498 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 42, NO, 2 , APRIL 1993

Two-way Time Transfer Experiments Using an INTELSAT Satellite in a Inclined

Geostationary Orbit Fujinobu Takahashi, Kuniyasu Imamura, Eiji Kawai, Chang Bok Lee, Dong Doo Lee, Nak Sam Chung,

Hiroo Kunimori, Taizoh Yoshino, Toshimichi Otsubo, Atsushi Otsuka, and Tadahiro Gotoh

Abstract-The Communications Research Laboratory (CRL) started a new two-way time transfer experiment program in 1988 based on a recommendation of the ConfCrence GCnCrale des Poids et Mesures (CGPM) in 1987. Korea Research Insti- tutes of Standards and Science (KRISS) and CRL, Japan agreed in 1990 to an intergovernmental program of cooperation in the two-way time transfer experiments using satellites of the Inter- national Telecommunications Satellite Organization (INTEL- SAT). CRL prepared two sets of Ku-band communications sys- tems including a microwave time and ranging experiment (MITREX) modem and a compatible prototype modem (I-mo- dem) developed by CRL.

CRL and KRISS had the opportunity to use INTELSAT sat- ellites for the two-way time transfer experiment. CRL per- formed two kinds of INTELSAT experiments: the ranging (RG) experiment and the Japanese domestic (JD) experiment. CRL and KRISS also performed a Japan-Korea (JK) joint interna- tional experiment. This work describes the results of these three experiments.

I. INTRODUCTION THE field of two-way time transfer experiments, the r Communications Research Laboratory (CRL) has

played several important roles such as the first trans-Pa- cific experiments via the ATS-1 satellite in 1979 [ l ] and the first Ka-band experiment using the Japanese Com- munication Satellite (CS) [2]. The use of very small ap- erture terminals (VSAT) at Ku-band has already been ex- plored at National Institute of Standards and Technology (NIST) [3] while comparisons between two-way and GPS have been performed between the United States Naval Observatory (USNO) and NIST [4]. The present position regarding two-way time transfer experiments world-wide is summarized in the review paper by Kirchner [5]. It was recommended by the ConfCrence GCnCrale des Poids et Mesures (CGPM Resolution 4) in 1987, that “the respon- sible national and international bodies support experi-

Manuscript received June 10, 1992; revised September 8, 1992. F. Takahashi, K. Imamura, E. Kawai, H. Kunimori, T . Yoshino, A.

Otsuka, and T. Gotoh are with the Communications Research Laboratory, Koganei, Tokyo 184, Japan.

C. B. Lee, D. D. Lee, and N. S. Chung are with Korea Research Insti- tute of Standards and Science, Taedek Science Town, Taejon 305-606, Republic of Korea.

T. Otsubo is with Hitotsubashi University, Kunitachi, Tokyo 186, Ja- pan.

IEEE Log Number 9207037.

ments over telecommunications satellite links for the study of synchronization techniques. ” The International Tele- communications Satellite Organization (INTELSAT) sat- ellite system is the most realistic candidate as a provider of international two-way communication channels. Thus CRL began in 1988 a new international two-way time transfer program using INTELSAT satellites. The Korea Research Institute of Standards and Science (KRISS) and CRL agreed in 1990 to an intergovernmental program of cooperation to accomplish two-way time transfer using the satellites of INTELSAT.

Since the two-way time transfer technique using spread spectrum signals is already well established, the remain- ing research and technical factors are how to improve the precision, how to compensate for the unknown delay off- sets of each part of transfer links and how to handle the two-way data effectively. And another part of our INTEL- SAT program was to negotiate the scientific use of the satellite links suitable for our experiments with INTEL- SAT Headquarters (HQ). For our experiments mentioned in the following sections, CRL received very strong sup- port from Kokusai Densin Denwa Co. (KDD) in negoti- ation with INTELSAT/HQ. In regard to the JK experi- ment, KRISS received the support from Korea Telecommunications (KT) , the Korean agent of INTEL- SAT. The encouragement from KDD and KT was inval- uable in making available the commercial INTELSAT links to realize these international experiments. In respect of the JK experiment, this paper covers only the results of preliminary data analysis following the recent comple- tion of the experiment.

