The Research on Carrier Phase Time Transfer of

BeiDou Navigation Satellite System

GUANG Wei, ZHANG Pengfei, YUAN Haibo, LI Wei ZHANG Qin, HUANG Guanwen, Zhang Rui
National Time Service Center (NTSC), CAS Chang’an University
Key Laboratory of Time and Frequency Primary Standards College of Geology Engineering
Xi’an China Xi’an China
guangwei@ntsc.ac.cn

Abstract—GPS/GLONASS Common view, all in view and PPP since the BDS open service. The performance of BDS time
Time Transfer technology are widely used in the remote time transfer on CV is at statistical uncertainty about 2 ns for the code
comparison. With the benefit of the high precise measurement on observation and the uncertainty less than 1ns for the smoothing
carrier phase, the PPP time transfer technology which based on the code, the frequency stability is at the e-14 level for an averaging
carrier phase data has been used in the International time time of one day [2][3]. As the BDS system is developing, the
comparison for TAI computation. In this Paper, The Carrier Phase widely use for time transfers on BDS system is testing and in
Time Transfer on BeiDou Navigation satellite system (BDS) is progress yet.
implemented. The technology is referenced with GPS carrier phase
time transfer method Precise Point Position (PPP) and combined In this paper, the carrier phase time transfer based on BeiDou
with dual-frequency observation from the BeiDou Navigation Satellite System is implemented with the same technology of
Satellite system. The broadcast ephemeris and precise orbits and GPS Precise Point Position. As the IGS (International GNSS
clock data are used for calculation and comparison. The Service) provide the precise obits and the precise clock products,
performance of this time transfer technology is analyzed on the the GPS PPP time transfer can get a better precision. For the BDS
respect of precise, frequency accuracy and stability. Compared with satellite, precise obits and clock data cannot get from IGS yet, the
the BDS Common View and GPS Common View, the result of broadcast ephemeris data is used for BDS PPP time transfer. The
BeiDou carrier phase time transfer is verified and shows its better precise obits and the clock data of BDS Satellite is got from one
performance.
of our civil data analysis center. The principle of PPP and its
Keywords—Time Transfer, Carrier Phase, Precise Time Transfer,
application in time comparison is described in section II. After
BeiDou Navigation System that, the data processes method in BDS carrier phase precise time
transfer is discussed. In section III, we compared different
INTRODUCTION satellite obits used in BDS PPP algorithm to show the affection of
The Carrier Phase time transfer technology is researched satellite obits and clock error on precise time transfer. To confirm
server decade. At present, the most of the time and frequency the accuracy of calculation, BDS Common View and GPS
laboratory are using GPS Precise Point Position and TWSTFT Common View is compute for comparison with technical CGTTS
links for participating TAI Calculation. As the Precise of phase [4]. The performance of different time transfer technology is
measurement is higher than code measurement in GNSS receiver, analyzed on the respect of precise, frequency accuracy and
the time transfer based on carrier phase is improved from stability at the end of paper.
nanoseconds to hundreds picosecond, and its frequency stability THE PRINCIPLE OF CARRIER PHASE TIME TRANSFER
is reaching e-15 level at an averaging time of one day[1].
Traditional Precise Point Position is usual used in geodetic
The BeiDou Navigation Satellite System is the Chinese survey. For time and frequency transfer, receivers in different
developing Navigation system. The BDS start providing position time laboratory are connected to their time and frequency
and timing service for the B1I signal at the end of 2012, and the references generating, as the 1PPS signal and 5/10 MHz
open service signal B2I is published in December 2013. The frequency signal. Based on the reference impulse and frequency
space constellation of BDS consists of five Geostationary Earth signal, receivers acquire phase and code observations from all
Orbit (GEO) satellites, twenty-seven Medium Earth Orbit (MEO) satellites in view at dual-frequency. Each station calculates its
satellites and three Inclined Geosynchronous Satellite Orbit clock difference between the time UTC (k) and the reference,
(IGSO) satellites. There are 5 GEO, 4 MEO and 5 IGSO BeiDou such as IGST and etc. The stations which will compare their
navigation satellites in orbit when we prepare for this paper. time difference minus the time difference (UTC (k) - RefT), can
The common view (CV) Time transfer based on BDS system get their time different UTC (k1)-UTC (k2). The figure1 show
has been researching by using code and phase smoothed code the PPP time transfer principle.

978-1-4799-5252-6/14/$31.00 ©2014 IEEE 113

 And N w means wide wavelength and wide ambiguity of the
combination.
In the paper data processing, the follow equations are used to
remove unsuitable phase and code observation data [6].

