  STE-QUEST is a fundamental science mission which is considered for launch
within the Cosmic Vision programme of the European Space Agency (ESA). Its main
scientific objectives relate to probing various aspects of Einstein's theory of
general relativity by measuring the gravitational red-shift of the earth, the
moon and the sun as well as testing the weak equivalence principle to
unprecedented accuracy. In order to perform the measurements, the system
features a spacecraft equipped with two complex instruments, an atomic clock
and an atom interferometer, a ground-segment encompassing several
ground-terminals collocated with the best available ground atomic clocks, and
clock comparison between space and ground via microwave and optical links. The
baseline orbit is highly eccentric and exhibits strong variations of incident
solar flux, which poses challenges for thermal and power subsystems in addition
to the difficulties encountered by precise-orbit-determination at high
altitudes. The mission assessment and definition phase (Phase-A) has recently
been completed and this paper gives a concise overview over some system level
results.
