Fisrt application of the T2L2 ground to space time transfer : Characterisation of the DORIS USO.

The Time Transfer by Laser Link experiment is a joint CNES and OCA space mission that shall allow ground to ground time transfer with an expected stability of about 1 ps over 1,000 s and 10 ps over one day and an accuracy in the 100 ps range. The T2L2 instrument has been successfully launched with the Jason-2 space vehicle on the 20th of June 2008 and switched on 5 days later. It is been undergoing continuous operation since this date. The first days of operation have been devoted to functional tests, followed by a first evaluation of the performances of both the space instrument and the whole system. First results are very promising, preliminary data analysis shows a short term time stability of some tens of ps for an integration time of 1s, without any compensation of the behavior of the instrument. The next step, in progress, is improving of data processing, with the introduction of the relativistic compensation and of the numerical model of the instrument. That shall allow us to perform rapidly ground to space and ground to ground time transfer with the expected performances. Among science objectives of the mission, a first application of T2L2 shall be to precisely characterize the DORIS Ultra Stable Oscillator (USO) aboard Jason 2, independently from the DORIS system and with a better precision. By using an hydrogen maser as ground clock, relative stability of both T2L2 and DORIS USO shall allow to ldquoreadrdquo the USO frequency for integration time of some tens of seconds. This paper will present the first restitution of the USO frequency thanks to the T2L2 ground to space time transfer.