º¼ ½°
1
Nw Nw  ª¬ N w,i  N w
i i 1
i ° i 1
¾ (4)
1
V i2 V i 1  ª¬( N w,i  N w i 1 )  V i 1 º¼ °
2 2 2

i °¿
N w ,i  N w i 1
t 6V iˈN w,i 1  N w,i d 1

Figure 1.  GPS PPP time transfer principe diagram Only one method cannot detect the cycle slip well. We need
combine different cycle slip detect algorithm for the data
The Observations of receiver can be written as the follow preprocessing. The other cycle slip detect algorithm is the
equation for the ion-free code and phase measurements: Ionospheric Residual which can be expressed as:

 ­° Pi U  cdtr  cdTs  d orb  dtrop  dion / Pi  d mult / Pi  H pi   ­° Lion O1I1  O2I2 O1 N1  O2 N 2  I f 1  I f 2

® (5)
°̄ Li U  cdtr  cdTs  d orb  dtrop  dion / )i  Oi Ni  d mult / )i  H )i ®
°̄ I diff Lion (ti 1 )  Lion (ti )
Each Station computes its time difference dtr by PPP
After that, the Geometry-Free algorithm is also used in this
method. This time difference which is the bias of local time and paper for cycle detection. The mathematical expression canbe
the reference time can be express as: written:

­ 'Tutc1 Tutc1  IGST / Re f_T Lgf L1  L2 (6)

 ®  
¯ 'Tutc 2 Tutc 2  IGST / Re f_T The quadratic polynomial fitting of least square method is
used to calculate the residual. The follow is satisfy it means the
The time difference between the two station is: current epoch is slipped.

 'T1,2 Tuct1  Tutc 2 'Tutc1  'Tutc 2   ( Lg ,i  Qi )  ( Lg ,i 1  Qi 1 ) ! 6(O2  O1 ) (7)

( Lg ,i 1  Qi 1 )  ( Lg ,i  Qi )  1
DATA PROCESS METHOD
The reason of the high precise time transfer is that the PPP ParametersEstimation
data process procedure use the precise phase observation. The The Parameters will be estimate are the position of the
carrier phase measurement is formed by initial ambiguity, integer receiver in the ECEF coordinate, clock bias, and the ambiguity of
cycle and the fractional cycle. When the GNSS signal is locked each satellite phase observation. Extension Kalman Filter (EKF)
by the receiver, the phase is increased continuously; otherwise the is adopted for the parameters estimation. The linearized
cycle slips is formed. If the data arc without cycle slip, the observation equation can be written:
difference between code and carrier phase is a constant called
ambiguity. The one of the procedures is the cycle slip detecting.
Lk H k X k 1  B( X k  X k 1 )  Wk  H (8)
CarrierphaseCycleslipdetection
The most conventional method for cycle slip detection is The procedure the EKF is as follows:
Melbourne-Wübbena (M-W) combination with the double
frequency observation. The equation of this combination is as P0 E[( x0  xˆ0 )( x0  xˆ0 )T ]
follows [5]:
Xk Ak X k 1
( f1 L1  f 2 L2 ) ( f1 P1  f 2 P2 ) Ak 1 Pk 1 AkT1  Qk 1
LM Lw  Pn  Pk
 f1  f 2 f1  f 2  
Pkc1 H KT ( H k Pkc1H KT  Rk )
Ow N w Kk

The L is the carrier phase measurement for the distance; f xk xk 1  K k ( yk  H k Ak xk 1 ) ½

¾
P ( I  K k H k ) Pkc1 ¿
mean the associated frequency, P means code measurement. Ow 

114
 Figure 2.  EKF filter procedure (&()&RRUGLQDWH5HVLGXDO
 
;D[LV <D[LV =D[LV
It needs several epochs to get the precise coordinate in the 

5HVLGXDO>P@
EKF parameters estimation called convergence time. The 

receiver clock bias in this period is unsuitable. When we used this 
data for time transfer, it will make the clock bias jump between  
the two days. In order to repair EKF convergence time of   
7LPH+RXU>VLQWHUYDO@
 

estimation, the forward and backward filter is applied. The time 87& 176& 5()7
comparison data in the convergence time is inaccuracy should be 

&ORFN'LIIHUHQFH>QV@
delete. This may lead the time transfer result un-continually every 

estimated period. So the two wards EKF is necessary for the 

parameters estimation. The figure shows the only forward filter 
and combined forward and backward filter method used in the 
position estimation.   
7LPH+RXU>VLQWHUYDO@
 

7KH)RUZDUG(.)
  Figure 5.  Beidou PPP using precise obits and clock bias.
5HVLGXDO>P@


()&)&RRUGLQWH5HVLGXDO
  
;D[LV <D[LV =D[LV
;D[LV <D[LV =D[LV 

5HVLGXDO>P@
 
    

7LPH+RXU>VLQWHUYDO@
7KH)RUZDUGDQGEDFNZDUG(.) 
 
