  Cold-atom interferometers commonly face systematic effects originating from
the coupling between the trajectory of the atomic wave packet and the wave
front of the laser beams driving the interferometer. Detrimental for the
accuracy and the stability of such inertial sensors, these systematics are
particularly enhanced in architectures based on spatially separated laser
beams. Here we analyze the effect of a coupling between the relative alignment
of two separated laser beams and the trajectory of the atomic wave packet in a
four-light-pulse cold-atom gyroscope operated in fountain configuration. We
present a method to align the two laser beams at the $0.2 \ \mu$rad level and
to determine the optimal mean velocity of the atomic wave packet with an
accuracy of $0.2\ \textrm{mm}\cdot\textrm{s}^{-1}$. Such fine tuning constrains
the associated gyroscope bias to a level of $1\times
10^{-10}~\textrm{rad}\cdot\textrm{s}^{-1}$. In addition, we reveal this
coupling using the point-source interferometry technique by analyzing
single-shot time-of-flight fluorescence traces, which allows us to measure
large angular misalignments between the interrogation beams. The alignment
method which we present here can be employed in other sensor configurations and
is particularly relevant to emerging gravitational wave detector concepts based
on cold-atom interferometry.
