Army Communicator
United States Army Signal Center, Fort Gordon, GA

Radio-traffic analysis' contributions

by Joseph Browne

The Japanese bombing of the Philippines Dec. 8, 1941, came as a shock, even though some Americans were braced for other attacks following the Pearl Harbor infamy the previous day. After nearly destroying the U.S. fleet in Hawaii, the Japanese focused on the rows of B-17s and P-40s parked neatly in the midday sun at Clark Field. MacArthur's air force was destroyed on the ground without a fight.

On that day, Lt. Howard Brown, a radio-intelligence veteran attached to 2d Signal Service Company in Manila, changed the Army intercept unit's mission from Japanese diplomatic to potentially more lucrative air-force communications and began reconstructing Japanese tactical military nets. Thus began U.S. Army radio-traffic analysis in World War II.

By war's end, several thousand men were sigint specialists in the Pacific and Europe, contributing much to the final victory.

Here we look at how this all came about.

The Southwest Pacific

As a corporal in 10th Signal Service Company, Fort Santiago, Manila, in the early 1930s, Brown became familiar with the Japanese Kana code. The small Manila station intercepted Japanese press broadcasts and any Kana code it could hear to sample communications activity in the Pacific. Besides commercial traffic, the station intercepted Japanese army and navy messages.

Brown helped design a course at the signal school, Fort Monmouth, N.J., on intercepting the Kana code in September 1933. He left the Army in 1936 to work for a Manila commercial radio station, but later applied for and received an Army commission. He had been a second-lieutenant operations officer for only 10 months when the Japanese attacked U.S. forces in Hawaii, the Philippines, Hong Kong, Malaya and the Dutch East Indies.

The radio-intelligence men in Manila wasted little time, intercepting a Japanese air-to-ground net passing bearings to bombers on their way to Manila. The Japanese, however, continued bombing the Philippines' major military installations before their invasion force landed at Lingayen, 100 miles west of Manila, early Dec. 22, 1941.

Brown's station was evacuated Christmas Eve, and the men moved to Corregidor with the rest of MacArthur's staff. Radio intelligence was halted and Brown's men reassigned, but the lieutenant convinced his superiors to allow him to reopen an intercept station in Corregidor's Malinta Tunnel — about 100 feet long, with an average temperature of 95 degrees, jammed full of men and women under siege by the Japanese.

The makeshift station monitored enemy air-reconnaissance nets. Brown's men passed sigint alerts to the air-defense unit which, based on those reports, shot down six Japanese reconnaissance planes. Brown's tip-offs included predicted time and direction, so the artillery's anti-aircraft guns were loaded and cocked to shoot at the aircraft, called "Foto Joe," as they tried to gain altitude following a pass over the rock.

Traffic analysis of the Japanese air-to-ground net revealed a control and 14 outstations. The control station was at Sama, Hainan Island, with one outstation in Indo-China, one outstation near Hong Kong, and the other 12 unlocated. The net, which combined operations and aircraft-ferry information, included two Japanese naval stations.

The men in Malinta Tunnel, by reconstructing the nets, followed the Japanese land-invasion force's progress from Damortis, Lingayen to Zamboanga and Davao. Although Japanese codes weren't broken until 1943, Brown and his men were able to infer where the Japanese might move next, providing critical information to MacArthur as he performed a double-retrograde maneuver to move his forces out of Luzon into the Bataan peninsula and onto the island of Corregidor.

The move was to buy time until reinforcements could reach the hard-pressed Filipino-American garrison. But reinforcements were never sent, and the Philippines eventually succumbed. (See related article on the fall of Corregidor.)

Retreat to Australia

MacArthur left Corregidor for Australia to lead the defense of the Southwest Pacific from a larger operations base. Brown arrived there in mid-April 1942, helping set up a joint Australian-American intercept station at Townsville, Queensland. The lieutenant was also instrumental later in creating the sigint organizations attached to the Army Air Forces (radio squadron, mobile) in the Pacific.

Some of the Philippines' survivors later joined 126th Radio Intelligence Company at Townsville, following battles in the southwest Pacific for the next several years. The 126th was eventually placed under operational control of U.S. Air Force Far East in June 1943 to support 5th Air Force.

