  Euclid is an ESA Cosmic-Vision wide-field-space mission which is designed to
explain the origin of the acceleration of Universe expansion. The mission will
investigate at the same time two primary cosmological probes: Weak
gravitational Lensing (WL) and Galaxy Clustering (in particular Baryon Acoustic
Oscillations, BAO). The extreme precision requested on primary science
objectives can only be achieved by observing a large number of galaxies
distributed over the whole sky in order to probe the distribution of dark
matter and galaxies at all scales. The extreme accuracy needed requires
observation from space to limit all observational biases in the measurements.
The definition of the Euclid survey, aiming at detecting billions of galaxies
over 15 000 square degrees of the extragalactic sky, is a key parameter of the
mission. It drives its scientific potential, its duration and the mass of the
spacecraft. The construction of a Reference Survey derives from the high level
science requirements for a Wide and a Deep survey. The definition of a main
sequence of observations and the associated calibrations were indeed a major
achievement of the Definition Phase. Implementation of this sequence
demonstrated the feasibility of covering the requested area in less than 6
years while taking into account the overheads of space segment observing and
maneuvering sequence. This reference mission will be used for sizing the
spacecraft consumables needed for primary science. It will also set the
framework for optimizing the time on the sky to fulfill the primary science and
maximize the Euclid legacy.
