Korean War – Air Force

Intercepts of Soviet-built MiG fighter aircraft radio traffic confirmed the long-held suspicion that Russians were controlling the air defense of North Korea and Manchuria, not the Chinese or the North Koreans: “…we were actually monitoring the Soviet Air Force fighting the American Air Force, and we were listening to the Soviet pilots being directed by Soviet ground controllers to fight American pilots.

We were fighting our own little war with the Soviets.”(38) That information, from pilots’ and ground controllers’ plain language conversations and T/A on callsigns and procedures, gave the policy makers firm information upon which to make a decision of enormous proportions—whether to confront the Soviets with a distinct possibility of starting WWIII or to let the respective air forces continue the fighting under false pretenses. Obviously the U.S. policy makers chose the latter course of action.

Vietnam War

Army – Infiltration from North to South Vietnam, 1964–1973

One of the great controversies during the Vietnam War involved the number of communist troops infiltrating into South Vietnam from the North. As early as 1964 there was a wide and strident difference of opinion between Military Assistance Command, Vietnam (MACV), in Saigon and the Joint Chiefs of Staff in Washington. At that time Saigon wanted to prove that the war was being won and proceeded to “prove” that the threat from North Vietnam was being met by inflicting casualties. Headquarters in Washington, however, stated that the number of enemy troops coming south was much greater than MACV was estimating. Another point of major disagreement was whether the troops coming south were only South Vietnamese repatriates or regular North Vietnamese troops.

As the controversy continued in 1964, T/A with RDF provided significant information. Traffic analysts were studying some communications from the People’s Army of Vietnam (PAVN) and followed the transit of their 325th Division from southern North Vietnam into the northern highlands of South Vietnam. This arrival of a regular infantry division represented a major escalation in the North Vietnamese involvement in the south, a development many military and political factions found hard to accept.

Another less precise yet important contribution of T/A to the infiltration dilemma was derived by assessing the volume of messages on a civil network in North Vietnam used by new recruits to send messages home as they left for the trek south. It had been determined that it took about four months for a soldier to make the trip from north to south. When the volume of messages surged in the north, arrival of more troops in the south could be anticipated within four months.

The big breakthrough in the production of SIGINT on this target came in late 1967/early 1968 when analysts gained access to low powered North Vietnamese communications serving their General Directorate of Rear Services (GDRS). The GDRS was responsible for managing a system designed to move troops and materials out of North Vietnam and, either through the demilitarized zone (DMZ) or through Laos, into South Vietnam. The move south went through a number of way stations, known as Trams, which normally were located a day’s walk from each other. These Trams often were run by families, but they had radios and did communicate. Messages from the Trams had four-digit indicators. The first digit identified the individual’s destination in South Vietnam, and the next three digits identified the soldier’s unit. These texts contained information on the size of the units moving on the trail, the ratio of officers to enlisted, the need for fuel, numbers of sick and wounded, etc. This information essentially resolved the dispute between MACV and Washington on the subject of the numbers and timing of troop movements to South Vietnam.(39)

The Battle of Dak To

In October of 1967 the 330th Radio Research Battalion was located on a hill in Vietnam’s western highlands. The unit, protected by sandbags, barbed wire, and watch towers, was listening to Vietnamese transmitters in the immediate area. At the time the Army personnel manning the station had a sense of foreboding based mainly on the “feel” of recent Communist radio activity but without definable evidence.

A civilian from NSA had just arrived at the unit to support its efforts. When he arrived at the location, he found a group of bright dedicated soldiers working diligently 24/7 under extremely challenging physical circumstances. The target was difficult; the Communists used daily changing callsigns, frequencies, and schedules. Further, they were using low-power transmitters, making intercept difficult.

A chat with the chief traffic analyst was the first step in developing a broad assessment of the situation. The first observation was that “the whole ball of wax was coming apart.” Specifically a new North Vietnamese command station appeared which talked to Hanoi, was more active than anyone else around, contacted the highest North Vietnamese echelon in South Vietnam, operated at night when other Vietnamese transmitters were down, and had just moved 77 kilometers to a new location. Then the 330th unit’s chief linguist stated that many Communist elements were moving and realigning with other elements throughout the area. All of this activity indicated that an attack was in the offing, but one key element still was missing: the Vietnamese Communists had not given any indications that they had reconnoitered the area—normally a prerequisite to an attack.

