synchronization of distant laser stations thanks to time ... · synchronization of distant laser...
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Synchronization of distant Laser stations thanks to Time Transfer by Laser Link:
Proposal for a dedicated campaign
A.Belli1,2, P. Exertier1, E. Samain1, C.Courde1, J.M.Torre1, F. Vernotte2
1 Géoazur Valbonne, France
2 Observatoire de Besançon UTINAM, Besançon, France
Email: [email protected]
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Outline
• The Time Transfer by Laser Link (T2L2) Experiment
• The Common View (CV) Time Transfer
• The Non Common View (NCV) Time Transfer issues
• Space Effect on the on-board Oscillator and model
• The NCV Time Transfer
• Computation principle
• First Result
• A new Dedicated Campaign
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THE TIME TRANSFER BY LASER LINK (T2L2) EXPERIMENT
• Launched in 2008, on-board Jason-2 at 1335 km
• Synchronized ultra stable clocks using SLR
Optical
detection
Timing
DORIS (Doppler Orbitography and Radiopositioning
Integrated on Satellite) USO
• pass over station 10 to 15 min (Torbital ~ 110 min)
• 5 to 6 per day,
• dates optical events at the picosecond resolution from laser stations (ILRS)
T2L2
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Common View (CV) Time Transfer
Grasse
Matera
CV
The instability of the oscillator is removed
[Exertier et al., ASR 54(11), 2014]
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Non Common View Time Transfer (NCV)
Rely on the stability of the oscillator
Grasse
Simosato
• Between ground stations ( > 3000 km)• A model to explain the frequency variations (see Belli et al., ASR SI
Doris, 2015)• the frequency accuracy of the model was estimated at ~5.10-13,
which roughly corresponds to a stability of 5 ns @ 10,000 s• NB: Orbital period of Jason-2 : 6700 s
~1500 s
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From the space environment effect on the USO, to a frequencymodel
USOY
X
Z
Jason-2
Radiations(protons trapped in the SAA)
Thermal variation
in part due to pass through the
Earth’s shadow
AgingRelativistic effects
(Einstein effect)
Earth
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• Complete model on 10 days.
• RMS in the range 4.5 to 6.5 10-13
Oscillator model on 10-day
Temperature + radiation + local drift due to aging
[Belli et al. accepted 2015 ASR]
Days since the beginning of the mission (January 2013)
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The NCV computation Principle
1
Selecting a Master Station (A reference Station)
2
Fitted to UTC(OP) by a calibrated GPS link
Integration of the frequency model
Synthetic Coordinate Time Scale fitted to Grasse local time
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The NCV computation Principle
3
control on every pass over Grasse
(integrated phase - observed phase)
4
Time Transfer Relative differences (Grasse - Station(i))
This operation give the error propagation of the integrated model
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First Results (Global Network)
Pass 1
Pass 2 Pass 3Pass 4
1st dateError propagation of the frequency model : linear drift of -50 ns on 30,000 s
~862 ns
~856 ns
Common ViewCommon View
~-128 ns
~-41 ns
~-138 ns
~-205 ns
~-125 ns~-45 ns
UTC(OP) - Grasse
thanks to GPS calibrated link
[Laaz-Bourez 2013]
~200 ns (MJD 56299)
Tim
eT
ransfe
r R
ela
tive d
iffe
rences
(Gra
sse -
Sta
tion(i))
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A new Dedicated Campaign:
• 4 stations involved (Grasse, Herstmonceux,
Koganei and Changchun)
• Winter to Spring 2016
• 2 by 2 in common view (control)
• Calibration needed
• expected results : Non common view time transfer
at the level of several nanoseconds (accuracy)
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First results (4 stations)Without calibration
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Conclusions
ACKNOWLEDGMENTS
• CNES
• Astrogeo team of OCA
• International Laser Ranging Network
• Labex FIRST-TF, Besançon University & Region Franche Comté
• Success in the frequency model integration over 30,000 s (around 4 - 5
revolutions of Jason-2 )
• Synthetic time scale is elaborated each day, which is fitted to Grasse Observatory
local time (Grasse - UTC(OP) ~200 ns) permanently measured / GPS
• The laser intercontinental network (ILRS) is currently not synchronized to the 50 -
100 ns level (necessary for the International Terrestrial Reference Frame (ITRF)
estimation)
• It is the first space optical time transfer over intercontinental distances !
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Thank you for your attention !
T2L2 website : http://www.geoazur.fr/t2l2/en/data/v4/
Grasse SLR station January 2015