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T2L2 ready to put Einstein’s theory to the test

6 November 2008
The T2L2 instrument on board Jason-2 is working well after a 1st series of tests to compare the DORIS instrument’s clock with atomic clocks here on Earth. Scientists are continuing to calibrate the instrument.

Passenger on board Jason-2

The ancients used water clocks to tell the time. Today, we synchronize atomic clocks using an instrument flying on an ocean-observing satellite.
Fine-tuning T2L2 in the laboratory before launch on 20 June. Credits: CNES/E. Grimault.

Fine-tuning T2L2 in the laboratory before launch on 20 June. Credits: CNES/E. Grimault.

The T2L2 instrument (Time Transfer by Laser Link) was launched on board the Jason-2 satellite on 20 June. This CNES mission, conducted in collaboration with the Côte d’Azur Observatory, will use laser signals to synchronize clocks. It is already functioning with a network of 25 ground laser ranging stations and atomic clocks, out of the 50 such stations operating around the globe.

“The ground stations send laser pulses to Jason-2,” explains Philippe Guillemot, T2L2 Operations Project Leader at CNES. “T2L2 receives the signals, which are partially reflected back to the transmitting stations.

“Each step is time-tagged very precisely to determine the time difference between the orbiting clock and clocks on the ground. Everything is then synchronized using complex algorithms.”



T2L2 laser in the laboratory. Credits: CNES/E. Grimault.

T2L2 laser in the laboratory. Credits: CNES/E. Grimault.

Initial results are already very encouraging and T2L2 appears to be working like a charm.

In September, it provided a 1st independent comparison with the DORIS instrument’s clock, which is not an atomic clock but an ultrastable quartz oscillator. This oscillator is a key component of DORIS, which helps to determine Jason-2’s orbit precisely and thus enhance altimetry mission performance.

Two new experiments

T2L2 undergoes final tests in the laboratory. Credits: CNES/E. Grimault.

T2L2 undergoes final tests in the laboratory. Credits: CNES/E. Grimault.

Scientists are now working to refine their models. The next step will involve 2 new experiments: the 1st, at the end of the year, will fire laser beams simultaneously from 2 stations operated by the Côte d’Azur Observatory.

“The aim is to refine the instrument’s calibration, as we already know the result: there should be no difference between the two,” says Philippe Guillemot. For the 2nd experiment, in 2009, the Paris Observatory will conduct a series of laser firings to validate T2L2’s performance.
T2L2 undergoes final tests in the laboratory. Credits: CNES/E. Grimault.

T2L2 undergoes final tests in the laboratory. Credits: CNES/E. Grimault.

CNES will be providing expert engineering support for these experiments, working with the Paris Observatory and the Côte d’Azur Observatory, which will analyse the data.

There are many immediate applications for atomic clock synchronization, including banking operations and geolocation systems like GPS. Ultimately, the experiment will also serve a wide range of science missions, particularly in fundamental physics. And one day it may even be used to test some of the principles of Einstein’s theory of relativity.

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