Optical clocks have demonstrated uncertainties in the low 10^-18 (18th digits !) region in fractional frequency, after integration times of order 1,000 seconds. This makes them prime candidates for applications in navigation, geodesy, and fundamental physics. In particular, chronometric geodesy i.e. determining geopotential differences between distant locations by measuring the frequency difference of clocks is expected to significantly contribute to studies in geophysics, sea level variations, geoid determination and related subjects. According to Einstein’s theory of general relativity a clock will be redshifted by 10^-18 when changing its height above the geoid by about 1 cm, so modern optical clocks can be used for centimetric geodesy which is competitive and complementary to the best space geodesy techniques available (CHAMP, GRACE, GRACE Follow-On, …).

To implement chronometric geodesy one requires two ingredients: Transportable high performance clocks, which can be easily deployed in the field at locations where a geopotential measurement is of interest (e.g. the ROYMAGE project coordinated at SYRTE), and medium distance links that are easily deployed and allow comparing the clock in the field to a distant reference clock. Currently, only optical methods using the carrier phase or femtosecond pulses, allow comparing optical clocks without degrading their performance. Such methods have been implemented in optical fibre links and more recently through free space, the latter being particularly useful for chronometric geodesy.

Free space links over distances > 100 km require an airborne relay. A SYRTE-CNES team has recently demonstrated an optical link via an airborne retroreflector (a tethered balloon at 300 m altitude) reaching a stability of 8x10^-19 after 16 s averaging, the best result on such links reported so far. The ground terminal is easily transportable and deployed to be locked and fully operational in less than 30 min. This zero-baseline experiment is a first step towards genuine point-to-point clock comparisons first over short distances via a low altitude relay, and ultimately using a stratospheric platform at 20 km altitude allowing clock comparisons over several 100 km baselines.

Fractional frequency stability with phase noise compensation "on" and "off" for the balloon link and an equivalent ground link (corner cube payload fixed on ground 300 m from the terminal).

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On chronometric geodesy

Get in touch : Peter.wolf_at_obspm.fr