  The definition of Simon tensor, originally given only in the Kerr spacetime
and associated with the static family of observers, is generalized to any
spacetime and to any possible observer family. Such generalization is obtained
by a standard "3+1" splitting of the Bianchi identities, which are rewritten
here as a "balance equation" between various spatial fields, associated with
the kinematical properties of the observer congruence and representing the
spacetime curvature.
