PRESENT AND FUTURE FREQUENCY AND TIMING CAPABILITIES OF THE DEEP SPACE NETWORK

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PRESENT AND FUTURE FREQUENCY AND TIMING CAPABILITIES OF THE DEEP SPACE

NETWORK

P. Kuhnle, R. Sydnor

To cite this version:

P. Kuhnle, R. Sydnor. PRESENT AND FUTURE FREQUENCY AND TIMING CAPABILITIES OF THE DEEP SPACE NETWORK. Journal de Physique Colloques, 1981, 42 (C8), pp.C8-373-C8-381.

�10.1051/jphyscol:1981844�. �jpa-00221740�

JOURNAL DE PHYSIQUE

CoZZoque C8, suppl6ment au n012, Tome 42, d6cembre 1981 page C8-373

PRESENT AND FUTURE FREQUENCY AND TIMING CAPABILITIES OF THE DEEP SPACE NETWORK

P.F. Kuhnle and R.L. Sydnor

Communications Systems Research S e c t i o n

J e t Propulsion Laboratory, C a l i f o r n i a I n s t i t u t e o f Technology, Pasadena, California, U.S.A.

Abstract.- The world-wide Deep Space Network (DSN), with f a c i l i t i e s i n t h e United S t a t e s , Spain, and A u s t r a l i a , uses atomic frequency s t a n d a r d s t o gen- e r a t e s t a b l e frequency and timing s i g n a l s t o meet t h e t e l e m e t r y and t r a c k i n g requirements f o r space e x p l o r a t i o n missions. Outer p l a n e t n a v i g a t i o n r e q u i r e s doppler accuracy of 30 micrometers per second, range accuracy of two m e t e r s , angle accuracy of a few nanoradians ( u s i n g Very Long B a s e l i n e I n t e r f e r o m e t r y (VLBI) t e c h n i q u e s ) , and high r e l i a b i l i t y . These needs impose s t a b i l i t y r e - quirements on frequency s t a n d a r d s of p a r t s i n 1014 over s e v e r a l hours and 1012 a t one second. I n a d d i t i o n , a low d r i f t r a t e i s n e c e s s a r y so a s t o mini- mize t h e number of measurements needed t o m a i n t a i n c l o c k s y n c h r o n i z a t i o n t o

100 nanoseconds and frequency s y n t o n i z a t i o n t o 3 x 10-l3 between t h e t h r e e complexes. The J e t P r o p u l s i o n Laboratory (JPL), which o p e r a t e s t h e DSN f o r t h e N a t i o n a l Aeronautics and Space A d m i n i s t r a t i o n (NASA), uses ensembles of a c t i v e hydrogen masers, cesium beam frequency s t a n d a r d s , and high r e s o l u t i o n c l o c k s t o achieve t h e r e q u i r e d performance r e l i a b l y . Time s y n c h r o n i z a t i o n between complexes i s c u r r e n t l y achieved by t h e use of VLBI t e c h n i q u e s , t r a v e l i n g c l o c k s , and Loran-C. Use of t h e Global P o s i t i o n i n g S a t e l l i t e s (GPS) i s under a c t i v e i n v e s t i g a t i o n a s a l e s s c o s t l y and more a c c u r a t e a l t e r n a t i v e . This a r t i c l e covers t h e above requirements, equipment, t e c h n i q u e s , and more s t r i n g e n t requirements of t h e 1990 time frame.

I . I n t r o d u c t i o n . - The United S t a t e s NASA DSN i s a world-wide ground s t a t i o n

s p a c e c r a f t t r a c k i n g network. This network i s t h e t r a c k i n g and communication l i n k with the s p a c e c r a f t t o s t u d y t h e o t h e r p l a n e t s i n t h i s s o l a r system. Some of t h e s e missions have been Voyager, Viking, Mariner s e r i e s , Pioneer s e r i e s , and H e l i o s . The DSN c o n s i s t s of t h r e e complexes, each w i t h t h r e e t o f o u r antennas. The complexes a r e l o c a t e d i n Spain, A u s t r a l i a , and t h e United S t a t e s .

