  We present the study of the internal dynamics of the intriguing galaxy
cluster Abell 1703, a system hosting a probable giant radio halo whose
dynamical status is still controversial. Our analysis is based on unpublished
spectroscopic data acquired at the Italian Telescopio Nazionale Galileo and
data publicly available in the literature. We also use photometric data from
the Sloan Digital Sky Survey. We select 147 cluster members and compute the
cluster redshift $\left<z\right>\sim 0.277$ and the global line-of-sight
velocity dispersion $\sigma_{\rm v}\sim 1300$ km/s. We infer that Abell 1703 is
a massive cluster: $M_{200}\sim 1-2\times 10^{15}$ $M_{\odot}$. The results of
our study disagree with the picture of an unimodal, relaxed cluster as
suggested by previous studies based on the gravitational lensing analysis and
support the view of a perturbed dynamics proposed by recent works based on
Chandra X-ray data. The first strong evidence of a dynamically disturbed
cluster comes from the peculiarity of the BCG velocity with respect to the
first moment of the velocity distribution of member galaxies. Moreover, several
statistical tests employed to study the cluster galaxies kinematics find
significant evidence of substructure, being Abell 1703 composed by at least two
or three subclumps probably caught after the core-core passage. In this
observational scenario, the suspected existence of a radio halo in the centre
of this cluster is not surprising and well agrees with the theoretical models
describing diffuse radio sources in clusters.
