n c y Sharing

The e x t r e m e l y s h a r p d i r e c t i v i t y of m o d e s t size a n t e n n a s operating in the u p p e r p a r t of the space band l e a d s to a second benefit; the sharing or s i m u l t a n e o u s u s e of a frequency by two or m o r e p r o g r a m s in the s a m e g e o g r a p h i c a l a r e a . S e l e c -tion of the d e s i r e d p r o g r a m is a c c o m p l i s h e d by d i r e c t i o n a l d i s c r i m i n a t i o n , in favor of the d e s i r e d signal and a g a i n s t the u n d e s i r e d signals which o r i g i n a t e from o t h e r d i r e c t i o n s . F r e q u e n c y s h a r i n g i s p r e s e n t l y p r a c t i c e d by s u r f a c e m i c r o w a v e l i n k s , and extension to s a t e l l i t e s m e r e l y expands the p r i n c i p l e from two d i m e n s i o n s to t h r e e . In o r d e r to take the g r e a t e s t p o s s i b l e advantage of frequency s h a r i n g , it is n e c e s s a r y to e x e r c i s e c o n s i d e r a b l e planning and control over t r a n s m i t t e r power l e v e l s and ground t r a n s m i t t e r l o c a t i o n s .

A c o m m u n i c a t i o n s link which o p e r a t e s through a s a t e l l i t e b a s e d r e l a y h a s the potential of a v e r y high r e l i a b i l i t y , p a r t i c u l a r l y when o p e r a t e d on f r e q u e n c i e s

somewhat r e m o v e d from the edges of the space band. Other long r a n g e c o m -m u n i c a t i o n s links such a s r a d i o , c a b l e s , e t c . , a r e subject to i n t e r f e r e n c e and p e r t u r b a t i o n s , p r i n c i p a l l y from s o l a r and i o n o s p h e r i c activity. Since the i o n o s p h e r e

i s e s s e n t i a l l y t r a n s p a r e n t to s p a c e - b a n d f r e q u e n c i e s , even m a j o r p e r t u r b a t i o n s of the m e d i u m have little or no effect on the link. At s p a c e - b a n d f r e q u e n c i e s , p r o p a g a t i o n is v e r y n e a r l y that of ideal f r e e - s p a c e conditions.

B e s i d e s p o i n t t o p o i n t c o m m u n i c a t i o n s , s a t e l l i t e s m a y a l s o be u s e d to p r o -vide a signal to an u n l i m i t e d n u m b e r of r e c e i v e r s within a l a r g e a r e a . Such c o v e r a g e m a y be for the p u r p o s e of t r a n s m i t t i n g p r o g r a m s to conventional ground s t a t i o n s for r e b r o a d c a s t , or even d i r e c t l y to home r e c e i v e r s , or it m a y be for such s e r v i c e s a s new^s w i r e s , w e a t h e r information, p o l i c e , m i l i t a r y or other one-w^ay information d i s s e m i n a t i o n c h a n n e l s .

B . ORBIT CONSIDERATIONS

1. C o v e r a g e A r e a

The size of the c o v e r a g e a r e a , i . e . , the a r e a on the e a r t h ' s surface from which the s a t e l l i t e is s i m u l t a n e o u s l y v i s i b l e , is d e t e r m i n e d by the s a t e l l i t e altitude at any given m o m e n t . The m a x i m u m surface r a n g e between ground stations i s given by:

R = 2 r cos'-^ (r / r )* . . . ( 1) e e o'

w h e r e r i s the r a d i u s of the e a r t h , r the o r b i t r a d i u s , and cos i s in r a d i a n s . e ' o

R i s also equal to the d i a m e t e r of the c o v e r a g e c i r c l e for b r o a d c a s t c o n s i d e r -a t i o n s . It should be noted th-at the c o v e r -a g e -a r e -a thus d e s c r i b e d i s not g e n e r -a l l y fixed with r e s p e c t to the e a r t h , but m o v e s at a r a t e d e t e r m i n e d by s a t e l l i t e

a l t i t u d e .

2. C o v e r a g e T i m e

The length of t i m e for which a s a t e l l i t e i s visible from a given ground station i s dependent on both o r b i t p a r a m e t e r s and ground station location with r e s p e c t to the o r b i t a l p l a n e . If the o r b i t a l p e r i o d is s h o r t c o m p a r e d to one s i d e -r e a l day, the c o v e -r a g e du-ration f-rom c i -r c u l a -r o -r b i t s m a y be a p p -r o x i m a t e d by:

A t = - = ^ c o s ~ (r / r c o s a ) . . . (2a)

w h e r e T is the o r b i t a l p e r i o d , r the e a r t h r a d i u s , r the o r b i t r a d i u s , and C!

^ ' e ' o ' the latitude of the ground station with r e s p e c t to the o r b i t a l p l a n e .

F o r c i r c u l a r e q u a t o r i a l o r b i t s of any p e r i o d , the c o v e r a g e duration is given exactly by:

A t = yril^r) ^ ° ^ " ( r ^ / r ^ c o s a ) . . . ( 2 b )

w h e r e A t and T a r e in s i d e r e a l d a y s , and 01 is the e a r t h longitude of the ground s t a t i o n . If the d i r e c t i o n of the s a t e l l i t e is the s a m e a s the e a r t h ' s r o t a t i o n , T is p o s i t i v e .

F o r a p e r i o d of one s i d e r e a l day, the c o v e r a g e t i m e b e c o m e s infinite. This is the s p e c i a l c a s e of the s t a t i o n a r y s a t e l l i t e , which r e v o l v e s in s y n c h r o -n i s m with the r o t a t i o -n of the e a r t h . A s a t e l l i t e i-n such a-n o r b i t m a i -n t a i -n s a constant b e a r i n g and elevation a s seen from any ground point within the c o v e r a g e a r e a ; in this s p e c i a l c a s e this is a fixed a r e a on the s u r f a c e of the e a r t h .

3. T r a c k i n g Rate

III. RECEIVING INSTALLATION CONSIDERATIONS

A r a d i o c o m m u n i c a t i o n link m a y b e s t be e x a m i n e d by f i r s t c o n s i d e r i n g the r e c e i v i n g i n s t a l l a t i o n .

A. TYPICAL RECEIVING INSTALLATION

F i g u r e 1 r e p r e s e n t s a typical r e c e i v i n g i n s t a l l a t i o n , which c o n s i s t s of an antenna, t r a n s m i s s i o n l i n e , v a r i o u s components of the r e c e i v e r , and the output t r a n s d u c e r . A r e c e i v e d signal u n d e r g o e s v a r i o u s l o s s e s and p e r t u r b a t i o n in the c o u r s e of p r o c e s s i n g for u l t i m a t e r e p r o d u c t i o n in intelligible f o r m . A s s u m e that C w a t t s of signal a r e i n t e r c e p t e d by the antenna and d e l i v e r e d to a perfectly m a t c h e d t r a n s m i s s i o n l i n e . The antenna will a l s o r e c e i v e a c e r t a i n amount of r a d i a t i v e n o i s e ( N , ) , e i t h e r n a t u r a l or a r t i f i c i a l in origin, and will d e l i v e r t h i s n o i s e to the t r a n s m i s s i o n l i n e . In p r o p a g a t i o n along the l i n e , both signal and noise will suffer attenuation by the s a m e amount due to line l o s s e s . A c e r t a i n amount of n o i s e (N-,) is g e n e r a t e d in the line due to t h e r i n a l agitation of the line l o s s r e s i s t a n c e . The signal and n o i s e is then d e l i v e r e d to the f i r s t stage of the r e c e i v e r p r o p e r , u s u a l l y a r e s i s t i v e load which g e n e r a t e s n o i s e (N,) due to t h e r m a l agitation. At t h i s point, the r e m a i n i n g signal and the total n o i s e a r e amplified by a n e c e s s a r i l y i m p e r f e c t a m p l i f i e r which c o n t r i b u t e s additional noise (N.) due to shot effect, spontaneous e m i s s i o n , and other m e c h a n i s m s .

