Amethod for estimating the airspeed, angle of attack, and sideslip without using a conventional, pitot-static air data system is presented. The method relies on measurements from Global Positioning System, an inertial measurement unit, and a low-fidelity model of the aircraft's dynamics, which are fused using two cascaded extended Kalman filters. In the cascaded architecture, the first filter uses information from the inertial measurement unit and Global Positioning System to estimate the aircraft's absolute velocity and attitude. These estimates are used as the measurement updates for the second filter in which they are fused with the aircraft dynamics model to generate estimates of the airspeed, angle of attack and sideslip. Methods for dealing with the time and interstate correlation in the measurements coming from the first filter are discussed. Simulation and flight-test results of the method are presented. Simulation results show that the root mean square error of the airspeed estimate is less than 1 m/s. The nominal errors from the flight test on airspeed, angle of attack, and sideslip are less than 2.5 m/s, 2 deg, and 1 deg, respectively. Factors that affect the accuracy, including the implication and impact of using a low-fidelity aircraft model, are discussed.