The DSN u s e s frequency and time a s t h e b a s i c measuring elements f o r naviga- t i o n of s p a c e c r a f t and f o r t h e performance of VLBI, r a d i o s c i e n c e , and space exper- iments. The p r e c i s i o n needed t o perform measurements imposes s t r i n g e n t timing re- quirements on t h e Frequency and Timing Subsystem (FTS) of t h e DSN. The p r e s e n t and f u t u r e needs and c a p a b i l i t i e s of t h e FTS a r e d e s c r i b e d i n t h i s a r t i c l e , a s w e l l a s performance d a t a on t h e system components being i n s t a l l e d . A system block diagram i s presented ( f i g u r e 1 ) t o show t h e o v e r a l l f u n c t i o n of t h e FTS.

11. Background and H i s t o r y of t h e DSN Frequency Standards. - The DSN has used atomic o s c i l l a t o r s a s frequency r e f e r e n c e s and c l o c k s s i n c e t h e e a r l y 1960's. I n 1961 cesium beam s t a n d a r d s were used i n a Venus r a d a r experiment; i n 1964 rubidium vapor s t a n d a r d s , developed under c o n t r a c t t o JPL, were i n o p e r a t i o n a l use; t h e

f i r s t experimental hydrogen masers were used i n t h e DSN i n 1968; by 1975 t h e f i r s t hydrogen maser was o p e r a t i o n a l i n t h e DSN; by 1983 each s t a t i o n w i l l o p e r a t e w i t h

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981844

TO COMPLEX MONITOR AND CONTROL I 100 MHz FREQUENCY REFERENCE ::aTREF.

1

FREQUENCY

I

BATTERY BA~K-UP

-

FREQ ST0 SWITCH t COHERENT REFERENCE GENERATOR

AUXl LlARY REFERENCE MONITOR CLOCKS 141 - I I I EXT TIME 5 MHz CLOCK TIME CODE TIME CODES SIMULATIU4 MASTER REF O REF. FREQ. INSERTION

-

TRANSLATORS AND TIMING 1128 MAX. I

(I - TlME RCVR LORAN-C. N, GPS STABILITY ANALYZER REFERENCE

ANTENNA CONE1 S I

I

CABLE STABILIZER TRANSMInER

-1

I

-

RECEIVER I USERS I

two hydrogen masers and two cesium beam s t a n d a r d s ; t h e DSN of 1990 w i l l i n c o r p o r a t e even more advanced s t a n d a r d s , p o s s i b l y trapped i o n s , s u p e r c o n d u c t i n g c a v i t i e s , and hydrogen masers.

This e v o l u t i o n a r y growth was a r e s u l t of t h e needs of t h e v a r i o u s m i s s i o n s and t h e i n c r e a s i n g s o p h i s t i c a t i o n of t h e n a v i g a t i o n and t h e experiments a s t h e s p a c e c r a f t f l i g h t s moved from t h e moon t o Venus, Mars, and t h e o u t e r p l a n e t s .

111. Needs and C a p a b i l i t i e s o f t h e DSN (1981-19861.- Performance r e q u i r e m e n t s of t h e DSN FTS a r e shown i n T a b l e s I and 11. These a r e t h e most s t r i n e e n t r e a u i r e m e n t s

-

from a l l m i s s i o n s i n t h i s time p e r i o d . I n a d d i t i o n , t h e FTS must meet o t h e r re- q u i r e m e n t s , a s follows:

1 ) G e n e r a t i o n and d i s t r i b u t i o n of t h e r e f e r e n c e f r e q u e n c i e s needed i n a s t a t i o n .

2) G e n e r a t i o n and d i s t r i b u t i o n of t h e v a r i o u s time codes and t i m i n g p u l s e s needed i n a s t a t i o n .