11. TIME TRANSFER SYSTEMS Two-way time transfer is a method to determine the

absolute time difference between two stations by cancel- ling the propagation delays in common two-way paths (see Fig. 1) . With the two-way time transfer method, both ground stations transmit time signals to each other via a communications satellite. This is a unique point com- pared to other (passive) one-way time transfer methods such as the Global Positioning System (GPS), the Geo- stationary Meteorological Satellite (GMS) or Loran-C. The most important factor in one-way time transfer is how

0018-9456/93$03.00 0 1993 IEEE

1

TAKAHASHI et al . : TWO-WAY TIME TRANSFER EXPERIMENTS 499

INTELSAT SATELLITE

180' E 177' E

. , ?-

TRANSCEIVER I

RX IF TX IF

COMPUTER 1

I

TRANSCEIVER 2

COMPATIBLE 2 MODEM SQ

(I-MODEM) RX IPPS

STOP

TIME INTERVAL COUNTER

GPl0

COMPUTER 2

CLOCK 2

TABLE I

EXPERIMENTS, (B) CRL + KRISS AND (C) KRISS -+ CRL LtNK BUDGETS OF CRL'S TWO-WAY TIME TRANSFER SYSTEM. (A) RG

CRL to CRL to KRISS Earth Station CRL KRISS toCRL

a

b

d

e

f g

C

h

i

k

1 m n

P

9 r

j

0

Earth station transmit EIRP

Uplink path loss (dB) Uplink tracking loss (dB) Satellite G/T at beam edge

Uplink C/T (dBW/K) (a - b -

Gain of lm2 antenna (dBi/m2) Power flux density arriving

at satellite (dBW/m2) ( a - b + f )

Transponder saturation flux density toward the earth station (dBW/m2)

Input back-off (dB) (g - h) Output back-off (dB) Total transponder saturation

Downlink EIRP (dBW) (k + j) Downlink path loss (dB) Downlink tracking loss (dB) Earth station G/T (dB/K) Downlink C / T (dB/K) (1 - m

Total C/No (dBHz) Estimated time uncertainty (ns)

(dBW)

(dB/K)

c + d)

EIRP (dBW)

- n + o )

52.4

207.5 1 .o 0.0

- 156.1

44.5 - 110.6

-79.0

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43.0

16.9 205.3

1 .o 20.1

- 169.3

59.3 0.6

53.5 52.4

207.5 207.5 1 .o 1 .o 0.0 0.0

-155.0 -156.1

44.5 44.5 -109.5 -110.6

-79.0 -79.0

-30.5 -31.6 -25.0 -26.1

43.0 43.0

18.0 16.9 205.3 205.3

1 .o 1 .o 20.1 19.1

- 168.2 - 170.3

60.2 58.3 0.5 0.6

Fig. 1 . Block diagram of CRL's two-way time transfer system.

to measure the propagation delay time accurately. Two- way transfer does not require such precise measurements of propagation delay, because of the almost complete can- cellation of propagation delay times for each path direc- tion. For complete cancellation, both path and frequency would have to be the same. In a real situation, these two conditions are not completely satisfied. In addition, com- pensation for the Sagnac effect [6] is needed to recover the differences.

