The 7th Meeting of International Committee on GNSS
Performance Evaluation of BeiDou
Satellite Clocks in Orbit and Time Offset
Monitoring
Han Chunhao,Lin Yuting,Cai Zhiwu,
Xiao Shenghong and Wang
Xianglei
Beijing Satellite Navigation Center
November 2012
1、TWTT between satellites and stations
Time synchronization of satellite clocks is the base
of satellite navigation
The main errors of satellite time synchronization
are caused by time transfer and satellite clock
prediction
The prediction error of a clock is determined
mainly by its frequency instability
1、TWTT between satellites and stations
Two Way Time Transfer is used
in BeiDou system
{xSi (tSr),tSr}
The uncertanty of clock difference
{xSi (tSe),tSe}
usually consists of random noises
( type A) and a system bias( type B)
„ The uncertainty of type A is less
than 0.3ns, and that of type B is
less than 1.5ns at present
{xRi (tRe),tRe}
{xRi (tRr ),tRr}
2、Evaluation of Satellite Clock Stability
¾ The measurement errors and relativistic effects must
be taken into account in satellite clock stability
evaluation
¾ The following time model is used in BeiDou satellite clock
prediction and its perfomance evaluation
x ( t ) ≡ T(t) − BDT(t)
= a0 + a1 (t − t0 ) + a2 (t − t0 ) + ξ (t) +Δtgrav
2
Δt grav
p
p
r r&
2
= −2 μ ae sin E / c = −2 xS ⋅ xS / c
2
2、Evaluation of Satellite Clock Stability
Frequency accuracy：1.06e-11
Frequency drift：-7.35e-14
2、Evaluation of Satellite Clock Stability
Frequency accuracy：1.65e-11
Frequency drift：-1.57e-13
2、Evaluation of Satellite Clock Stability
Frequency accuracy：1.28e-11
Frequency drift：1.83e-13
2、Evaluation of Satellite Clock Stability
2、Evaluation of Satellite Clock Stability
2、Evaluation of Satellite Clock Stability
Frequency stability of satellite clocks
GEO-1
GEO-3
GEO-4
GEO-5
IGSO-1
IGSO-2
IGSO-3
IGSO-4
IGSO-5
Stability
7.31E-14 5.52E-14 7.58E-14 9.17E-14 8.13E-14 5.95e-14
(10000s)
7.94e-14
8.53e-14
8.98e-14
Stability
6.71E-14 2.90E-14 3.83E-14 5.66E-14 9.38E-14 3.07e-14
(1day)
2.53e-14
3.91e-14
4.45e-14
2、Evaluation of Satellite Clock Stability
Beidou on-board clock Allan variance frequency stability evaluate result
-10
10
GEO-1
GEO-3
GEO-4
GEO-5
IGSO-1
IGSO-2
IGSO-3
IGSO-4
IGSO-5
MEO-3
MEO-4
-11
y
Allan Frequency Stability σ (τ)
10
-12
10
-13
10
-14
10
-15
10
0
10
1
10
2
10
3
10
Averaging Time（ seconds）
4
10
5
10
6
10
3、 Satellite clock performance in orbit
4、Time offset between GPS and Beidou
The time offset between BDT and GPST is
monitored by a GPS multi-channel dual
frequency receiver with reference signal of
BDT
4、Time offset between GPS and Beidou
Std=9.5ns
a0= -75.6ns
a1= 8.6e-15
a2= -1.1e-20/s
2012-10-01 to 2012-10-21
5、Summary
a) The type A uncertainty of TWTT between satellites
and stations is less than 0.3ns, and type B is less than
1.5ns at present.
b) The frequency stability of BeiDou satellite clocks in
orbit is about
3.8×10-14~9.2×10-14 @10000s
2.2×10-14~7.4×10-14 @1day
5、Summary
c) The performance of BeiDou satellite clocks in
orbit is a little better than in the ground
vacuum pots.
d) The time offset between GPS and BeiDou is less
than 100ns, and the frequency offset is about
1e-14 in last month.
Thank you!