1、 Recommendation ITU-R TF.1153-4 (08/2015) The operational use of two-way satellite time and frequency transfer employing pseudorandom noise codes TF Series Time signals and frequency standards emissions ii Rec. ITU-R TF.1153-4 Foreword The role of the Radiocommunication Sector is to ensure the ratio
2、nal, equitable, efficient and economical use of the radio-frequency spectrum by all radiocommunication services, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted. The regulatory and policy functions of the Radiocom
3、munication Sector are performed by World and Regional Radiocommunication Conferences and Radiocommunication Assemblies supported by Study Groups. Policy on Intellectual Property Right (IPR) ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Annex 1 of
4、Resolution ITU-R 1. Forms to be used for the submission of patent statements and licensing declarations by patent holders are available from http:/www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent informat
5、ion database can also be found. Series of ITU-R Recommendations (Also available online at http:/www.itu.int/publ/R-REC/en) Series Title BO Satellite delivery BR Recording for production, archival and play-out; film for television BS Broadcasting service (sound) BT Broadcasting service (television) F
6、 Fixed service M Mobile, radiodetermination, amateur and related satellite services P Radiowave propagation RA Radio astronomy RS Remote sensing systems S Fixed-satellite service SA Space applications and meteorology SF Frequency sharing and coordination between fixed-satellite and fixed service sys
7、tems SM Spectrum management SNG Satellite news gathering TF Time signals and frequency standards emissions V Vocabulary and related subjects Note: This ITU-R Recommendation was approved in English under the procedure detailed in Resolution ITU-R 1. Electronic Publication Geneva, 2015 ITU 2015 All ri
8、ghts reserved. No part of this publication may be reproduced, by any means whatsoever, without written permission of ITU. Rec. ITU-R TF.1153-4 1 RECOMMENDATION ITU-R TF.1153-4 The operational use of two-way satellite time and frequency transfer employing pseudorandom noise codes (Question ITU-R 250/
9、7) (1995-1997-2003-2010-2015) Scope TWSTFT has been recognized as the most precise and accurate means for remote clock comparisons and is thus widely used in the time and frequency community, including institutions and organizations affiliated with telecommunication administrations. In view of the p
10、rogress in performance of the atomic clocks compared via TWSTFT it was found necessary to calculate corrections applied to the measurement results with a higher accuracy than considered necessary in previous editions. As the Earth is not perfectly spherical, it is considered as an ellipsoid at first
11、 approximation. For a given location, there are a single longitude and two latitudes: the geocentric latitude and the geodetic latitude. The current version takes proper care of this in the calculation of the Sagnac correction. Keywords Two way satellite time and frequency transfer, TWSTFT, CDMA, Sa
12、gnac correction, timescale comparison The ITU Radiocommunication Assembly, considering a) the demonstrated high-accuracy for time and frequency comparisons using the two-way satellite time and frequency transfer (TWSTFT) method as expressed in Question ITU-R 250/7; b) the well-established use of TWS
13、TFT systems in global networks using telecommunication satellites, predominantly in the Ku band (10.7-14.5 GHz), in support of, but not limited to, the realization of Coordinated Universal Time (UTC); c) that other frequency bands are becoming important; d) that some TWSTFT links have been repeatedl
14、y calibrated so that time transfer with a systematic uncertainty below 1 ns can be achieved; e) that theoretical background is available to calculate the corrections for the effect of the propagation delay through the troposphere and the ionosphere, the correction for the Sagnac-effect, and other re
15、ciprocity factors; f) that TWSTFT is nowadays performed in networks with more than ten participating stations using code division multiple access (CDMA); g) the need for standardizing: measuring procedures; data processing; formats for the exchange of data and relevant information between participat
16、ing stations and interested bodies, such as the International Bureau for Weights and Measures (BIPM), 2 Rec. ITU-R TF.1153-4 recommends 1 that the measuring and data processing procedures for accurate time and frequency transfer via TWSTFT be followed as outlined in Annex 1; 2 that the data formats
17、for the exchange of the relevant data between participating stations and interested bodies should be as outlined in Annex 2. Annex 1 Procedures for TWSTFT 1 Introduction TWSTFT using geostationary telecommunication satellites has been proven to be the most appropriate means of comparing time-scales
