Thirteenth Air Force radio countermeasures operations, 1944-45.

Author:Cahill, William
 
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It was a beautiful evening with light haze as May 7, 1945 drew to a close in the southwestern Pacific. Lieutenant Everett re-checked the settings on the three APT-1 radar jammers, then powered them up when Consolidated B-24L 44-41464 was 60 miles out from the target. Tuned to 190, 194 and 197 MHz, the jamming pattern covered the Imperial Japanese Navy Type 12 air search radars and Mark IV searchlight radars that were protecting the Japanese naval base at Soerabaja, Java. Eight minutes later the Liberator was within 40 miles of the target--time for the waist gunners to start dispensing 'rope,' very long strips of aluminum foil one half inch wide and 400 feet long. Suspended from a paper parachute, 'rope' was used against radars below 350 MHz in frequency. With their controlling radars blinded, the Japanese searchlight crews vainly scanned the skies with their beams looking for the inbound bombers they knew were out there. Lt Olsen, piloting Liberator #464, switched his landing lights on to draw the searchlights. As soon as they came close, he switched them off. Olsen continued the game of chicken as six aircraft piloted by his squadron mates barreled in at low altitude, bearing down on the merchant ships at anchor throughout the harbor. These bombers received only inaccurate anti-aircraft fire when attacking one of the best protected Japanese naval bases in the region--a direct result of the past two years of work undertaken by the Allies to understand and counter the Japanese use of the electromagnetic spectrum.

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The South West Pacific Area

The South West Pacific Area (SWPA) was the Allied supreme military command that oversaw operations in the Philippines, Borneo, Dutch East Indies, Australia, New Guinea, and the western part of the Solomon Islands. Commanded by General Douglas MacArthur, the SWPA came into being on April 18, 1942, replacing the failed American-British-Dutch-Australian Command (ABDA) and, after the fall of the Philippines in May 1942, became the sole US force in the region. Airpower under MacArthur was organized under a combined command--Allied Air Forces--with Australian and US air components. Australian forces were organized under the RAAF Command which by October 1944 had fielded the Australian First Tactical Air Force. US forces would eventually comprise three numbered air forces.

Fifth Air Force was created from the remnants of Philippine air power and the Far East Air Force in February 1942, while the Thirteenth Air Force was assigned to SWPA under a re-formed Far East Air Forces in June 1944 followed by Seventh Air Force in July 1945.

The fight for New Guinea was the defining action of the first part of the war for the South West Pacific Theater. First occupied by the Japanese in January 1942, the battle for the island would be waged for the remainder of the war. The Allied campaign to retake the island started with the defense of Port Moresby and the landing at Milne Bay on the eastern end of the island. The campaign turned into a series of amphibious end runs by MacArthur's forces as the Allies fought westward, with goals of reducing the Japanese base at Rabaul and securing territory to support the liberation of the Philippines.

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Japanese Radar Development through 1944

Like many nations in the 1930s, Japanese technical research was split between Army and Navy factions. The Imperial Japanese Navy's (UN) Naval Technical Research Department started their initial work into what became radar in 1935. Neither the Naval General Staff nor the Bureau of Naval Construction displayed much interest in the effort, so development was slow. (1) A visit in March 1941 to Germany that included inspection of German and captured British radars revealed the state of radar development to Japan, spurring interest and development. Tests in September 1941 led to the production of the first IJN land-based radar, the Mark I Model 1 (Type 11), in early 1942. (2) A more mobile radar, the Mark I Model 2 (Type 12), appeared at the end of 1942 with the smaller Mark I Model 3 (Type 13) following in 1943. (3)

The Imperial Japanese Army (IJA) was also sponsoring radar development in the 1930s. The Type A continuous wave radar system was initially deployed on the Japanese Home Islands in 1941, providing longer range but poorer resolution compared to a pulsed radar system. An IJA technical commission spent many months in Germany in 1940, and their summary reports coupled with Japanese research resulted in the production of the Tachi-6 search radar in 1942; the more mobile Tachi-7 followed in 1943. Searchlight and fire control radars based on captured US and British radars appeared with the Tachi-1, -2 and -3 sets in 1943. (4)