 
5HVLGXDO>P@

    
 7LPH>VLQWHUYDO@
87& 176& ,*67
;D[LV <D[LV =D[LV 
  &ORFNGLIIHUHQFH>QV@
    
7LPH+RXU>VLQWHUYDO@ 

Figure 3.  the residual of the position by forward and backward EKF parameters 
estimation

    
THE EXPERIMENTS OF BDS PPP 7LPH>VLQWHUYDO@
In the experiment, the observation pseudorange and the
Figure 6.  GPS PPP using IGS obits and clock products.
carrier phase at the frequency B1and B2 is adopt from Beidou
monitoring receiver at time laboratory NTSC’s. The monitoring Compared with the figure 5 and figure 6, the residual of
receiver is connected with 1PPS and 10 MHz signal generating position is as well as GPS. The RMS of coordinate is in mm level
from UTC (NTSC). This input signal is applied to synchronize booth the GPS and BDS. However, for the clock bias, the GPS
the internal receiver time to an external time source. PPP is better than BDS. This may be caused by precise obits and
(&()&RRUGLQDWH5HVLGXDO clock products, as the BDS tracking station is much less than

;D[LV <D[LV =D[LV
 GPS for the precise products calculation. And we do not correct
the DCB delay and the antenna phase bias for the BDS satellite.
5HVLGXDO>P@


THE PERFORMANCE OF BDS PPP TIME TRANSFER
 
By using the PPP method, the local time laboratory can get
     their local time difference with reference time. To analyze the
7LPH+RXU>VLQWHUYDO@
87& 176& %'7
performance of carrier phase time transfer of BeiDou navigation
 satellite system, the period of the observation data of NTSC and
&ORFNGLIIHUHQFH>QV@

BSNC is selected from Dec.16 to Dec.20, 2013. Figure 7 shows

 the Beidou Precise Point Position time transfer using the
broadcast ephemeris. Figure 8 shows the same data processing
method using precise satellite obits. The two figures show the

     advantage the precise obits and clock used in time transfer.
7LPH+RXU>VLQWHUYDO@

Figure 4.  Beidou PPP using broadcast ephemeris.

Figure 5 shows the precise Beidou satellite obits and clock

data which provide by the civil data analysis center used in the
Beidou PPP calculation.

115
 87& 176& 87& %61&

)UHTXHQF\6WDELOLW\

 
%'6333
&ORFNGLIIHUHQFH

 *36&9
%'6&9




   

6WELOLW\
7LPH>0-'PLQ@

Figure 7.  BDS PPP time transfer using broadcast ephemeris



87& 176& 87& %61&

7LPH'LIIHUUQFH>QV@



     
    
$YHUDJH7LPH W>QV@

     
Figure 10. The frequency of different time transfer reslut
0-'>PLQLQWHUYDO@
TABLE I.  THE ALLAN DEVIATION OF DIFFERENT TIMETRANSFER RESULT
Figure 8.  BDS PPP time transfer using precise satellite obits
Tau(s) GPS CV BDS CV BDS PPP
There are other time links which are GPS CV and BDS CV 960 1.49E-12 2.26E-12 1.88E-13
between the two stations of NTSC and BSNC. The same period 1920 8.45E-13 1.43E-12 1.28E-13
result is calculated for comparison.
3840 4.49E-13 6.76E-13 9.67E-14
87& 176& 87& %61&
 
7680 2.99E-13 3.31E-13 7.32E-14
15360 1.74E-13 1.99E-13 5.59E-14
7LPHGLIIHUHQFH>QV@


30720 1.19E-13 1.14E-13 4.45E-14
 61440 7.00E-14 5.97E-14 2.68E-14


CONCLUSION
*36&9 %'6&9 %'6333

      The time transfer on carrier phase of Beidou Navigation
0-'
Satellite System is implemented using the broadcast ephemeris
Figure 9.  Different links of time transfer between UTC(NTSC) and and precise orbits and clock data. The results show the precise
UTC(BSNC) orbits and clock products have much advantage in the time
transfer obviously. From the time comparison between the UTC
The comparison between different time transfer method (NTSC) and UTC (BSNC), the precision of BDS PPP is much
shows that the precision of BDS PPP is high than that of BDS CV better than that of CV links. The frequency stability shows that
and GPS CV. To evaluate the performance of the time transfer, BDS carrier phase time transfer is about tenth of traditional
the Allan Deviation is calculated for the BDS CV, GPSCV and common view time transfer as well as GPS. At present, the BDS
BDS PPP methods. Figure 9 shows the frequency stability of PPP is not widely used in transfer. The precise products of BDS
different time transfer results. The table 1 lists the value of the is not as well as GPS limited for the monitoring stations. With the
Allan Deviation. We can find the BDS PPP short time frequency development of the Beidou system and progress of the precise
stability can reach 1E-13/960s, E-14/day. product, time transfer based on BDS PPP technology will play an
important role in the remote time transfer among the laboratories.
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[5] H. M. Peng, C. S. Liao, “GPS Smoothed P3 code for Time Transfer,” EFTF, [7] Witchangkoon, B. Elements of GPS Precise Point Positioning. PhD Thesis,
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