SpaceOps2006

1
SpaceOps2006
High Accuracy Clock Synchronization
Experiment and Results from Preliminary Experime
nt
Shinichi Nakamura, Nobuo Kudo, and Seiji
Katagiri

2
Introduction (1/2) Position of HAC Experiment
Future
Roadmap of JAXA Positioning Technology Plan
QZS
Quasi-Zenith Satellite System

Japanese version of GPS
At Present
High Accuracy Clock Synchronization Experiment
Past
JAXA has developed
New Technology1 Exact time control of the satelli
te
Precise Orbit Determination System (ADOES-2, ALO
S)
New Technology2 High accurate clock synchronizati
on
between satellite and ground stations
GPS
SLR
3
Introduction (2/2) About ETS-8
Engineering Test Satellite-8 (ETS-8)
is a geosynchronous orbit satellite, which
carries a cesium atomic clock and is able to sen
d positioning signals (time reference signal) si
milar to GPS signals in L band and S band.
4
Purpose of HAC Experiment(1/2)

• The ETS-VIII experiment will be performed to
  obtain/evaluate basic technologies necessary for
  implementation of GPS-like space-based
  positioning missions in the future.
• JAXA will obtain following Key Technologies.
• ?Atomic Clock for Space Use
• ?High Accuracy Time Synchronization
• ?High Accuracy Orbit Determination

5
Purpose of HAC Experiment(2/2)
6
HAC Experiment systems
Actual HAC experiment system consists of
satellite monitoring stations,
SLR equipment and analysis center.
7
HAC Experiment Items 1

• The following technologies will be demonstrated
  on the ETS-VIII space-based positioning
  experiment.
  
• The Atomic Clock Performance in Orbit and Clock
  Synchronization Technique
  
• Clock Synchronization between Satellite and
  Ground Station
  
• a. Clock Synchronization using
  Navigation Signal and SLR
  
• technique
• synchronization accuracy better
  than 1nsec.
  
• b. Clock Synchronization using Inverse
  GPS Technique synchronization accuracy using
  than 10nsec in real time

8
GPS Technique and Inverse GPS Technique
In the left image, measurements of code-phase
arrival time from at least four
satellites are used to estimate the four
quantities receiver position in three
dimensions (X,Y,Z) and GPS time (T).
In the right image, Measurements of code-phase
arrival time from at least four stations are use
d to estimate four quantities transmission
position in three dimensions (X,Y,Z) and GPS tim
e (T).
9
Configuration of HAC Experiment(Ground Station)
10
HAC Experiment Items 2

• Clock management
• Evaluation of the Frequency Stability of an
  Atomic Clock evaluate the frequency stability
  
• Two -sample standard deviation (s(t) t1E3
  to 8E4) Clock drift rate
  
• In this HAC experiment, the properties of the
  atomic clock onboard (frequency stability,
  temperature dependency, and aging degradation)
  are understood, and the management skill of
  atomic clock is acquired.

11
HAC Experiment Items3

• Clock synchronization between satellite and
  ground station (atomic clock mode)
  
• the reference signal is generated by the
  satellite-mounted atomic clock unit.
  
• This experiment will be performed in two
  cases non-realtime and realtime synchronization.
  
  
• Target accuracy is within 1 nsec and 10
  nsec for nonreal time and real time analysis,
  respectively.
  

12
Image of Atomic Clock mode
13
HAC Experiment Items
2.2 High Accuracy Orbit Determination Technique
(1) Ranging and Error Estimation Technique
a. Ranging Experiment ranging accuracy
using navigation signal is less than
30cm. b. PN Code investigate
suitable chip rare to the future positioning
system. chip rate variation 1.023M
cps, 1.705Mcps, 3.410Mcps, 5.115Mc
ps (2) High Accuracy Orbit Determination Te
chnique a. Orbit Determination using Naviga
tion Signal orbit accuracy is better
than 30m in real time.(target)
b. Orbit Determination using SLR Date
orbit accuracy is better a 10m.
(target)
14
HAC Experiment Items 4

• Clock synchronization between satellite and
  ground station (relay clock mode)
  
• JAXA developed and installed another atomic clock
  unit in a ground station (SMS1Masuda).
  
