A three part experiment was conducted to develop and compare time transfer techniques. The experiment consisted of (1) a very long baseline interferometer (VLBI), (2) a high precision portable clock time transfer system between the two sites, and (3) a television time transfer. A comparison of the VLBI and traveling clock shows each technique can perform satisfactorily at the five nsec level. There was a systematic offset of 59 nsec between the two methods, which we attributed to a difference in epochs between VLBI formatter and station clock. The VLBI method had an internal random error of one nsec at the three sigma level for a two day period. Thus, the Mark II system performed well, and VLBI shows promise of being an accurate method of time transfer. The TV system, which had technical problems during the experiment, transferred time with a random error of about 50 nsec

Angerhofer, P. E.

Johnston, K. J.

Kaplan, G. M.

Klepczynski, W. J.

Matsakis, D. N.

Santini, N. J.

Spencer, J. H.

Waltman, E. B.

English

NASA Technical Reports Server

COMPARISON OF VLBI,  TV, AND TRAVELING CLOCK TECHNIQUES FOR TIME TRANSFER 
J. H. Spencer,  E. B. Waltman, K. J. Johnston,  N. J. S a n t i n i  (E. 0. 
Hulburt  Center f o r  Space Reseach, Naval Research Laboratory,  
Washington, D. C. 20375) 
W. J. Klepczynski,  D. N, Matsakis, P. E. Angerhofer, G. H. Kaplan 
(U. S. Naval Observatory,  Washington, D. C. 20375) 
ABSTRACT 
A t h r e e  p a r t  experiment w a s  conducted t o  develop and compare 
t i m e  t r a n s f e r  techniques.  The experiment cons i s t ed  o f :  I. a 
V L B I  between Maryland Po in t  Observatory a t  Rivers ide ,  Mary- 
land and NRAO a t  Green Bank, West Vi rg in i a ;  11. a high  pre- 
c i s i o n  p o r t a b l e  c lock  t i m e  t r a n s f e r  system between t h e  two 
sites coordinated by t h e  U. S. Naval Observatory i n  Wash- 
ing ton ,  D. C . ;  and 111. a t e l e v i s i o n  t i m e  t r a n s f e r  between 
t h e  U. S .  Naval Observatory and t h e  Maryland Po in t  Obser- 
v a t o r y  us ing  a l o c a l  Washington, D.  C. t e l e v i s i o n  s t a t i o n ,  
WTTG. 
A comparison of t h e  VLBI and t r a v e l i n g  c lock  shows each 
technique can perform s a t i s f a c t o r i l y  a t  t h e  f i v e  nsec  level. 
There w a s  a sys t ema t i c  o f f s e t  of 59 nsec  between t h e  two 
methods, which we a t t r i b u t e d  t o  a d i f f e r e n c e  i n  epochs 
between VLBI fo rma t t e r  and s t a t i o n  clock.  
The VLBI method had an i n t e r n a l  random e r r o r  of one nsec  a t  
t h e  three-sigma level f o r  a two-day per iod .  Thus, t h e  Mark 
I1 system performed w e l l ,  and VLBI  shows promise of being an 
a c c u r a t e  method of t i m e  t r a n s f e r .  
The TV system, which had t e c h n i c a l  problems dur ing  t h e  
experiment,  t r a n s f e r r e d  t i m e  w i th  a random e r r o r  of about 50 
nsec.  
INTRODUCTION 
An experimental  t i m e  t r a n s f e r  w a s  conducted between the hydrogen maser 
c locks  a t  t h e  Maryland Po in t  Observatory of t h e  Naval Research Laboratory 
(NRL) and t h e  Nat iona l  Radio Astronomy Observatory* (NRAO), and t h e  U. S. 
*The Nat iona l  Radio Astronomy Observatory is  operated by Associated 
U n i v e r s i t i e s ,  Incorpora ted ,  under c o n t r a c t  w i t h  t h e  Nat iona l  Science 
