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Jikitanchiki - Japanese

Airborne Magnetic Detector

By August 1944, the Japanese Combined Fleet had lost its air power and its submarine force had been neutralized. American carrier-based air attacks compelled a major change in merchant convoy shipping routes between the Empire and bases in the Netherlands East Indies (NEI). The convoy route from the Empire to Singapore was changed from the western coast of the Philippine Islands to the eastern coast of mainland Asia. Convoys carrying petroleum from NEI possessions to the Empire were made smaller, and the number and strength of escorts were increased; some of these convoys were escorted by destroyers and cruisers, in addition to kaibokans (escort vessels), submarine chasers, and armed transports. These petroleum convoys were routed along the Asian coastline. Attacks from American submarines continued to pose the greatest threat to this commerce in the South China Sea and adjoining waters. 1

By November 1944, the First Escort Squadron, with headquarters at Takao, on Formosa's southwest coast, commanded a total of sixty vessels comprised of approximately four destroyers, forty-five coastal defense ships, four mine sweepers, two submarine chasers, and two armed transports. In addition, a large portion of Japanese antisubmarine air power was situated at aerodromes near Takao. The 901st Air Group had established a force of 170 aircraft dedicated to convoy protection. The force was comprised of long-range flying boats, land-based fighters, carrier-type attack planes, land attack planes, seaplanes, and spotters. The air squadron's mission was to protect merchant shipping in the area from latitude 8° North to 27° North, covering the entire South China Sea, and from the western shore of the Philippines to the Asiatic coastline. 2

The Japanese navy first utilized an airborne magnetic detection instrument (MAD) (also often referred to as a magnetic anomaly detector) for antisubmarine warfare during the middle of 1944. They called the instrument Jikitanchiki. The MAD was a device that an aircraft could use to detect a submerged submarine some 500 feet directly beneath the plane. Expert Japanese pilots flew MAD-equipped planes 30 to 40 feet above the surface. Pilots with average skills flew at an altitude of 150 to 200 feet. 3 Aircraft equipped with the detectors generally probed for submarines ahead of convoys. They would also investigate reported submarine contacts. 4 The MAD device was used during daylight hours when submarines were more likely to be submerged. At night when submarines ran on the surface, radar was used to detect and intercept them. A Japanese staff officer estimated that between August 1944 and July 1945, eleven American submarines were sunk using the MAD device - four in Empire waters and seven in the South China Sea. About a third of the 901st Air Group aircraft were equipped with MAD; another third were equipped with radar. A small number of planes had both. The remaining aircraft had neither. 5

The MAD equipment was very heavy and seriously overloaded the planes equipped with it. Protective steel armament had to be removed from the aircraft to improve magnetic sensitivity. All 901st Air Group aircraft were equipped with bombs that could be set to explode at various depths. Some aircraft were also equipped with a circular-run torpedo for use against submarines. This torpedo, equipped with a contact exploder, would be dropped two hundred yards ahead of a submarine, and it would run in a reducing spiral for four complete cycles as it sank to six hundred feet. 6

At night when a submarine was detected with radar the airplane homed in on the boat until it was within 500 meters, at which time they completed their run by dead reckoning. The MAD device and radar were improved technically and operationally during the war. Beginning in February 1944, the circular-run torpedo was used against American submarines, but after four to five months of unsuccessful attacks it was abandoned. Two MAD-equipped planes were normally used to escort a convoy. When a submarine was located, the procedure followed was to bomb the submarine first, then report and mark its location. One pilot would remain on station until relieved or low on fuel. 7

As part of his interrogation, Lieutenant Commander T. Okamoto used the following diagrams to describe the deployment of MAD-equipped aircraft during convoy coverage and the procedure followed when the MAD device detected a submarine. 8

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The Japanese officer's estimate of the number of American submarines sunk between August 1944 and July 1945 due to use of the MAD device is grossly wrong. Details of the attack responsible for the sinking of the USS Trigger (SS-237) suggest she was detected by a MAD-equipped aircraft, which vectored surface forces to her location. A MAD-equipped plane may have been involved in the sinking of the USS Harder (SS-257), depending on which version of her final moments is correct. Both versions are partially based on English translations of Japanese documents, however only one of them involves a plane.

