The Block 1 very long baseline interferometer (VLBI) operated by the Deep Space Network (DSN) to make weekly measurements of the relative epoch and rate offsets between the time standards in the global network of DSN stations is discussed. The precision of these measurements routinely achieves sub-microsecond levels for epoch offset and accuracies of better than one part in 10 to the 12th power for rate offset. The implementation of the phase calibrator system permits absolute measurement of epoch offset to better than 10 nanoseconds. With the near-real-time play-back and on-line storage of VLBI data, the Block 1 system typically produces clock parameters within 48 hours from the time of observation

Roth, M. G.

English

NASA Technical Reports Server

INTERCONTINENTAL CLOCK SYMCHRONIZATION 
WITH THE BLOCK I VLBI SYSTEM 
Mitche l l  G. Roth, Jet Propuls ion Laboratory,  Pasadena CA 
ABSTRACT 
The Block I Very Long Base l ine ln t e r f e romete r  (VLBI) 
operated by t h e  Deep Space Network (DSN) is used t o  
make weekly measurements of t h e  r e l a t i v e  epoch and 
rate o f f s e t s  between t h e  time s tandards  i n  t h e  g loba l  
network of DSN s t a t i o n s .  Over t h e  p a s t  year ,  t h e  
p rec i s ion  of t h e s e  measurements has  r o u t i n e l y  achieved 
sub-microsecond levels f o r  epoch o f f s e t  and accu rac i e s  
of b e t t e r  than  one p a r t  i n  1012 f o r  ra te  o f f s e t .  The 
implementation of t h e  phase c a l i b r a t o r  system w i l l  
permit abso lu t e  measurement of epoch o f f s e t  t o  b e t t e r  
than  1 0  nanoseconds. 
back and on-l ine s to rage  of VLBI d a t a ,  t h e  Block I 
system t y p i c a l l y  produces c lock  parameters wi th in  48 
hours from t h e  time of observat ion.  
With t h e  near-real-t ime play- 
INTRODUCTION 
The Block I VLBI (Very Long Base l ine  In te r fe rometry)  system 111 has 
been developed a t  JPL t o  provide Deep Space S t a t i o n  (DSS) c lock  
synchronizat ion,  p la t form parameter determinat ion (UT1 and polar  
motion-UTPM) and spacec ra f t  naviga t ion  i n  n e a r l y  real time. The 
accuracy goa l s  f o r  t h e  system i n  t h e  1981 time frame cal l  f o r  measure- 
ment of epoch o f f s e t  t o  150 n s ,  rate o f f s e t  t o  0.6 p s / s ,  and UTPM t o  
30 cent imeters  i n  each component. During c r i t i c a l  events ,  such as 
p lane tary  encounters,  t h e  r e s u l t s  are t o  be a v a i l a b l e  wi th in  24 hours 
from t h e  t i m e  of d a t a  a c q u i s i t i o n .  With t h e  implementation of t h e  
phase c a l i b r a t o r  system, now n e a r l y  completed, t h e  accuracy of t h e  
epoch o f f s e t  measurements w i l l  improve t o  b e t t e r  than  1 0  nanoseconds. 
From 1980 December t o  1981 August, system development w a s  c a r r i e d  out  
concurren t ly  wi th  observa t ions  designed t o  monitor c lock  parameters 
and UTPM. This  r e p o r t  desc r ibes  t h e  s t e p s  involved i n  conducting and 
processing t h e  observa t ions  and t h e  r e s u l t s  t h a t  w e r e  obtained f o r  t h e  
c lock  parameters,  
Block I VLBI System 
The Block I system is implemented a t  t h e  Deep Space Network (DSN) 
64 meter r a d i o  t e l e scopes  loca ted  a t  Madrid, Spain (DSS 63)  Canberra, 
Aus t r a l i a  (DSS 4 3 )  and Goldstone, Ca l i fo rn ia  (DSS 14 ) .  The hardware 
489 
https://ntrs.nasa.gov/search.jsp?R=19820012643 2020-03-21T16:32:57+00:00Z
conf igu ra t ions  a t  a l l  t h r e e  s t a t i o n s  are i d e n t i c a l ,  cons i s t ing  of S and 
X-band r e c e i v e r s ,  phase c a l i b r a t o r s ,  channel mul t ip l exe r s ,  analog 
t o  d i g i t a l  conve r t e r s  and t a p e  r eco rde r s ,  as shown in  Figure 1 f o r  
a p a i r  of s t a t i o n s .  The receivers u s e  maser f r o n t  ends t o  achieve  
system temperatures  of 25OK over  a 40 MHz bandpass a t  %band and 
100 MHz at  X-band. Up t o  4 channels  may be recorded i n  each band by 
t ime mul t ip lex ing  t o  o b t a i n  a maximum spanned bandwidth of 40 MHz. 
