Review of Two-Way Satellite Time and Frequency Transfer

MAPAN - Journal of Metrology Review of Society Two-Way of India, Satellite Vol. 21, Time No. and 4, 2006; Frequency pp. 243-248 TransferReview of Two-Way Satellite Time and FrequencyTransferMICHITO IMAETime and Frequency DivisionNational Metrological Institute of Japan(NMIJ), AISTTsukuba, Ibaraki, 305-8563, Japane-mail :michito.imae@aist.go.jp[Received : 25.10.2006 ]AbstractTWSTFT (Two Way Satellite Time and Frequency Transfer) is one of the most precise and accuratetechniques for comparison of the frequency standards located at remote sites. Its history is quite longand 1 ns level of the time transfer precision was already experimentally obtained at early time of the1970s. As it needs expensive facilities and satellite fee, and it seems too early for the performance ofthe frequency standards at that time, TWSTFT didn't become a popular technique for the globaltime and frequency transfer. And GPS common-view method has became the main tool of theinternational time and frequency transfer method because of its good cost performance of userequipment in 1980s. In 1990s, due to the development of precise and accurate frequency standards,such as fountain type Cs primary standards, more accurate and precise time and frequency transfertechniques have became necessary. According to such demands, several time and frequency instituteshave started to research and develop the TWSTFT system and a working group on TWSTFT underthe CCTF was established to discuss about the technical issues and operation of the TWSTFT link.Nowadays TWSTFT is one of the main tools of the international time and frequency transfer techniquesfor the TAI calculation.1. IntroductionTime and frequency standards have the smallestuncertainties among all of metrological standardsavailable today, with the world's top-level primaryfrequency standards offering uncertainty better than1×10 -15 [1, 2]. The generation and keeping of TAI(International Atomic Time) and UTC (CoordinatedUniversal Time ) are performed at the BIPM (BureauInternational des Poids et Mesure) by using themutual comparison data of approximately 300 ofcesium atomic clocks and hydrogen-maser typefrequency standards located at about 58organizations worldwide. TAI and UTC are used not© Metrology Society of India, All rights reserved.only the base of the legal times of each countries butalso the Key Comparison Reference Value in the fieldof time and frequency metrology for the MRA(Mutual Recognition Arrangement). Nowadays TAIand UTC have uncertainty of better than 1 × 10 -15 forrealizing the definition of SI second and stability of4 × 10 -16 at an averaging time of 40 days [3]. Theperformance improvement of the time and frequencytransfer techniques is one of the most importantelements for the improvement of the uncertainty ofTAI and UTC.2. Brief History of TWSTFT MethodIn the 1970s to begging of 1980s, several timetransfer experiments have been performed using243

Michito ImaeTWSTFT method, such as experiments usingSymphony and Hermes satellite between Europe,North America, and Asia [4-6], using ATS-1 satellitebetween Kashima in Japan and Mojave in USA [7],and Japanese domestic experiment using 30/20 GHzband [8]. Especially ATS-1 experiment realized 1 nsorder of measurement precision using spreadspectrum technique and it led to verification of theSagnac effect, a phenomenon predicted under thetheory of relativity [7].Although the TWSTFT method was recognizedas considerably superior to other methods in termsof uncertainty, it was not put into practice until the1990s, when it began to be applied to the internationaltime links used in the establishment of TAI, becauseTWSTFT method needs expensive communicationsatellite link, expensive user equipments. On thecontrary at same time the GPS common-view method[9,10] which needs inexpensive user equipmentbecame a popular tool for the global time transferlink for the TAI calculation.However, with recent improvements in theaccuracy of primary frequency standards andimprovements in the performance of atomic clocksused for TAI, the conventional GPS common-viewmethod has come to be seen as insufficient in termsof time transfer precision. In the latter half of the1990s, the need for research and development of amore precise methods of time transfer method beganto be addressed in a number of international forums,such as the CCTF (Consultative Committee for Timeand Frequency), the ITU-R (Radio CommunicationFig.1. Principle of TWSTFT method244