N- and N , a r e g e n e r a l l y t r e a t e d a s an "effective r e c e i v e r input n o i s e " , N J , which is given by:

N ' = k T B F . . . (3) 3 s ^ ' w h e r e T is the a b s o l u t e a m b i e n t t e m p e r a t u r e , B the effective n o i s e bandwidth of the s y s t e m , k B o l t z m a n n ' s constant, and F the s y s t e m n o i s e f i g u r e . N J i s

5 O s o m e t i m e s given a s :

N • = k T B . . . (4)

•i r

% 1 - en I o 00 T R A N S M I S S I O N ANTENNA LINE ^LOSSES ^LOSSES

i i

r

t

r

SIGNAL TREATMENT DETECTOR

POSTDETECTION OUTPUT SIGNAL TREATMENT TRANSDUCER

w h e r e T is an effective r e c e i v e r n o i s e t e m p e r a t u r e . The l a t t e r notation is useful in the a n a l y s e s of m a s e r s and other l o w - n o i s e s y s t e m s , and h a s the

advantage of p e r m i t t i n g s i m p l e s u m m a t i o n s of line l o s s n o i s e and antenna n o i s e s if t h e s e f a c t o r s a r e not n e g l i g i b l e .

Beyond the f i r s t a m p l i f i e r , the p r e d e t e c t i o n s i g n a l / n o i s e r a t i o C / N does not change a p p r e c i a b l y , p r o v i d e d the f i r s t a m p l i f i e r h a s a r e a s o n a b l e gain. Succeeding a m p l i f i e r s a r e u s u a l l y a t l e a s t a s good a s the f i r s t a m p l i f i e r , p a r -t i c u l a r l y w h e r e -the f i r s -t a m p l i f i e r is a d o w n - c o n v e r -t e r . I n -t e r m e d i a -t e a m p l i f i e r s then function at a l o w e r frequency, w h e r e low n o i s e f i g u r e s a r e m o r e r e a d i l y

obtainable. In the b e s t of r e c e i v e r s , h o w e v e r , s o m e quantity of unwanted n o i s e inevitably a p p e a r s in the output.

B . INFORMATION TREATMENT AND IMPROVEMENT FACTORS

Most modulation s c h e m e s provide a c e r t a i n d e g r e e of redundancy in the t r a n s m i t t e d i n f o r m a t i o n . In detection, w h e r e the w a v e f o r m of the o r i g i n a l information is r e c o n s t r u c t e d , some of t h i s redundancy m a y be u t i l i z e d to i m -prove the s i g n a l / n o i s e (S/N) r a t i o of the final output s i g n a l . It is a l s o p o s s i b l e in s o m e c a s e s to take advantage of s t a t i s t i c a l knowledge of n o i s e and modulation w a v e f o r m s to i m p r o v e the output S/N r a t i o or to o t h e r w i s e i n c r e a s e the a c c e p t -ability of the output s i g n a l . Information t r e a t m e n t s c h e m e s a r e d i s c u s s e d in Appendix B .

C. SIGNAL/NOISE RATIO REQUIREMENTS

The o v e r a l l quality of the r e p r o d u c e d signal is m e a s u r e d by the output S/N r a t i o , i. e. , the S/N r a t i o at the output t r a n s d u c e r . This r a t i o m a y be g r e a t e r or l e s s than the p r e d e t e c t i o n S/N r a t i o , depending upon the information t r e a t m e n t t e c h n i q u e s employed (see Appendix B).

The output S/N r a t i o r e q u i r e d for a c c e p t a b i l i t y v a r i e s with the type of signal, with the n a t u r e of the information t r a n s m i t t e d , and to s o m e extent, with the

RECEIVER 162712 watts F i g u r e 2. P r o p a g a t i o n Between I s o t r o p i c Antennas NO ENERGY WASTED I REFLECTOR \ j - . N \ REFLECTOR Gr TRANSMITTER (Pf watt* ) \ RECEIVER , „ P f G t G , 2 I b i r ^ r ^

IV. PROPAGATION

Unlike surface links w^hich depend on the p r e s e n c e of a reflecting o r r e f r a c t i n g i o n o s p h e r e for l o n g - r a n g e t r a n s m i s s i o n s , s a t e l l i t e links u s e f r e q u e n c i e s at which the i o n o s p h e r e is n e a r l y t r a n s p a r e n t . P r o p a g a t i o n in a s a t e l l i t e link is prinnarily governed by g e o m e t r i c , f r e e - s p a c e or l i n e - o f - s i g h t c o n s i d e r a t i o n s .

A. IDEAL PROPAGATION IN A VACUUM

An i s o t r o p i c antenna in free space, r a d i a t i n g P w^atts of R F energy, h a s a s p h e r i c a l wave front that is a p p r o x i m a t e l y flat in the vicinity of any point r e a s o n -ably distant f r o m the r a d i a t o r , and the e n e r g y flux m a y be calculated f r o m geo-m e t r i c c o n s i d e r a t i o n s :

^t 2

P^ = 2 w a t t s / m . . . . (5) 4Trr

If an i s o t r o p i c r e c e i v i n g antenna is employed, its effective c a p t u r e a r e a is given by:

A = ^ . . . . ( 6 ) c 4Tr ^ '

F i g u r e 2 i l l u s t r a t e s this hypothetical s y s t e m . It is now p o s s i b l e to calculate the r e c e i v e d pow^er, and, if the r e c e i v e r c h a r a c t e r i s t i c s a r e known, the output S/N r a t i o .

B. ANTENNA GAINS

A m o r e p r a c t i c a l e x a m p l e is i l l u s t r a t e d in F i g u r e 3. A r e f l e c t o r is employed to d i v e r t some of the w^asted t r a n s m i t t e r output tow^ard the r e c e i v i n g antenna, and the r e c e i v e d pow^er is thus i n c r e a s e d . The i n c r e a s e r a t i o is the gain of the t r a n s -m i t t i n g antenna, G.. In this r e p o r t , gains a r e r e f e r r e d to an i s o t r o p i c r a d i a t o r , and not the s t a n d a r d half-wave dipole of frequent e n g i n e e r i n g u s a g e .

The s a m e p r i n c i p l e is applicable to r e c e i v i n g a n t e n n a s , and the r e c e i v i n g gain of an antenna (G ) is equal to its gain as a t r a n s m i t t i n g antenna. It follows *A p e r f e c t l y i s o t r o p i c antenna is not r e a l i z a b l e , but s e r v e s a useful t h e o r e t i c a l

that a r e c e i v i n g antenna w^ith gain will have a p r e f e r e n t i a l r e c e i v i n g d i r e c t i o n . The c a p t u r e a r e a of a gain antenna m a y be e x p r e s s e d a s :

A = ~ - . . . . (7) c 47r ^ ' T r a n s m i t t i n g and r e c e i v i n g antenna a r e i n t e r c h a n g e a b l e , i . e . , an antenna will have the s a m e d i r e c t i v i t y p a t t e r n w h e t h e r u s e d for t r a n s m i t t i n g or r e c e i v i n g . Antenna c h a r a c t e r i s t i c s a r e d i s c u s s e d in m o r e detail in Appendix D.

