Chronometric levelling

Measurement of the relative frequency difference between two optical clocks, connected by fibre links, due to the gravitational redshift. The frequency difference is directly proportional to the height difference between the two clock locations. This method provides differences of physical heights independent of the distance. Recent clocks achieve uncertainties of 10^-18 which corresponds to 1 cm in height.

For more information:
Müller, J., Dirkx, D., Kopeikin, S., Lion, G., Panet, I., Petit, G., Visser, P.: High Performance Clocks and Gravity Field Determination. ISSI book on High Performance Clocks, Space Science Reviews 214:5, 2018, DOI: 10.1007/s11214-017-0431-z

Delva, P., Denker, H., Lion, G. (2019). Chronometric Geodesy: Methods and Applications. In: Pützfeld, D., Lämmerzahl, C. (eds) Relativistic Geodesy. Fundamental Theories of Physics, vol 196. Springer, Cham. https://doi.org/10.1007/978-3-030-11500-5_2