Radio Direction Finding (RDF) was a technology used by both the US and Japan to locate enemy ships and track their movements. Large military forces have a need to keep in contact with their headquarters and supply trains, and since the early 1900s (check this) have done so with powerful radio transmitters and receivers. With the right eqipment, listeners can hear these broadcasts and take a bearing on it's signal. With one station, only the direction can be determined, but with multiple, the bearings from each will intersect and plot a point on a map with the location of the transmitting vessel.
While this sounds simple in theory, there were many things that complicated the RDF process. The first was radio's characteristics and how it was affected by external forces such as atmospheric and magnetic conditions. Radio is essentially a wave like one sees on the water. Some waves are stronger than others and can travel farther. Higher frequency waves have less power than low-frequency and are line-of-sight; due to the curvature of the earth a receiver that is within range of a high-frequency radio might not hear the signal because it is "below" the horizon and blocked from receiving it. Low-frequency (LF) waves are more powerfull and hug the earth, which would appear to give them a longer range.
High-frequency (HF) radio waves have a special characteristic that extends their range far beyond line of sight, and that is that they bounce off of surfaces. As high-frequency radio waves propigate outwards they will hit the ionosphere (Look up info on the ionosphere) an area of our atmosphere that consists of charged particles, turning it into a floating reflective surface that bounces the HF back down, and thus actually extends its range farther than LF radio waves.
But because it relies upon the atmosphere to extend its range, HF signals are also more subject to signal loss due to atmosphereic conditions. Because of the signal bounce, a receiver might not be able to receive the signal if it is passing overhead on its way up to the ionoshpere. Sunlight passing through the ionosphere changes its characteristics and certain frequencies are unusable in the day time. Solar storms can wipe them all out. The solution to these problems was the construction of multiple RDF stations that would operate together as a "net." One station might not be able to make out the call sign of the transmitter but would be able to take a directional fix and and frequency and could notify other stations operating in the same net to listen Each side also used radio deception to try and throw off accurate plots of vessels so that they would appear to be elsewhere. Radio Direction Finding is just that, a large antennae was used to determine which direction a transmission was coming from. But there were serious limitations in the technology at first, making it less science than art that had to be interpretted, and operators had to be both intelligent and watchfull, in order to both keep and active search and then when a signal was found record data and traffic.
Weather played a large part in what was heard and how accurate each station could be. Certain times of the year signals could travel far, and others not far at all. Signals could be a couple of degrees off due to weather, potentially locating a ship far away from its actual location. To prevent this, each station would take fixes on known stations periodically to make a note of the variations in direction.
Each station could only report the bearing of a contact as well as the signal strength, but not distance. But when two or more stations took bearings on the same contact the intercept line from each station would intersect when drawn on a map, giving a much more accurate idea of where the contact was located. {Write more here}
By December of 1941, the US Navy had created a network of stations broken down into different groups. The "West Coast Strategical High Frequency Direction Finder Net" was a collection of five stations across the west coast and Alaska; Station AE at NAS Sitka, Alaska; Station SAIL on Bainbridge Island, Washington; Station TARE at Point St. George near Eureka in Northern California; Station Z at Point Arguello, California; and Station ITEM at San Diego, California, with Station S serving as the Net Control Station. A second group was the "Pacific Strategic High Frequency Direction Finder Net," headquartered at Pearl Harbor and in charge of stations such as Station KING at Dutch Harbor, Alaska, in the Aleutians; Somoa (designator?), Midway (Designator?), Lualualei, Oahu (Designator?),
Station SAIL, or Station S as it was sometimes referred to, was serving as Net Control, and could direct the other stations to focus on a transmission when noticed to better pinpoint the location. {Write more here}
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RDF Images:
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| The above images are from records of the 13th Naval District held at Seattle NARA and are of Station AE (NAS Sitka's RDF hut while temporarily installed for testing at Station S (Bainbridge Island). | |
LINKS:
Pearl Harbor Revisited: U.S. Navy Communications Intelligence, 1924-1941
History of Communications - Electronics in the United States Navy
Pearl Harbor: Who Deceived Whom? -Naval History Magazine (Registration req'd)
RDF Documents:
Operating Instructions, Strategic Direction Finder Operators. October 31, 1941
RDF Station Logs for November and December:
November, 1941
Station S Report for Month ending 30 November 1941
Dutch Harbor Report for Month ending 30 November 1941
Station AE Report for Month ending 30 November 1941- PROOFREAD & Citation
December, 1941
Station S Report for Month ending 30 December 1941 - In Progress - PROOFREAD
Dutch Harbor Report for Month ending 30 December 1941 - Not transcribed yet.
Station AE Report for Month ending 31 December 1941