Page 71 - 卫星导航2021年第1-2合期

Han et al. Satell Navig (2021) 2:18 Page 9 of 10

 dTT
 exceed 50 ns (Zhang and Cai, 2018). Terefore, the difer-
 ≡ 1 − L G ence between BDT and TT is negligible when being used
dTCG

(16) as time reference. Te time ofset between BDT and UTC
L G

 TCG − TT = (JD TT − T 0 ) × 86400 is shown in fgure (Fig. 1).

1 − L G
where JD is the Julian Days at the time of TT, and Conclusions
TT
BDS adopts a technique for time synchronization com-
−10
L G ≡ 6.969290134 × 10 (17) pletely diferent from GPS, GLONASS, and Galileo.
T 0 ≡ 2443144.5003725
TWTFT technology is used in BDS to directly measure
Te starting point of TT is International Atomic Time the clock ofsets between satellites and ground stations.
(TAI) 00:00:00 of January 1, 1977. T 0 is the corresponding Terefore, BDS is essentially a precision time measure-
number of Julian days. At this moment, TT is diferent ment and time synchronization system for a large-scale
from TAI by 0.0003725d = 32.184s , so the relationship space near the Earth. General relativity is the basic the-
between TAI and TT can be written as: oretical framework for the data processing in BDS. Te
spatial reference of BDS is BDCS, and the time reference
TT = TAI + 32.184s (18) is BDT. BDCS and BDT are the realizations of GTRS and
TT in BDS, respectively. Te BDS ground station coor-
And then the relationships between BDT, TAI and TT
can be approximately expressed as: dinates are consistent with ITRF2014 at the centimeter
level. Te s cale diference is ap proximately 1.1 × 10 .
−8
BDT = TAI + BDT PPS − 33s
BDT is maintained by the master control station of BDS.
(19) −14
TT = BDT + BDT PPS + 65.184s Te frequency deviation of BDT is less than 2 × 10 ,
and the time ofset is less than 50 ns relative to TAI or
Here BDT PPS is the time deviation of the second pulse UTC.
of BDT relative to TAI or UTC, expressed as: In addition to BDCS and BDT, GCRS and BCRS are
also involved in the operation of BDS. Te observation
(20)
BDT PPS = BDT PPS − TAI PPS model of time synchronization and precise orbit deter-
BDT is realized in the form of composed clock by an mination is established within the GCRS framework. Te
ensemble of atomic clocks of the master control station, coordinate transformation between BDCS and GCRS
and aligned with UTC through the National Time Ser- is consistent with IERS. In the autonomous operation
vice Center (NTSC) of the Chinese Academy of Sciences mode without the support of ground master control sta-
and China National Institute of Metrology (NIM). Te tion, EOP is obtained by means of long-term prediction
clock ensemble consists of several active hydrogen maser and on-board observation. Observation models for on-
and cesium atomic clocks. A new time scale algorithm board astrometry should be established within the BCRS
derived from ALGOS algorithm is adopted. Te time dif- framework.
ferences between BDT and UTC(k)/UTC are monitored Acknowledgements
continuously by the links of GNSS common view and The authors wish to thank the editor and the reviewers, whose comments
TWTFT. When it is necessary, frequency adjustment will helped improve this paper enormously.
be introduced into BDT to keep the consistency between Authors’ contribution
BDT and UTC. Under normal circumstances, the fre- HC carried out the space–time reference system studies. LL participated in the
quency deviation of BDT relative to TAI or UTC is less space–time references of BDS and drafted the manuscript. CZ carried out the
than 2 × 10 −14 , and the absolute time deviation does not Local inertial reference system studies. LY participated in the BDT time system
studies and helped to draft the manuscript. All authors read and approved the
fnal manuscript.
Authors’ information
Han Chunhao graduated with a Ph.D. in Astronomy from Nanjing University in
1990. He is a researcher at Beijing Satellite Navigation Center, and concur-
rently a professor at the University of Information Engineering and Shanghai
Astronomical Observatory. Mainly engaged in the research of satellite naviga-
tion system, time and frequency control and relativistic space–time reference
system.
Liu Li is a senior engineer at Beijing Satellite Navigation Center, mainly
engaged in data processing such as precise orbit determination and time
synchronization of satellite navigation.

Fig. 1 the time ofset between BDT and UTC