GPS receivers are heavily used in aviation for both visual and instrument flights. There is a need for a backup system so that en route and non-precision approaches can be flown without reliance on ground based systems should GPS be unavailable. This project developed a prototype which used barometric pressure, airspeed, low-cost inertial sensors, three-axis magnetometer along with high-fidelity geomagnetic models, and an optical ground speed sensor to provide backup position and navigation solutions during GPS outages and improve the GPS solution when the signal is available.