Radio Direction Finding in WWII

In peace and war there is an obvious benefit to knowing where potential enemies are. On land tools such as spies and scouts exist, but a simple spy cannot report the location of a fleet once it disappears over the horizon and the sheer scale of the open sea can make effective scouting impossible. It is with situations like this that military forces turn to technology and extend their reach by other means.

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 1900's have done so with powerful radio transmitters and receivers. With the right equipment, 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 propagate outwards they will hit the ionosphere, an area of our atmosphere that consists of charged particles that effectively turn 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 atmospheric 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 ionosphere. Sunlight passing through the ionosphere changes its characteristics and certain frequencies are unusable in the day time. Solar storms can wipe them all out. Each station could only report the bearing of a contact as well as the signal strength, but not distance, so exact position could never be plotted.

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 frequency and could notify other stations operating in the same net. 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. Thus, a more accurate fix might be obtained and information that one station missed another might pick up.

But there were serious limitations in the technology at first, making it less science than art that had to be interpreted, and operators had to be both intelligent and watchful in order to both keep an active search and then when a signal was found record data and traffic. Each side also used radio deception to try and throw off accurate plots of vessels so that they would appear to be elsewhere. As part of the attack on Pearl Harbor, for example, the Japanese kept the regular radio operators from the aircraft carriers sailing to Hawaii off of their ships, in Tokyo, sending out false traffic so their unique styles would be recognized and recorded as being in the home islands.

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 signal direction. Over time, a better understanding of the atmosphere and its effects was built up.

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, Somoa, Midway (after September, 1941), and Lualualei, Oahu .

In the years since the attack on Pearl Harbor, some have claimed that commands within the US military were had advanced knowledge of the attack and were even tracking the Kido Butai carrier force on its journey to the Hawaiian Islands.


RDF Images (Click to enlarge):
The above images are from records of the 13th Naval District held at Seattle NARA and are of Station AE's (NAS Sitka) RDF hut while temporarily installed for testing at Station S (Bainbridge Island). The above photo is Station K, Dutch Harbor, during the attack on June 4th. The full shot can be seen on this Dutch Harbor attach image gallery.

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)
History File for Station T, Point St. George, California
A Cryptologic Veteran's Analysis of "Day of Deceit" - Philip H. Jacobsen Lieutenant Commander, USN (ret.)

RDF Documents:
Operating Instructions, Strategic Direction Finder Operators. October 31, 1941
Analysis of Radio Direction Finder Tracking Methods - 1938 instructions for Shipboard use.


RDF Station Logs for November and December:

November, 1941
Station S (Bainbridge) Report for Month ending 30 November 1941
Station King (Dutch Harbor) Report for Month ending 30 November 1941
Station AE (Sitka) Report for Month ending 30 November 1941- PROOFREAD & Citation

December, 1941
Station S (Bainbridge) Report for Month ending 30 December 1941 - In Progress - PROOFREAD
Station King (Dutch Harbor) Report for Month ending 30 December 1941 - In Progress
Station AE (Sitka) Report for Month ending 31 December 1941