3 ) S y n c h r o n i z a t i o n of t h e t h r e e complexes w i t h i n t h e DSN t o 50 microseconds w i t h knowledge of time w i t h i n 10 microseconds and between t h e m a s t e r c l o c k bank a t Goldstone and t h e N a t i o n a l Bureau of S t a n d a r d s (NBS) t o 50 microseconds w i t h know- l e d g e of time t o w i t h i n 5 microseconds.

4 ) S y n t o n i z a t i o n of t h e t h r e e complexes t o 3 x 10-13.

5 ) Monitor and c o n t r o l of t h e FTS through each s t a t i o n ' s m a s t e r c o n t r o l com- p u t e r t o e n s u r e proper o p e r a t i o n and t o change c o n f i g u r a t i o n a s n e c e s s a r y .

6 ) O p e r a t i o n a l r e q u i r e m e n t s on r e l i a b i l i t y , a v a i l a b i l i t y , m a i n t a i n a b i l i t y , t r a i n i n g

,

F i g u r e 1 shows a block diagram of t h e FTS designed t o meet t h e above r e q u i r e - ments.

Frequency Standards.- F i g u r e 2 shows t h e A l l a n v a r i a n c e performance r e q u i r e m e n t s f o r s t a b i l i t y , a s w e l l a s t h e performance of t y p i c a l hydrogen masers a s measured over t h e l a s t t h r e e y e a r s . I n o r d e r t o a c h i e v e t h i s performance, t h e hydrogen masers must be housed i n an i s o l a t e d , t e m p e r a t u r e - c o n t r o l l e d room w i t h 0.05OC v a r -

i a t i o n i n t e m p e r a t u r e . The measured performance u p t u r n a t 103 s e c o n d s , a s shown i n f i g u r e 2, i s b e l i e v e d t o be caused p a r t l y by aging and p a r t l y by environmental e f f e c t s . The new masers being b u i l t f o r t h e DSN have d e s i g n improvements t o reduce t h e a g i n g e f f e c t s . They w i l l be t e s t e d i n t h e I n t e r i m Frequency S t a n d a r d T e s t F a c i l i t y (IFsTF) a t J P L t o determine i f t h e d e s i g n g o a l of 2-3 t i m e s improvement over t h e p r e s e n t 1 x 10-14/day t y p i c a l a g i n g r a t e h a s been o b t a i n e d . A s e t of two hydrogen m a s e r s (one prime, t h e o t h e r a backup) and two backup cesium beam s t a n d - a r d s w i l l be used a t each complex t o a c h i e v e t h e r e q u i r e d r e l i a b i l i t y and a l l o w continuous m o n i t o r i n g of frequency s t a b i l i t y . The requirements on t h e secondary s t a n d a r d s a r e n o t a s s t r i n g e n t a s t h e y a r e on t h e prime s t a n d a r d . F i g u r e 3 shows t h e requirement w i t h t y p i c a l performance o f t h e cesium beam s t a n d a r d s . These s t a n d - a r d s r e a d i l y meet t h e r e q u i r e m e n t .

Reference Frequency D i s t r i b u t i o n . - The e x i s t i n g frequency s y n t h e s i s and d i s t r i b u - t i o n equipment w i l l be expanded from 155 t o 210 o u t p u t p o r t s t o s u p p o r t a d d i t i o n a l u s e r s . Seven 100-MHz r e f e r e n c e frequency o u t p u t p o r t s w i l l be added t o improve t h e s t a b i l i t y of t h e r e c e i v e r and t r a n s m i t t e r .

An a c t i v e c a b l e compensator w i l l b e used t o d i s t r i b u t e r e f e r e n c e f r e q u e n c i e s t o t h e antenna a r e a f o r VLBI measurements, with equipment p r o p a g a t i o n d e l a y known

