  The ESA mission "Space Optical Clock" project aims at operating an optical
lattice clock on the ISS in approximately 2023. The scientific goals of the
mission are to perform tests of fundamental physics, to enable space-assisted
relativistic geodesy and to intercompare optical clocks on the ground using
microwave and optical links. The performance goal of the space clock is less
than $1 \times 10^{-17}$ uncertainty and $1 \times 10^{-15} {\tau}^{-1/2}$
instability. Within an EU-FP7-funded project, a strontium optical lattice clock
demonstrator has been developed. Goal performances are instability below $1
\times 10^{-15} {\tau}^{-1/2}$ and fractional inaccuracy $5 \times 10^{-17}$.
For the design of the clock, techniques and approaches suitable for later space
application are used, such as modular design, diode lasers, low power
consumption subunits, and compact dimensions. The Sr clock apparatus is fully
operational, and the clock transition in $^{88}$Sr was observed with linewidth
as small as 9 Hz.
