How are the atomic clocks synchronised between worldwide VLBI telescopes? I know that VLBI needs precise synchronised atomic clocks to calculate the baseline distance between the radio telescopes at cm level. But i cant´t find any information how the atomic clocks are kept synchronised over long distances (several thousand kilometres around the world). As far as i know GPS is not accurate enough.... How are atomic clocks kept synchronised on all VLBI telescopes ?

Thanks a lot :)
They don’t have to synchronize the atomic clocks at each telescope.  Instead, they synchronize the collections using GPS (so the telescopes observe the same target in the same frequency range at the same time), as shown in the diagram from this paper.

When the data is brought together, they use differences between the arrival times of quasar data to calculate relative antenna locations.  From this paper from NASA:

During geodetic VLBI observations, signals emitted by distant sources
of radio frequency energy (quasars) are received and recorded at
several antennas. At each antenna (VLBI station) a very stable
frequency standard (hydrogen maser) provides a reference signal that
enables time tagging of the quasar signals as they are being recorded.
For each VLBI experiment, correlation of the time-tagged, recorded
information from the participating antennas yields the differences
among the arrival times of any specific quasar radio wave at the
antennas. These time differences are used to calculate the locations
of the antennas with respect to each other.