18、and atomic frequency standards with an uncertainty in time of less than 1 ns and with relative uncertainty for frequency of about 1 part in 1015 at averaging times of one day. This is why TWSTFT is widely used in the international network of time-keeping institutions supporting the realization of In
19、ternational Atomic Time (TAI) and UTC by the International Bureau for Weights and Measures (BIPM, Bureau International des Poids et Mesures). Such activities are performed under the auspices of the Working Group on TWSTFT of the Consultative Committee for Time and Frequency (CCTF). For the same reas
20、ons, TWSTFT has inter alia been chosen or proposed as the primary means to synchronize the elements of the ground segment of global navigation satellite systems. Many other applications can be envisaged. This Recommendation builds on well-established practice currently in use in networks comprising
21、institutes in Europe and the United States of America, in Europe and Asia, and also within the Asia-Pacific Region, operating in support of the BIPM. It shall, however, remain sufficiently open to adapt for new applications and services. Radio links have been used to transfer time from one clock to
22、another for a long time. In radio links, however, the signal delays are changing with distance, ionosphere, troposphere, temperature, earth conductivity and so on. To cancel these influences to first order the two-way scheme has been introduced: at both clock sites the time signals are transmitted a
23、t nominally the same instant and on both sides the signal from the other clock is received and its time of arrival is measured. After the exchange of the measured data, the difference of the two clocks is calculated. The delays cancel due to the reciprocity, to first order, of the signal paths. The
24、accuracy of the result then depends on the residual effects due to the incomplete reciprocity. Some of these effects are well understood and others are still under study. In some cases corrections for these effects can be used to improve accuracy. 2 Brief description of the exchanged signals TWSTFT
25、is based on the exchange of timing signals through geostationary telecommunication satellites, as is schematically shown in Fig. 1. It is done by transmission and reception of radio-frequency (RF) signals, containing pseudorandom noise (PRN) codes binary phase-shift keying (BPSK) modulated on the in
26、termediate frequency (IF) by a modem. The phase modulation is synchronized with the local clock, and the modem generates a one-pulse-per-second (1PPS) output, synchronous with the BPSK sequence, and named 1PPSTX. This signal is the realization of a time-scale named TS(k). Each station uses a dedicat
27、ed PRN code for its BPSK sequence in the Rec. ITU-R TF.1153-4 3 transmitted signal. The receiving equipment generates the BPSK sequence of the remote stations and reconstitutes a 1PPS tick from the received signal, named 1PPSRX. The difference TI(k) between the two 1PPS signals is measured by a time
28、-interval counter (TIC). Following a pre-arranged schedule, a pair of stations lock on the code of the corresponding remote station for a specified period, called a session, measure the signals time of arrival, and store the results. After exchanging the data records the difference between the two c
29、locks can be computed. In the remainder of Annex 1, details of the data reduction and the treatment of systematic effects are elaborated. FIGURE 1 TWSTFT principle T F . 1 1 5 3 - 0 1SP T (1 )Sat el l i t eSP T (2 )SP D (2 )SC U (2 )SP U (2 )SP (1 )USC U (1 )SP (1 )DSC D (2 )SC D (1 )Rec ei v erT ra
30、n s mi t t erMo d u l at i o n D em o d u l at i o nMo d emT X (1 ) RX (1 )T I(1 )T i me i n t erv alco u n t er1 PP ST XT i me s cal e/-cl o ckG ro u n d s t at i o n at s i t e 11 PP SR XRec ei v erT ran s mi t t erMo d u l at i o n D em o d u l at i o nMo d emT X (2 ) RX (2 )T I(2 )T i me i n t e
31、rv alco u n t er1 PP ST XT i me s cal e/-cl o ckG ro u n d s t at i o n at s i t e 21 PP SR XFigure 1 illustrates the various signal delays and explains how the time difference between the clocks at stations 1 and 2 can be determined. The various acronyms have the following significance. TS(k): Loca
32、l time-scale, physically represented by the 1PPSTX signal generated by the modem, k being 1 for station 1 and 2 for station 2 TI(k): Time interval reading, the counter gate is opened by a 1PPS signal related to the local transmit signal and closed by a 1PPS signal related to the received signal; sho