JOURNAL DE PHYSIQUE

Sampling Time

Table 1

FREQUENCY STANDARD REQUIREMENTS 1981-1986

A l l a n Variance

Primary Secondary

Standard Standard

1 second 1 x 10-l2 1 x 10-11

lo4 seconds 1 x 10-l4 3 x 10-13

12 hours 1 x 10-l4 3 x 10-13

10 days 1 x 10-l3 3 x 10-13

Power S p e c t r a l D e n s i t y o f Phase: BW = 1 Hz f 10 Hz Reference

D i s t r i b u t i o n Frequency

Primary Standard

Secondary S t a n d a r d 5 MHz B e t t e r t h a n -116 dB B e t t e r t h a n -110 dB 10 MHz B e t t e r t h a n -110 dB B e t t e r t h a n -104 dB 50 MHz B e t t e r than -96 dB B e t t e r t h a n -90 dB 100 MHz B e t t e r than -90 dB B e t t e r t h a n -84 dB

T a b l e I1 TIMING REQUIREMENTS

1981-1986

Timing P u l s e s

Rate 1, 10, 100, 1000 PPS

~ i t t e r ( l ) 10 n s RMS ( l u ) R i s e t i m e 15 ns

S e t t a b i l i t y 100 ns s t e p s

( 1 ) 2 ns RMS (1u) f o r s e l e c t e d u s e r s

Time Codes

36-bit b i n a r (ms of day, day of y e a r ) 30-bit BCO(Z~ (s e c o f day, day of y e a r )

46-bit b i n a r y ( 3 ) (pet of ms, ms of d a y , day of y e a r )

( 2 ) E x i s t i n g u s e r s only.

( 3 ) S e l e c t e d u s e r s only.

_ _rl_î <-, ; J » s i i i » i ; i i s t i i ! i i i < i i i « * i ? ) i ) I DU ? 3 * *> i i l ï i I 1 i ï i i i ) i in FT 1 ' '—'•''•') 1 1 '—' III')—~ 1 1 1—i i i • i : ! !—t. '. ' "i 1 1 1—i t _. i i l ni i i i i i t t iU xC I I I i : J- \. O PRIMARY FREQUENCY STANDARD REQUIREMENTS 1981 TO 1986 £ \ '//////A MEASURED HYDROGEN MASER PERFORMANCE FOR THE DSN ~JL3 '////S. *AJS%- _S, - j i J-ta •IU )- AAA/A/A'//^^ - V " ~~ " —— ~— • jul l-*//V//-/•/•//JV -A _#^ - >. £// f / S /////// f>^ . . ^ _^ - , ///////////////\ x. _^ *~* ^*%s/s s • / //////h^ ^W ^f ^fif'/' r** 1/1 Xt^ *//-/////^//^_ x. __^_^ '•••<'• M ,._ • • ^&'A'AAA'AA&K. • sj/rT*///***--,/,,, i.. ^<£'AAAAAAAAA*ÏT^- • _^///-</////.i///// *. «._ _ ^CAAA'AA/AAsVSk ^r?///s/////s/.\///// CM * ^^,AAAAAAAAA,7 ^. ^0r?//////ssf//// //s// v_- i - - - - ^^/AAAAAAAAA? ^w - - - -^r?'"""/'"///* •••••• - ^CAAAAAAAA, 'STV + JtrT'r/sss/ssssss//// v///y b<-~ ^^'/AAA'AAtVS*. -__" fyr/rs/s,/////s/////s.i/,//, ^V."/'/, '/S"<r^^ _^>//////////-////////////'/ •///// X^x/ V///////7V J__^^'"*'//s//////////////s////y// •//?/ ^^x •//////////^Wfc___ ^^/^^//////s//s/ //s//s////ss///////// • >^____*^ ' - ^\**t*/f**sssss/sj}>l M%wr77/ /, / s / / / / s / s / / / / s ?? s / , / / / , / / / / / , / ,-V^** r ^/ ////////// ///////. J - _ 10*^ >^///////////////, ///////////////////////////_>^^ - *- ^_//-/Vy'//V//••••/-, ////f//f////////f//// //____#! '^^ — I__5 ^^///s/////s//. /////S//////////S///1 ^^ -i _-, ^^ i- • •• — -• —•• ' ^t/y/////^./^ Vi'1«>'iS'i<yy/y>')V>'y»jw^ - • ... --. — —• " - •• - » J.U ï- - ^t_//v •///'//'/•, •///////s//////jji^^ •• i-— ^/_///, y////^_v^»^^ "I" | , | 1 | , i i,| | j | | , , ,,| ..I . I I I I J llj 1 I I I I i ni . I « t 1 i i i il I I I 1 • i .. 1 10 102 103 104 105 106 T, seconds ? Figure 2. Allan Variance Requirements Compared to Actual Performance ^J

n CO W .. a 4

1

5

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ldllX~.

