1. A guiding system for a projectile provided with at least one piloting actuator and comprising measuring means for measuring physical parameters, a guidance computer and controlling means for controlling said piloting actuator, said projectile having to follow a reference trajectory in first flight instants and said guidance computer comprising: navigational means for determining an attitude, a position of the projectile upon a flight, and a goal position; guidance means for providing guidance orders from the attitude, the position and the goal position determined by said navigational means; and piloting means for establishing control orders, intended for said controlling means, from said guidance orders; wherein said guidance computer further comprises estimation means for estimating, in the course of flight, the attitude and an aerodynamic speed of the projectile, as well as variations of wind speed, from said guidance orders, said reference trajectory and measurements obtained by said measuring means, using a model of dynamic behaviour of the projectile and a model of wind dynamics.