Foundation. 
Naval Observatory 's  master c lock  t o  develop and compare t i m e  t r a n s f e r  
231 
https://ntrs.nasa.gov/search.jsp?R=19820012630 2020-03-21T16:35:46+00:00Z
techniques.  The experiment cons i s t ed  of t h r e e  p a r t s :  i) a very  long 
b a s e l i n e  in t e r f e romete r  (VLBI)  between t h e  Maryland Po in t  Observatory 
a t  Rivers ide ,  Maryland and NRAO a t  Green Bank, West Vi rg in i a ;  ii) a 
high  p r e c i s i o n  p o r t a b l e  c lock  system t i m e  t r a n s f e r  between t h e  t h r e e  
sites; and iii) a t e l e v i s i o n  t i m e  t r a n s f e r  between t h e  U. S. Naval 
Observatory and t h e  Maryland Poin t  Observatory us ing  t e l e v i s i o n  
channel  5 i n  Washington, D. C . ,  WTTG. The purpose of t h e  experiment 
w a s  t o  compare each method of t i m e  t r a n s f e r ;  t o  e v a l u a t e  t h e  e r r o r s ,  
i n t e r n a l  and sys temat ic ,  of each method; and t o  i s o l a t e  problem areas 
i n  t h e  VLBI technique,  a t i m e  t r a n s f e r  technique t h a t  i s  n o t  f u l l y  
developed, b u t  has  t h e  most promise. 
It is  i n t e r e s t i n g  t o  n o t e  t h a t  whi le  one method t r a n s p o r t s  h igh  
p r e c i s i o n  atomic c locks  between sites, t h e  o t h e r  two methods have "no 
moving par t s" .  E l e c t r o n i c  equipment remains s t a t i o n a r y  i n  c o n t r o l l e d  
environments. These two methods r e fe rence  t o  e x t e r n a l  r a d i o  s i g n a l s ,  
one l o c a l  and man made, t h e  o the r  a celestial r a d i o  galaxy o r  quasar .  
The TV system, whi le  s impl i e r ,  h a s  a f i x e d  frequency and t h e r e f o r e  
cannot overcome propagat ion de lay  v a r i a t i o n s .  The r e fe rence  s i g n a l  of 
t h e  VLBI system, however, only propagates  through one t o  two atmospheres 
and can be  a t  f requencies  s e l e c t e d  f o r  b e s t  r e s u l t s .  It has  t h e  
g r e a t e s t  p o t e n t i a l .  
THE VLBI  TECHNIQUE 
The VLBI experiment w a s  conducted February 19-22, 1980 a t  1670 MHz 
between t h e  26-meter antenna of t h e  Naval Research Laboratory and t h e  
NRAO 43-meter antenna wi th  a bandwidth of 2 MHz. The Mark I1 d a t a  
record ing  system w e  used has  been descr ibed  by Clark  (1973). 
s y n t h e s i s ,  a technique which improves t h e  a b i l i t y  t o  determine t h e  
de l ay  between t h e  two antennas by t h e  r a t i o  of synthes ized  t o  a c t u a l  
bandwidth, w a s  no t  used i n  t h e s e  observa t ions ,  which w e r e  designed t o  
be  a b a s i c ,  proto-type experiment.  Bandwidth s y n t h e s i s  could improve 
t h e  i n t e r n a l  e r r o r s  i n  d i r e c t  p ropor t ion  t o  bandwidth, b u t  w a s  n o t  
compatible wi th  t h e  o the r  observa t ions  t h a t  occurred s imultaneously.  
Bandwidth 
The Mark I1 r eco rd ing  system w a s  used because,  whi le  i t  is  no t  now t h e  
most advanced system a v a i l a b l e ,  it is t h e  most widely used system f o r  
VLBI experiments.  Therefore  i t  has  t h e  g r e a t e s t  p o t e n t i a l  f o r  d i s t r i -  
bu t ing  t i m e  around t h e  world,  i f  i t  can t r a n s f e r  t i m e  w i th  adequate  
accuracy. 
