The present invention describes methods for programming trigger time of a projectile (60) based on remaining flight time to a target (P) after the projectile (60) is airborne. The actual muzzle (Vo) and flight speeds (V1, V2, etc.) are independently determined and compared to those used by the ballistic computer (30), and a better estimate of trigger time is accordingly used to activate detonation of the projectile (60). In one embodiment, a Kalman algorithm is used to provide a better estimate of the projectile's flight speeds obtained by independent methods to provide the better estimate of the trigger time.