C. COMMUNICATIONS E Q U A T I O N - FORM I

P r o p a g a t i o n calculation m a y be s u m m a r i z e d in the C o m m u n i c a t i o n s Equation, which e x p r e s s e s r e q u i r e d t r a n s m i t t e r power in t e r m s of signal r e q u i r e m e n t s ,

r e c e i v i n g p a r a m e t e r s , and g e o m e t r i c c o n s i d e r a t i o n s . F o r the c a s e w h e r e the s y s t e m i s dominated by the n o i s e figure of a r e c e i v e r o p e r a t i n g at roonn t e m p e r a -t u r e (290°K), -the C o m m u n i c a -t i o n s Equa-tion -t a k e s -the f o r m :

P = 20 log r + 20 log f + 10 log B + L + C/N + F

L O - [ G ^ + G^ + 171.6], dbw , . . . ( 8 ) w h e r e r = d i s t a n c e (km) f = c a r r i e r frequency (Mc) B = noise bandwidth (cps) L = m i s c e l l a n e o u s l o s s e s (db) C / N = p r e d e t e c t i o n s i g n a l / n o i s e r a t i o (db) F = s y s t e m noise figure (db) s G , G = antenna gains (db)

L includes a b s o r p t i o n l o s s e s in the p r o p a g a t i o n path (often negligible for s p a c e band f r e q u e n c i e s ) , d i s s i p a t i v e and m i s m a t c h l o s s e s in t r a n s m i s s i o n lines p r e

Equation 8 is applicable only to s y s t e m s w h e r e n o i s e e x t e r n a l to the r e c e i v e r i s negligible in connparison to N ' a s given by Equation 3. This r e s t r i c t i o n i s satisfied by a l a r g e c l a s s of r e c e i v i n g i n s t a l l a t i o n s which include l o w c o s t e q u i p -ment, such a s c o n s u m e r home r e c e i v e r s , or p o r t a b l e s y s t e m s w h e r e advanced equipment such a s m a s e r s a r e not conveniently u s a b l e .

D. PERTURBATIONS, LOSSES, AND EXTERNAL NOISE

T h e r e a r e a n u m b e r of p e r t u r b a t i o n s to w^hich s p a c e - b a n d p r o p a g a t i o n s a r e subject, and they d e t e r m i n e the p r a c t i c a l low^- and high-frequency b o u n d a r i e s of the s p a c e - b a n d . Some of the i m p o r t a n t p e r t u r b a t i o n s a r e d i s c u s s e d in the follow-ing s e c t i o n s .

1. L o w - F r e q u e n c y P e r t u r b a t i o n s

N e a r the low^frequency end of the s p a c e b a n d (100 Mc), the chief p r o p a gation p e r t u r b a t i o n s a r e F a r a d a y effect, e x t e r n a l n o i s e , and i o n o s p h e r i c a b s o r p -tion. It i s a l s o difficult to obtain l a r g e signal bandwidths or antenna gains at t h e s e f r e q u e n c i e s , and it is t h e r e f o r e i m p r a c t i c a l to o p e r a t e h i g h c a p a c i t y s y s -t e m s in -t h i s region. T h e r e a r e a few^ applica-tions in -t h i s region, however, which m e r i t a further a n a l y s i s of p r o p a g a t i o n p e r t u r b a t i o n s .

a. F a r a d a y Rotation

F a r a d a y effect o c c u r s w^hen p r o p a g a t i o n t a k e s place through a m e d i u m such a s the i o n o s p h e r e , w h e r e free e l e c t r o n s and a m a g n e t i c field a r e p r e s e n t , and r e s u l t s in a r o t a t i o n of the p o l a r i z a t i o n plane of a l i n e a r l y p o l a r i z e d wave. The F a r a d a y angle is given by:

^3 l i _ ^ _ ! ^ o 1^ / n H d r , . . . ( 9 ) * 87r m'^C € o o o r SI - • ^ - 9 7 x 1 0 - ^ '~ f2

negligible. ' At lower f r e q u e n c i e s , however, Xlr m a y amount to s e v e r a l complete 4 r o t a t i o n s , and is a s e n s i t i v e function of rapidly v a r y i n g i o n o s p h e r i c p a r a m e t e r s . It is thus i m p o s s i b l e to continuously align a l i n e a r l y p o l a r i z e d r e c e i v i n g antenna with the incoming w^ave, and a v a r i a b l e l o s s equal to cos ilr r e s u l t s .

This l o s s m a y be m i t i g a t e d by using a c i r c u l a r l y p o l a r i z e d antenna at e i t h e r the t r a n s m i t t e r or the r e c e i v e r ; in e i t h e r c a s e a constant l o s s of 0.5 (3 db) r e s u l t s . P o l a r i z a t i o n - d i v e r s i t y r e c e p t i o n , in w^hich c r o s s - p o l a r i z e d a n t e n n a s combine signals in a s p e c i a l r e c e i v e r , can c i r c u m v e n t F a r a d a y l o s s e s ; however, such a r e c e i v e r is fairly complex and e x p e r i e n c e s a somew^hat g r e a t e r total e x t e r -n a l -noise i-nput. The l o s s due to F a r a d a y r o t a t i o -n m a y be c o m p l e t e l y avoided by using c i r c u l a r l y p o l a r i z e d antennas of the s a m e s e n s e at both t r a n s m i t t e r and

r e c e i v e r .

b. E x t e r n a l Noise

With high quality l o w - n o i s e r e c e i v e r s , it is not a p p r o p r i a t e to i g n o r e the effects of e x t e r n a l n o i s e , since this m a y be the dominating factor in d e t e r -mining the s y s t e m S/N r a t i o . For s y s t e m s in this c a t e g o r y it is m o s t convenient to d e a l with effective n o i s e t e m p e r a t u r e s a s defined by Equation 4, and c o n t r i b u -tions to the s y s t e m n o i s e t e m p e r a t u r e f r o m s o u r c e s e x t e r n a l to the r e c e i v e r .

F i g u r e 4 show^s the t e m p e r a t u r e contribution due to c o s m i c noise for n a r r o w -5 6

b e a m antennas pointed at the n o i s i e s t and q u i e t e s t r e g i o n s of the sky. ' The a v e r a g e sky t e m p e r a t u r e is m u c h c l o s e r to the m i n i m u m than the maximunn, the l a t t e r o c c u r r i n g only n e a r the d i r e c t i o n of the g a l a c t i c c e n t e r . A s t r o n o m i c a l bodies such as the sun and moon a l s o r a d i a t e t h e r m a l n o i s e . F i g u r e 5 shows the

7

noise t e m p e r a t u r e of the u n d i s t u r b e d sun. The moon b e h a v e s n e a r l y like a b l a c k -body r a d i a t o r , with s u r f a c e t e m p e r a t u r e s ranging f r o m 120 to 400° K, depending on the lunar p h a s e . At e x t r e m e l y high f r e q u e n c i e s ('^25 Gc), surface t e m p e r a -t u r e s a r e m o d e r a -t e d (210 -to 300°K) by wha-t a p p e a r s -to be a r a d i o - -t r a n s p a r e n -t

7 l a y e r of t h e r m a l l y insulating dust.

