One of the earliest radio navigation systems was Radio Direction Finding (RDF). Introduced in the 1920s, RDF allowed pilots to determine the direction of a ground-based radio transmitter.
Author from ch. 1, p. 1. https://siris-libraries.si.edu/ipac20/ipac.jsp?&profile=liball&source=~!silibraries&uri=full=3100001~!133318~!0#focus ...
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