A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Joe Suligoy
Date: 2019 Apr 21, 14:49 -0700
Hello, nice list you have here!
This is an interesting subject; I've been interested in early air navigation for a while and can contribute some tidbits.
Amelia Earhart did indeed try and fail at RDF, probably because of a lack of understanding (she left her most important antenna behind in Florida). She wasn't the first civilian to use it though, unless by civilian you mean not an airline either. Pan Am had been using both VHF and HF direction finding for a few years at that point.
As a matter of fact, HF direction finding (HfDf or "huffduff") was invented by a man named Hugo Leuteritz specifically for the inauguration of Pan Am's Pacific service - in 1935. The development of this was a secret project shared only with the U.S. Navy. Pan Am knew they were going to need another nav method to supplement DR and celestial on the long crossings to find isolated atolls like Wake island. It worked a little differently than modern RDF though; because of the antenna size given the HF band and the day's technology, the aircraft didn't receive - it transmitted. Radio operators at land-based stations (Alameda, Honolulu, Midway island, Wake Island, Guam, and Manila) took DF bearings on the Clipper and then radioed the bearing and time back to the Clipper.
So in November of 1935, the China Clipper, a Martin M-130 flying boat, became the first aircraft to cross an ocean in commercial service. She navigated primarily by DR / celestial, but the HfDf provided another important layer of redundancy. Interestingly, her navigator was Fred Noonan, he was Pan Am's chief navigator at that time, and he distrusted the HfDf, even computing celestial landfall procedures using a sun LOP to find the atolls. Makes you wonder if this distrust carried over to his ill-fated flight with Earhart two years later, after Pan Am fired him because of his drinking problem...
Anyway, regarding more conventional airborne RDF, here is an electronic copy of the AAF's WWII-era navigator training manual. It has a section on the RDF techniques of the day, including using commercial broadcast stations (and is just a neat text if you like this stuff). Yes, this one is in the public domain, no copyright ;).