Fig. 1 shows a block diagram of CRL's two-way time transfer system [7]. We introduced two sets of Ku-band (uplink: 14 GHz, downlink: 11 GHz) transportable anten- nas and transceivers for access to the INTELSAT satel- lites. We used 14.030/10.980 GHz for RG and two fre- quency slots of [14.032, 14.037 GHz]/[10.982, 10.987 GHz] for the JD and JK experiments. These systems are allowed access to the INTELSAT satellites. Both antenna diameters are 1.8 m (or equivalent offset-type). The an- tenna beam width is about 1" at 14 GHz. Both system have low-noise amplifiers with 2.7 dB noise figure. The output signal is upconverted from 70 MHz IF signal and fed to a 4 W transmitter. The link budgets for our system is shown in Table I. From the table we obtain an esti- mated carrier to noise spectral density, C/No, of about 59 dBHz which is consistent with observed C /No value. Table I also gives the estimated uncertainty in time trans- fer, in nanoseconds.

In our series of experiments, CRL/HQ in Tokyo is al- ways the master station with one set of our system and we transported the other set to the other clock site (CRL/ Kashima for the JD experiment and KRISS for the JK ex- periment, as shown in Fig. 2). The IF signals at 70 MHz communicate between the transmitters/receivers and two types of time transfer modems. We introduced one set of a microwave time and ranging experiment (MITREX) modem type 2500 A, developed by Stuttgart University in Germany [8]. The modem transforms the 1 PPS of the local clock into a digital bit stream with the chip-rate of 2.5 MHz and a length of 4 ms and provides up to eight spread-spectrum codes. One of the authors (Imamura, CRL) also developed an original equipment, "I-mo- dem," compatible with the MITREX modem. This mo- dem is a prototype modem for CRL's future upgraded two- way time transfer operations.

111. INTELSAT SPACE SEGMENT INTELSAT is a worldwide consortium of 119 member

nations that own and operate the global commercial com- munications satellite system. INTELSAT operates satel- lites in three regions: the Atlantic Ocean (AO), the Indian Ocean (IO), and the Pacific Ocean (PO). In our experi- ments, we used the Pacific Ocean satellites comprising satellites at 174" E, 177" E and 180" E longitude.

Each satellite can be accessed by C-band and Ku-band communications links. For Ku-band access, INTELSAT offered two independent spot beams for each satellites.

I I

5 0 0 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 42, NO. 2. APRIL 1993

/( ?/+ cover area /J'

ttime Fig. 2. Locations of participating stations and an example of estimated Ku-band west spot beam pattems. The authorized beam

pattem data of IESS-405 [lo] are used for the beam pattem envelopes in this figure.

Each spot beam covers a region of about 1200 km diam- eter on the earth's surface. Fig. 2 shows the example of a Ku-band west spot beam pattern for the Pacific satel- lites. The beam boundary (3 dB down) covers the major parts of Japan and the eastern part of Korea. In the series of our experiments mentioned below, we used the IN- TELSAT V(F-3) satellite at 180" E for the RG experi- ment performed in 1989 and the INTELSAT V(F-3) sat- ellite at 177" E for the JD and JK experiments. The 180" E satellite is in a well controlled orbit and its inclination can be as small as 0.03". Meanwhile, the 177" satellite is not controlled in the north-south direction by reason of fuel shortage and its inclination has become larger than 2".

As mentioned in the above section, our antenna beam- width is about 1 " and we had to direct our ground antenna to the satellite manually. The large inclinations intro- duced the important considerations in our systems as mentioned in the following sections.

IV. EXPERIMENTS We performed three kinds of experiments. Table I1

summarizes the conditions of these experiments particu- larly about the locations of ground stations and the effects of daily variations due to the inclination.

A. Ranging (RG) Experiment The first ranging experiment was performed in Nov.

1989 using the small inclination 180" E satellite [7]. For the RG experiment, we used the MITREX modem type 2500 A. The purpose of the RG experiment was to study the performance of the MITREX modem and to under- stand the operational conditions for INTELSAT utiliza- tion.

B. Japanese Domestic (JD) Experiments The Japanese Domestic (JD) experiments between To-

kyo and CRL/Kashima using the 177" E satellite were performed in March 1992.

