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Improvement and New Results of the Laser Time Transfer on MEO satellites
Meng Wendong, Zhang Haifeng, Yang Huafeng, Chen Wanzhen, Wang Jie, Ivan Prochazka , Zhang Zhongping
Shanghai Observatory, CAS China Academy of Space Technology Czech Technical University


Goals of Laser Time Transfer (LTT)
· Evaluate the performance of the space rubidium clocks with respective to the ground hydrogen maser, dedicated for the Compass system · Testing of the Relativity theory


LTT Payloads onboard Chinese Navigation satellites
April 13, 2007 The first LTT payload onboard the COMPASSMEO (altitude 21500km) was launched, and LTT experiment started. Aug 1, 2010 The second new LTT payload onboard the COMPASS-IGSO1(altitude 36,000km) was launched, and obtained the LTT data at the first attempt April 10, 2011 The third LTT payload onboard COMPASS-IGSO3 was launched April 30, 2012 The fourth LTT payload onboard COMPASSMEO3 was launched (altitude 21500km)


Design of New LTT Payload Detector LTT payload design
Simply designed Single photon sensitive Radiation resist Cold and hot vacuum environment Low noise

Improvement
Gate/Un-gate Mode Two Channel Two FOV Narrow bandwidth filter


Block Diagram of LTT Module


Diagram of Laser Detector

Gate

Silicon K14 SPAD (QE 20-30%, radiation resist) Gate/Un-gate circuit -- to reduce the effect of the dead time of SPAD when the noises are strong.


Design of New Detector
Field of view
MEO3 23°/17° The OLD MEO 28° The bigger one is for nighttime (low noise) experiment, the smaller is for daytime (high noise) experiment (but to be restricted to laser track IGSO satellites in daytime ).
IGSO Altitude ( °) Incidence angle ( °) Slant range (km) 90 0 36000 50 5.55 37264 30 7.48 38837 90 0 21500 MEO 50 8.45 22686 30 11.42 24129


Narrow bandwidth filter
BARR Associates, Inc. -- Diameter 25mm Bandwidth 3.8nm --Transmittance >75% at 532nm The OLD MEO Bandwidth 8nm


Dual-SPAD detector 300g, <1W, 105в70в50mm Field of View: 28°, 8.8nm bandwidth filter
(for COMPASS-MEO):


New LTT Detector
New Dual-SPAD detector
(for IGSO1/IGSO3/MEO3)

500g, <2W,105в70в80mm Two Field of View: 15°/ 11°for IGSO, 23°/17°for MEO, 4nm bandwidth filter Gate/Un-gate mode


vacuum experiment


Detector NEW Big FOV NEW Small FOV OLD
8 0% 7 0%

Precisionrms,ps 1 2 3 .1 1 2 0 .2 1 4 7 .2 1 3 3 .1 1 9 4 .7 1 4 9 .5

points 458 437 389 409 442 667

Rate 74% 73% 75% 71% 47% 43%
250

200 6 0% 5 0% 4 0% 3 0% 2 0% 50 1 0% 0% NE W Bi g F OV NE W S m l l a F OV Ra t e OL D 0 100 150

Pr ec i s i on r m , ps s


Specifications of Compass Dedicated SLR Station
Receiving telescope : 1000mm Transmitting telescope: 300mm Active-active mode-locked Nd:YAG laser: 150mJ@532nm, 250ps pulse width; 20Hz TargetsGEO/IGSO/MEO, 20000km-40000km Ranging precision :2~3cm Daylight tracking capability, up to GEO orbit Used for Laser Time Transfer Designed and built by the SHAO during 2006-2009 1 Meter SLR telescope made by Shanghai Observatory


SLR Real-time tracking interface for IGSO1 (2010.09.21)


The real time LTT data processing for COMPASS-MEO3


The primary LTT data processing for COMPASS-MEO3


Noise(ns) 0-25 25-50 50-75 75-100 >100 total Mean of noise

Points 1805 1968 993 935 1077 6778 58.65ns

Rate 26.63% 29.04% 14.65% 13.79% 15.89% 100.00%

Noise(ns) Points Rate 0-25 935 8.11% 25-50 832 7.22% 50-75 693 6.01% 75-100 682 5.91% 100-200 2005 17.39% 6384 55.36% 200 total 11531 100.00% Mean of 396.58ns noise

Noise of Compass-MEO

Noise of Compass-MEO3


The comparison of the LTT experiments' results
Satellite Date 2008-1-10 COMPASS-MEO 2008-1-10 88 326 6 .1 7 % Time(minutes) 31 Points 827 Rate 4 4 .4 6 % Situation Inside the Earth's shadow Outside the Earth's shadow Outside the Earth's shadow

COMPASS-MEO3

2012-7-16

21

434

3 4 .0 4 %


Results of LTT Experiment (Compass-MEO3)


Summary
Some improvements were made for new LTT Detector , such as gate mode increased , two different FOV used, etc. LTT experiment on Compass MEO3 was successful carried out at the precision of better than 300ps. The frequency drift and stability are about 2.447E-11 and 4.749E-14 respectively. The LTT design improvement makes the noise much lower, the detection rate much bigger, and the experiment much efficiency. Through LTT between satellite and ground, time synchronization for different stations on ground in the Chinese regions or beyond China will be carried out in the future.


Thank you


Results of LTT Experiment (Compass-IGSO1)


Results of LTT Experiment (Compass-IGSO3)