"A range of new applications will be enabled by ultra-precise optical clocks, some of which by using them in space, near or far distant from Earth. They cover the fields of fundamental physics (tests of General Relativity), time and frequency metrology (comparison of distant terrestrial clocks, operation of a master clock in space), geophysics (mapping of the gravitational potential of the Earth), and potential applications in astronomy (local oscillators for radio ranging and interferometry in space).We propose to (1) develop two “engineering confidence“ ultra-precise transportable lattice optical clock demonstrators with relative frequency instability < 1×10-15/root(tau)1/2, inaccuracy < 5×10-17, one of which as a breadboard. They will be based on trapped neutral Ytterbium and Strontium atoms. Goal performance is about 1 and 2 orders better than today’s best transportable clocks, in inaccuracy and instability, respectively. The two systems will be validated in a laboratory environment (TRL 4) and performance will be established by comparison with laboratory optical clocks and primary frequency standards.(2) We will develop the necessary laser systems (adapted in terms of power, linewidth, frequency stability, long-term reliability, and accuracy), atomic packages with control of systematic (magnetic fields, black-body radiation, atom number), where novel solutions with reduced space, power and mass requirements will be implemented. Some of the laser systems will be developed towards particularly high compactness and robustness. Also, crucial laser components will be tested at TRL 5 level (validation in relevant environment).The work will build on the expertise of the proposers with laboratory optical clocks, and the successful development of breadboard and transportable cold Sr and Yb atomic sources and ultrastable lasers during the ELIPS-3 ESA development project “Space Optical Clocks (SOC)”."