An implementation of multi-frequency shift keying (MFSK) providing a robust modulation system for air/ground communications

The author describes a prototype modem built to overcome the Doppler shift problems resulting primarily from the motion of the aircraft, and thus to permit the use of this robust modulation method on ground/air circuits. He also describes how the effects of other HF problems (particularly the lack of diversity in the aircraft) have been minimised, and presents the results obtained from extensive trials