Review of Two-Way Satellite Time and Frequency TransferSector of the International TelecommunicationsUnion), and more. In response, time and frequencystandards institutes in many countries have begunto investigate methods of achieving higher precisionin time transfer.3. Basic Principle of TWSTFTThe principle of TWSTFT method is illustratedin Fig. 1 [11]. Each of the two time-transfer stationstransmits a signal to the other via satellite, and eachstation measures the arrival time of the signal fromthe other station with reference to the local stationclock. The time difference between the referenceclocks of the two stations can be determined basedon the difference between the measurement resultsof the two stations.The measured values at the two stations can beexpressed by the following formulas. Here, the lastterms of the two formulas represent the effect of theEarth's rotation as predicted under the theory ofrelativity (the Sagnac effect), with A denoting the areabetween the projection of the satellite/Earth stationson the equatorial plane and the center of the Earthand ω denoting the angular rotational speed of theEarth. From the difference between Eqs.(1) & ( 2) thetime difference of the reference clocks of the twostations can be estimated, using the followingformula.T A= ∆T+T TA+T UA+T SA+T DB+T RB+2ωA/c 2 (1)T B= - ∆T+T TB+T UB+T BA+T DA+T RA+2ωA/c 2 (2)∆T = (T A-T B)/2 - T TA+T TB- T UA+T DA- T SA+T SB- T DB+T UB- T RB+T RA- 4 ωA/c 2 (3)The second term and the sixth term on the rightside of Eq. 3 represent a delay time difference ininternal uplink and downlink at stations A and B,respectively, which cannot be canceled out butnormally represent fixed biases. The third term andthe fifth terms represent propagation delay time insatellite uplink/downlink for station A and stationB, respectively; these terms are nearly canceled outin light of the assumption that the signals follow thesame path. The fourth term represents internalsatellite delay time, and can be canceled out basedon the traditional use of frequency spreadmodulation for time transfer, and also based on theuse of a single frequency band for the signals of thetwo stations.Through the above-described relationships,almost all signal delay can be canceled out for thetwo stations under the two-way satellite system. Thissystem thus enables highly precise time transfer in anumber of ways, specifically including the following.(i) The method does not depend on satellite positionalerror and movement.(ii) The method does not depend on positional errorrelative to the Earth.(iii) The method allows for measurement featuringrelatively high circuit quality.4. Equipments for TWSTFT4.1 Earth StationUsually we use a satellite terminal called VSAT(Very Small Aperture Terminal) type which areconsist of a dish antenna of 1.8 - 2.6 m, a poweramplifier which has several W of output power, alow noise amplifier, up and down converters. All ofpresent TWSTFT links use Ku-band signal for thecommercial communication satellite, so the up linkfrequency is around 14 GHz and the down linkfrequency is around 12 GHz. The modems describedin the next section need the quality of thecommunication channel of about 50 - 60 dBHz, andthe time transfer performance of sub-ns level can beobtained.Even in the case of TWSTFT system, the signaldelay variation in which the path is not common forthe up link and down link causes the time transfererror. Especially the environment temperaturevariation effects largely. To compensate such effects,the measurement technique of the internal delayvariation of the earth station using a satellitesimulator has been introduced [12]. To reduce thetemperature effect, minimization of the temperaturevariation methods are also considered.By using such techniques, sub-ns level timetransfer has been realized.4.2 TWSTFT DeviceFor TWSTFT, the special equipments called"modem" are used. They usually use the pseudorandom noise (PRN) code modulation technique touse the same frequency band of the satellitetransponder to minimize the delay variation in thesatellite.245

Michito ImaeFig. 2.Concept of simultaneous time transfer among the participatingstations using the multi-channel modemIn Europe and USA, MITREX [13] modem orSATRE modem [14] developed by German companiesare mainly used. In Asian TWSTFT links, another typeof modem which is developed in Japan [15,16] ismainly used. This type of modem can make theTWSTFT operation among up to 7 stationssimultaneously. And it can realize all pairs of the timetransfer. Fig. 2 shows the concept of this modem.Unfortunately, they don't have compatibility, so theinter operation using both types modem is impossible.5. Present Status of TWSTFT Network5.1 TWSTFT links in Europe and USAMore than ten European institutes and NIST andUSNO in USA are participating the daily TWSTFTusing Intelsat satellite IS707. They use 3.5 MHz ofbandwidth for Europe to Europe, USA to Europe,and Europe to USA, respectively. The dailymeasurements are performed 12 times a day, and foreach time they make several combinations of timetransfer pairs concurrently using same frequencyband [17-18].5.2 TWSTFT links in Pacific Rim RegionIn the Pacific Rim region, NICT, NMIJ, KRISS,NTSC, TL, NMIA, SPRING are performing theTWSTF. Except for NMIA in Australia, 6 institutesuse JCSAT-1B satellite. For among NMIA, NICT, TL,and KRISS use the PAS-8 for the TWSTFT links [19].For the JCSAT-1B links, the multi-channelmodem is used and the time transfer of every onehour, namely 24 times a day, has been performed.5.3 TWSTFT Links between Asia and Europe andbetween Asia and USAThe efforts for establishing the links betweenPacific Rim region and another region has beenstarted recently.246

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