We present a transportable optical clock (TOC) with ${}^{87}\mathrm{Sr}$. Its complete characterization against a stationary lattice clock resulted in a systematic uncertainty of $7.4\times {10}^{-17}$, which is currently limited by the statistics of the determination of the residual lattice light shift, and an instability of $1.3\times {10}^{-15}/\sqrt{\tau }$ with an averaging time $\tau$ in seconds. Measurements confirm that the systematic uncertainty can be reduced to below the design goal of $1\times {10}^{-17}$. To our knowledge, these are the best uncertainties and instabilities reported for any transportable clock to date. For autonomous operation, the TOC has been installed in an air-conditioned car trailer. It is suitable for chronometric leveling with submeter resolution as well as for intercontinental cross-linking of optical clocks, which is essential for a redefinition of the International System of Units (SI) second. In addition, the TOC will be used for high precision experiments for fundamental science that are commonly tied to precise frequency measurements and its development is an important step to space-borne optical clocks.

• Received 20 September 2016

DOI:https://doi.org/10.1103/PhysRevLett.118.073601