Providing global communication, navigation and positioning with outdoor systems might be challenging in indoor and deep indoor situations, due to the limited penetration capabilities of the radio signals into buildings. The use of drones as radio source platforms and low radio frequencies might help alleviate the problem due to improved propagation characteristics in terms of elevation angle and coupling into construction materials. This paper explores air-to-indoor propagation based on an operational proof-of-concept navigation system working at 133, 401.5 and 500 MHz in two different building scenarios. The results demonstrate the favorable radio propagation conditions experienced by the setup, which resulted in viable outdoor transmission of navigation signals into indoor and deep indoor locations.