In c a s e s w h e r e the angular extent of a d i s c r e t e noise s o u r c e i s s m a l l e r than the antenna beamwidth, the noise contribution is r e d u c e d by the r a t i o of solid angles subtended. It is a l s o often i m p o r t a n t to include noise contributed by a n tenna side lobes, which m a y be focused on a d i s c r e t e noise s o u r c e of high i n t e n

10

e

10*

SOLAR BLACKBODY TEMPERATURE

J I I I I I I I J I I I I I I I

0.1 1,0 10. FREQUENCY ( Gc)

c. I o n o s p h e r i c A b s o r p t i o n

F i g u r e 6 shows the d i s s i p a t i v e l o s s e s suffered in p r o p a g a t i o n through the 7

i o n o s p h e r e . T h e s e l o s s e s a r e g e n e r a l l y s m a l l except for p r o p a g a t i o n paths n e a r l y tangent to the e a r t h , and b e c o m e negligible at f r e q u e n c i e s above a fe\^^ hundred Mc. However, the effect of t h e s e l o s s e s is a u g m e n t e d by an i n c r e a s e in s y s t e m n o i s e . Since the l o s s i s d i s s i p a t i v e , it m a y be t r e a t e d a s a r e s i s t i v e network w^hich g e n e r a t e s t h e r m a l agitation n o i s e in i t s equivalent r e s i s t a n c e s .

For t h e s e s m a l l a t t e n u a t i o n s , the noise contribution is well a p p r o x i m a t e d by adding 7° K to the effective antenna t e m p e r a t u r e for each 0.1 db of d i s s i p a t i v e attenuation.

2. H i g h - F r e q u e n c y P e r t u r b a t i o n s

N e a r the h i g h f r e q u e n c y end of the s p a c e band (20 Gc), the chief p r o p a -gation p e r t u r b a t i o n s a r e t r o p o s p h e r i c a b s o r p t i o n s and the n o i s e s a r i s i n g f r o m t h e m . E q u i p m e n t is a l s o s t r o n g l y s t a t e - o f - a r t limited, and it a p p e a r s unlikely that any e x t e n s i v e u s e of the r e g i o n above 20 Gc can be m a d e d u r i n g the n e a r future.

a. A t m o s p h e r i c A b s o r p t i o n

F i g u r e 7 shows a b s o r p t i o n through the a t m o s p h e r e , due p r i m a r i l y to its n o r m a l w a t e r vapor and oxygen content. A s s o c i a t e d with this a b s o r p t i o n is a

7 sky t e m p e r a t u r e contribution show^n in F i g u r e 8.

b. A b s o r p t i o n Through Rainfall

Falling r a i n in the path of h i g h - f r e q u e n c y p r o p a g a t i o n m a y c a u s e v e r y l a r g e additional l o s s e s . F i g u r e s 9 and 10 i l l u s t r a t e the l o s s e s i n c u r r e d for r a i n

-7 8 9

fall, in t e r m s of d b / k m of v e r t i c a l r a i n depth. ' ' The antenna n o i s e contribution f r o m this s o u r c e i s a l s o about 7°K p e r 0.1 db of l o s s , for s m a l l l o s s e s and low b a s i c antenna t e m p e r a t u r e s .

3. C o m p o s i t e Sky T e m p e r a t u r e

100

0.1 1.0 10

FREQUENCY (Gc)

1,000 r :

1.0

FREQUENCY (Gc)

10 h-Q. o < tr o E \ Q I 0.1

1 1 1 1 1 1 1 M

u

hx^

^ ^

^y

J

^y

1

^y

y

y

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^y

H

HORIZONTAL EXTENT OF RAIN (km)

F i g u r e 9. P a t h L o s s Due to Rainfall (Light, 0.1 c m / h r )

1 1 1 1 1 1 1 1 -— : / / /

/ y

- / y>^

y^ /

if

^Y

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y

^y

^Y

V

HORIZONTAL EXTENT OF RAIN (km)

and upper edges of the band. A u t h o r i t i e s differ a s to the p r o p e r m a n n e r in which to deal with the effective sky t e m p e r a t u r e , and often include d i s c r e t e n o i s e s o u r c e s and antenna s i d e - l o b e effects. The t r e a t m e n t p r e s e n t e d h e r e d e a l s w^ith these i t e m s s e p a r a t e l y but includes light rainfall and i o n o s p h e r i c noise a s u s u a l p e r t u r b a -tions in s a t e l l i t e c o m m u n i c a t i o n p r o b l e m s . F i g u r e 12 show^s what may be expected a s a m i n i m u m sky t e m p e r a t u r e .

4. L a r g e D i s s i p a t i v e L o s s e s

Any path l o s s w^hich is d i s s i p a t i v e (as opposed to g e o m e t r i c d i s p e r s i o n ) w^ill behave a s a d i s s i p a t i v e r e s i s t a n c e network and will contribute n o i s e to the network output if the effective t e m p e r a t u r e of the network i s above the effective input t e m p e r a t u r e to the network. For s m a l l l o s s e s and low^ input t e m p e r a t u r e s , e a c h 0.1 db of l o s s in a network at r o o m t e m p e r a t u r e will contribute about 7° K to the s y s t e m noise t e m p e r a t u r e . For l a r g e l o s s e s , or w h e r e the l o s s o c c u r s at o t h e r than r o o m t e m p e r a t u r e , the network output t e m p e r a t u r e is given by;

^ o = ( ^ - i r ) ^ L + i r T i > • • • ( 1 0 )

w h e r e L = (power at input)/(power at output), T,. = t e m p e r a t u r e of l o s s network, 7

and T. = n o i s e t e m p e r a t u r e at input of l o s s network. F o r l a r g e l o s s e s the output t e m p e r a t u r e a p p r o a c h e s the l o s s t e m p e r a t u r e .

5. C o m m u n i c a t i o n s Equation — F o r m II

The C o m m u n i c a t i o n s Equation nnay now be m o r e conveniently stated a s :

P = 20 log r + 20 log f + 10 log B + L + C/N

+ 10 log Tg - [G^ + G^ + 196.2], dbw , • • • ( H )

w h e r e L includes all s y s t e m l o s s e s , d i s s i p a t i v e or o t h e r w i s e , and T is the sum of the r e c e i v e r and s o u r c e noise t e m p e r a t u r e s . S o u r c e noise t e m p e r a t u r e h e r e m e a n s the effective noise t e m p e r a t u r e p r e s e n t e d to the input t e r m i n a l s of the

r e c e i v e r , usually by a t r a n s m i s s i o n line, and is the s u m of all e x t e r n a l noise t e m p e r a t u r e contributions, modified by any l o s s e s which may o c c u r in the t r a n s m i s s i o n line (see L a r g e D i s s i p a t i v e L o s s e s , I V D 4 ) . F o r m II of the C o m m u n i

1,0 0 0 1 0 0 o I . 0 10 F R E Q U E N C Y ( G c ) 100

V. EQUIPMENT CONSIDERATIONS

Cost, convenience, and attainable quality of e l e c t r o n i c equipment a r e major f a c t o r s in d e t e r m i n i n g r e q u i r e d s a t e l l i t e c o m m u n i c a t i o n s s y s t e m p a r a m e t e r s . Equipment c o n s i d e r a t i o n s a r e thus an a n a l y s i s of a v a i l a b l e s t a t e - o f - t h e a r t , and a s e l e c t i o n of a n e a r o p t i m u m c o m p r o m i s e . C e r t a i n combinations of r e q u i r e -m e n t s frequently occur, or a r e frequently excluded; e . g . , t h e r e is little value in using a 10°K m a s e r in a low frequency s y s t e m w h e r e e x t e r n a l noise m a y run s e v e r a l thousand d e g r e e s . S i m i l a r l y , t h e r e is little point in c o n s i d e r i n g c o s m i c and side lobe noise w h e r e the s y s t e m is c o n s t r a i n e d to u s e poor quality (F = 15 db) r e c e i v e r s .

