  Over the last decade of the 20th century and the first few years of the 21st,
the uncertainty of atomic clocks has decreased by about two orders of
magnitude, passing from the low 10^-14 to below 10^-16, in relative frequency .
Space applications in fundamental physics, geodesy, time/frequency metrology,
navigation etc... are among the most promising for this new generation of
clocks. Onboard terrestrial or solar system satellites, their exceptional
frequency stability and accuracy makes them a prime tool to test the
fundamental laws of nature, and to study gravitational potentials and their
evolution.
  In this paper, we study in more detail the requirements on orbitography
compatible with operation of next generation space clocks at the required
uncertainty based on a completely relativistic model. Using the ACES (Atomic
Clock Ensemble in Space) mission as an example, we show that the required
accuracy goal can be reached with relatively modest constraints on the
orbitography of the space clock, much less stringent than expected from "naive"
estimates. Our results are generic to all space clocks and represent a
significant step towards the generalised use of next generation space clocks in
fundamental physics, geodesy, and time/frequency metrology.