Records are scare, but it appears Japan started to deploy radars to its overseas conquests in mid-1942. The first Tachi 6 overseas deployment was to an airfield in Sumatra in late 1942, followed up with an additional 20 sets to Palambang, Sumatra between December 1942 and spring 1944. At least four sets were deployed to Java as well. The Tachi 7 started to deploy in 1943, with 20 sets being shipped to the Philippines in October 1944 - the first IJA radars deployed to this former US territory. (5) Though close to 50 IJA radars were shipped to the Southwest Pacific Area, on average less than 20 were deployed at any given time, the majority likely in depot for future use or to act as reserves for those in the field. The majority of the Japanese radars deployed overseas were the IJN Type 11, 12 and 13. The IJN initiated deployment of radars shortly after the Type 11 went into production, with one set being on Guadalcanal prior to the US invasion in August 1942. (6)

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The Japanese air surveillance network in the SWPA consisted of thinly scattered belts of radars to provide early warning of approaching raids. (7) As the tactical situation changed, the IJA or IJN would shift their radar sites around to increase coverage of critical avenues of approach. As the US moved into the southern Philippines, the Japanese rapidly re-oriented their air surveillance coverage to better screen their southern flank. (8) Many Japanese radar posts had more than one radar set assigned to them. A review of the February 1945 electronic order of battle for the Netherlands East Indies shows this to be the case with approximately 15 percent of the sites, with a slightly smaller percentage of sites in the Philippines following this rule. (9) Some Japanese radar sets could also receive US Identification, Friend, or Foe (IFF) signals, a factor likely taken into account when equipment was deployed. The IJN Type 13, along with some variants of the Type 12, the Type 11-3-K, and land based adaptations of the Air Mark VI Mod 4 radars operated in the same frequency band as the US Type III IFF transponders that came into universal service in the Pacific in last 1943. These radars could and did trigger an IFF response that enabled the Japanese to monitor Allied aircraft not only medium altitude but also allowed them to track aircraft at altitudes lower than the radars could normally pick up targets at ranges out to 300 kilometers. (10)

While the concepts and countermeasures taken by the adversaries were no different than that of the European Theater, the critical difference was knowledge of the adversary's radar technology. In the Pacific, the Allied knowledge of Japanese radar in 1942 was nonexistent, and the Allies spent the first few years of the war understanding the adversary's technology and tactical employment of radar. Key to this analysis in the SWPA was Section 22. (11)

Section 22

Founded in Brisbane, Australia in July 1943, Section 22 reported to the Chief Signal Officer of General MacArthur's General Headquarters of the Southwest Pacific Area (GHQ SWPA). Originally named "Radio and Radar Countermeasures Division" until November 1943, Section 22 combined all Allied efforts--not only US Army Air Forces and US Navy but also British Commonwealth and Dutch personnel--in a coalition effort to understand the Japanese use of radar and develop countermeasures to ensure success of Allied operations. (12) Section 22 had its origins with the primary communications intelligence organization in SWPA, the Central Bureau, which among other tasks had been plotting the locations of Japanese visual observer posts. When these posts started to get radar sets, it was natural that a new organization attached to GHQ SWPA should take over the work. Geographically separated subordinate elements called "Field Units" provided information to the headquarters through a myriad of reports. Some of the intelligence was actual 'hands-on' exploitation of captured Japanese equipment while other was electronic intelligence--intercepts of Japanese radar signals from air, land or sea-based platforms. Section 22 collated all information and circulated weekly and monthly reports on Japanese radar technology and current radar deployment locations. (13) Section 22 also requisitioned and assigned radio counter-measures (RCM) personnel and equipment. As the theater expert in radar countermeasures, 4Section 22 was also involved in the development and installation of radar interception and jamming equipment and worked closely with US based organizations such as the Radio Research Laboratory at Harvard University. But all of this was in the future; the first task assigned to the unit was to determine the type, accuracy and location of Japanese radar stations. With the theater being a broad expanse of water populated with many small islands, the easiest and fastest method to accomplish such a task was with an airborne platform. (14)

Initial SWPA RCM Activities

History is never clean and simple with clear cut start and stop dates. The emergence of airborne signals intelligence (SIGINT) operations in the Pacific in the early days of the Second World War was no different. Though records of such...

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