This paper is concerned with combining the information from a ground-based radio navigation aid (the Decca Navigator) with self-contained sources of dead-reckoning such as air data and doppler. The essentially complementary relationship between D.R. devices and a ground-reference radio system can be exploited so as to produce an airborne navigational system whose merit exceeds that of either component individually. There is, of course, nothing new in a proposition of this kind. It is suggested that a combination such as that of Decca and a self-contained dead-reckoning device can usefully be described as a ‘compound’ system since the whole is formed, to quote the relevant entry in Webster's dictionary, ‘by the aggregation of otherwise independent elements’.