  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