E q u i p m e n t c o n s i d e r a t i o n s m a y be conveniently divided into four c a t e g o r i e s : 1) Up-link (ground t r a n s m i t t e r and s a t e l l i t e r e c e i v e r )

2) Down-link ( s a t e l l i t e t r a n s m i t t e r and ground r e c e i v e r ) 3) C r o s s - l i n k s ( s a t e l l i t e - t o - s a t e l l i t e )

4) A c c e s s o r y s u b s y s t e m s .

A. U P - L I N K

The u p - l i n k is u s u a l l y not a limiting factor in a c o m m u n i c a t i o n s s a t e l l i t e s y s t e m , since l a r g e quantities of cheap power a r e u s u a l l y r e a d i l y a c c e s s i b l e on the ground and l a r g e , h i g h - g a i n antennas m a y be used. The S/N r a t i o in the u p - l i n k m a y thus be adjusted to w h a t e v e r level is r e q u i r e d m e r e l y by i n c r e a s i n g the ground t r a n s m i t t e r power a n d / o r t r a n s m i t t i n g antenna gain. A p o s s i b l e e x -ception to this g e n e r a l i z a t i o n would be the m i l i t a r y s a t e l l i t e c o m m u n i c a t i o n s s y s t e m , w h e r e p o r t a b l e field units might be both p o w e r - and a n t e n n a - s i z e limited. Some l i m i t s m a y be i m p o s e d on ground t r a n s m i t t e r p o w e r s by i n t e r -f e r e n c e c o n s i d e r a t i o n s , e i t h e r to o t h e r s a t e l l i t e s y s t e m s or to s u r -f a c e s y s t e m s s h a r i n g the s a m e s p e c t r u m a l l o c a t i o n s . Careful attention to t r a n s m i t t e r l o c a -tion and antenna side lobe r e s t r i c t i o n would p r o b a b l y p e r m i t the u s e of any ground t r a n s m i t t e r power n e c e s s a r y from a viewpoint of u p - l i n k S/N r a t i o .

Satellite r e c e i v i n g s y s t e m s a r e expected to have an effective s y s t e m t e m -p e r a t u r e of about 3000°K (F = 10 db), at l e a s t in the n e a r future. Cryogenic

s

or p a r a m e t r i c a m p l i f i e r s in s p a c e . By 1965 to 1970, h o w e v e r , we can expect to have 200° p a r a m e t r i c a m p l i f i e r s suitable for u s e in s a t e l l i t e s .

E v e n highly d i r e c t i v e s a t e l l i t e r e c e i v i n g a n t e n n a s m u s t view at l e a s t two m a j o r d i s c r e t e n o i s e s o u r c e s , the e a r t h and the sun. F o r t u n a t e l y , the sun s u b -tends only a s m a l l solid angle and is in view of a n a r r o w - b e a n n s a t e l l i t e antenna only a s m a l l f r a c t i o n of the t i m e . The sun view m a y be avoided c o m p l e t e l y by s e l e c t i n g a r e c e i v i n g beamwidth l e s s than the angle subtended by the e a r t h . F o r v e r y w i d e - b e a m a n t e n n a s , the s u n ' s t e m p e r a t u r e c o n t r i b u t i o n is r e d u c e d by the r a t i o of s o l a r solid angle to b e a m solid angle. C a r e m u s t a l s o be e x e r c i s e d to avoid having the sun in a m a j o r side lobe of the antenna, since this might a l s o c o n t r i b u t e s u b s t a n t i a l n o i s e . The e a r t h view is unavoidable and c o n t r i b u t e s a n o i s e t e m p e r a t u r e of 250 to 300°K.

P o w e r c o n s u m e d by the s a t e l l i t e r e c e i v e r is fairly independent of everything except r e q u i r e m e n t s for c o m p l e x i n f o r m a t i o n p r o c e s s i n g . In the future it m a y b e c o m e d e s i r a b l e to i n c o r p o r a t e c o m p l e x switching and logic functions in the s a t e l l i t e , but this would be for highly flexible s y s t e m s such a s d i r e c t global telephone s e r v i c e , or for a m i l i t a r y s y s t e m with high r e s i s t e n c e to j a m m i n g . At p r e s e n t , h o w e v e r , it a p p e a r s that all n e c e s s a r y i n f o r m a t i o n p r o c e s s i n g and switching m a y be p e r f o r m e d m o r e e c o n o m i c a l l y at the a s s o c i a t e d ground t e r -m i n a l s .

The power r e q u i r e m e n t of s a t e l l i t e r e c e i v e r s m a y be c o n s i d e r e d to have an upper limit of 10 w a t t s ; a c t u a l power consumption, using m o d e r n s o l i d - s t a t e e l e c t r o n i c s , is e x p e c t e d to be m u c h l e s s . An a l t e r n a t i v e viewpoint is to c o n -s i d e r the r e c e i v e r a -s an a m p l i f i e r c o m p r i -s i n g p a r t of the -s a t e l l i t e t r a n -s m i t t e r , and to include the r e c e i v e r p o w e r consumption in calculating the o v e r a l l effi-c i e n effi-c y of the t r a n s m i t t e r . F o r h i g h - p o w e r s a t e l l i t e t r a n s m i t t e r s the r e effi-c e i v e r power r e q u i r e m e n t b e c o m e s negligible in c o m p a r i s o n to the power d e m a n d of the final a m p l i f i e r i n the t r a n s m i t t e r .

It will p r o b a b l y be d e s i r a b l e to u s e the s a m e antenna for s i m u l t a n e o u s t r a n s m i s s i o n and r e c e p t i o n at both ground and s a t e l l i t e s t a t i o n s . This p l a c e s

m a y be r e a l i z e d by using opposite s e n s e s of c i r c u l a r p o l a r i z a t i o n for t r a n s m i s sion and r e c e p t i o n . H e l i c a l antennas a r e l e s s suitable for s i m u l t a n e o u s t r a n s m i s s i o n and r e c e p t i o n , since they a c c e p t only the s a m e p o l a r i z a t i o n a s is t r a n s -mitted, and can be designed to cover only a r e l a t i v e l y n a r r o w frequency r a n g e .