Indiv idua l  channels  have bandwidths of 250 KHz and are d i g i t i z e d  by 
s ign ,  time tagged and then recorded a t  500 Kb/s on d i g i t a l  tape.  
frequency and t iming systems a t  t h e  s t a t i o n  are con t ro l l ed  by hydrogen 
masers wi th  frequency s t a b i l i t i e s  of Af/f < lo-% 
are replayed from t h e  s t a t i o n s  t o  JPL over t h e  NASCOM wide band d a t a  
l i n e s  a t  56 Kb/s, where they  are s to red  on t h e  d i s c s  of t h e  VLBI 
processor  f o r  c ross -cor re la t ion .  When t h e  d a t a  from both s t a t i o n s  have 
been rece ived  a t  JPL, u sua l ly  wi th in  10  hours of t h e  observa t ions ,  they 
are cross-cor re la ted .  The r e s u l t i n g  c o r r e l a t i o n  func t ion  is analyzed 
by t h e  pos t - co r re l a t ion  sof tware of t h e  VLBI processor  t o  determine 
c lock  parameters and UTPM. 
The 
The recorded d a t a  
Observation Procedures 
Beginning i n  1980 weekly VLBI observing se s s ions  w e r e  scheduled on both 
t h e  California-Spain and Cal i forn ia-Aust ra l ia  base l ines  f o r  t h e  purpose 
of monitoring c lock  parameters and UTPM and f o r  Block 1 system develop- 
ment Eind t e s t i n g .  I n  most ca ses  both base l ines  w e r e  observed i n  t h e  
s a m e  24 hour per iod.  Estimation of c lock  parameters r e q u i r e s  only a 
s i n g l e  base l ine ,  but UTPM estimates r e q u i r e  two base l ines  t o  sepa ra t e  
t h e  t h r e e  components. 
For each base l ine ,  7 t o  13 e x t r a g a l a c t i c  r ad io  sources  from t h e  JPL 
naviga t ion  source ca t a log  [2] w e r e  observed f o r  200 seconds each. The 
sources  t o  be observed w e r e  chosen t o  g ive  an observat ion schedule 
which would minimize t h e  c o r r e l a t i o n s  between t h e  c lock  and UTPM 
parameters.  For most of t h e  1981 observa t ions ,  t h e  scheduling of 
s e s s ions  w a s  governed by DSS a v a i l a b i l i t y .  However, during t h e  per iod 
from D W  136 through DOY 200, a s i d e r e a l  schedule w a s  employed, which 
permit ted t h e  same sources  t o  be observed each week f o r  10 consecut ive 
weeks. The s i d e r e a l  schedule cons i s t ed  of a set of 1 2  sources  observed 
i n  t h e  s a m e  sequence each t h e .  
s i d e r e a l  rate of approximately 4 minutes pe r  day so t h e  s a m e  sources  
could be used f o r  each session.  
t inquishing between random and systematic  e r r o r s .  
The observing time s h i f t e d  a t  t h e  
This  schedule w a s  very u s e f u l  f o r  d i s -  
I n i t i a l l y ,  t h e  d a t a  w e r e  acquired in 4 !%band and 4 X-band channels 
f o r  bandwidth syn thes i s  wi th  a maximum spanned bandwidth of 30 MHz i n  
each band. To improve t h e  s i g n a l  t o  n o i s e  r a t i o s ,  t h e  conf igura t ion  
w a s  changed t o  3 channels  per  band wi th  a maximum span of 40 MHz on 
1981 DOY 107. 
490 
Data w e r e  recorded a t  each s t a t i o n  on d i g i t a l  t a p e  a t  t h e  rate of 
500 Kb/s. 
f o r  t ransmission t o  JPL via t h e  56 Kb/s wide band d a t a  l i n e s .  