We made use of the large-inclination 177" E satellite for the JD and JK experiments. Because our system is mainly designed for the inclination smaller than 0.1 " , we had to compensate for three major inclination effects: (1) antenna tracking, (2) Doppler tracking, and (3) carrier-to- noise ratio (C/N ratio) change caused by daily shift of the satellite spot-beam. Our experience may be useful in the design of future INTELSAT time transfer systems with large inclination satellites.

For the JD experiment we transported one set of our system to CRL/Kashima from CRL/HQ. We observed the 177" E satellite for a total of 48 hours. Both stations used hydrogen maser frequency standards having better stabil-

TAKAHASHI et al.: TWO-WAY TIME TRANSFER EXPERIMENTS 501

TABLE I1 SUMMARY OF THE CONDITIONS FOR THE THREE EXPERIMENTS, RG, JD AND JK

( 1 ) RG (2) JD (3) JK Experiments (Ranging) (Japanese Domestic) (Japan-Korea)

Date Purpose

Ground stations

Standard clocks Satellites Inclination Daily variation

Tracking Doppler C /N change

November 1989 First INTELSAT utilization Ranging error estimation CRLIHQ, Tokyo

Cs standard 180" E 0.03"

fixed f10 Hz < 1 dB

March 1992 First two-way measurement Effects of large inclination CRLIHQ, Tokyo CRL/Kashima H masers 177" E 2.1"

Az f 3", El f 3" * 1 KHz 3 dB

April 1992 First intemational experiments Single-beam coverage CRL/HQ, Japan KRISS, Korea Cs standards 177" E 2.8"

Az f 3.1", El = k3 .1" + 1 KHz 6 dB

ity than a cesium standard over periods of less than one day. As shown in a later section, there is, however, no significant difference between the results for the stations equipped with H-masters or cesium standards.

C. Japan-Korea (JK) Joint Experiment The first international INTELSAT two-way time trans-

fer experiments were performed between Japan and Korea using the 177" E satellite in April 1992 following the JD experiment. Before the JK experiment, we transported one of our systems to KRISS, Korea, just as in the JD exper- iment. This was our first international transportable IN- TELSAT experiment for two-way time transfer. On the JK experiment, a heavy rain storm in Tokyo and the large inclination created problems for the MITREX modem, decreasing the amount of data to 30 hours from the planned total of 50 hours.

V. DISCUSSION A. RG Experiment

Fig. 3 shows the residuals after the least square analysis for the ranging data measured at CRL/HQ. The rms error is about 0.43 ns which is consistent with the theoretical error of 0.6 ns with 59 dBHz C/No. The square root of the Allan variance in the interval between 1 s and 200 s is shown in Fig. 7. For larger interval we have used a special least square method to adjust and eliminate the orbital daily range variations, as given by Zhao [9].

RG experiment C 0 3 .- ns CI RMS error = 0.43 ns n

Systematic errors L L

h

-3

Time from 13:30 UT, Nov. 16, 1989. Fig. 3. Residuals of ranging (RG) experiments by least-squares analysis.

2) the need to track precisely the local frequency shifts for excursions larger than 1 kHz from the center lo- cal frequency because the I-modem was also de- signed for small Doppler compensation, and

3) the need to obtain the maximum ground segment performance to recover the decrease of satellite sig- nal due to drift of the spot-beam by reason of the orbit inclination. This factor is, however, not so large in the JD experiment since both Tokyo and Kashima are located near the center of the spot-beam as shown in Fig. 2 [lo].

The square root of the Allan variance for the JD exper- iment is given in Fig. 7. Since we used H-masers for the experiment, we can neglect the instabilities of the clocks themselves.

