We present a transportable optical clock (TOC) with $^{87}$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. The measurements confirm that the systematic uncertainty is reduceable to below the design goal of $1 times 10^{-17}$. The instability of our TOC is $1.3 times 10^{-15}/sqrt{(tau/s)}$. Both, the systematic uncertainty and the instability are to our best knowledge currently the best achieved with any type of transportable clock. For autonomous operation the TOC is installed in an air-conditioned car-trailer. It is suitable for chronometric leveling with sub-meter resolution as well as intercontinental cross-linking of optical clocks, which is essential for a redefiniton of the 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 it is a first step to space borne optical clocks
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