In his book about the USS Gudgeon (SS-211), Mike Ostlund speculates that she may have been detected and bombed by a MAD-equipped plane in April 1944. However, the MAD device did not go into use until the middle of 1944. 9.

On November 14, 1944, while running submerged and tracking an enemy convoy in the Luzon Strait, the USS Halibut (SS-232) was detected and bombed by a MAD-equipped plane. She was forced down to 420 feet by the explosions. Damages to her hull were extensive. Fortunately, the Japanese left her for dead and she made it back to Saipan. Her hull was found to be so badly damaged that she was retired from service. At Saipan, Halibut's skipper compared notes with the captain of one of her wolf pack consorts who had been tracking the same convoy. He had observed three planes flying in continuous circles ahead of and over the convoy. 10

The sinking of the USS Trigger is the only instance in which it can be reasonably argued that the MAD device was used to sink an American submarine during the Pacific War. Evidence relating to the USS Harder and the USS Gudgeon is speculative at best. The USS Halibut survived a brutal attack by a MAD-equipped plane and her damages demonstrated how effective the MAD technology could be if used properly.

Of the sixteen American submarines listed as lost due to unknown causes on my United States Submarine Losses in World War II webpage, only three were lost during the time period and in the areas in which the MAD device was being used. The submarines are the USS Kete, the USS Snook, and the USS Escolar. After the war, no Japanese records were found to explain their losses.

In September 1945, U. S. Navy intelligence experts began searching through the Japanese records seized at the naval ministry in Tokyo. Among other things, it was clearly established that while the Japanese were very capable at detecting submerged American submarines with sonar, they consistently failed to destroy them. The reason for this was a lack of persistence and skill in Japanese antisubmarine personnel. Because of these flaws, they were quick to accept the flimsiest of evidence as proof of a sinking. The records seized from the naval ministry showed that the Japanese believed they had sunk 486 American submarines during the course of the war, more than the total number in the entire American fleet. 11

The Japanese did not develop a feasible design for a radar set suitable for aircraft until mid-1944. Prior to July 1944, an antisubmarine patrol plane equipped with radar was a rarity. The maximum range at which the radar could detect a surfaced U.S. submarine was ten miles. During daylight hours, most pilots preferred to use their own eyesight because it had a greater range. Due to a severe shortage of planes, antisubmarine sweeps beyond the immediate convoy routes were not possible and often unarmed training flights were dispatched over outlying sea areas in an attempt to discourage any submarines that happened to be in the area. Airborne antisubmarine patrols after dark and attacks on submarines at night were extremely unusual. A postwar investigation found that Japanese airborne radar technology was about four years behind that of the United States. 12

Pips & Blips Articles Index


1. Interrogation of Japanese Officials Volume 1, U. S. Strategic Bombing Survey, 1946. Interrogation of Rear Admiral Shigetada Horiuchi, IJN, Chief of Staff, First Escort Fleet, from May 1944 to December 1944; Commanding 901st Air Group, Formosa from January 1945 to September 1945.

2. Ibid.

3. Roscoe, Theodore, United States Submarine Operations in World War II, p. 212.

4. Sturma, Michael. Death at a Distance: The Loss of the Legendary USS Harder, p. 174.

5. Ibid., Interrogation of Lieutenant Commander T. Okamoto, IJN, a naval aviator. Staff Officer of First Escort Fleet Air Squadron and General Headquarters, Grand Escort Fleet.

6. Holmes, Wilfred J. Undersea Victory: The Influence of Submarine Operations on the War in the Pacific, p. 357.

7. Interrogation of Japanese Officials Volume 2, U. S. Strategic Bombing Survey, 1946. Interrogation of Captain S. Kamide, IJN, Commanding Officer of the 901 Air Flotilla, from September 1943 to December 1944, and Senior Staff Member of this flotilla from December 1944 until the end of the war.

8. Interrogation of Lieutenant Commander T. Okamoto.

9. Ostlund, Mike, Find 'Em, Chase 'Em, Sink 'Em: The Mysterious Loss of the WWII Submarine USS Gudgeon, p. 353-355.

10. Holmes, Undersea Victory: The Influence of Submarine Operations on the War in the Pacific, p. 402-403.

11. Sasgen, Peter, T. Hellcats: The Epic Story of World War II's Most Daring Submarine Raid, p. 252.

12. Parillo, Mark P. The Japanese Merchant Marine in World War II, p. 111.

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