* SECONDARY STANDARD REQU I REMENTS

1981- 1986 - MEASURED PERFORMANCE ON CES 1 UM BEAM FREQUENCY STANDARDS WITH lo- l2 HIGH PERFORMANCE TUBES. 1 10 lo2 lo3 1 o4 lo5 lo6

seconds

*- - -*

-

-13 -10

b 10- l4 10- l5

- --- *,

t o l e s s than 1 nanosecond and phase s t a b i l i t y of l e s s then 1.1 picoseconds ( f o r root-mean-square (RMS) sampling times l e s s t h a n 10 seconds).

Time and Timing D i s t r i b u t i o n . - The e x i s i n g d i g i t a l c l o c k w i l l be r e p l a c e d w i t h a master c l o c k with m a j o r i t y v o t i n g of a l l time codes and timing p u l s e s generated by

t h e t h r e e time code g e n e r a t o r s to meet t h e performance requirements of Table I1 and provide high r e l i a b i l i t y .

A modified I R I D G ( ~ ) time code w i l l be s u p p l i e d t o each s e r i a l - t o - p a r a l l e l time code t r a n s l a t o r which i n t e r n a l l y g e n e r a t e s a time code from an e x t e r n a l 5-MHz r e f e r e n c e . This time code i s synchronized t o t h e incoming modified IRIG-G time code. The r e q u i r e d output time codes and pulse t r a i n s a r e generated synchronously with t h e IRIG-G code t o g i v e day of year i n m i l l i s e c o n d s . The propagation d e l a y t o each of t h e 128 t r a n s l a t o r s a t each s i t e can be a d j u s t e d to w i t h i n 100 nanoseconds of epoch a t d i s t a n c e s up t o 1000 meters from t h e master clock. This d i s t a n c e i s s u f f i c i e n t to reach each of t h e co-located antennas a t a l l t h r e e complexes.

Each complex r e q u i r e s s i m u l a t i o n time ( a time o f f s e t up t o one year from UTC, Universal Time, Coordinated) f o r t r a i n i n g and d i a g n o s t i c purposes. This w i l l be provided by a d d r e s s i n g t h e a p p r o p r i a t e s e t of t r a n s l a t o r s t o read s i m u l a t i o n time while t h e balance of t h e complex w i l l remain on r e a l time (UTC) f o r m u l t i p l e mission c a p a b i l i t y .

Time Synchronization.- A bank of seven cesium beam frequency s t a n d a r d s i s maintain- ed a t t h e Goldstone complex f o r both time s y n c h r o n i z a t i o n and frequency syntoniza- t i o n . A cesium beam t r a v e l i n g clock i s t r a n s p o r t e d from Goldstone t o t h e United S t a t e s NBS f o r t h e time s y n c h r o n i z a t i o n a t 60-day i n t e r v a l s . Time s y n c h r o n i z a t i o n between t h e t h r e e g l o b a l complexes i s performed by t r a v e l i n g cesium beam c l o c k s from t h e United S t a t e s Naval Observatory (USNO) and by VLBI, w i t h d a i l y checks per- formed u s i n g Loran-C i n Spain and t h e "TV Mutual ~ o o k " ( ~ ) i n A u s t r a l i a w i t h t h e Division of N a t i o n a l Mapping. The accuracy of t h e s e t e c h n i q u e s f o r time synchroni- z a t i o n i s more t h a n adequate t o meet t h e DSN requirements.

