Optimal traceability to the SI second through TAI

The frequency of a secondary frequency standard needs to be determined in the SI, by comparison to that of the Cs hyperfine transition. When no direct comparison to such a primary frequency standard is possible, the SI second can also be accessed through TAI. Based on the experience of several recent experiments, we propose an approach providing optimal access to the SI second through TAI and show that it can be used for any laboratory even those not participating to UTC. We estimate the presently achievable uncertainty and show how it could be improved under realistic considerations in the coming years.