“…For the problem with variable propagation time, a linearized state and time transition matrix method is available in the literature [17][18][19]. Starting from the conventional STM, an additional term is added to include time perturbations and compute the associated time partial derivative [17], so that the corresponding STM terms for a time perturbation can be obtained. This turns out to be useful in linear guidance or covariance analysis with multiple event triggers [18,19].…”