This paper is the sixth chapter of a series on the history of air navi
gation systems which have appeared from time to time in the pages of t
he Journal since 1988. Part II of this chapter will appear in the next
issue of the journal. The author acknowledges the assistance of Bath
College of Higher Education and, in particular, Dr Terence Rodgers, Th
e Royal Aeronautical Society, The Royal Air Force Museum, and Philip S
axon of the Royal Air Force Historical Society.