Frequency Syntonization.- The p r e s e n t method of frequency s y n t o n i z a t i o n u s e s VLBI time synchronization a t weekly i n t e r v a l s . The r e s o l u t i o n of t h e s e measurements i s 30 t o 40 nanoseconds so t h a t the s y n t o n i z a t i o n i s r e a d i l y achieved a t t h e 1 x 10-l3 l e v e l . I n t h e near f u t u r e t h e DSN p l a n s t o use time s y n c h r o n i z a t i o n r e c e i o e r s a t t h e t h r e e complexes and a t NBS t h a t u t i l i z e t h e Global P o s i t i o n i n g S a t e l l i t e s . Pre- liminary t e s t s conducted by NBS f o r t h e DSN i n d i c a t e t h a t 10- t o 100-nanosecond time s y n c h r o n i z a t i o n i s a c h i e v a b l e , a l l o w i n g both time s y n c h r o n i z a t i o n and f r e - quency s y n t o n i z a t i o n requirements t o be met more a c c u r a t e l y , economically, and r e l i a b l y than by the use of VLBI.

I V . Needs and C a p a b i l i t i e s of t h e DSN (1986-1990) Frequency Standards.- F u r t h e r missions to t h e o u t e r p l a n e t s a r e planned which w i l l r e q u i r e b e t t e r n a v i g a t i o n and measurement c a p a b i l i t y i n t h e DSN. Experiments on g r a v i t y waves and r e l a t i v i t y a r e

( 1 ) IRIG-G d e f i n i t i o n : t h e format f o r s e r i a l time code t o t h e n e a r e s t 10 m i l l i - seconds defined by t h e I n t e r r a n g e I n s t r u m e n t a t i o n Group now renamed a s t h e Range Commanders Council

.

( 2 ) This method i s used t o determine t h e time d i f f e r e n c e of two c l o c k s by s i m - u l t a n e o u s l y measuring t h e time of a r r i v a l of t e l e v i s i o n v e r t i c a l synchroniza- t i o n p u l s e s a t each c l o c k and c o r r e c t i n g t h e r e c e i v e d d a t a f o r propagation delays from t h e source t o each u s e r .

JOURNAL DE PHYSIQUE

a l s o planned. Based on t h e s e p l a n s , t h e m i s s i o n r e u i r e m e n t s on t h e frequency s t a n - d a r d s i n t h e DSN f o r an A l l a n v a r i a n c e of 3 x 10-I? from 100 seconds t o 30 days w i t h a g o a l o f 1 x 10-l7 from 100 seconds t o 25,000 seconds. F i g u r e 4 shows t h e

t y p i c a l performance of t o d a y ' s a c t i v e hydrogen masers t e s t e d a t t h e JPL IFSTF and t h e r e q u i r e d f u t u r e frequency s t a b i l i t y performance. I t i s e v i d e n t t h a t frequency s t a n d a r d s w i l l be r e q u i r e d t h a t a r e not a v a i l a b l e today. JPL i s developing equip- ment and s u p p o r t i n g work i n s e v e r a l a r e a s t o meet t h e s e r e q u i r e m e n t s . Cryogenic a c t i v e hydrogen m a s e r s may meet t h i s requirement i n t h e s h o r t - t e r m s t a b i l i t y r e g i o n (e.g., t o 10,000 s e c o n d s ) and p o s s i b l y a t l o n g e r a v e r a g i n g times on t h e o r d e r of days. F e a s i b i l i t y o f t h i s approach has n o t been demonstrated and o t h e r methods may be n e c e s s a r y t o s a t i s f y t h e r e q u i r e m e n t s . Superconducting c a v i t i e s may be n e c e s s a r y t o a c h i e v e t h e s h o r t - t e r m performance g o a l . Continuous hydrogen maser c a v i t y t u n i n g t o t h e hydrogen l i n e frequency may reduce t h e long-term d r i f t . Trapped i o n f r e - quency s t a n d a r d s may s a t i s f y t h e requirement f o r l o n g a v e r a g i n g t i m e s . A combina- t i o n of s e v e r a l o f t h e above t y p e s which a r e phase locked t o g e t h e r , making an ensemble, may be n e c e s s a r y . A l l t h e above a r e b e i n g a c t i v e l y pursued a t JPL o r elsewhere under JPL s u p p o r t .

