In the search for an economical means of precise time transfer, the NRC Time Laboratory decided to adapt the techniques used by radio astronomers in an experiment to compare the phases of the local oscillators at widely separated VLBI stations. The objective is to design a system which would use commercial satellites, and which would be of reasonable cost for the ground stations and for operations. Two satellite ground stations were installed at NRC about 100 m from the Time Laboratory. For the preliminary experiment, a channel on the Anik Al 6/4 GHz satellite was made available by TELESAT Canada. Two tones were transmitted + or - MHz from the suppressed carrier. The difference frequency of 32 MHz was recorded using narrow band receivers. A low level 1 MHz phase modulation was added to identify the 32 MHz cycle, giving 1 microsec ambiguity in the time transfer. With less than 1/4 W in each tone, the EIRP is 43 dB below that of a normal TV Earth station, and no frequency dispersion is required. The measurements taken each second for the 32 MHz have an rms scatter of 1 ns

Boulanger, J. S.

Costain, C. C.

Daams, H.

English

NASA Technical Reports Server

C.C. Costain, H. Daams and J.-S. Boulanger 
National Research Council o f  Canada 
Ottawa, Canada 
ABSTRACT 
I n  the search f o r  an economical means o f  precise t ime 
transfer, the NRC Time Laboratory decided t o  adapt, f o r  
time transfer,  the techniques used by radio astronomers i n  
an experiment t o  compare the phases o f  the loca l  
osc i l l a to rs  a t  widely separated VLBI stations. The 
object ive i s  t o  design a system which would use commercial 
sa te l l i tes ,  and which would be o f  reasonable cost  f o r  the 
ground sations and f o r  operations. 
Two s a t e l l i t e  ground stat ions have been ins ta l l ed  a t  NRC 
about 100 m from the Time Laboratory. The antennas are 
3 m i n  diameter, and the transmit ter  power i s  1W. 
prel iminary experiment, a channel on the Anik A1 6/4 GHz 
satel 1 i t e  was made avai 1 ab1 e by Tel esat Canada. 
For the 
Two tones were transmitted i: 16 MHt from the suppressed 
carr ier .  
using narrow band receivers. A low level  1 MHz phase 
modulation was added t o  i den t i f y  the 32 MHz cycle, g iv ing 
1 vs ambiguity i n  the time transfer. With less than % W 
i n  each tone, the EIRP i s  43 dB below tha t  o f  a normal TV 
earth stat ion, and no frequency d i  spersion i s  required. 
The dif ference frequency o f  32 MHz was recovered 
The measurements taken each second f o r  the 32 MHz have an 
rms scat ter  o f  1 ns. The transponder o f  the f a i l i n g  Anik 
A1 (now out  o f  service) was 15 dB down on normal 
performance, so tha t  much bet ter  resu l ts  are expected on 
the Anik A3 l a t e r  t h i s  year. 
It i s  three years since I reported here, i n  1979, on the two-way s a t e l l i t e  
time transfer between NRC/NBS, NRC/USNO and USNO/NBS using the Hermes CTS 
satel  1 i te, and the NRC/LPTF t ransfer  using the Symphonie satel  1 i te. The 
Hermes experiment ended July 1, 1979 a f t e r  one year, and the Symphonie 
experiment ended Ju ly  1, 1982 a f t e r  four years. 
The PTB i n  Braunschweig jo ined the Symphonie experiment i n  February 1980, 
and I would l i k e  t o  make a b r i e f  report  on the resul ts  o f  the past two 
years, because the resul ts  are relevant. 
31 
https://ntrs.nasa.gov/search.jsp?R=19830027081 2020-03-21T00:51:53+00:00Z
t o  
compare the frequencies o 
clock Csl e The resu l ts  a 
CsV was evaluated a t  WD 44512, w i th  a resu l t ing  increase i n  frequency o f  
4 x There was l i t t l e  change i n  the frequency o f  CsV a t  WD 44865 
f o r  the next evaluation i n  September 1981. The regular variat ions, over 
the central period, we a t t r ibu ted  t o  the reversal o f  
every 35 days, because there appeared t o  be a corre la  
reversal dates and the change frequency of the order of 5 x 10-14. However 
the PTB assures us t h a t  from t h e i r  in ternal  evaluation t h i s  could not 
occur, so i t  must be a t t r ibu ted  t o  noise. 
This curve, and the f a c t  t ha t  the curve for the previous three years a lso 
l i e s  i n  t h i s  2 us interval, gives a long term agreement between the PTB and 
NRC standards o f  2 x 10-14. Measurement o f  frequency dif ferences t o  
1 x 10-14 on a monthly basis was achieved, except f o r  the l a s t  year, where 
f o r  two periods there were numerous equipment fa i lures and changes i n  
