T2L2 Time Transfer by Laser Link
General comments :
T2L2 data are processed on a daily basis, as soon as they are delivered
by the T2L2 Instrument Mission Centre (“CMI”, located in CNES, Toulouse, France)
on the one hand, and are available at the international data centers on the other hand
(EDC
for EUROLAS Data Center in DGFI and
CDDIS
for Crustal Dynamics Data Information System in NASA).
The T2L2 data processing is based on 5 different levels ;
it is developed by the Scientific Mission Center
(“CMS”, located in Geoazur/CNRS-OCA-UNS, in Sophia-Antipolis, France).
Levels I to III are dedicated to the processing of onboard and ground data,
and to the computation of orbital parameters/corrections
(as the satellite attitude, the control of range residuals, etc.)
which result in the indentification process of the “Triplets”.
Then, levels IV and V are dedicated
to the Ground to Space time transfer processing
(fine corrections, analysis of noise, filtering, etc.) and to calculation
of the Ground to Ground link between 2 involved SLR stations, respectively.
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STATION (Satellite Laser Ranging data) :
this section provides information about the activity of involved laser stations when they shot the Jason-2 satellite —
it is equipped with a laser reflector together with the T2L2 detector —: for each pass, we give the number of shots and echoes.
this section provides information about the activity of laser stations when they shot at Jason-2 / T2L2 : for each pass, number of shots and number of echoes.
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BOARD (T2L2 data) :
this section provides information about the activity of T2L2 / DORIS (the onboard time scale used to date the laser pulses by T2L2 (picosecond resolution)
is based on the DORIS Ultra Stable Oscillator): monitoring of the frequency offset computed from a local GPS pulse per second, number of optical pulses acquired
(at this step, the instrument is not able to know from which SLR station a given pulse has been transmitted) and instrument's internal parameters.
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ENERGY (Link budget) :
this section provides, for a set of passes of a given station, the number of shots, the number of "triplets" and the energy of the laser pulses measured by T2L2.
It is possible to plot, for each observed satellite pass, the energy level which was received for each detected laser shot.
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GROUND TO SPACE (Time transfer) :
this section provides an overview of the available ground to space time transfers, between a given SLR station (a given pass) and the satellite (the on-board clock of Jason2).
It allows to plot:
- the long-term monitoring of the UTC(station) relative to an onboard (local) GPS pulse per second, and
- the time transfer itself, by pass: (up) the residuals after a linear fit, and (down) the Allan time stability (Sigma_x (tau)) for data (black circles) and theoretical 1/N**(1/2) (green circles).
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GROUND TO GROUND (Time transfer, currently only in Common View) :
this section provides plots about the ground to ground time transfer in Common View between 2 SLR stations :
a global plot which represents the time transfer for the chosen period of time (limit at 10 days), and a dedicated plot for each available common pass.
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JASON-2 FORECASTING (Ephemeris) :
this section provides simple Jason-2 ephemeris (UTC date, elevation & azimuth), from a given SLR station or from any geographical position.
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