They went through the Imperial Japanese Navy Zero with a fine-toothed comb. First, it was American soldiers taking souvenirs from the plane that had crashed at Fort Kamehameha near Pearl Harbor. Then, in the days after the attack of December 7, 1941, the U.S. Army thoroughly examined the fighter aircraft. Both failed to recognize the significance of one piece of equipment, an American-made Fairchild RC-4 radio compass.
The overlooked compass in that Japanese Zero was an important component of what was a complete Japanese Imperial Navy radio beacon system, which Japan employed in a variety of ways. It is best documented in the early sea battles that took place in 1941 and 1942.
The Fairchild compass was the brainchild of Geoffrey Kruesi, a Swiss-born inventor who immigrated to the United States in 1920. The radio compass was the receiver component of a two piece navigation system. A land-based transmitter was the other half. The transmitter could be dedicated for navigation or even a broadcast station. In either case, the aircraft tuned into the transmitter, which is best thought of as a beacon.
Using the compass, the pilot tuned into the beacon and adjusted his two antennas by listening for maximum signal. A compass indicator on his instrument panel told him in which direction the beacon was. Sometimes, but not always, he could tune into multiple beacons each in turn and determine his position.
But the growing aviation industry had a problem in the 1930s. How does one navigate long distances over large, open seas such as the Pacific Ocean that have no beacons?
The U.S. Bureau of Standards tested the Fairchild compass over the Pacific Ocean off the coast of California in 1935. The Bureau set up temporary ship-borne beacons. The compass passed the test and the Bureau suggested that all airlines carry it. The U.S. Army welcomed the successful test as they had just placed an order for 500 compasses.
The U.S. Army was not the only military service interested in the compass. The Imperial Japanese Navy was, too. They issued requirements for a new fighter aircraft in October 1937. The new fighter aircraft specifications required that it carry a radio compass. The aircraft was the Mitsubishi AM6, better known as the Zero. The compass was the RC-4 purchased by the Japanese.
It is not known how many RC-4's the Japanese purchased. After the U.S. embargo on the sale of aircraft parts to Japan in 1940, the RC-4 was manufactured locally in Japan. It was known as the “Kruesi” and more officially as the Type 1 Aircraft Radio Directional Finder and Homing Device No. 3. It had two bands that were selected by a switch: 170-460 kilohertz (kHZ) or 450-1200 kHz. It was carried on the various light aircraft of the navy: fighters, attack, and scout aircraft. Larger aircraft such as flying boats carried a compass based on ones designed by the German Telefunken company.
The Imperial Japanese navy employed the radio beacon system in a number of ways. In the simplest one, Japanese aircraft homed in on an American broadcast station to find its target. That Zero that was shot down on December 7thhad its radio compass tuned to 760 kHz, a Honolulu radio station with the call sign KGU. At least some of the Zero fighters preparing for the attack on Pearl Harbor trained in homing in on a radio station by practicing on JOGK(790 kHz) in Kumamoto on the island of Kyushu in Japan. This was in the vicinity of their training bases on the northern part of the island.
Commander Fuijida, leading the first Japanese attack that morning, homedin on another Honolulu station, KGMB on 590 kHz. It is not clear why the compasses were tuned to two different stations. Tuning the compass to 760 kHz appears to have been deliberate as there were no Japanese stations operating on 760 at that time.
Ironically, American aircraft were also homing in to Hawaii broadcast stations that morning. U.S. Navy Douglas SBD Dauntless dive bombers from the American aircraft carrier Enterprise were practicing their navigation homing skills on KGU. American Army B-17 bombers arriving from California homed in on KGMB.
In the other variations, Japanese naval aircraft and vessels served as medium-frequency radio beacons.* An aircraft was always either a receiver or beacon in them. Documented frequencies were in the 300 and 400 kHz portion of the radio spectrum. Instead of voice, the Japanese transmitted in Morse code. The Japanese believed their non-directional beacons had a range of about 100 miles (162 kilometers) air to ground. The radio compass was the receiver in aircraft and similar types of receivers were carried aboard ships.
Japanese attack aircraft did use air navigation to return to their aircraft carriers. They also carried a medium-frequency transmitter as an additional navigation aid.** The aircraft trailed a long antenna of about 240 feet (70 meters) to transmit. The aircraft carrier received the beacon and now knew which direction the plane was. The Japanese recognized that there was a disadvantage as it slowed the speed of the plane quite a bit. The trailing antenna could also break at high speed. The ship replied using voice communications in the high frequency (HF) range (3,0000 to 30,000 kHz) to tell the plane which in direction to head in. They practiced this method both in training and in battle.
