Abstract: The present invention refers to an instrumentation system of an aircraft by means of GPS (Global Positioning System). In particular, it refers to a modular instrumentation system for aircraft, preferably airplanes, based on the GPS, and to the relative method. In one embodiment the modular instrumentation system for aircraft comprises: four antennas (A1-A4) connected to four GPS receivers (GPS1-GPS4) that supply in output the attitude data and the angular velocities; a data acquisition card (S, C) that receives, memorizes and processes said attitude data and said angular velocities coming from said data acquisition card (S, C) and supplies data relating to the board instruments of an aircraft; visualization means (V) of said data relating to the board instruments.