The T2L2 instrument on board the Jason-2 satellite has enabled a unique experiment to compare atomic clocks using laser pulses.

Atomic clocks are regularly compared to synchronize them or gauge their performance. This involves a ‘time transfer’ operation based on transmitting electromagnetic signals from satellites to the clocks being compared, which may be in space or on the ground.

Using optical signals rather than the microwave signals used until now could yield a gain in accuracy of one to two orders of magnitude. To demonstrate this, the Time Transfer by Laser Link experiment (T2L2) is using a space instrument built by CNES flying on the Jason-2 satellite in combination with a ground network of laser-ranging stations linked to very-high-quality atomic clocks.

The principle behind the experiment is based on propagating laser pulses between clocks to be synchronized. Each ground laser-ranging station transmits a train of pulses to Jason-2. The T2L2 instrument detects and timestamps these light pulses, while a retroreflector relays them back to the stations. With these three timestamps—transmission, arrival at the satellite and return—it is possible to calculate the time difference between the two clocks.

From 2008 to 2014, the T2L2 instrument executed some 600 million timestamps, showing that it is possible to remotely compare clocks with a degree of uncertainty better than 500 picoseconds. The clock frequency of the DORIS oscillator on Jason-2 has also been validated with unprecedented precision. But above all, T2L2 has paved the way for new experiments, notably for the future European Laser Timing link (ELT) that will be part of the Atomic Clock Ensemble in Space (ACES) scheduled to be launched to the International Space Station in 2016. The T2L2 experiment is currently conducting other optical telecommunications tests.