The observa t ions  w e r e  c o r r e l a t e d  on t h e  NRAO processor  i n  Char lo t tes -  
v i l l e ,  V i rg in i a ,  and then  f u r t h e r  reduced wi th  a series of programs 
developed f o r  t h e  TI-ASC computer a t  t h e  Naval Research Laboratory.  
The sources  used f o r  t h i s  s tudy  w e r e  3C273, 312345, 3C120, 3C84, and 
4639.25. W e  assumed t h e i r  p o s i t i o n s  as given by Wade and Johnston 
(1977). The i n i t i a l  v a l u e  of t h e  NRL-NRAO b a s e l i n e  w a s  t h a t  given by 
232 
a 1976 V L B I  (Waltman et a l .  1982) ,  but  i t  w a s  la ter  updated by less 
than  one meter t o  remove sys temat ic  d i f f e r e n c e s  between sources .  
Because VLBI t r a n s f e r s  t i m e  by measuring t h e  t i m e  of a r r iva l  of wave- 
f r o n t s  from a d i s t a n t  celestial  r a d i o  source  such as a r a d i o  galaxy o r  
quasar ,  t h e  r e f e r e n c e  t i m e  of t h e s e  wavefronts must b e  t i e d  t o  t h e  
l o c a l  s t a t i o n  c lock ,  a hydrogen maser s tandard .  This  i s  done by 
measuring t h e  wavefront de l ay  from f u d i c i a l  marks on t h e  t e l e scope  
through t h e  r e c e i v e r s ,  c a b l e s ,  and backends t o  t h e  l o c a l  v ideo  t ape  
r eco rde r .  Unfor tuna te ly ,  f o r  t h e  2 MHz bandwidth of t h e  Mark I1 
recorde r ,  t h e  fundamental c lock  of t h e  t ape  is t h e  4 MHz fo rma t t e r  
c lock.  W e  found it  t o  be  impossible  t o  p r e c i s e l y  measure t h i s  c lock  
wi th  r e s p e c t  t o  t h e  5 MHz s t a t i m  clock  us ing  s tandard  equipment. 
The de lay  between v ideo  t a p e s  w a s  found by performing a Four ie r  t r ans -  
form i n  l a g  t o  g e t  t h e  bandpass a t  t h e  peak f r i n g e  rate frequency. A 
s t r a i g h t  l i n e  f i t  t o  t h e  phase a c r o s s  t h e  bandpass then y ie lded  t h e  
de lay .  Af t e r  t h e  d a t a  w a s  co r rec t ed  f o r  atmospheric e f f e c t s ,  r e t a rded  
b a s e l i n e ,  antenna geometry, and e a r t h  t i d e s ,  a leas t - squares  s o l u t i o n  
f o r  t h e  c lock  rate and o f f s e t  w a s  found us ing  pa i red  observa t ions  of 
t h e  s a m e  source from February 20 and 21. The u s e  of pa i r ed  scans  on 
consecut ive  days minimized t h e  e r r o r  con t r ibu ted  by t h e  atmosphere, 
source  s t r u c t u r e ,  and o t h e r  r e p e a t a b l e  e r r o r  sources  and allowed t h e  
b a s e l i n e  t o  be determinea more p r e c i s e l y .  
W e  measured t h e  c a b l e  de l ays  through t h e  receivers t o  t h e  f o r m a t t e r s  
a t  each t e l e scope  and co r rec t ed  f o r  t h i s  e f f e c t .  The r e s u l t  is  t h a t  
t h e  t i m e  d i f f e r e n c e  f o r  NRL-NRAO i s  shown i n  F igure  1 a f t e r  removal of 
a s t r a i g h t  l i ne  wi th  a s l o p e  of 5.509 psec per  second and an  o f f s e t  
of -14.397 t 0.005 psec a t  0 UT on 2 1  February. In  t h i s  f i g u r e  t h e  
d i f f e r e n t  symbols i n d i c a t e  t h e  source of t h e  d a t a .  The sources  are 
coded: 3C273 observa t ions  = ;  3684 by X ;  36345 by 0 ; and t r a v e l i n g  
c lock  by @. The one sigma e r r o r s  are shown f o r  each 3 minute scan of 
t h e  V L B I  d a t a  except  near February 20 where t h e  one e r r o r  b a r  a t  12:30 