B. JD Experiment The JD experiment was the first CRL two-way time

transfer experiment using INTELSAT satellite. The lo- cations of CRL/HQ and CRL/Kashima are shown in Fig. 2 and their separation is about 120 km. In this experiment we used H-masers at both stations. Their stability- is better than over a period of lo00 s. Use of the inclined 177" E satellite presents the following three problems:

c. JK Experiment The JK experiments was the first, joint, two-way time

transfer experiments under the cooperation agreement be- l ) the necessity to track the satellite manually using

the Ku-band (fixed direction) antenna because it was designed for an inclination smaller than 0.1 " ,

tween CRL, Japan and KRISS, Korea. The JK experiment was performed one month after the JD experiment and it had almost the same difficulties as have been mentioned

I 1

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502

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 42, NO. 2, APRIL 1993

rms=O. 9411s

. . . . .

t 4 n s

f 2 n s

Ons

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U n i v e r s a l Time ( A p r i l 2 3 , 1992) Fig. 4. 1 PPS example residuals of JK experiments for 12 min (720 points) data.

10 ns

I KST

Time in UT and Japan / Korea standard time Fig. 5 . Long-term observed data (100 point averaged data) given by daily time scales.

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TAKAHASHI et al.: TWO-WAY TIME TRANSFER EXPERIMENTS 503

a U L: a k a ccr ccr .rl Q

-2150

- 2200 n

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CO -2250 CO

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1 8 1 9 2 0 2 1 2 2 2 3 2 4 2 5

D a t e ( A p r i l , 1 9 9 2 ) i n J a p a n K o r e a s t a n d a r d t i m e Fig. 6. Time transfer data comparison between JK two-way data and conventional GPS data.

in regards to the JD experiment. In the JK experiment, the difficulty due to the problem 3) mentioned above was more pronounced than in the JD experiment because KRISS is located near the edge of the Ku-band spot beam for the 177" E satellite. The area of the spot-beam also drifts about 1,000 km around the Japanese Islands in the north-south direction (see Fig. 1). Thus the satellite beam gain directed to KRISS is usually smaller than that di- rected to Tokyo.

Figs. 4-6 show the preliminary results of the JK ex- periment. Fig. 4 shows the residuals for 12 min (720 points) data. The rms value of the residuals is 0.94 ns. The rms value is somewhat larger than the theoretical value of 0.55 ns due to the MITREX's ambiguity errors and the low-pass characteristics of the I-modem. This means that the JK experiments attained nano-second-level time transfer between Japan and Korea using the INTEL- SAT satellite. Fig. 5 shows the long-term observed data (100 s data averaged) given by daily time scale. Around every noon (12 h JapanIKorea Standard Time) the MI- TREX modem failed to lock because the pseudo-range rate was larger than about 100 ns/s. At night both MITREX and I-modem failed to obtain data because the low C / N ratio caused the modems to unlock. We confirmed that the night time C / N ratio was almost 6 dB smaller than the daytime value.

10-l0

1 0-l2 L t

0 RG 0 J D 0 J K

8

2 6

10-13 1 0

1 o o 1 o1 1 o 2 1 o3 10 A v e r a g e T i m e ( s e c )

Fig. 7. Comparisons of stability (square root of the Allan variance) among the three experiments: RG, JD and JK.

From Fig. 6 we can successfully confirm the clock dif- ference between both national standard time laboratories with precision at the ns-level and better than for a con- ventional GPS time transfer. The long-term stability for

504 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 42, NO. 2, APRIL 1993

one day or longer is worse because a heavy rain storm occurred in Tokyo and accentuated the effects due to the

frequency drift and the degradation of the C / N ra- tio.

satellite inclination. Fig. 7 shows the comparison of the Allan variance between three experiments. We can con- firm the variance difference between RG and JD/JK ex- periments but we can not see the difference between JD (H-maser) and JK (cesium) experiments.

The experiment was unique because the single spot beam from the satellite covered both stations. In future international experiments, we may use this single beam method to reduce the use of satellite transponders for the time transfer between neighboring countries. The concept of a single beam and the large inclination effects qualify as new subjects for study in future INTELSAT experi- ments.

