  The ultimate goal for future neutrino facilities is the determination of CP
violation in neutrino oscillations. Besides $| U(e3) | \ne 0$, this will
require precision experiments with a very intense neutrino source and energy
control. With this objective in mind, the creation of monochromatic neutrino
beams from the electron capture decay of boosted ions by the SPS of CERN has
been proposed. We discuss the capabilities of such a facility as a function of
the energy of the boost and the baseline for the detector. We compare the
physics potential for two different configurations: I) $\gamma=90$ and
$\gamma=195$ (maximum achievable at present SPS) to Frejus; II) $\gamma=195$
and $\gamma=440$ (maximum achievable at upgraded SPS) to Canfranc. We conclude
that the SPS upgrade to 1000 GeV is important to reach a better sensitivity to
CP violation iff it is accompanied by a longer baseline. In both Setups, the
gain in the CP violation sensitivity with a previous knowledge of $| U(e3) |$
is apparent.
