The paper describes an Attitude & Heading Reference System able to achieve a high accuracy level by using a Kalman filter, which integrates the measurements coming from low cost inertial sensors, magnetometric sensors and a GPS receiver. The errors correction on attitude and heading angles, calculated through the integration of the gyroscopes measurements,is performed on the basis of their alternative estimation. Such values are provided by a model which uses magnetometric measurements and accurate estimation of the gravity vector,calculated by purging the aircraft acceleration effect from the accelerometers measurements.An estimation of aircraft acceleration is obtained tanks to velocity information coming from GPS measurements.