The 126th provided intercept and traffic-analysis support for MacArthur's jump into Hollandia, Dutch New Guinea, in early 1944.

Signal Security Agency

The Army's Signal Security Agency opened a cryptographic school in October 1942 at Vint Hill Farms, Warrenton, Va., to train cryptanalysts, traffic analysts and related technicians. Traffic analysis initially concentrated on the Pacific theater. The first challenge was to solve code numbers indicating message- center placenames in Japanese army messages.

Traffic analysts' contributions to U.S. military intelligence began with locating military message centers attached to Japanese unit headquarters in the Pacific. Coupled with communications-net reconstruction, analysts were able to identify troop locations, chain of command and order of battle. As we weren't reading Japanese army messages until well into 1943 and then only spasmodically, the analysts' contribution was critical to the Pacific war effort.

By June 1943, traffic analysts had solved practically all 12 Japanese message-number systems used on the military networks of imperial headquarters in Tokyo and the Southern Field Force. The analysts quickly improved, solving the Japanese water-transport organization message-number system within a month of its April 1944 introduction. Half of it was in hand in the first 48 hours.

Studies of variations in the enemy's army-traffic volumes and patterns of station activity were used to warn of impending Japanese army activities. Enemy divisions south of Manchuria were located through a painstaking analysis of communications between field units and headquarters. Movements of field units were tipped by readdressals of messages and compromises in unit code names.

Analysis of Japanese communications helped track a convoy to Manila on its route between Shanghai and New Guinea. On board were the Japanese 32d and 35th Infantry Divisions, bound to meet MacArthur's advances in April 1944. The convoy was intercepted near Luzon and virtually destroyed by U.S. submarines. Nine merchant ships and 12 escorts were sunk. Four thousand enemy troops and loads of equipment were lost at a time when MacArthur was penetrating the southern Japanese defenses at Hollandia.

Army sigint field units

Gen. Omar Bradley's 12th Army Group came into being Aug. 1, 1944, in Normandy, and with it Gen. George Patton's 3d U.S. Army was unleashed on the Germans. Two of Patton's corps were put into action south of Avranches in France. With Patton's corps were two signal service companies: the 3253d, attached to XV Corps, and the 3254th to VIII Corps.

Patton also had a sigint unit supporting his headquarters, 118th Signal Radio Intelligence Company, twice as large as companies assigned to corps.

Traffic analysts read intercept logs, identified units, reported their activities and kept order-of-battle and personality files up-to-date based on identifications from communications intelligence. They also posted various situation maps and did plotting. In short, traffic analysts were the hub of sigint units.

By January 1944, the German army's back was broken following their failed Ardennes offensive. Patton was free to strike into Germany; in January, the 17 divisions of 3d U.S. Army moved an average of 100 miles in subzero temperatures. This Army was responsible for more than 130,000 enemy casualties (killed, wounded, prisoners) during the Battle of the Bulge.

Traffic analysts helped keep 11 Panzer Division located around Saarburg, and determined 130 Panzer Lehr elements' presence around Bitburg when ground sources couldn't reach that far behind the lines.

3250th and V Corps

One of the most active sigint units, the 3250th, responsible for supporting Patton's V Corps, participated in the hard fighting of June and July 1944 in western France. From early August until early September, the company moved 10 times in 261 miles, laid 143 miles of wire, sent 2,924 messages, intercepted 2,687 enemy messages and copied an average 28 nets a day.

The Germans opened up a 50-mile offensive from Monschau to Echternach in the Ardennes at dawn Dec. 16. From that day until Christmas Day, the 3250th suffered nearly 20 percent casualties, including four killed in action.

During action in Western Europe, the 3250th operated 12 receivers covering the frequency spectrum between 100 and 3500 kilohertz all day every day within 10 miles of the front during some of the most mobile operations the U.S. Army ever conducted.

Mr. Browne works for the National Security Agency.

(Editor's note: this article was adapted from Cryptologic Quarterly.)

Last modified on:
May 08, 2006

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