  In this paper, we give a new covariation spectral representation of some non
stationary symmetric $\alpha$-stable processes (S$\alpha$S). This
representation is based on a weaker covariation pseudo additivity condition
which is more general than the condition of independence. This work can be seen
as a generalization of the covariation spectral representation of processes
expressed as stochastic integrals with respect to independent increments
S$\alpha$S processes (see Cambanis (1983)) or with respect to the general
concept of independently scattered S$\alpha$S measures (Samorodnitsky and Taqqu
1994). Relying on this result we investigate the non stationarity structure of
some harmonisable S$\alpha$S processes especially those having periodic or
almost-periodic covariation functions.
