Instrumentation & Measurement Magazine 25-6 - 20

diurnal variations, which
are the dominant uncertainty
source of TW link.
And, so far, there is no definite
conclusion about its
dominant origin [4].
To lower the uncertainty
of TWSTFT, Software-Defined
Radio (SDR) TWSTFT
and a few digital TWSTFT
modems supporting both
code-phase and carrierphase
measurements have
been under development in
recent years [5]. In the current
international TWSTFT
network, the Europe-to-Europe
and Europe-to-USA
links have been running
stably on the Telstar 11N
satellite [6], using 1.7 MHz
and 1.6 MHz bandwidths,
respectively. Under the coordination
of the Consultative
Committee for Time and Frequency (CCTF) Working Group on
TWSTFT and Eurasian timekeeping laboratories, the Asia-to-Europe
TWSTFT links using 2.5 MHz bandwidth on the Express-80
satellite have been reestablished since June 2021. The following six
timekeeping laboratories have participated in routine TWSTFT
measurements: National Time Service Center (NTSC), National
Institute of Metrology (NIM) and Korea Research Institute of
Standards and Science (KRISS) in Asia; and the Physikalisch-Technische
Bundesanstalt (PTB), Russian metrological institute of
technical physics and radio engineering (SU) and Consortium of
laboratories in Poland (PL) in Europe, where PTB is the pivot laboratory
specified by BIPM in the international time comparison
network. The Asia-Europe TWSTFT network is shown in Fig. 1.
In this paper, we discuss the principle of TWSTFT. Then, we
compare the TWSTFT with GPS precise point positioning (GPS
PPP) time transfer solution, an independent high-accuracy
method for time comparisons on the long and short baselines.
The stability evaluation results of different links are presented
and discussed, and finally, the conclusions are provided.
Principle of TWSTFT
A TWSTFT time transfer link requires two ground stations to
simultaneously transmit and receive time signals through a
transponder on the same geostationary satellite. The time signals
originate from each station's reference time UTC (k) (k
refers to different laboratories). Fig. 2 shows the principle of
TWSTFT between ground stations A and B. The main hardware
devices include atomic clocks that generate time signals,
modems that modulate and demodulate time signals, the upconverter
and down-converter that convert high-frequency
and intermediate-frequency signals to each other, time interval
counters that measure local delays, power amplifiers
20
Fig. 2. Principle of TWSTFT between ground station A and ground station B.
that amplify transmitted signal, and low-noise amplifiers that
amplify the received signal. The time signal is carried by pseudorandom
noise (PRN) modulated by a Satellite Time and
Ranging Equipment (SATRE) device. Because each station has
a unique PRN code based on the code-division multiple-access
(CDMA) principle, multiple pairs of TWSTFT links can be
performed simultaneously. Each station transmits the modulated
signal through the very small aperture terminal (VSAT)
and measures the arrival time of the signal received from the
remote station with respect to the local reference clock. After
subtracting the link calibration corrections, the clock difference
between the two stations can be obtained [7], [8].
The modem measurements at station A are determined by:
T T TT T T T
    
     
T TT T T T T
TIA TSA TSB TXB SGUB STUB SIUB
SCUB SPTB SGDA STDA SIDA SCDA RXA
(1)
The modem measurements at station B are determined by:
TIB TSB TSA TXA SGUA STUA SIUA
T TT T T T T
    
     
T T T T TT T
SCUA SPTA SGDB STDB SIDB SCDB RXB
(2)
The symbols in the equations are defined in Table 1, where
k is station A or station B.
The time difference between the two clocks can be expressed
by (1) minus (2):
TSA TSB
T T TT T T 
0.5(TT T T
  ) 0.5(
SCDB SCUB ) 0.5(
0.5( TIA TIB
  
SPTA SPTB ) 0.5(T T

0.5(T T  
0.5(
T T TT T

IEEE Instrumentation & Measurement Magazine
SCDA SCUA
SGUA SGDA ) 0.5(T T


SGUB SGDB
STUA STDA ) 0.5(T T TT )
SIUB SIDB ) 0.5(
  TXA RXA ) 0.5( TXB RXB
STUB STDB) 0.5( SIUA SIDA
T )
(3)
September 2022
)
)
Instrumentation & Measurement Magazine 25-6

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