Antenna choice is f u r t h e r l i m i t e d by the a c c u r a c y with which attitude c o n -t r o l can be m a i n -t a i n e d . This does no-t a p p e a r -to be a s e v e r e p r o b l e m a s far as h i g h - q u a l i t y ground s t a t i o n s a r e c o n c e r n e d . Automatic t r a c k i n g of the antenna is p o s s i b l e , but this i m p o s e s additional s u b s y s t e m r e q u i r e m e n t s which m a y p r o v e awkward, p a r t i c u l a r l y in the s a t e l l i t e or in p o w e r - and s i z e - l i m i t e d

ground s t a t i o n s . When p r e c i s e attitude c o n t r o l cannot be nnaintained, the antenna beamwidth m u s t be b r o a d e n e d to c o m p e n s a t e for attitude v a r i a t i o n s . This, of c o u r s e , r e s u l t s in a r e d u c t i o n of u s a b l e gain. (See Section V-D for a d i s c u s s i o n of attitude c o n t r o l . )

B. DOWN-LINK

The down-link i s the w e a k e s t link in m o s t s a t e l l i t e c o m m u n i c a t i o n s s y s t e m s and, t h e r e f o r e , the link which m o s t influences the o v e r a l l s y s t e m S/N r a t i o . Available t r a n s m i t t e r power is l i m i t e d by c o n s i d e r a t i o n s of s a t e l l i t e weight and

s i z e , and for b r o a d c a s t s y s t e m s the design m u s t include allowance for p o o r l y o p e r a t e d and m a i n t a i n e d r e c e i v i n g s t a t i o n s of m e d i u m to low^quality. The ground r e c e i v i n g i n s t a l l a t i o n m u s t a l s o be designed to p e r f o r m s a t i s f a c t o r i l y in the face of p o s s i b l e i n t e r f e r e n c e from n e a r b y s o u r c e s .

F o r t u n a t e l y , both n a t u r a l and m a n - m a d e i n t e r f e r e n c e die out r a p i d l y above 100 Mc. Nonincidental m a n - m a d e i n t e r f e r e n c e from s u r f a c e s e r v i c e s c a n be avoided by c a r e f u l planning of s p e c t r u m a l l o c a t i o n s and by imposing tight quality r e g u l a t i o n s on p o t e n t i a l s o u r c e s of i n t e r f e r e n c e such as d i a t h e r m y equipment, r a d a r ovens, e t c . , which o p e r a t e in other s p e c t r u m r e g i o n s .

a v a i l a b l e in m i l l i o n s of m a s s - p r o d u c e d h o m e r e c e i v e r s . F u r t h e r , a s y s t e m d e s i g n e r cannot expect a p r i v a t e individual to invest m u c h money, skill, or effort in the i n s t a l l a t i o n and o p e r a t i o n of a s o p h i s t i c a t e d antenna s y s t e m .

Antenna c o n s i d e r a t i o n s a r e d i s c u s s e d in Appendix D. Table I s u m m a r i z e s p r e s e n t and p r o j e c t e d r e c e i v e r noise f a c t o r s , with c o s t s indicating the a p p l i c a -bility of each type to s y s t e m s of high, i n t e r m e d i a t e , and low ground i n v e s t m e n t .

Satellite t r a n s m i t t i n g equipment m a y have a v a r i e t y of f o r m s . Except for v e r y low-power t r a n s m i t t e r s , the final r a d i o - f r e q u e n c y (RF) amplifier stage of

the t r a n s m i t t e r will c o n s u m e m o s t of the APU output. T r a n s m i t t i n g equipment with power outputs of 20 kw and m o r e a r e c u r r e n t l y available, and power outputs of o v e r 100 kw should be a v a i l a b l e within a few y e a r s . F a i r l y long t r a n s m i t t e r l i f e t i m e s have a l r e a d y b e e n a t t a i n e d (thousands of h o u r s ) , and d e m o n s t r a t e d lifetimes of 10 to 30,000 h o u r s a r e expected s h o r t l y . Table II s u m m a -r i z e s p e -r t i n e n t c h a -r a c t e -r i s t i c s of v a -r i o u s p o s s i b l e s a t e l l i t e t -r a n s m i t t e -r s .

TABLE I

RECEIVER NOISE FIGURES* Type P r e s e n t f r e q u e n c y - m o d u l a t e d (FM) t u n e r 1965 F M t u n e r P r e s e n t TV u l t r a h i g h f r e -quency (UHF)t 1965 TV tunnel diode (TD) Advanced TD T r a v e l i n g wave tubes (TWT) Advanced TWT P a r a m e t r i c a m p l i f i e r s (PA) Cooled P A Advanced PA (cooled) M a s e r s Advanced m a s e r s Fs (db) 6 to 10 ' - 3 10 to 20 5 T r 1,200 to ~ 6 0 0 3,000 3,000 to 30,000 900 300 600 to ~ 4 0 0 100 to 50 to < 5 0 10 to ~ 2 1,000 200 100 100 Cost and Complexity low l o w low low low high m e d i u m m e d i u m high high v e r y high v e r y high *Noise t e m p e r a t u r e s do not include e x t e r n a l s o u r c e s of n o i s e .

F o c u s e d e l e c t r o n b e a m tubes such a s k l y s t r o n s , m a g n e t r o n s , and t r a v e l i n g -w a v e - t u b e s (TWT's) a p p e a r to be the m o s t p r o m i s i n g devices for the g e n e r a t i o n of R F p o w e r a t s p a c e - b a n d f r e q u e n c i e s . Efficiencies a s high a s 60% a r e quoted in the l i t e r a t u r e , but t h e s e a r e beann efficiencies and do not include power r e q u i r e d to o p e r a t e f i l a m e n t s , focusing s o l e n o i d s , e t c . P e r m a n e n t m a g n e t a s s e m -blies m a y be u s e d at low power l e v e l s , thus saving a p p r e c i a b l e p o w e r . At R F outputs of s e v e r a l kw the s u b s i d i a r y loads a r e g e n e r a l l y negligible c o m p a r e d to the b e a m power input. N e v e r t h e l e s s , it a p p e a r s that the o v e r a l l r a t i o of R F output to APU load s e l d o m e x c e e d s 30% in p r a c t i c a l o p e r a t i n g d e v i c e s .

It m a y be g e n e r a l l y s t a t e d t h a t t r a n s m i t t e r weight is an i n v e r s e function of frequency and a d i r e c t function of power level. Some indication of t h e s e r e l a -tionships i s given by the weight figures in Table II. A factor not indicated in Table II is the dependence of efficiency and power capability of a t r a n s m i t t e r upon the c h a r a c t e r of the s i g n a l to be t r a n s m i t t e d . F o r AM s i g n a l s , the power capability of the t r a n s m i t t e r is r e d u c e d 3 to 6 db, and the o v e r a l l efficiency is l i m i t e d to about 25%.

Solid s t a t e d e v i c e s such a s t r a n s i s t o r s and v a r a c t o r s do not a p p e a r feasible for outputs g r e a t e r than a few w a t t s n e a r the low frequency end of the s p a c e band. Tiinnel diodes a p p e a r to offer future p o s s i b i l i t i e s for h i g h p o w e r , h i g h -frequency output.

A final factor of significance in d e t e r m i n i n g APU r e q u i r e m e n t is the efficiency of p o w e r conditioning equipment. This equipment, which includes t r a n s -f o r m e r s , r e c t i -f i e r s , r e g u l a t o r s , and -f i l t e r s , is r e q u i r e d to c o n v e r t the APU out-put to the v a r i o u s v o l t a g e s and c u r r e n t s r e q u i r e d by the e l e c t r o n i c equipment, p r i n c i p a l l y the t r a n s m i t t e r . P o w e r conditioning equipment m a y be expected to have an efficiency of 80 to 90%. O v e r a l l efficiency of the t r a n s m i t t e r m a y then be e s t i m a t e d at 20% for AM s i g n a l s and 25% for F M .