i s  r e p r e s e n t a t i v e .  
h 
The sources  of e r r o r  are u n c e r t a i n t y  i n  t h e  fo rma t t e r  c lock  synchroni- 
z a t i o n ,  u n c e r t a i n t y  i n  t h e  cab le  de l ay  measurements, and t h e  i n t e r n a l  
VLBL random e r r o r s .  W e  estimate t h e  e r r o r  i n  t h e  cab le  de l ay  measure- 
ments t o  be 2 nsec  (1 sigma). The random e r r o r  of t h e  V L B I  f i t  w a s  1 
nsec  a t  t h e  t h r e e  sigma level. Thus, t h e  random i n t e r n a l  e r r o r s  i n  
t h e  V L B I  s o l u t i o n  made a n e g l i g i b l e  c o n t r i b u t i o n  t o  t h e  e r r o r  ass igned 
t o  t h e  f i n a l  VLBI r e s u l t .  
While each  fo rma t t e r  w a s  "synchronized" t o  t h e  s t a t i o n  c lock ,  t h i s  is  
u s u a l l y  t o  only a f r a c t i o n  of a microsecond. 
f o r  the o f f s e t  between t h e  fo rma t t e r  1 pu l se  pe r  second and t h e  s t a t i o n  
c lock ,  b u t  w e  do n o t  know t h e  t iming between t h a t  fo rma t t e r  1 pps and 
A c o r r e c t i o n  w a s  made 
233 
t h e  i n t e r n a l  4 MHz c lock  used t o  record and t i m e  t h e  v ideo  s i g n a l s  on 
tape .  
THE TRAVELING CLOCK TECHNIQUE 
A h igh  p r e c i s i o n  p o r t a b l e  c lock  t i m e  t r a n s f e r  system c o n s i s t i n g  of 
f o u r  cesium c locks  ( t h r e e  of which w e r e  "super tubes") and a d a t a  
a c q u i s i t i o n  system w e r e  t r anspor t ed  by van between t h e  t h r e e  observa- 
t o r i e s  almost cont inuous ly  dur ing  t h e  per iod of t h e  VLBI observa t ions .  
This  r e s u l t e d  i n  one measurement pe r  day a t  each r a d i o  observa tory  and 
t h r e e  p e r  day a t  t h e  USNO Master Clock. 
w e r e  intercompared cont inuous ly  us ing  an HP-9815 computer t h a t  a l s o  
t r a v e l e d  i n  t h e  van i n  o rde r  t o  form a mean-time scale through which 
w e  monitored t h e  performance of the i n d i v i d u a l  cesium c locks .  Com- 
pa r i son  of t h e  mean t i m e  scale, based on t h e  fou r  cesium c locks ,  wi th  
t h e  USNO Master Clock ind ica t ed  t h a t  i t  remained s t a b l e  over t h e  per iod 
of t h e  experiment.  F igure  2 shows t h e  behavior of each of t h e  fou r  
c locks  compared t o  t h e  mean a f t e r  a best f i t  s t r a i g h t  l i n e  has  been 
s u b t r a c t e d ,  Clock 2 is n o t  a super  tube.  The r e s u l t s  of t h e  t r a v e l i n g  
c lock  are summarized i n  Table  1 and the  c lock  d i f f e r e n c e  of NRL-N 0 
i s  p l o t t e d  i n  F igure  1 a f t e r  an o f f s e t  of - 1 4 , 3 3 9  k 3 . 4  nsec  a t  0 
on February 2 1  and a c lock  rate of 5.509 psec pe r  second have been 
removed . 
The f o u r  t r a v e l i n g  c locks  
P UT 
THE TV TIME TRANSFER TECHNIQUE 
It i s  p o s s i b l e  t o  t r a n s f e r  t i m e  by synchronizing t o  t h e  t e l e v i s i o n  
s i g n a l  t r ansmi t t ed  by a l o c a l  commercial s t a t i o n .  In  t h e  Washington, 
D. C.  area, channel  5, WTTG, b roadcas ts  a s i g n a l  t h a t  i s  phase locked 
t o  a cesium frequency s tandard  and monitored d a i l y  by t h e  Naval Obser- 
va tory .  Since t h i s  s i g n a l  can be rece ived  a t  t h e  Maryland P o i n t  