the CRC terminal used by the NRC. 
Except f o r  these periods, t ransfers between the three sations normally gave 
closures o f  2 2ns - 10 ns. The 10 ns bias resul ted from the change i n  
frequency and the satel  1 i t e  transponder which was necessary f o r  the 
LPTF/PTB transfer. Because the three transfers were run consecutively over 
a per iod o f  an hour, i t  was also necessary t o  determine the re la t i ve  rates 
of the stat ion clocks and extrapolate t o  a common time. 
These resul ts  o f  1 o r  2 parts i n  1014 are i n  marked contrast t o  the 
seasonal var iat ions of 3 x 10-13 which are evident i n  comparing our 
frequency t o  tha t  o f  the USNO v i a  Loran C. While Loran C has been, and 
remains, very useful t o  us, it has been obvious f o r  some t ime tha t  we need 
a be t te r  system f o r  precise t i m e  dissemination, and tha t  need i s  re f lec ted  
i n  many o f  the papers t o  be presented a t  t h i s  meeting. 
It has also been obvious from the f i r s t  t ha t  both the Hermes and Symphonie 
sa te l l  i t e  time t ransfer  experiments had a 1 i m i  ted  1 i fet ime. 
f o r  a resonably economic means t o  achieve precise t i m e  transfer, we decided 
t o  experiment w i th  methods used by the radio atronomers t o  establ ish phase 
coherent 1 inks between VLBI stat ions a t  the Naval Research Laboratories, 
Washington, Algonquin Park, Ontario and Penticton, B r i t i s h  Columbial. 
t h i s  experiment two CW tones were exchanged over each path, and the beat 
notes recorded a t  each s tat ion were subsequently compared t o  determine the 
r e l a t i v e  phases o f  the loca l  osc i l l a to rs  a t  the three stations. They 
recent ly achieved an rms noise of 25 ps for 0.1 s averaging time. 
I n  searching 
I n  
For the NRC time transfer experiment, and we hope f o r  u l t imate operational 
use, we have set  up two transmi t-receive satel  1 i t e  ground stat ions about 
100 meters from the Time Laboratory, wi th several t r i a x  cables connected 
d i r e c t l y  t o  the Time Laboratory f a c i l i t i e s .  The antennas are 3 m i n  
32 
he transmit ter  
Anik A1 6/4 GH 
P o f  our stat ions i 
and so frequency d i  
noi se preamp1 tf i e r  preceded the narrow band recei  ver. 
I n  our experiment the i n i t i a l  CW tones were 2 16 MHz from the c a r r i e r  
frequency, wi th  the ca r r i e r  suppre 
receiver, wi th  70 MHz IF, the f req  
a lMHz bandwidth f i l t e r ,  and a 54 MHz VCO w i th  a ramp search and 15 Hz 
bandwith i n  the acquis i t ion loop. The ramp search was necessary t o  
accommodate a possible 20 kHz o f f s e t  a r is ing  from the s a t e l l i t e  
transponder, the Doppler s h i f t  and the receiver loca l  osc i l la to r .  For the  
received 54 MHz, the signal t o  noise was -37 dB f o r  1 MHz bandwidth o r  23 
dB Hz. The 32 MHz was recovered from the 86-54 signal w i th  a c rys ta l  
o s c i l l a t o r  i n  the loop, because only a Doppler s h i f t  o f  1 t o  2 Hz i s  t o  be 
expected. 
d 35 dB below the t o  
cy o f  54 MHz was rec 
For cycle i d e n t i f i c a t i o n  o f  the 32 MHz, two methods were t r ied .  I n  the  
f i r s t ,  the 2 16 MHz tones were switched t o  A- 16.5 MHz a t  a 10 kHz rate, and 
33 MHz was recovered as well  as the 32 MHr as above. This gave a 1 MHz 
signal wi th  l u s  ambiguity i n  the time transfer. 
MHz signal was used t o  phase modulate the 16 MHz a t  a low modulation index, 
and the 1 MHz was recovered d i rec t l y .  The resu l ts  obtained are shown i n  
Figure 2. The upper dots are the measurements taken each second o f  the 1 
MHz locked signal. The middle section gives the measurements o f  the 32 MHz 
signal, and the lower set o f  measurements are from a Canada/France t rans fer  
using the Symphonie sa te l l i t e .  It can be seen tha t  the qua l i t y  o f  the CW 
tone transfer i s  not  as good as the Symphonie results, but  i t  should be 
mentioned tha t  the performance o f  the Anik A1 s a t e l l i t e  was 15 dB below 
normal. 
I n  the second method, a 1 
I n  Figure 3, the resu l ts  o f  another run are presented, w i th  a 1 i n e  g i v ing  
the 10 point  moving average. The 1 MHz resu l ts  are adequate t o  i d e n t i f y  a 
cycle o f  the 32 MHz, and the 32 MHz resul ts  show tha t  sub-nanosecond 
precis ion was obtained. 