C. CROSS-LINKS

Inter s a t e l l i t e r e l a y a p p e a r s o p e r a t i o n a l l y a t t r a c t i v e at f i r s t glance, since it offers the p o s s i b i l i t y of linkage between antipodal points without intervening ground r e l a y s . When one c o n s i d e r s this s y s t e m m o r e c l o s e l y , however, it b e -c o m e s l e s s a t t r a -c t i v e b e -c a u s e :

1) i n t e r s a t e l l i t e d i s t a n c e s m a y be v e r y l a r g e

2) not one, but two low quality r e c e i v e r s m u s t be included in the s y s t e m , one in each s a t e l l i t e

3) attitude c o n t r o l r e q u i r e m e n t s a r e v e r y s e v e r e since high gain antennas a r e d e s i r a b l e . Two additional axes of c o n t r o l a r e r e q u i r e d for each c r o s s - l i n k o p e r a t e d .

In g e n e r a l , the c r o s s - l i n k suffers from at l e a s t e v e r y w e a k n e s s of the o r d i n a r y downlink, and p o w e r i s r e q u i r e d for s i m u l t a n e o u s o p e r a t i o n of both the c r o s s -link and the u l t i m a t e down--link.

Since antipodal r e l a y can be a c c o m p l i s h e d by an intervening ground s t a t i o n / s , i n t e r s a t e l l i t e r e l a y does not a p p e a r a t t r a c t i v e for use in the i m m e d i a t e future.

D. ACCESSORY SUBSYSTEMS

P r o b a b l y the m o s t i m p o r t a n t of s u b s y s t e m functions is attitude c o n t r o l . In a s a t e l l i t e this c o n s i s t s of maintaining the beam axis of the antenna, and s o m e t i m e s the whole v e h i c l e , in the p r o p e r d i r e c t i o n . Long-lived attitude c o n t r o l s a r e usually g y r o s c o p i c and c o n s u m e e l e c t r i c p o w e r . A v e r a g e power d r a i n for such a s y s t e m might r a n g e from 10 to 200 w a t t s , depending on the vehicle m a s s , and including r e q u i r e m e n t s of a u x i l i a r y c o m m a n d and c o n t r o l c i r c u i t s .

At ground i n s t a l l a t i o n s , attitude c o n t r o l m e a n s t r a c k i n g both a z i m u t h and elevation of the antenna to follow the motion of the s a t e l l i t e . P r o v i s i o n of t r a c k -ing ability is quite expensive, usually constitut-ing at l e a s t half of the t o t a l antenna cost. In the c a s e of s m a l l and inexpensive a n t e n n a s , t r a c k i n g ability m a y cost m a n y t i m e s a s m u c h a s the antenna p r o p e r . P r e s e n t l y a v a i l a b l e antenna r o t a t o r s cost $25 or m o r e and p r o v i d e m a n u a l c o n t r o l of a z i m u t h only. T r a c k i n g ability ^Initial e s t a b l i s h m e n t of attitude r a t e s would p r o b a b l y be done with r e a c t i v e gas j e t s . It does not a p p e a r p o s s i b l e to p e r m a n e n t l y e s t a b l i s h d e s i r e d r a t e s , and

is thus not e c o n o m i c a l l y feasible for home b r o a d c a s t r e c e i v e r s . T h e s e f a c t o r s e m p h a s i z e the d e s i r a b i l i t y of a s t a t i o n a r y orbit, for which the ground antenna position need only be adjusted during initial i n s t a l l a t i o n .

Another s u b s y s t e m of p o s s i b l e i m p o r t a n c e in c o m m u n i c a t i o n s s a t e l l i t e s is s t a t i o n - k e e p i n g f a c i l i t i e s . T h e s e a r e n e c e s s a r y w h e r e the m a i n t e n a n c e of one or m o r e specific orbit p a r a m e t e r s is e s s e n t i a l to the function of the s y s t e m , as in the c a s e of a s t a t i o n a r y orbit depending on fixed ground a n t e n n a s . Orbit p e r t u r b a t i o n s make p e r m a n e n t e s t a b l i s h m e n t of any specific orbit p r a c t i c a l l y i m p o s s i b l e , and p r o v i s i o n m u s t be made for o c c a s i o n a l application of t h r u s t for orbit c o r r e c t i o n s . F o r h i g h - p o w e r e d s a t e l l i t e s , t h e s e t h r u s t s m a y be provided by t e m p o r a r i l y switching the APU output to an e l e c t r i c p r o p u l s i o n device. In any event, a u x i l i a r y c o m m a n d and c o n t r o l c i r c u i t s a r e r e q u i r e d .

VI. MISCELLANEOUS CONSIDERATIONS

T h e r e a r e a n u m b e r of f a c t o r s , a s i d e f r o m t r a n s m i s s i o n and o r b i t c o n s i d e r a -t i o n s , which m a y influence s y s -t e m o p e r a -t i o n s . Some of -t h e s e a r e no-ted in -the following s e c t i o n s .

A, TIME DELAYS IN PROPAGATION

T r a n s m i s s i o n s a r e p r o p a g a t e d at the finite speed of 3 x 10 k m / s e c , and for s o m e of the paths to be c o n s i d e r e d t h e r e i s an a p p r e c i a b l e delay between t r a n s -m i s s i o n and r e c e p t i o n . In the c a s e of s t a t i o n a r y s a t e l l i t e s , the delay i s 0.27 sec for a p a t h l e n g t h o f 40,000 k m . F o r one-way t r a n s m i s s i o n s this delay i s of no s e r i o u s c o n s e q u e n c e . In twoway l i n k s , h o w e v e r , the delay m a y have significant c o n -s e q u e n c e -s . C o n -s i d e r an a t t e m p t to c o n t r o l a m e c h a n i -s m -such a -s a r e m o t e

m a n i p u l a t o r by a c o m m a n d and information link r e l a y e d via a s t a t i o n a r y s a t e l l i t e . The position r e p o r t e d to the o p e r a t o r l a g s the t r u e position by 0. 27 s e c , and an additional 0. 27 s e c m u s t e l a p s e before the o p e r a t o r ' s c o n t r o l o p e r a t i o n s b e c o m e effective at the m a n i p u l a t o r . This is m o r e unfavorable than lengthening the o p e r a t o r ' s r e a c t i o n t i m e by 0. 54 s e c .

A second i m p o r t a n t effect of propagation delays is found in o p e r a t i o n of

c o m m o n - c a r r i e r telephone c i r c u i t s . In itself, a delay of l e s s than a few seconds offers no s e r i o u s c o n v e r s a t i o n i m p e d i m e n t . T h e r e i s , h o w e v e r , a c e r t a i n

amount of coupling between t r a n s m i t and r e c e i v e c i r c u i t s which a r i s e s a s a r e s u l t of the n a t u r e of s t a n d a r d telephone c i r c u i t design, and t h i s c a u s e s s o m e of the r e c e i v e d signal to be r e t r a n s m i t t e d a s an echo. An echo delay of a few t e n t h s of a second i s e x t r e m e l y annoying, leading t o drawing, s t a m m e r i n g , a n d often t o complete d i s r u p t i o n of speech ajid thought. A v a r i e t y of t e c h n i q u e s exist for m i t i g a t i n g or eliminating the echo effect, with v a r y i n g d e g r e e s of s u c c e s s . It i s g e n e r a l l y a g r e e d that new, i m p r o v e d t e c h n i q u e s can and will be developed to deal with t h i s p r o b l e m at t h e ground t e r m i n a l s .

