Airborne gravimetric and altimetric measurements depend heavily on the determination of the instantaneous position and attitude of the aircraft. Here, a methodology to process raw inertial data together with GPS data collected from at least two antennas/receivers in the aircraft is presented. This methodology is primarily aimed at the attitude determination, but has potential to be applied in precise position determination as well. The main idea behind the methodology is to process GPS carrier phase measurements in differential mode between a pair of aircraft antennas to obtain precise heading angles. These, together with doppler velocity readings and approximate position estimates, serves to maintain stability of the attitude angles obtained from filtering raw inertial measurements. On the other hand, the availability of precise attitude estimates from the inertial sensors allows keeping phase integer ambiguities fixed through cycle slips and satellite configuration changes. As a side effect, it is also possible to evaluate the quality of each phase measurement in order to exclude readings corrupted by noise or multipath effect. This is important for absolute position determination where the aircraft GPS measurements are processed against a reference station, namely when long baselines are used. The integration of displacement estimates computed from inertial data is potentially helpful to solve and keep integer phase ambiguities between the aircraft and the reference station. This methodology is being employed to process data from the Azores campaign of the AGMASCO project.