The results are presented of clock synchronization experiments using OMEGA transmissions from North Dakota on 13.10 kHz and 12.85 kHz. The OMEGA transmissions were monitored during April 1974 from NASA tracking sites located at Madrid, Spain; Canary Island; and Winkfield, England. The sites are located at distances between 6600 kilometers (22,100 microseconds) to 7300 kilometers (24,400 microseconds) from North Dakota. The data shows that cycle identification of the received signals was accomplished. There are, however, discrepancies between the measured and calculated propagation delay values which have not been explained, but seem to increase with distance between the receiver and the transmitter. The data also indicates that three strategically located OMEGA transmitting stations may be adequate to provide worldwide coverage for clock synchronization to within plus or minus two (2) microseconds

Chi, A. R.

Wardrip, S. C.

English

NASA Technical Reports Server

RECENT FIELD TEST RESULTS USING OMEGA 
TRANSMISSIONS FOR CLOCK SYNCHRONIZATION 
A. R. CHI 
s. e. WARDRIP 
Goddard Space Flight Center 
ABSTRACT 
This paper presents the results of clock synchronization experiments 
using OMEGA transmissions from North Dakota on 13. lOkHz and 
12.85 kHz . The OMEGA transmissions were monitored during April 
1974 from NASA tracking sites located at Madrid, Spain; Canary Island; 
and Winkfield, England. The sites are located at distances between 
6600 kilometers (22, 100ps) to 7300 kilometers (24,400 ps) from Nor# 
Dakota. 
The data shows that cycle identification of the received signals was ac- 
complished. There are, however, discrepancies between the measured 
and calculated propagation delay values which have not been explained, 
but seem to increase with distance between the receiver and the trans- 
mitter. The data also indicates that .three strategically located OMEGA 
transmitting stations may be adequate to provide worldwide coverage 
for clock synchronization to within * two (2) microseconds. 
INTRODUCTION 
The field tests were conducted from April 1 through April 22, 1974. The ob- 
jectives of these tests were to determine the adequacy of unique OMEGA trans- 
missions for time determination at distances greater than 60OOkm from the 
transmitter and to collect experimental data that would indicate the minimum 
number of sites, appropriately located, required for worldwide clock synchro- 
nization to within k2 ps . 
Receiving transmissions from the OMEGANorth Dakota(N. D.) station, data were 
collected at Madrid, Spain (7185 km) ; Canary Island (7298 km) ; and Winkfield, Eng- 
land (6626 km) . Previous test results conducted at Greenbelt, Md. (1934km) ; 
Washington, D.C. (1922km) and Rosman, N.C. (1794km)were reported at the 1973 
PTTI Meeting.(l) Further tests will be conducted during September-October 1975 
at Hawaii andAustralia.(2) Transmissions from OMEGA stations N. D. andHawaii 
will  be used. Transmissions from 0MEGAN.D. will be monitored at Kauai, Ha- 
waii (5926 km) and from both N. D. and Hawaii will be monitored at Orroral, Aus- 
tralia (14,408 km and 8443 km respectively from the transmitters). 
187 
https://ntrs.nasa.gov/search.jsp?R=19750019114 2020-03-19T21:06:41+00:00Z
The use of the OMEGA system for time transmission not only will augment ex- 
isting systems such as Loran-C by providing an additional capability in the north- 
ern hemisphere, but would also provide coverage in the southern hemisphere 
where Loran-C transmitters do not exist and time synchronization is limited to 
within *25 ps, 
PROPAGATION DELAY MEASUREMENT 
If the phase velocities for two VLF signals (fl and f, ) are v1 and v2 respectively, 
