Direction finding

"Direction finding (DF) refers to the establishment of the direction from which a received signal was transmitted. This can refer to radio or other forms of wireless communication. By combining the direction information from two or more suitably spaced receivers, the source of a transmission may be located in space via triangulation. This is called a cross-cut or fix."[1]

The Coast Guard LORAN station on Nikumaroro is an example of a sophisticated navigation system based, in part, on DF principles.

On the flight from Lae to Howland, Earhart and Noonan planned to use the simplest form of radio direction finding: find where the transmission was coming from and fly toward it. On the flight from Oakland to Honolulu, Manning and Noonan took seven radio bearings.[2]

From the Chater Report:

"Miss Earhart and Captain Noonan spent a considerable time in the radio office and as previously mentioned it was learned that neither of them could read morse at any speed but could only distinguish letters made individually slowly and repeated often; in that case their direction finding apparatus would be useless or misleading unless they were taking a bearing on a station using radiophone which could give the station position on voice. We understand the 'Itasca' was to do this but if the plane was unable to pick up the 'Itasca' it is doubtful if the direction finder would be any use to her."