Beginning on 1981 DOY 86, t h e  d a t a  were logged d i r e c t l y  onto t h e  d i s c  
s to rage  u n i t s  of t h e  VLBI processor .  
f i r s t  recorded on magnetic t a p e  when rece ived  a t  JPL and then  read 
i n t o  t h e  VLBI processor .  
t h e  t a p e  record ings  w e r e  used as a backup c a p a b i l i t y  i n  t h e  event of a 
system malfunction. 
The t o t a l  on-source t i m e  per  base l ine  w a s  nominally 2000 
seconds, corresponding t o  10 9 b i t s .  These d a t a  requi red  about 5 hours  
P r i o r  t o  DOY 86, t h e  d a t a  w e r e  
With t h e  advent of d i r e c t  d i s c  playbacks,  
Data Reduct ion 
The f i r s t  s t e p  i n  t h e  reduct ion  of t h e  da t a  w a s  t h e  c ros s -co r re l a t ion  
of  t h e  two streams of da t a .  
simultaneous c ros s -co r re l a t ions  over a b i t  stream alignment (BSA) de lay  
range of 32 microseconds. 
i s  t h e  epoch o f f s e t  between t h e  s t a t i o n  c locks ,  t h i s  o f f s e t  must be 
known accura t e ly  enough t o  p l a c e  t h e  c o r r e l a t i o n  func t ion  i n  t h e  32 
microsecond window. I f  t h e  o f f s e t  w a s  no t  known, t h e  c o r r e l a t o r  
opera tor  would search  f o r  t h e  c o r r e l a t i o n  func t ion  i n  s t e p s  of 32 
microseconds. Once found, t h e  c o r r e l a t i o n  func t ion  w a s  cen tered  by 
ad jus t ing  t h e  epoch o f f s e t ,  and a da ta  f i l e  conta in ing  t h e  cross-  
c o r r e l a t i o n  func t ion  and instrumental  phase c a l i b r a t i o n s  w a s  produced. 
The Block I c o r r e l a t o r  performs 16 
Since t h e  p r i n c i p a l  component of BSA de lay  
The next  s t e p  i n  t h e  processing,  phase t r ack ing ,  es t imated t h e  
c o r r e l a t i o n  func t ion  parameters for each r ad io  source.  The c o r r e l a t i o n  
func t ion  f o r  each d a t a  channel is completely descr ibed by fou r  
parameters:  amplitude,  phase, phase rate, and BSA delay.  These w e r e  
determined by a least squares  f i t  t o  t h e  c o r r e l a t o r  output .  
syn thes i s  (BWS) de l ays  w e r e  ca l cu la t ed  from t h e  phase d i f f e r e n c e s  
between t h e  d a t a  channels  in each band. The BSA delays ,  BWS de lays ,  
and s i n g l e  channel phase rates were the’YLBI obsepvables from which 
t h e  c l s c k  and UTIYM parameters w e r e  estimated. 
Bandwidth 
I n  t h e  f i n a l  s t e p  of process ing ,  t h e  c lock  and UTPM parameters w e r e  
es t imated from t h e  VLBI observables .  It is important t o  n o t e  t h a t  
ins t rumenta l  phase c a l i b r a t i o n  i s  requi red  f o r  t h e  abso lu te  measure- 
ment of epoch o f f s e t .  
observables  are corrupted by ins t rumenta l  de lays .  However, t h e  BSA 
delays  may be used t o  measure r e l a t i v e  epoch o f f s e t s  conta in ing  an 
unknown b i a s  due t o  t h e  ins t rumenta l  de lays ,  provided that t h e  
ins t rumenta l  de lays  are cons tan t  i n  time. This  approach w a s  used t o  
s b t a i n  t h e  epoch o f f s e t  d a t a  presented i n  t h i s  r e p o r t ,  Although t h e  BtJS 
de lays  are inhe ren t ly  much more p r e c i s e  than  t h e  BSA delays ,  they  are 
u s e l e s s  f o r  epoch o f f s e t  measurements without phase c a l i b r a t i o n  
because of t h e  d i f f i c u l t y  i n  reso lv ing  BWS c y c l e  ambigui t ies .  
unca l ib ra t ed  BWS delays  can be used f o r  UTPM estimates. 