then the propagctjon delays for a path length D are: 
D = -  
t P l  v1 
and 
Since the two signals were emitted in phase at time tm at 
signals as received at a station at time t, relative to the 
isfy the following conditions(3): 
the transmitter, the 
station clock must sat- 
where t 
receiving station clock, and tpy and 
The received cycle of each carrier, nl and q is determined from the phase 
measurements Anl and An,. 
and tWz are the time of signal reception of f ,  and f2 relative to the mi are  measured propagation delay values, 
The ambiguity identification of n for T ,  (20 milliseconds) is considered a priori 
knowledge or determined from the theoretical predicted propagation delay (see 
table 1) which is in error by much less than 4 milliseconds. The receiver de- 
sign ‘actually requires the ambiguity resolution to 4 milliseconds (see cycle iden- 
tification p. 193 and reference 2). 
Equations (3) and (4) can be used to calculate the measured propagation delay if 
the receiving station clock time is known relative to the transmitter clock. Hav- 
ing measured the propagation delays via portable clock, the received VLF sig- 
nals can be used to synchronize a receiving station clock. 
188 
Table 1 
Predicted Delay ( p s )  NASA 
Tracking 
Sites t P  1 tP2 (13.10kHz) (12.85kHz) 
Madrid 23,974.8 23,972.1 
Canary Island 24,346.3 24,343.5 
Winkfield 22,113.0 22,110.5 
Ambiguity Determination 
n nr, ( P S I  
1 20,000 
1 20,000 
1 20,000 
EXPERIMENTAL DATA AND PROCESSING 
V L F  phase data at each station visited were recorded in both analog and digital 
form as shown in Figure 1. The analog data was replotted for the 13.1 and 12.85 
M z  signals to demonstrate the diurnal phase signature for each station as shown 
in Figures 2, 3, and 4 (solid line). The predicted phase data furnished by C . P. 
Kugel of the Naval Electronics Laboratory Center (NELC) is the dotted curve. 
It can be seen from these figures that the agreement between the observed and 
predicted phase is rather good. 
I 
Figure 1. VLF signa3 phase data as recorded by the 
OMEGA Timing Receiver in analog (left) and digital 
(right) form. 
189 
MEASURED AND PREDICTED DIURNAL PHASE OF 
13. I KHz OMEGA N DAKOTA TO MADRID, SPAlN 
I2 24 I2 24 12 24 I2 24 12 24(M) 
3 APRIL 4 APRIL 5 APRIL 6- 7APRILif974) 
MEASURED AND PREDICTED DIURNAL PHASE O f  
12.85KHz OMEGA N.DAI(OTA TO MADRID, SPAIN 
Figure 2 .  Diurnal phase records of OMEGA 
North Dakota transmitted signals as received at 
Madrid, Spain on April 3 to 7, 1974. 
I 
7 A r n l L  OAPRIL 9APRlL  IO lR l lL  11 APt?IL i 19741 
MEASURED AND PREDICTED DIURNAL PHASE OF 
13. I KHr OMEGA N.DAKOTA TO CANPRY ISLAND. SPAIN 
MEASURED AND PREDICTED DIURNAL W E  OF 
12.85 K H Z  OMEGA N. DAKOTA TO CANARY ISLAND, SPAIN 
Figure 3. Diurnal phase records of OMEGA North 
Dakota transmitted signals as received at Canary 
Island, Spain on April 6 to 11, 1974. 
ORIGINAL PAGE 18 
OF POOR QUALITY 190 
I I I I I 1  
4 
’. 
L I I . I . , . . , , I  -%E I , I I . I a I . I . . . I . I . I . I . I . I . I . . . . J . J 
12 24 I2 24 I2 24 I2 24 I2 24tGMT) 
16 APRIL 17 am.  i8 MRIL t S M I L  20 aPRn (19741 
MEASURED AND PREDICTED DIURNAL PHASE OF 
13. I KHz OMEGA N. DAKOTA TO WINKFIELD, ENGLAND 
--roe*su(KD 
. . . , . , , , * . I  “ “ ‘ ~ t . , ,  1 1 1 1 1 , 1 * 1 , ,  1 1 , , . , , , , . ,  I , ‘ , # , I * I , I  
t2 24 0 24 I2 24 12 24 I2 2UW) 
I6 APRIL I? am& 18 M I L  19 hPRk 20 APRIL (19741 
MEASURED AND PREDICTED DIURNAL PHASE OF 
12.85 KHz OMEGA N. DAKOTA TO WINKFIELD, ENGLAND 
Figure 4. Diurnal phase records of OMEGA North 
Dakota transmitted signals as received at Winkfield, 
England on April 16 to 20, 1974. 