It is not known if the Japanese used this variation at Pearl Harbor. The Japanese communications plan ordered that transmissions to aid in navigation “should be avoided for as long was possible.” At least three of the Japanese attack aircraft shot down at Pearl Harbor had their medium-frequency transmitter set to 458 kHz.
The Japanese aircraft carrier Zuikaku directed overdue planes to use this method by transmitting on 359 kHz during the Battle of Coral Sea from May 4-8, 1942. The Zuikaku used it to recall aircraft that attacked the American tanker Neosho and the destroyer Sims during the battle. They also used it to recall their aborted evening strike. Japanese communications plans called for the same variation to be used during the initial air attacks on the Aleutian Islands on June 3-4, 1942.
Japanese float plane scout aircraft also transmitted beacons. Rear Admiral Matsunaga ordered a scout plane shadowing the British warships Prince of Walesand Repulse to transmit a beacon that directed in attack aircraft that subsequently sunk the two ships on December 10, 1941. An aircraft from the cruiser Tone guided the attack that sunk the British cruisers Cornwall and Dorsetshire on April 5, 1942.
The Japanese communications plan for the Battle of Coral Sea instructed scout aircraft of the 5th Cruiser Division to transmit a beacon on 423 kHz. The plan also called for using two cruisers if necessary with one determining the direction of the scout and another the direction of the attack aircraft.
The Japanese cruiser Tone repeatedly ordered its #4 scout aircraft to transmit a beacon to guide scout aircraft from the cruiser Chikuma during the Battle of Midway (June 4-7, 1942.) A scout aircraft from the Chikumatransmitted a beacon during the battle to guide strike aircraft from the Japanese carrier Hiryu.
Vessels also transmitted beacons. The Chikuma transmitted a beacon to recover her #2 and #4 scout aircraft during the Battle of Midway. Submarines served as beacons during both of the “K Operations,” which were long-distance reconnaissance and bombing missions of Pearl Harbor conducted by Japanese H8K flying boats. For
K-1, the Japanese submarine I-9 served as a beacon at a point about 1,300 miles (2,000 kilometers) southwest of the flying boats' refueling point in the French Frigate Shoals. The plan for the aborted K-2 prior to the Battle of Midway called for the Japanese submarine I-71 to transmit a beacon for an hour from about the same location.
Aircraft carriers transmitted beacons as well. Fighter aircraft were the most dependent on these as they had neither a dedicated navigator or medium frequency transmitter. For independent flight or at distances of less than 100 miles (162 kilometers) from the carrier, fighters were to use beacons. When returning from an attack, they were to rendezvous with the attack aircraft to facilitate their return. Failing this, they were to return using beacons. There are a few documented cases of this occurring.
Japanese fighter aircraft returning from Pearl Harbor were to home in on a beacon transmission from the carrier Soryu.
Aircraft from the carrier Ryujo used this system to return after their first attack on Dutch Harbor on Amaknak Island, Alaska on June 3, 1942. It was also used by both the Ryujo and the Junyo on their follow-up attack on Dutch Harbor the following day with all aircraft reportedly returning safely.*** A war-time American account paints a different picture of the beacon system in the Dutch Harbor attack. The Ryujo retreated into the fog and many planes ended up in the sea because of it.
The commander of the Japanese carrier Akagi says they did not use beacons in this way at the Battle of Midway. He added that the ship would transmit a message only if requested by the pilot.
The beacon system had some weaknesses. The Japanese copies of the Fairchild were poorly manufactured with inferior parts so they did not work well. They needed to be calibrated and adjusted before leaving the factory and this was not always done. So the copied sets did not work that well and the pilots did not have confidence in them. The system just used a line of bearing, that is, a single heading. This simply gave a direction, not a location. It would have been more accurate if the Japanese used multiple receivers and/or beacons to determine locations. While their regulations called for using such a “cross-bearing method,” they found it too difficult to execute in practice so they did not do so.
Compared to Allied navigation systems such as YE-ZB, the Japanese radio beacon system did not provide anywhere near as precise information for aircraft returning to an aircraft carrier. But at a time when both sides were struggling to have their carrier attack aircraft find targets and return, the Japanese Imperial Navy made good use of their radio beacon system.
* The Japanese referred to this transmissions as low frequency or long wave, but technically they were medium frequency as defined today: 300-3,000 kHz.
** These transmitters were crystal controlled and could transmit on a single frequency in the range of 300-500 kHz.
*** They did lose aircraft, such as what become known as the “Akutan Zero,” which captured by the Americans and found to have onboard a Fairchild radio compass in the 450-1200 kHz band position. The Japanese source says the attack was on Adak (sic, actually Amaknak) Island.
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