B . DELAYED PROGRAM TRANSMISSION

T h e r e a r e s e v e r a l o c c a s i o n s w h e r e an intentionally i n t r o d u c e d delay in t r a n s -m i s s i o n -m a y be o p e r a t i o n a l l y a d v a n t a g e o u s . An e x a -m p l e of t h i s technique i s the

a suitable position is r e a c h e d . The effective s u r f a c e r a n g e of low altitude

s a t e l l i t e s m a y thus be i n c r e a s e d . Different r e c o r d i n g and playback s p e e d s m a y be employed, and it is p o s s i b l e to r e c o r d an extended p r o g r a m into the r e c o r d e r in a few m i n u t e s a s the s a t e l l i t e p a s s e s over a ground c o n t r o l s t a t i o n . The r e c o r d i n g m a y then be r e p l a y e d at n o r m a l speed, providing p r o g r a m m a t e r i a l until the s a t e l l i t e p a s s e s within r a n g e of a n o t h e r (or the s a m e ) ground c o n t r o l

station. A low altitude s a t e l l i t e could thus provide b r o a d c a s t s in a p p r o p r i a t e language a s it p a s s e s in s u c c e s s i o n over different g e o g r a p h i c a l a r e a s . The bandwidth capability of p r e s e n t r e c o r d e r s suitable for space is somewhat l i m i t e d . It a p p e a r s l i k e l y however, that r e c o r d e r s capable of dealing with t e l e v i s i o n signal bandwidths will soon be a v a i l a b l e .

C. PINPOINTING

VII. EXAMPLE CALCULATIONS

A n u m b e r of e x a m p l e c a l c u l a t i o n s have been c a r r i e d out to i l l u s t r a t e the m e t h o d s developed, to indicate a p p r o p r i a t e s e t s of a s s u m p t i o n s , and to p r o v i d e quantitative indication of the amounts of o n - b o a r d power r e q u i r e d for v a r i o u s types of s e r v i c e . No p a r t i c u l a r effort h a s been made to indicate optimum or e c o n o m i c a l s y s t e m d e s i g n s , the intent h e r e being to provide e x e r c i s e s of i l l u s t r a -tive v a l u e . Unless o t h e r w i s e indicated it is a s s u m e d that the down-link i s the s y s t e m limiting f a c t o r .

A " w o r s t - c a s e " philosophy h a s been adopted; i . e . , d i s t a n c e s , antenna orientation e r r o r s , e t c . , a r e a s s u m e d to have the m a x i m u m v a l u e s d i c t a t e d by c o n s i d e r a t i o n s of o r b i t g e o m e t r y . No m a r g i n h a s been allowed for unknown f a c t o r s . In s u r f a c e - l i n k design, it h a s been c u s t o m a r y to i n s e r t m a r g i n allow-a n c e s rallow-anging from 10 to 50 db, depending on the n allow-a t u r e of the s y s t e m allow-and the c o m m u n i c a t i o n s r e l i a b i l i t y d e s i r e d . This i s n e c e s s a r y b e c a u s e of the l a r g e n u m b e r of p e r t u r b a t i o n s and i n t e r f e r e n c e s to which a s u r f a c e link is subject. Satellite c o m m u n i c a t i o n s , on the other hand, is subject to only a few fairly well defined p e r t u r b a t i o n s .

T h e r e a p p e a r to be two schools of thought on the subject of m a r g i n allowance in s a t e l l i t e c o m m u n i c a t i o n s . Both g r o u p s a g r e e that s m a l l m a r g i n should be r e -quired, but one group contends that a m a r g i n allowance of a few tenths of a db i n d i c a t e s " r a t h e r sloppy d e s i g n . " The o t h e r group feels that s a t e l l i t e p r o p a -gation is not yet sufficiently u n d e r s t o o d , and that a m a r g i n of about 6 db should be included after a l l know^n f a c t o r s have been c o n s i d e r e d .

s a t e l l i t e b r o a d c a s t i n g , but the e r a of difficulties should not e n d u r e so long, nor should it be a s s e v e r e a s was the c a s e with e a r l y TV.

A. E X A M P L E A - F M BROADCASTING FROM A LOW ORBIT 1. Mission Specifications

P r o v i d e a b r o a d c a s t signal of at l e a s t PASSABLE'" quality, r e c e i v a b l e by p r e s e n t typical home F M r e c e i v e r s , from a s a t e l l i t e in a 2 - h r c i r c u l a r p o l a r o r b i t . The p r o g r a m m a y c o n s i s t of both spoken and m u s i c a l m a t e r i a l and should m e e t the fidelity s t a n d a r d s of p r e s e n t United States F M b r o a d c a s t p r a c t i c e .

2. A n a l y s i s

P r e s e n t FM b r o a d c a s t p r a c t i c e s call for the t r a n s m i s s i o n of modulating signal components up to f r e q u e n c i e s of 15 k c . The modulation index m^, i s 5.0, and above t h r e s h o l d the F M i m p r o v e m e n t factor i s 3 m,. = 75 = 18.7 d b . ' An additional i m p r o v e m e n t due to s t a n d a r d p r e e m p h a s i s (see F i g u r e B-5), of 19 db can be added, but it is unlikely that many r e c e i v e r s can r e a l i z e m o r e than 15 db of t h i s p o t e n t i a l . The total detection i m p r o v e m e n t i s thus about 34 db and the r e -quired p r e d e t e c t i o n C/N is 20 - 34 = -14 db in a 30 kc bandw^idth. This figure is far below the p r e d e t e c t i o n 13 db t h r e s h o l d , how^ever, and m o s t of the F M i m -p r o v e m e n t i s l o s t in the f i r s t 3 db below t h r e s h o l d . It thus a -p -p e a r s n e c e s s a r y to p r o v i d e a p r e d e t e c t i o n C/N of about 10 db in the p r e d e t e c t i o n bandwidth of 200 kc for PASSABLE r e c e p t i o n , and E X C E L L E N T r e c e p t i o n will be e x p e r i e n c e d w^ith C/N = 13 db or g r e a t e r .

Since the a p p a r e n t motion of a s a t e l l i t e in a 2 - h r o r b i t is fairly rapid, and since a r e c e i v i n g antenna t r a c k i n g s y s t e m does not a p p e a r e c o n o m i c a l l y feasible for home i n s t a l l a t i o n s , a low gain r e c e i v i n g antenna m u s t be s e l e c t e d . A half-wave dipole h a s a beamwidth of 78° in the h o r i z o n t a l d i r e c t i o n and can be made fairly uniform in a v e r t i c a l plane by p r o p e r a d j u s t m e n t of antenna height. A r e c e i v i n g antenna gain of about 2.2 db can thus be r e a l i z e d for s a t e l l i t e p a s s e s in the antenna n o r m a l p l a n e , and the gain will be no l e s s than -0.7 db for p a s s e s within 39° of this p l a n e . The d i r e c t i o n of the antenna m u s t be aligned to coincide with the d i r e c t i o n of the s a t e l l i t e ground point t r a c k for n e a r - o v e r h e a d p a s s e s of *See Appendix C for quality definitions