This manual is the first volume of the Radio Nav series published by Marques Aviation Ltd. This title describes the fundamental principles of the radio signal, non-directional beacon (NDB) navaid and automatic direction finder (ADF) avionics for radio navigation. The manual contains an extensive range of exam-type multiple-choice questions and practical exercises. The exercises have been designed for the student pilot to carry out mental arithmetic calculations and to promote a sense of situational awareness so that you know where the aircraft is heading and its position relative to the ground station at all times.