A method for the inertial navigation of a projectile equipped with a mediocre-quality inertial init that is fired from a carrier equipped with a precision inertial unit includes, before firing the projectile, the biases of the accelerometers and of the gyrometers of the inertial unit of the projectile are determined using the inertial unit of the carrier. During the inertial navigation of the projectile, the measurements of the accelermeters and of the gyrometers output by the inertial unit of the projectile are corrected by the biases determined before launching.