Observatory,  about  70 k i lome te r s  from t h e  t r a n s m i t t e r ,  i t  can be used 
t o  t r a n s f e r  t i m e  from t h e  USNO t o  t h e  Maryland Po in t  Observatory.  
Unfor tuna te ly ,  as shown i n  Figure  3 ,  t h e  behavior  of t h e  cesium s tandard  
a t  t h e  t r a n s m i t t e r  w a s  erratic dur ing  t h e  per iod  of t h e  experiment.  
We are t h e r e f o r e  l i m i t e d  t o  s imultaneous measurements a t  bo th  observa- 
t o r i e s  i n  o r d e r  t o  e l i m i n a t e  t h e  l a r g e  randomness of t h e  t r a n s m i t t e r  
s i g n a l .  Th i s  reduces  t h e  d a t a  t o  four  measurements where t h e  TV and 
van can be  d i r e c t l y  compared. These r e s u l t s  are shown i n  Table 2. 
The d a t a  p o i n t  f o r  February 19 is  almost 900 nsec  d i f f e r e n t  from t h e  
mean of t h e  o t h e r  t h r e e  (approximately 19 s tandard  d e v i a t i o n s ) .  W e  
c an  n o t  e x p l a i n  t h i s  except  t o  p o i n t  ou t  the erratic behavior  of t h e  
TV cesium s tandard  f o r  t h i s  day and t h a t  dur ing  t h i s  per iod  t h e  TV 
s i g n a l  was f r e q u e n t l y  n o t  locked t o  t h e  s tandard  as a r e s u l t .  Because 
t h e  c lock  rate d i f f e r e n c e s  a t  t h e  t r a n s m i t t e r  and Maryland Po in t  
Observatory are n o t  of d i r e c t  i n t e r e s t ,  t h e  most i n t e r e s t i n g  method t o  
compare van and TV t i m e  t r a n s f e r  methods is  t o  compute t h e  d i f f e r e n c e  
234 
of USNO master c lock  minus Maryland Po in t  Observatory v ia  TV and v ia  
" t rave l ing"  clock.  The r e s u l t  of 230.119 psec  is l a r g e l y  due t o  t h e  
f r e e  space  propagat ion de lay ,  bu t  a l so  inc ludes  antenna,  cab le ,  and 
receiver de lays  of t h e  system at t h e  Maryland P o i n t  Observatory.  The 
s tandard  d e v i a t i o n  of t h i s  mean i s  50 nsec ,  much l a r g e r  than  t h e  e r r o r  
of t h e  van a l o n e  o r  when compared t o  VLBI. W e  t h e r e f o r e  conclude t h a t  
f o r  a 70 k i lometer  t ransmiss ion ,  50 nsec r e p r e s e n t s  t h e  s h o r t  term (1- 
3 day) s tandard  d e v i a t i o n  of a t i m e  t r a n s f e r .  Seasonal  e f f e c t s  could 
b e  expected t o  degrade t h i s  va lue  f u r t h e r  f o r  t i m e  t r a n s f e r s  t h a t  last  
longer .  
DISCUSSION 
For t h e  NRL-NRAO t i m e  t r a n s f e r ,  t h e  f i na l  c lock  rate determined from 
t h e  VLBI experiment i s  i n  agreement wi th  t h e  t r a v e l i n g  c lock  rate 
c a l c u l a t e d  f o r  t h e  s a m e  two-day t i m e  per iod .  The VLBI d a t a  y ie lded  a 
c lock  rate of 5.483 _+ 0.013 psec pe r  second (2 sigma e r r o r ) ;  t h e  
t r a v e l i n g  c lock  experiment gave 5.5 f 1.0 psec  pe r  second, which is  
w i t h i n  t h e  quoted e r r o r .  
Incorpora t ing  t h e  r e s i d u a l s  of F igure  1 i n t o  t h e  o f f s e t s  removed, w e  
f i n d  t h e  c lock  comparison between NRL and NRAO has  an unexpected 
sys temat ic  o f f s e t  of 59 nsec  between t h e  V L B I  and t r a v e l i n g  c lock  
methods of t i m e  t r a n s f e r .  This  i s  probably due t o  t h e  d i f f e r e n c e  i n  
epochs of t h e  4 MHz fo rma t t e r  c lock  used t o  w r i t e  t h e  v ideo  t a p e  and 