I n  t h e  absence of phase c a l i b r a t i o n ,  t h e  de lay  
However, 
In t h i s  
491 
x 
context ,  t h e  r e s u l t i n g  epoch o f f s e t s  are meaningless,  but t h e  corre-  
sponding sigmas are r e p r e s e n t a t i v e  of t h e  va lues  that would be obtained 
i f  t h e  BWS delays  w e r e  p roper ly  ca l ib ra t ed .  Frequency s tandard rate 
o f f s e t s  w e r e  es t imated from t h e  X-band phase rate observables .  
Clock Synchronization Resul t s  
Of t h e  55 observing se s s ions  conducted from 1980 Decemter t o  1981 
August, 36 produced u s e f u l  d a t a  f o r  c lock  synchronizat ion.  These 
r e s u l t s  are shown i n  F igures  2-5. 
naviga t ion  d a t a  a l s o  p l o t t e d  on these  f i g u r e s  and are i n  good 
agreement w i t h  t h e  c lock  synchronizat ion da ta .  The epoch o f f s e t  
u n c e r t a i n t i e s  ranged from 35 t o  about 200 n s  and t h e  rate o f f s e t  
u n c e r t a i n t i e s  va r i ed  from O J 3  t o  1.52 p s / s .  
without phase c a l i b r a t i o n  f o r  instrumental  delays.  Therefore,  an 
unknown b i a s  i s  p resen t  in t h e  repor ted  epoch o f f s e t  va lues .  
Comparisons wi th  t h e  o f f s e t s  measured by a t r a v e l l i n g  c lock  i n d i c a t e  
t h a t  t h e  b i a s  w a s  smaller than  one microsecond on both  base l ines .  The 
VLBI da t a  gave independent confirmation of two known c lock  anomalies. 
The s t eep  s lope  (1) in Figure 2 a t  t h e  end of 1980 w a s  due t o  t h e  use  
of a Cesium standard a t  Madrid (DSS 63). The anomalous p o i n t s  denoted 
by hollow symbols (2) on Figures  2 and 3 w e r e  caused by a 1 micro- 
second r e t a r d a t i o n  of t h e  Goldstone (DSS 14) epoch as measured by t h e  
Frequency and Timing Systems s tandards  labora tory .  The cause of t h e  
e r r a t i c  behavior (3) i n  Figure 2 is not  ye t  known. 
Epoch o f f s e t s  from Voyager 2 VLBI 
These d a t a  w e r e  processed 
The long t e r m  c lock  rate ca l cu la t ed  from t h e  s lope  of  t h e  epoch d a t a  
w a s  found t o  be .05 + .02 p s / s  on t h e  California-Spain base l ine ,  
excluding 1980 d a t a  and  1981 d a t a  a f t e r  DOY 165. All p o i n t s  w e r e  
equa l ly  weighted i n  t h e  linear regress ion  because it w a s  f e l t  that 
some of t h e  repor ted  u n c e r t a i n t i e s  w e r e  exaggerated by systematic  
e r r o r s  due t o  incons i s t enc ie s  between t h e  source ca t a log  p o s i t i o n s  
and t h e  precession model i n  t h e  r ap id  a n a l y s i s  software.  The RMS of 
t h e  r e s i d u a l s  w a s  92 nanoseconds. On t h e  Cal i forn ia-Aust ra l ia  base- 
l i n e ,  t h e  long t e r m  rate o f f s e t  w a s  0.51t .01 ps /s .  The RPlS of t h e  
r e s i d u a l s  was 89 nanoseconds. These va lues  agreed wi th  determinat ions 
made by LORAN on t h e  Californa-Spain base l ine  and by t r a v e l l i n g  c locks  
combined wi th  t e l e v i s i o n  t i m e  t r a n s f e r  on t h e  Cal i forn ia-Aust ra l ia  
base l ine .  On both base l ines  t h e  VLBI measurements w e r e  about 10 times 
more accura te .  The X-band rate da ta  f o r  t h e  California-Spain base l ine  
shown in Figure 4 had a mean of -0.14 + 0.20 p s / s  w i th  an RMS of 0.84 
ps / s .  While t h e  mean w a s  cons i s t en t  wi th  t h e  long term rate, t h e  
comparatively l a r g e  RPlS of t h e  r e s i d u a l s  i s  bel ieved t o  be caused by 
media e f f e c t s  and t h e  prev ious ly  mentioned problem concerning t h e  
precess ion  model. 
i n  Figure 5 exh ib i t ed  similar behavior. 
wi th  an RMS of 0.79 ps/s .  
