Book excerpt: This book is the offspring of Martin Cass's The VOR and ADF, first published in 1977 and which ran to three editions. Its aim is to introduce the theory required to operate and use the VOR (Very High Frequency Omni Directional Radio Range), ADF (Automatic Direction Finding) and RMI (Radio Magnetic Indicator) for their operational role in the air. There is also a chapter on DME (Distance Measuring Equipment). The text has been rewritten to make it compliant with the latest practices and regulations. It also includes new and improved diagrams and illustrations.