July 22, 2016

Exploitation

The exploitation of T2L2 experiment is jointly ensured by CNES and the Observatoire de la Côte d'Azur.

Two mission centres:

Two mission centres share the operational and scientific activities involved in the exploitation of the T2L2 system:

Under CNES responsibility, the Instrument Mission Center (CMI) is hosted by the COMS (Centre d'Opération des Missions Scientifiques: Scientific Mission Operations Centre). The CMI is the interface point with the Jason-2 ground segment which ensures the link with the satellite. The CMI is in charge of the preparation of the telecommands to be uploaded to the instrument in orbit. The CMI gathers and archives all the telemetry data as well as auxilliary data from the satellite or Jason-2 mission needed for a good exploitation of T2L2. These telemetry data are used in near real time (every 2 hours, after each satellite passage over a ground telemetry—telecommand station) to monitor the instrument. These data are also used in differed time to generate the "L0" and "L1" products which will be transmitted to the Scientific Mission Centre.

The Scientific Mission Centre (CMS) is operated by the Observatoire de la Côte d'Azur. It is in charge of producing and distributing the "L2" products to the scientific community. To produce them, it receives L0 and L1 products from the CMI and gathers data from laser stations that have echoes from T2L2/Jason-2. The first step consists in associating to each echo of a ground laser station the corresponding event recorded by T2L2 instrument. Then the triplet {date of departure of the echo, date of detection on board, date of return on ground} is thus reconstituted. Then, the numerical models developed by the scientific team around T2L2 enable to correct the data from the instrument imperfections and of the relativistic effects to reach at last the value of the time difference between the onboard clock and the ground clock.

Organization of T2L2 mission centers
Organization of T2L2 mission centres

The products delivered by T2L2:

The products delivered by T2L2 are separated in 4 categories:

  • Raw measurements: They are constituted of undecommuted telemetry data (Coded in binary). These data are not distributed.

  • Level 0 products (L0): They are constituted of decommuted telemetry data and translated in physical values. These data are not yet corrected by calibration data or instrumental models. The L0 are constructed from raw data by the CMI. The L0, unusable without the correction models of the instrument behaviour and the calibration data, are not distributed

  • Level 1 products (L1): They are the first exploitable data for scientific applications. For T2L2 they are triplets constituted by departure and return dates of the laser impulsions for the stations to which they are associated to the date recorded on board by the instrument. The construction of the level 1 products is performed in 3 steps:
    • Computing of a "rough" onboard date from the elementary data of the instrument as well as the models and calibration data of the dating chain. It is the last step conducted at the CMI (from software tools developed by the Observatoire de la Côte d'Azur); the data are then transferred to the CMS.
    • Reconstitution of the triplets, associating to each onboard event the ground event and the laser station from which it originates.
    • Correction of the onboard date in the triplet from the models and calibration data of the detection chain taking into account the laser station, the reference point of the onboard reflection and the reference point of the onboard detection geometry.
    The L1 products will be distributed to the user community (after a reservation period for priority exploitation by privileged partners of the T2L2 project).

  • Level 2 products 2 (L2): They are the first level of "scientific" products. In the T2L2 framework, the L2 products are represented by the elementary time transfer (for each laser pulse) between the ground clock and the onboard clock. These elementary time transfers will enable to reconstruct the behaviour of the onboard clock, to compare the ground clocks between themselves and to achieve all the objectives of the T2L2 mission. The L2 products will be distributed to the user community (after a reservation period for priority exploitation by privileged partners of the T2L2 project).

The products delivered by T2L2 are available on the Scientific Mission Centre's website