The f e a s i b i l i t y  o f  using 3 m terminals w i th  1 W power for  sub-nanosecond 
time transfer has therefore been demonstrated. However, we are not now so 
cer ta in  o f  the p r a c t i c a l i t y  o f  the system. One d i f f i c u l t y  ar ises from the 
fact  t ha t  commercial up-converters and down-converters have too much FM 
noise f o r  the narrow band system, and d i rec t  synthesis f o r  frequency a g i l e  
un i t s  becomes complex and expensive. 
A more serious d i f f i c u l t y  i s  the cycle ambiguity. While the present 1 us 
ambiguity could be reduced t o  10 us or  100 us, power i s  l o s t  w i th  each 
addi t ional  modul a t i  on frequency. The complexity a1 so makes i t  more 
d i f f i c u l t  t o  achieve a f u l l y  automated system. 
33 
r 
2, 
w i l l  have avai 1 ab1 e. 
I would l i k e  t o  suggest t ha t  an e f f o r t  should be made t o  agree on a 
standard PRN modulator and demodulator t h a t  could be used on any time 
t ransfer  experiment, geostationary sate1 1 
should be sub-nanosecond precision and accuracy, and econoqy. 
We would l i k e  t o  thank Telesat Canada f o r  making the experiment possible i n  
g i v ing  us access t o  the Anik A1 sa te l l i t e ,  and f o r  the very helpful  
assistance and advice o f  t h e i r  s t a f f  i n  the course o f  the experiment. 
, GPS, STIFT, 
1 S.H. Knowles e t  a1 "Time Transfer v ia Satel l i te-Link Radio 
Interferometry", Proc o f  the 11th Annual PTTI Meeting, p. 471, 
NOV. 27-29, 1979, NASA CP 2129. 
2 M. Imae e t  a1 "Time-Comparison Experiments wi th  a Small K-Band 
Antenna SSRA System v ia  a Domestic Geostationary Sa te l l i t e " ,  
CPEM 1982, page M-5. 
34 
n 
W 1,9 
3 ._ 
c 
8 
50 
e 40 
30 
N 
L 
ry g 20 
' 10 
8 
r* 
I) 
C 
" 0  
-10 
-20 
. .  
. . . 
c I I 
Fig. 1 - The absolute time difference UTC(NRC) - UTC(PT8) 
v i a  the Symphonie s a t e l l i t e .  
Synphonio : 0 = 0.47- 
-300 -200 -100 0 100 200 300 
Tim <~conds) - Arbitrav zero 
Fig. 2 - A comparison o f  the results usin CW tones via Anik A I  
wi th  the results using a 6 MHz v 3 deo band via Symphonie. 
35 
0 el 
W s 
-10 r) 
0 
C 
0 z 
-20 
-30 
-150 -100 -50 0 50 1 00 150 
Saconds 
32tlk vs 1Hz : 8 = 1.07 ns 
-1 50 -1 00 -50 0 50 100 150 
Seconds 
Fig. 3 - The 10 point running average and standard deviation o f  the 
recovered 1 M H t  and 32 MHt signals via Anik A1 . 
36 
. L. J ,  RUEGER, J H U / A P ~  
Have you obtained permission t o  use active transmitter links t o  the satel- 
1 i tes i n  other countries? 
DR. COSTAIN: 
Yes, I should qualify my statements by the fact tha t  we now have an 
experimental license t o  use a commercial satell i te.  There i s  probably 
rather a bigger step t o  get a commercial license t o  operate a ground 
station when i t  i s  an active s ta t ion transmitlreceive. In Canada we 
might  be able t o  do this by g i v i n g  t i t l e  of the station t o  TELESAT, who 
are the only ones allowed operate. And I t h i n k  i n  some countries i t  i s  
even more difficult. 
the motherhood t h a t  we have for systems, i t  might be a l i t t l e  more d i f f i -  
cult i n  Europe. 
can't be detected except by the compatible system. And one of the virtues 
i s  tha t  we cannot interfere w i t h  other systems, which makes i t  a l i t t l e  
more practical t o  get licensed. 
I t h i n k  t h a t  we could probably accomodate on this continent w i t h  
B u t  if we can keep the EIRP 40 dB down, i n  fact you really 
DR. WINKLER: 
I may add the important t h i n g  seems t o  be as Dr. Costain said, low power 
b u t  the second one, I believe, i t  i s  easier t o  ob ta in  licenses if  you can 
stick t o  a fixed schedule of 5 or 10 minutes a day; the same time, because 
then the primary user of such a frequency assignment can notify you if he 
needs t o  pre-empt. In this case, i t  seems t o  be easier t o  obtain licenses. 
And we have had no difficulty t o  ob ta in  an experimental 1 icense for  a peak 
power of ten watts, for the anticipated LASSO experiment, which unfortu- 
nately d id  not  materialize. 
DR. COSTAIN: 
Yes, also, this i s  one reason why we have very simple programming really 
i n  this type of experiment and one reason I want f u l l y  automated outfits 
so t h a t  we could do i t  a t  3 o'clock i n  the morning, and I don ' t  have t o  
be i n  the laboratory. 
37 


Two-way satellite time transfer using low power CW tones

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