  I rigorously analyze a proposal, introduced by D.R.Terno, about a spatial
localization observable for a Klein-Gordon massive real particle in terms of a
Poincar\'e-covariant family of POVMs. I prove that these POVMs are actually a
kinematic deformation of the Newton-Wigner PVMs. The first moment of one of
these POVMs however exactly coincides with a restriction (on a core) of the
Newton-Wigner selfadjoint position operator, though the second moment does not.
This fact permits to preserve all nice properties of the Newton-Wigner position
observable, dropping the unphysical features arising from the Hegerfeldt
theorem. The considered POVM does not permit spatially sharply localized
states, but it admits families of almost localized states with arbitrary
precision. Next, I establish that the Terno localization observable satisfies
part of a requirement introduced by D.P.L.Castrigiano about causal temporal
evolution concerning the Lebesgue measurable spatial regions of any Minkowskian
reference frame. The validity of the complete Castrigiano's causality
requirement is also proved for a notion of spatial localization which
generalizes Terno's one in a natural way.
