  We use beam position measurements over the first part of the AWAKE electron
beamline, together with beamline modeling, to deduce the beam average momentum
and to predict the beam position in the second part of the beamline. Results
show that using only the first five beam position monitors leads to much larger
differences between predicted and measured positions at the last two monitors
than when using the first eight beam position monitors. These last two
positions can in principle be used with ballistic calculations to predict the
parameters of closest approach of the electron bunch with the proton beam. In
external injection experiments of the electron bunch into plasma wakefields
driven by the proton bunch, only the first five beam position monitors
measurements remain un-affected by the presence of the much higher charge
proton bunch. Results with eight beam position monitors show the prediction
method works in principle to determine electron and proton beams closest
approach within the wakefields width ($<$1\,mm), corresponding to injection of
electrons into the wakefields. Using five beam position monitors is not
sufficient.