t h e  5 MHz s t a t i o n  frequency s tandard  as used i n  t h e  VLBI system. I n  
f u t u r e  experiments,  equipment must be developed t o  measure t h e  4 MHz 
fo rma t t e r  c lock  s i g n a l  re la t ive t o  t h e  5 MHz frequency s tandard .  
The comparison of t h e  TV t o  t r a v e l i n g  c lock  techniques between t h e  
USNO and NRL shows t h e  TV method t o  be good t o  % 50 nsec  over 70 
k i lome te r s  as given i n  t h e  last  sec t ion .  The s i m p l i c i t y  of t h i s  
method may make t h i s  technique a t t ract ive where p r e c i s e  t i m e  t r a n s f e r  
i s  n o t  requi red .  
CONCLUSIONS 
The Mark I1 VLBI system performed w e l l  i n  t h e  t ime- t ransfer  experiment,  
and a comparison of t h e  VLBI r e s u l t s  wi th  t h e  t r a v e l i n g  c lock  experiment 
w a s  u s e f u l  i n  i d e n t i f y i n g  p o s s i b l e  problem areas i n  developing t h e  
VLBI t echnique  of t i m e  t r a n s f e r .  
I n  t h e  f u t u r e  t h e  u s e  of bandwidth s y n t h e s i s  and b e t t e r  fo rma t t e r  
synchroniza t ion  should g r e a t l y  improve t h e  VLBI r e s u l t s ,  making t h e  
widely used Mark I f  system a promising technique f o r  t i m e  t r a n s f e r .  
The intercomparison of t h e  VLBI and t h e  four  t r a v e l i n g  c locks  shows 
each technique can perform s a t i s f a c t o r i l y  a t  t h e  5 nsec  level.  Th i s  
235 
suppor ts  claims made i n  t h e  p a s t  as t o  t h e  accuracy of t r a v e l i n g  
clocks.  
The t e l e v i s i o n  t r a n s f e r  system w a s  shown t o  work over 70 k i lometers  
wi th  s h o r t  t e r m  e r r o r s  of 50 nsec. 
The V L B I  method of t i m e  t r a n s f e r  had an i n t e r n a l  random e r r o r  t o  t h e  
s o l u t i o n  f o r  a two-day per iod of one nsec a t  t h e  t h r e e  sigma level. 
Thus, VLBI shows promise of being an accu ra t e  method of time t r a n s f e r .  
REFERENCES 
Clark,  B.G. 1973, Proceedings of t h e  IEEE,  $), 1242. 
Wade, C. M . ,  and Johnston,  K. J.  1977,  Astronomical Journa l ,  g, 791.  
Waltman, E. B . ,  Spencer, J. H . ,  Johnston, K. 3. 1982,  i n  prepara t ion .  
236 
TABLE 1 
NRL - N U 0  V i a  Travel ing Clock and VLBI 
Date T i m e  Txaveling Clock VLBI VLBI -Traveling Clock 
(v.lsec) (Frsec) ( w e e )  
-15.554 h m s  Feb. 18 08 54 44 
Feb. 19 09 29 00 -15.088 -15.141 -0. Q53 
Feb. 20 09 45 13 -14.619 -14.680 -0.061 
Feb. 21 09 28 28 -14.152 -14.209 -0.057 
Feb. 22 06 16  55 -13.742 -13.796 -0.054 
TABLE 2 
TV Experiment Compared t o  Traveling Clock Experiment 
D a t  ea USNQ -TV USNO -NRL 
V i a  TV V i a  Travel ing Clock Traveling Clock -TV 
(ttsec) (Frsec) (wee) (wet) 
Feb. 19 2.0003 -218.083 +11.154 229.237 
Feb. 20 U.678 -219.519 10,615 230.134 
Feb. 21 0.214b -220 081 10.075 230.156 
Feb. 22 -0,251 -220.531 9.536 230.067 b 
Corrected t o  17:OO UT. a 
bSee text. 
237 
I I I I I 
18 19 20 21 22 23 
DATE IN FEBRUARY 1980 
Fig. 1--VLBI and traveling clock results from NRL -NE?AO 
238 

c 
% 
DATE IN FEBRUARY 1980 
6 
Fig. 3--Behavior of the cesium standard a t  the TV station 
240 


Comparison of VLBI, TV and traveling clock techniques for time transfer

Comparison of VLBI, TV and traveling clock techniques for time transfer

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