Time Synchronization.- The 1986-1990 time s y n c h r o n i z a t i o n requirement i s t h a t t h e t h r e e complex c l o c k s be l e s s t h a n 10 nanoseconds a p a r t i n r e a l time. Two b a s i c problems e x i s t w i t h meeting t h i s requirement; f i r s t i s t h e a g i n g of hydrogen m a s e r s , second is t h e l a c k o f s u i t a b l e equipment f o r time t r a n s f e r between i n t e r -

c o n t i n e n t a l s i t e s .

Assuming t h a t t h e a g i n g r a t e and d i r e c t i o n a t a l l t h r e e s i t e s a r e n o t t h e same, t h e c l o c k s would r e q u i r e s y n c h r o n i z a t i o n every s e v e r a l days. Atomic frequency s t a n d a r d long-term frequency s t a b i l i t y and a g i n g must be improved t o meet t h i s requirement.

The GPS r e c e i v e r mentioned above may be c a p a b l e of s a t i s f y i n g t h i s r e q u i r e - ment based upon p r e l i m i n a r y d a t a . Another p o s s i b i l i t y is t h e u s e of t h e NASA Space

S h u t t l e w i t h an on-board hydrogen maser housed i n a proper environment. Using two- way r a n g i n g s e p a r a t e l y between each complex and t h e Space S h u t t l e may b e an i n t e r - e s t i n g s o l u t i o n t o time s y n c h r o n i z a t i o n . This concept h a s been proposed j o i n t l y by M a r s h a l l Space F l i g h t Center and Smithsonian I n s t i t u t e A s t r o p h y s i c a l Observatory b u t has y e t t o b e approved a s an experiment.

Phase Coherence.- A f u r t h e r requirement is f o r phase coherence and t i m i n g continu- i t y when frequency s t a n d a r d s a r e switched from t h e prime u n i t t o a backup u n i t . S i n c e no two frequency s t a n d a r d s a r e frequency o r phase c o h e r e n t , equipment i s being designed t o c o r r e c t t h e phase of each backup frequency s t a n d a r d t o p r o v i d e a continuous z e r o d e g r e e r e l a t i v e phase a n g l e between s t a n d a r d s a t t h e f r e q u e n c y s t a n d a r d s e l e c t o r s w i t c h .

A l l frequency and time d i s t r i b u t i o n systems w i t h i n t h e DSN w i l l a l s o have t o be improved by s e v e r a l o r d e r s of magnitude t o s u p p o r t t h e s e f u t u r e requirements t o t h e u s e r s who a r e c r i t i c a l l y dependent on t h i s performance.

Sunmary.- The p r e s e n t FTS i n t h e DSN i s b e i n g upgraded f o r t h e 1981-1986 time frame t o i n c l u d e two hydrogen maser frequency s t a n d a r d s t o meet t h e £ r e uency s t a b i l i t y r e q u i r e m e n t s of 1 x 10-14, s y n t o n i z a t i o n r e q u i r e m e n t s of 3 x 10- 19 w i t h i n t h e DSN, and c l o c k s y n c h r o n i z a t i o n t o w i t h i n 100 nanoseconds. Redundant c l o c k s and pro- g r a m a b l e time code t r a n s l a t o r s , a s w e l l a s m u l t i p l e frequency d i s t r i b u t i o n equip- ment a r e p a r t of t h i s system.

Work h a s begun on meeting t h e more s t r i n g e n t r e q u i r e m e n t s of 3 x 10-l6 f r e - quency s t a b i l i t y and l e s s t h a n 10-nanosecond time s y n c h r o n i z a t i o n by 1990, a s w e l l a s improved d i s t r i b u t i o n and s e l f m o n i t o r i n g .

7, seconds Figure 4. Allan Variance Requirements, 1986 To 1990