The digital data were grouped in time periods according to the diurnal phase rec- 
ord. This was done by taking the average of the data points obtained over 15 or 
20 minute intervals during the periods of sunrise, daytime, sunset, and nighttime 
(see table 2). Due to local interference problem in Madrid, only phase record 
during the daytime transmission period were collected. 
l 
With reference to the results given in table 3, An, is the phase difference of the 
received signal (13.10kHz) relative to the station clock and similarly for An, 
(12.85kHz). The difference of the two received signal phases is given as An,, 
which is An, - h, . 
C Y C U  DETERMINATION 
Combining equations (3) and (4): 
= (nl + A n , ) q  -(n2 + A I I ~ ) T ~ .  m Atp E At: = tpl - tP2  (5)  
Let: 
191 
Table 2 
I I 1 Night - 0310 - 0610 0010 - 0450 
Table 3 
Average of Measured VLF Signal Phase in April 1974 
* 554 Sunset 
Night - - .808 .164 .999 .556 
- - - .136 - 
* 
wd 
An,, = An, - An, 
then equation (5) becomes: 
192 
At: = (ni + Anl ) (72 + Ar) - (n2 + An2) r2 
= (nl - n2 + An, - An2) 7-2 + (nl + An1 )Ar 
:. A$ = (m + + (nl + Anl )AT 
Rewriting equation (6): 
Similarly: 
7 2  Atm 
nl +An, = -(m+An12) + 
Either equation (7) or  (8) can be used to determine the received carrier cycle of 
either f l  or  f 2  respectively. This is so because Ani, An2 and Anl2 are  measured 
quantities; m is the highest integer multiple of the beat frequency period within 
Values of (n) and (m) are therefore known. The values of (n) and (m) used for 
the stations visited in 1973 and 1974 are given in table 4. 
# the propagation delay value between the transmitter and the receiving station. 
The particular cycle of a carrier signalreceived at a station as calculated from equa- 
tion (7)or (8) is found by assuming a parametric value for At;, Le.,  At: = 0, 
1, 2, 3, 4, etc. ps .  After nl + An, has been calculated, the value is rounded to 
an integer by the following rule: 
{a) Round nl + An, downward to nl, if 
An, 50.44546 + 0.4455 + 0.446 +0.45 + 0.4 + 0 
@) Round n l +  An,upward to nl + 1, if 
An, 2 0.54546 -+ 0.5455 --* 0.546 --* 0.55 -+ 0.6 + 1 
The 0.1 cycle difference, between the upper bound of one cycle, nl , (0.44546) 
and lower bound of another cycle, nl f 1, (0.54646), is the wall thickness of the 
cycle well. This is the same as saying that 90% of the data is grouped correctly 
into a particular cycle and 10% of the data is grouped into eifher one cycle high 
or  one cycle low. 
193 
Table 4 
Station 
Rosman, N.C. 
USNO, Wash., D. C. 
GSFC, Md. 
NELC, Ca. 
Winkfield 
Madrid 
Canary Island 
Ambiguity Reduction 
Predicted 
Propagation Predicted 
Delay (PS) n m Propagation 
Delay trs) 
5994 0 1 1994 
642 1 0 1 2421 
5458 0 1 1458 
7388 0 1 3388 
22113 1 0 2113 
23974 1 0 3974 
24346 1 1 346 
PROPAGATION DELAY ANOMALY, Gtp" 
The knowledge of the propagation delay anomaly in the frequency range of 11.05 
kHz and 13.15 kHz is not accurate. This is because there is a lack of adequate 
phase velocity data in this frequency range. A more accurate method to deter- 
mine the propagation delay anomaly is perhaps by the use of the time measure- 
ment. Table 5 gives a comparison of predicted and measured propagation delay 
values. 
One area in which more research needs to be done is the determination or anal- 
ysis of propagation anomalies in the OMEGA frequency raqge where propagation 
predictions are not known to sufficient accuracy for microsecond clock synchro- 
nization. The propagation delays must be measured with a clock which is known 
relative to the clock at the transmitter. An alternate approach is to obtain an 
average of the measured. phase differences for f l  and f 2 for day and night time 