The rate d a t a  f o r  t h e  Cal i forn ia-Aust ra l ia  base l ine  
The mean w a s  0.41 - + 0.19 p s / s  
492 
The sources  of e r r o r  i n  t h e  epoch measurements are shown in Figure 6 f o r  
t h e  cases of unca l ib ra t ed  S-band BSA de lays  and c a l i b r a t e d  X-band BWS 
delays.  For BSA delays ,  t h e  l a r g e s t  e r r o r  is system no i se ,  a d i r e c t  
r e s u l t  of t h e  narrow 250 KHz channel bandwidths. 
and S band ionospheric  e f f e c t s  a l s o  c o n t r i b u t e  s i g n i f i c a n t l y  t o  t h e  
t o t a l  e r r o r  of 76 ns ,  which ag rees  f a i r l y  w e l l  wi th  RMS va lues  obtained 
i n  f i t t i n g  t h e  epoch d a t a  t o  a l i n e a r  model. 
r e s i d u a l s  in  t h e  d a t a  can be explained by not ing  that t h e  a c t u a l  number 
of sources  per  s e s s ion  w a s  somewhat smaller than  expected. 
The instrument phase 
The s l i g h t l y  l a r g e r  
With phase c a l i b r a t i o n  a t  X-band, t h e  BWS delays  provide an order  of 
magnitude improvement i n  accuracy. I n  t h i s  case t h e  dominant sources  of 
e r r o r  are t h e  source p o s i t i o n ,  t roposphere and geophysical models, 
f o r  a t o t a l  e r r o r  of 8.0. nanoseconds. Analysis  of a c t u a l  Bb?S d a t a  
i n d i c a t e s  t h a t  t h i s  level of accuracy is realistic. 
f i t t i n g  sof tware a t  J P L  t o  so lve  f o r  c lock  and UTPM parameters,  t h e  
epoch u n c e r t a i n t i e s  w e r e  t y p i c a l l y  5 nanoseconds and the rate 
u n c e r t a i n t i e s  w e r e  about 0.2 p s / s .  
s l i g h t l y  decreased by t h e  in t roduct ion  of phase c a l i b r a t i o n .  
Using t h e  master 
These u n c e r t a i n t i e s  should be 
Summary 
The Block I VLB'I system has provided relative epoch o f f s e t  measurements 
der ived from BSA de lays  wi th  a p rec i s ion  of b e t t e r  than  l O O n s , t y p i c a l l y  
wi th in  24 hours of d a t a  recept ion .  
mined from t h e s e  epochs have u n c e r t a i n t i e s  of about .01 ps / s .  
rates ag ree  wi th  LORAN determina t ions  and are about 10 times more 
accura te .  
anomalies. The precess ion  model Zn t h e  rap id  a n a l y s i s  sof tware should 
be updated t o  match t h e  source ca t a log  t o  ob ta in  t h e  f u l l  p rec i s ion  of 
the rate o f f s e t  measurements, which would be about 0.2 p s / s .  The 
implementation of t h e  phase c a l i b r a t i o n  system w i l l  a l l o w  t h e  BWS 
delays  t o  be used f o r  abso lu t e  epoch o f f s e t  measurements wi th  an 
accuracy of b e t t e r  than  10 nanoseconds. 
The long t e r m  rate o f f se t s '  de t e r -  
These 
The VLBI d a t a  has  hdependen t ly  confirmed known c lock  
Acknowledgements 
Sam Ward provided inva luable  a s s i s t a n c e  with t h e  a n a l y s i s  and i n t e r -  
p r e t a t i o n  of t h e  c lock  synchronizat ion d a t a ,  
Tom Runge, and Marshall Eubanks a s s i s t e d  with the c o r r e l a t i o n  and 
reduct ion  of t h e  VLBI da ta .  
John Lu'Valle, PhilHarmon, 
493 
References 
[l] "The DSN VLBI System Mark IV-85,'' W. D. Chaney, TDA Progress 
Report , 42-64, pp 61-76, 1981 May-June. 
"Development of a Radio-Astrometric Catalog By Means of Very Long 
Baseline Interferometry Observations," J. L. Fanselow, et al . ,  i n  
[ Z J  
Reference Coordinate Systems f o r  Earth Dynamics, E. M. Gaposhkin 
and R. Kolaczek (eds.), pp 351-357, Reidel (1981). 
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Intercontinental clock synchronization with the block 1 VLBI system

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