Design and Test of the T2L2 (Time Transfer by Laser Link) Optical Subsystem

We report on the design and test of the optical subsystem of the T2L2 (Time Transfer by Laser Link) space instrument. The T2L2 experiment, developed by OCA and CNES is a next generation optical time transfer system that will allow an improvement 1,2 by one to two orders of magnitude as compared to the performances of existing microwave time transfer systems like GPS or Two-Way. The principle is derived from satellite laser ranging (SLR) technology with dedicated space equipment embarked on the satellite Jason 2, scheduled for launch in mid-2008. Satellite Laser Ranging stations (connected to the clocks to be synchronized) emit short laser pulses towards the satellite where they are equally reflected and dated by an onboard event timer. The departure and return of the laser pulses are also timed in the laser stations. The time transfer is derived aposteriori from the data triplets (departure, satellite, return) acquired on the satellite and the respective laser stations. The T2L2 instrument consists of an optical and an electronic subsystem. The optical subsystem is designed such that its field of view (FOV) covers the whole earth for the Jason 2 orbit. It features a linear and a non-linear channel consisting of optical elements and avalanche photodiodes; the linear channel's purpose is threefold: it triggers the whole timing system and measures both the background light and the laser pulse energy. The non-linear channel is for precise timing. We report on the detailed construction of the optical assembly and an exhaustive calibration and performance test campaign in terms of metrology. This test campaign was performed in the clean-room facilities at CNES, Toulouse in March/April 2007 with a dedicated test bed featuring a mode locked laser, variable geometry for different incidence angles and a reference timing system.