transmissions. From these average values one can calculate the propagation 
delay, such as shown by Table 6. Table 6 gives the sample calculation of cycle 
determination for data obtained from Winkfield. 
194 
Table 5 
Madrid 
Canary 
Island 
Greenbelt 
Comparison of Predicted and Measured Propagation Delay Values in 
Microseconds for Indicated Values of tp's 
23,974.8 2.7 1 0 0 23,644.8 - - 
3 23,498.1 52 45 
24,346.3 2.8 1 1 0 24,585.7 - - 
3 24,508.6 57 59 
5 24,355.9 57 
6,458 - 0 1 2  6,463.8 84 84 
4 24,432.3 58 
I 
RESULTS 
The propagation delays for each station visited were measured. The results are 
given in Table 5. It is to be noted that for At, = 3 PS , the measured propagation 
delays for Winkfield and Madrid are lower than the predicted values; Canary 
Island is higher. 
A sample calculation of cycle determination of nl  for At ,  = 0, 1, 2, 3, 4 p s  is 
given in Table 6. The average of the measured propagation delays are 2097.5 
and 2137.3 ps respectively fo+ day- and night-time transmission paths. 
It is noted that the propagation anomaly €or a particular receiving station is de- 
termined only when the station clock is known relative to the transmitter clock. 
This information can be obtaiied by the use of a portable clock. During these 
field tests the OMEGA North Dakota station was measured relative to the U.S. 
Naval Observatory (USNO) Naster Clock (MC) via a portable clock. It was de- 
termined by the USNO that OMEGA North Dakota was slow relative to USNO-MC 
by 7.4 microseconds. Also, during the tests a portable clock referenced to the 
USNO-MC was used. All known biases such as clock differences must be taken 
into account in the calculation of the propagation delay values. Once these values 
are  established, microsecond time can be obtained since the only limitation is 
the phase stability of the received VLF signals. 
195 
Table 6 
Data 
No Date 
- - 
An, An,, 
Cycle Cycle 
1974 Apr ~ 29 29 28 
(Day Time 
Path) 
27 
27 
27 
27 
27 
27 
28 
27 
3 4  
27 
26 
26 
27 
26 
26 
27 
26 
1974 Apr 
0110 - 0530 
29 
29 
29 
29 
29 
30 
29 
I :I ~ :: I 0.955 10.566 
0.950 0.547 
(Night Time 
Path) 
28 27 
28 28 
29 28 
28 28 
28 28 
29 28 
28 27 
I I I 1 
n, For 
L 
tp": (PSI = 
- 
(27 + An,) 7,  
2099.2 
2100.0 
2099.8 
2099.1 
2144.1 
2137.6 
2133.9 
2133.6 
tp"l (Day) = (27 + An,) T,  = 2097.5 ps 
tg (Night) = (27 + An,) T ,  = 2137.3 ps 
CONCLUSIONS 
Besults based ,on the phase data collected during the field tests show that cycle 
deternoination of a received carrier signal can be determined and that the accu- 
racy of time measurement using two VLF signals is within *2 microseconds. 
For a worldwide time transmission system the field test results indicate that a 
.minimum of three transmission stations are needed. These stations which trans- 
mit the present dual frequency format must be selected from the OMF,GA Navi- 
gation System appropriately in order to provide the coverage. 
196 
i 
BIB UOGRAPHY 
1. Chi, A. R. and Wardrip, S. C . ,  Wlock Synchronization Experiments Using 
OMEGA Transmissions, '' Proc . 5th Annual NASA-DOD PTTI Planning Meet- 
ing, 369-392 (December 1973). 
2. Wilson, John J., Britt, James E . ,  and Chi, A. R . ,  "OMEGA Timing Re- 
ceiver, Design and System Test, '' Proc . 4th Annual NASA-DOD BTTI Plan- 
ning Meeting, 345-361 (November 1972). 
3. Chi, A. R . ,  Fletcher, Le A . ,  and Casselman. C. J .  "OMEGA Time Trans- 
missions and Receiver Design, '' Proc . National Electronics Conference VoI. 
27, 268-273 (October 1972). 
, 
19 7 


Recent field test results using OMEGA transmissions for clock synchronization

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