• If the experiment shows that the positioning
  accuracy of relay clock mode is accurate enough
  to use in GNSS, it will be possible to reduce the
  development cost of satellites because the
  maintenance of atomic clock on the ground is
  cheaper and easier than in orbit.

15
(No Transcript)
16
HAC Experiment Items 5
Positioning using geosynchronous and GPS.
(GPS Overlay) user positioning experiment with G
PS and ETS-VIII Navigation data.
evaluation of the user positioning
accuracy. validity evaluation of GEO naviga
tion satellite. Through this experiment, we can c
onfirm whether a geosynchronous satellite is use
ful for positioning system. We expect the accura
cy of positioning will increase by introducing
ETS-8 into GPS analysis, since the Dilution of
Precision (DOP) is improved by adding a
geostationary satellite.
17
Preliminary Experiment

• Because ETS-8 has not been launched yet, JAXA has
  been conducting preliminary experiments using GPS
  satellites as ETS-8. The purpose of the
  preliminary experiments is to synchronize the
  clock of SMSs via GPS satellites. As an analysis
  method of clock synchronization, we set the clock
  of SMS1 as the standard clock and estimate the
  difference between other SMSs and SMS1.

18
Preliminary Experiment

• GPS data during 3 days from21/02/2005 to
  24/02/05
  
• At first, we tried precise orbit determination of
  GPS.
  
• As the reference orbitIGS final orbit
• In order to estimate our accuracy, we calculated
  the difference between our analytical results and
  IGS final orbit.

19
Visible GPS
The number of visible GPS was around 14 from the
regional area of four SMS stations. ? Enough to
HAC Experiment However, at least three hours of
data are required for the determination
value to converge to a desired accuracy.
20
Result 1 the accuracy of the position of GPS
21
Result 1 the accuracy of the position of GPS

• Since all the four SMSs are located along the
  east side of the Pacific Ocean, GPS become
  periodically invisible from any of the four SMSs
  during the experiment. GPS satellites become
  temporarily invisible at regular 12-hour
  intervals.
• These conditions put a limit to the accuracy of
  our analysis using GPS data.
  
• As the result, the accuracy of the position of
  GPS is within 1.7m (radial), 1.4m (cross track),
  and 3.2m (along track). This result is not very
  accurate but useful.

22
the estimated SMSs clock
the estimated SMSs clock by comparing the
estimated SMSs clock using GUTS, which accuracy
is about 0.15 ns.
Table is showed next page
23
Difference between estimated SMS clock and SMS1
clock
Unit (n sec)
Results the clock differences are within 3 nsec (
RMS).
24
Discussion
Low accuracy of precise orbit determination and
clock synchronization is attributable to the
small number of receiving stations (only four
SMSs) and their geographically unbalanced
locations. HAC experiment system is still in th
e process of system tuning and does not provide
its best performance. We are going to perform
further preliminary experiments to optimize the
system.
25
Future plan

• JAXA installed a new SMS at Tsukuba Japan.
  Although the base length between SMS2 (Tokyo,
  Japan) and new SMS (Tsukuba, Japan) is not so
  long, the accuracy improvement is expected.
• ETS-8 will be launched in the near future. JAXA
  wants to carry out the HAC experiment using ETS-8
  as soon as possible. The experimental result will
  be reported in the next symposium.
• Thank you

26
Configuration of HAC Experiment(??????????)
Gurand Experiment Systems
ETS-VIII satellite signal
Navigation Messeage/ Pilot Signal
GPS satellite signal
SLR
TLM/CMD /RNG
SLR/Bi-directional Communication
JAXA SLR
Navigation Messeage/ Pilot Signal
TLM/CMD/RNG
SMS(Masuda)
SMS(Koganei)
SMS(KMITL)
SMS(QUT)
User Sta. (Tsukuba)
TX/RX/RNG Eqip.(USB)
RF Equip. (S-Band)
SLR(tentative)/Experimant data
Base-band Equip.
CCSDS Processing Eqip.
TLM/CMD
Operation Control Equip. SMS Control Equip.
Space-based positioning experiment software
Navigation Messeage
Data Trans.
Data Trans.
ETS-VIII USB Station
Master Control Station
TLM/CMD/RNG
ETS-VIII space-based positioning ground
experimant systems
ETS-VIII Tracing Control System
Navigation Masseage /TLM