Instrumentation & Measurement Magazine 25-6 - 21

Symbol
TTSk
TTIk
TTXk
TRXk
TSGUk
TSGDk
TSPTk
Table 1 - The meaning of the symbols in TWSTFT equations
Symbol
Description
local time scale
ground station modem measurements
ground station transmit channel delay
ground station receive channel delay
signal path uplink delay
signal path downlink delay
satellite path delay through the transponder
In the current Asia-Europe TWSTFT network, a Ku-band
transponder is used. The propagation delays through the atmosphere,
including geometric path delay, tropospheric delay
and ionospheric delay, are canceled due to the reciprocity of
the signal paths. When using the same satellite transponder,
the transponder delay can also be canceled. The Sagnac effect
is corrected by a specific model, and the equipment delay of
the ground stations can be accurately calibrated by using the
travelling calibrator [9], [10].
Data Analysis
UTC is calculated and published monthly. In this study, we selected
monthly data to evaluate the performance of TWSTFT
on the long and short baselines, respectively. We use the GPS
PPP link as a reference link. Compared to GPS PPP, the advantages
of TWSTFT are its high accuracy and long-term stability;
however, its short-term stability is poor due to the diurnal
variations of TWSTFT. The TWSTFT and GPS PPP equipment
of each laboratory employ an active hydrogen maser as
a reference clock, which is steered towards UTC by different
strategies. Table 2 shows the statistics of time links involved in
the calculation.
Calibration of the TWSTFT Link
The use of time comparison links in UTC calculation requires
calibration of the signal delays of the ground station. According
to the TWSTFT calibration guidelines, a TWSTFT link
can be calibrated through the GPS travelling calibrator or the
TWSTFT mobile station [11]. When there is no mobile calibration
station, we can use the calibrated GPS PPP link to calibrate
the TWSTFT link [12]. Specifically, the calibrated GPS PPP link
and uncalibrated TWSTFT link between the two stations have
Table 2 - The information of TWSTFT links
Link
NTSC-PTB
KRISS-PTB
PL-PTB
KRISS-NTSC
September 2022
Approximate
distance
7600 km
8300 km
700 km
1600 km
Comparison
period per
hour
twice
twice
twice
once
Baseline
type
long baseline
long baseline
short baseline
short baseline
the same reference source. The bias, or calibration result, is determined
by the average difference of the full month between
the TWSTFT results and GPS PPP solutions, and the TWSTFT
link can be aligned to GPS PPP link by using this bias. The calibration
result is obtained by:
[UTC k UTC k UTC k UTC k( )]TW
C Link PPP Link TWSTFT)
( )  ( )]
 
12 12
() (
[
PPP
where k1 and k2
)]TW
( )
is the calibration result,[UTC(k1)-UTC(k2
two stations.
Asia-Europe TWSTFT Link Results
We select TWSTFT and GPS PPP 30-day data (modified julian
date (MJD) 59543-59572) over different baselines for analysis,
respectively. The results for the two long baselines, the NTSCPTB
and KRISS-PTB Asia-Europe link, are shown in Fig. 3. The
results for the two short baselines, the PL-PTB Europe-Europe
link and KRISS-NTSC Asia-Asia link, are shown in Fig. 4. The
gap in GPS PPP solutions in MJD 59571 is due to missing data
from KRISS.
From the time comparison results of the different baselines
from Fig. 3 and Fig. 4 it can be seen that the TWSTFT results
are consistent with the GPS PPP solutions, indicating that
the TWSTFT results are accurate and stable and can be used
for UTC time transfer. We can also see that there are diurnal
variations in the TWSTFT results of some days with different
baselines, which are the dominant source that disturb the accuracy
of TWSTFT [13]. We do not yet know its cause and need
further research.
Performance Evaluation of TWSTFT Results
We employ modified Allan deviation (MDEV) to evaluate the
time link frequency instabilities at different averaging times
[14]. The MDEVs for the baselines of NTSC-PTB and KRISSPTB
are shown in Fig. 5, and the baselines of PL-PTB and
KRISS-NTSC are shown in Fig. 6.
Fig. 5 and Fig. 6 show that the link stability of the GPS PPP
solutions for averaging times less than 1 d over different baselines
is the highest due to the use of carrier phase data. The
IEEE Instrumentation & Measurement Magazine
21
(4)
represent two earth stations, respectively, C
)]PPP
GPS PPP solutions between two stations, and [UTC(k1
UTC(k2
is the calibrated
)is
the uncalibrated TWSTFT solutions between the
TSTUk
TSTDk
TSIUk
TSIDk
TSCUk
TSCDk
Description
tropospheric uplink delay
tropospheric downlink delay
ionospheric uplink delay
ionospheric downlink delay
Sagnac correction in the uplink
Sagnac correction in the downlink

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