CONCLUSION This series of INTELSAT experiments brought us both

research results and some organizational experience to re- alize future routine base time transfer by the low cost use of INTELSAT satellites. About JK experiment more de- tailed research work will be prepared by KRISS and CRL. The summary of major experimental results are as fol-

We attained a short-term precision of about 0.4 ns for the RG experiment and about 0.94 ns for both the JD and JK experiments. The short-term insta- bility of these experiments is larger than theory would indicate because of the MITREX ambiguity errors and the characteristics of the low-pass filter of the I-modem. The reason for the larger errors will be analyzed further. The preliminary results of the JK experiment shows an accuracy at about the 10 ns-level compared with the GPS method. The long-term stability of the JD/ JK experiments in the time scale of one day or longer is worse than the theoretical expectation. The main reasons for this are the large satellite in- clination and a heavy storm during the experi- ments. For the low-cost use of the INTELSAT satellite, we have to accept the consequences of the large inclination such as the large range rate, the local

On the basis of our experiments, we will pursue the Asian INTELSAT time transfer program in the future with CRL, Japan, KRISS, Korea and the Telecommunication Laboratory (TL), Taiwan. Both KRISS and TL have plans to install their two-way time transfer equipments. As CRL also has the potential for very long baseline interferom- etry (VLBI), whose time resolution is better than 0.1 ns, CRL will pursue the colocation experiments between two- way and VLBI.

ACKNOWLEDGMENT The authors wish to thank the staff of INTELSAT who

agreed to perform these experiments and those in KDD, Japan, particularly Mr. Ariizumi, KDD, and also in KT, Korea, who assisted in accessing for INTELSAT and ob- taining a Korean station license.

REFERENCES

[l] M. Yamamoto, K. Harada, and Y. Saburi, “A time comparison ex- periment performed by SSRA system via ATS-1,” J. Radio Res. Labs., vol. 23, no. 110, pp. 85-103, Mar. 1976.

[2] M. Imae, H. Okazawa, T . Sato, M. Urazuka, K. Yoshino, and Y. Yasuda, “Time comparison experiments with small K-band antenna and SSRA equipments via a domestic geostationary satellites,” IEEE Trans. Instrum. Meas., IM-32, pp. 199-203, Mar. 1983.

[3] D. A. Howe, “Ku-band satellite two-way timing using a very small aperture terminal (VSAT),” in Proc. 41st Annual Symp. Frequency Control, pp. 147-160, 1987.

[4] W. J. Klepczynski, P. J. Wheeler, W. Powell, J. Jeffris, A. Myers, R. T. Clarke, W. Hansen, J. Jespersen, and D. Howe, “Preliminary comparison between GPS and two-way satellite time transfer,” in Proc. 42nd Annual Symp. Frequency Control, pp. 421-477, 1988.

[5 ] D. Kirchner, “Two-way time transfer via communication satellites,” Proc. IEEE, vol. 79, pp. 983-990, July 1991.

[6] Y. Saburi, M. Yamamoto, and K. Harada, “High precision time comparison via satellite and observed discrepancy of synchroniza- tion,” IEEE Trans. Instrum. Meas., IM-25, pp. 473-477, Dec. 1976.

[7] K. Imamura and F. Takahashi, “Results of the test experiment of INTELSAT time transfer experiment,” J. Cum. Res. Lab, vol. 39, Mar. 1992.

181 P. Hartl, “A modem for microwave time and ranging experiments via telecommunication satellite,” MITREX 2500A manual, Jan. 1989.

[9] W. Zhao, K. Imamura, and F. Takahashi, “The least square analysis for range data using a MITREX modem,’’ J. Cum. Res. Lab., vol. 39, Mar. 1992.

[lo] INTELSAT Earth Station Standards (IESS), no. 405, rev. 4, p. 31, Dec. 1990.