33、rt-form designation of 1PPSTX 1PPSRX TX(k): Transmitter delay, including the modem delay RX(k): Receiver delay, including the modem delay SPU(k): Signal path uplink delay SPD(k): Signal path downlink delay SPT(k): Satellite path delay through the transponder SCU(k): Sagnac correction in the uplink S
34、CD(k): Sagnac correction in the downlink. 4 Rec. ITU-R TF.1153-4 The difference of the time-scale at station 2 from the time-scale at station 1 expressed by TS(1) _ TS(2) is determined as follows: The TIC reading at station 1 is: TI(1) TS(1) TS(2) TX(2) SPU(2) SCU(2) SPT(2) SPD(1) SCD(1) RX(1) The T
35、IC reading at station 2 is: TI(2) TS(2) TS(1) TX(1) SPU(1) SCU(1) SPT(1) SPD(2) SCD(2) RX(2) Subtracting the expression for station 2 from that for station 1, gives: TI(1) TI(2) 2 TS(1) 2 TS(2) TX(2) TX(1) SPU(2) SPU(1) SPT(2) SPT(1) SPD(1) SPD(2) RX(1) RX(2) SCD(1) SCU(1) SCD(2) SCU(2) The time-sca
36、le difference is thus given by the so-called two-way equation: TS(1) TS(2) 0.5 TI(1) (TIC reading at station 1) 0.5 TI(2) (TIC reading at station 2) +0.5 SPT(1) SPT(2) (Satellite delay difference) 0.5 SCD(1) SCU(1) (Sagnac correction for station 1) +0.5 SCD(2) SCU(2) (Sagnac correction for station 2
37、) +0.5 SPU(1) SPD(1) (Up/down difference at station 1) 0.5 SPU(2) SPD(2) (Up/down difference at station 2) TX(1) RX(1) (Transmit/receive difference at station 1) 0.5 TX(2) RX(2) (Transmit/receive difference at station 2). The last seven terms are the corrections for non-reciprocity. Except for the s
38、atellite delay difference SPT, the corrections can in principle be grouped in corrections per station. The non-reciprocity factors are further addressed in the following sections. 3 Causes of non-reciprocity and systematic uncertainty in TWSTFT 3.1 Non-reciprocity due to satellite equipment delays W
39、hen the satellite receive antenna, transponder channel and transmit antenna are common to both signal paths, the satellite signal delays are equal, i.e. SPT(1) SPT(2). This is not the case when different frequencies, transponders or different spot beams are used for the reception and/or transmission
40、s from each station, as is common for intercontinental links. In this case SPT(1) and SPT(2) or at least the difference SPT(1) SPT(2), designated as XPNDR(k), should be measured before the launch of the satellite or using another accurate method. 3.2 Sagnac-effect correction Due to the movement both
41、 of the earth stations and of the satellite around the rotation axis of the Earth during the propagation of a time signal to and from the satellite, a correction has to be applied to the propagation time of the signal. The Sagnac correction for the one-way path from satellite s to ground station k i
42、s given in a terrestrial reference frame which provides sufficient accuracy by: Rec. ITU-R TF.1153-4 5 SCD(k) ( / c2) Y(k) X(s) X(k) Y(s) where: X(k) : Geocentric x-coordinate of station (m) =(costan1(1 ) tan()+ ()cos()cos() X(s) : Geocentric x-coordinate of satellite (m) = R cosLA(s) cosLO(s) Y(k)
43、: Geocentric y-coordinate of station (m) =(costan1(1 ) tan()+ ()cos()sin() Y(s) : Geocentric y-coordinate of satellite (m) = R cosLA(s) sinLO(s) c : Speed of light 299 792 458 m/s : Earth rotation rate 7.2921 105 rad/s f : Flattening of the Earth ellipsoid 1/298.257222 a : Earth equatorial radius 6
44、378 137 m R : Satellite orbit radius 42 164 000 m LA(k) : Latitude of the station (rad) LO(k) : Longitude of the station (rad) H(k): Height of the station (m). As the Earth is not perfectly spherical, it is considered as an ellipsoid at first approximation. For a given location, there are a single l
45、ongitude and two latitudes: the geocentric latitude and the geodetic latitude. To convert from geodetic to geocentric coordinate, the following formula is used: )(c o s )(t a n )1(t a nc o s)(s i n)(t a n)1(t a ns i n)1(t a n)(d e td e t1d e td e t11kLAa kHkLAfkLAa kHkLAffkLAicg e oicg e oicg e oicg
46、 e og e o c e n t r i cFor geostationary satellites LA(s) 0 N, so: () = 2 (costan1(1 ) tan() +()cos()sin() () The total Sagnac correction SCT(1,2) for a measurement of the clock at station 2 with reference to the clock in station 1 is: SCT(1,2) 0.5 SCU(1) SCD(2) SCU(2) SCD(1) Furthermore, the sign o
47、f the Sagnac correction for the downlink is opposite to the sign of the Sagnac correction for the uplink due to the opposite propagation directions of the signals: SCU(k) = SCD(k), so that SCT(1,2) SCD(1) SCD(2) is valid. Example for a satellite at 43 W (317 E): LA(VSL) 5159 8 N, LO(VSL) 4 23 17 E,
48、difference in LO 4723 17, H(VSL) = 76.8 m, SCD(VSL) + 99.10 ns 6 Rec. ITU-R TF.1153-4 LA(USNO) 38 55 14 N, LO(USNO) 77 4 0 W, difference in LO 344, H(USNO) = 46.9 m, SCD(USNO) 95.22 ns SCT(VSLUSNO): SCD(VSL) SCD(USNO) 194.32 ns SCT(USNOVSL): SCD(USNO) SCD(VSL) + 194.32 ns VSL: Delft, the Netherlands, previously known as NMi Van Sw
