“…In the literature, most of contributions treat the case when the Gaussian process X(t), t ≥ 0 is a Brownian motion which allows to calculate the boundary non-crossing probability P (X(t) + f (t) < u, t ∈ [0, T ]), for some trend function f and two given constants T, u > 0 by various methods (see, e.g., [1,15]). For particular f including the case of a piecewise constant function, explicit calculations are possible, see, e.g., [17].…”