In this book, the problem of a ballistic target path on the re-entry phase is revisited. The altitude, velocity, and inverse ballistic coefficient of a target have been calculated with the help of the Extended Kalman Filter and particle filter. A particle filter is considered for the study because it has the potential to produce an accurate and computationally effective estimation of states. Particle filter combined with roughening particle filter, which is used to estimate the path of a falling body with the influence of air drag with both unknown and known beta coefficient. However, the method is unknown to signal processing and target path community.