U.S. troops fighting near Dak To, South Vietnam.

Throughout October, message activity showed that the North Vietnamese 1st Division was preparing for urgent operations, and they expressed concern that their activities would be detected by the U.S. Then on October 23 the missing piece fell into place; the expected North Vietnamese precombat reconnaissance had begun, indicating preparations for an attack. Based on the frequencies, callsigns, schedules, and a direction finding location of the North Vietnamese Military Intelligence link, U.S. analysts determined the attack would be somewhere in the Dak To area.

Further details followed quickly. On October 25 the 32nd Regiment was located in the Dak To area after having traveled 100 kilometers. On October 27 the 66th moved there, and on October 30 the 174th arrived. Translations of clear text messages also yielded useful information including instructions on conducting reconnaissance and the details of a new simplified signals plan. The North Vietnamese regularly instituted a new simplified signals plan just before beginning combat operations. Then one Vietnamese unit advised a subordinate to maintain secrecy before it was time to strike.

A report was issued by the 330th based upon all of the information it had gathered. In summary, the report advised that a major tactical thrust was in the offing, probably between October 30 and November 4 with the target in the Dak To area. Additional evidence came in on the day after the report was issued. An unmistakable pattern of communications was observed. Division headquarters established communications with combat units, reconnaissance began, combat units took positions, the simplified communications plan was instituted, and a tactical command post took control of combat units.

Based on the reports, U.S. forces took immediate action and disrupted the North Vietnamese execution of significant portions of their plan. B-57 air strikes were launched, and two U.S. battalions landed on two strategic hilltop positions. The battle of Dak To continued through late November and proved to be one of the largest battles of the war, but the U.S. had gained an important tactical advantage by disrupting the Vietnamese plan of attack. The overall North Vietnamese objective of this campaign was the destruction of two U. S. brigades. Although the fighting was fierce, their objective was denied.(40)

Air Force – Raid on Son Tay Prison

A number of U.S. Air Force, Marine, and Navy pilots were shot down during bombing raids and fighter combat in Vietnam. Many were captured and imprisoned in North Vietnam, some at a camp named Son Tay, located twenty miles northwest of Hanoi.

With assurance that prisoners were being held at this camp, planning began in April 1970 to mount an operation to free the prisoners.

Mock-Up of Son Tay Prison, North Vietnam.

Preparations for SIGINT support to this raid began in August 1970. Brigadier General Manor, commander of the operation, requested information to aid a safe incursion of low-flying helicopters from Tahkli Air Base in Thailand to Son Tay and their egress. General Manor also wanted all information indicating a possible capability of the North Vietnamese to interfere with the operation. Analysts concluded that, if the raiders used the proposed route and did it at night, the North Vietnamese would have no capability to interfere.

SIGINT not only provided a key input to planning the raid but also provided critical information during the incursion. Extraordinary measures were taken to ensure that all collection and analytic assets were employed. Further, special rapid communications were set up to pass the information to those running the operation and to those in the Pentagon overseeing the activity. The select group in the Pentagon convened in the National Military Command Center and included the secretary of defense, the chairman of the Joint Chiefs of Staff, and selected four and three-star generals. The NSA representative to the Pentagon also was there.

During the course of the raid the NSA Representative was briefing this group on the support being provided by SIGINT. Then an incident occurred illustrating just how important negative information can be. An officer entered the room and announced that General Manor had declared a MiG alert, indicating that North Vietnamese fighter aircraft could be threatening the operation. Everyone turned to the NSA Representative, who had just assured the group that there was no MiG threat. He based his judgment on analysis that had identified all night-qualified North Vietnamese pilots, where they were spending the night, and the absence of any activity from those airfields. Further, he had the best communications connections with the field, and he stayed with his position, reiterating “No MiGs”. After a few more tense moments in the room, a courier entered the room with the news, “Cancel MiG alert”.

Although the mission itself was well planned and executed and the SIGINT support to the military operation was excellent, tragically the mission failed, as the prisoners had been moved, undetected by U.S intelligence. There is some speculation that a Caucasian journalist had visited the camp a month earlier. This might have led the North Vietnamese to remove the prisoners from that location as a precautionary measure.(41)

Notes 38-41

38. Matthew M. Aid, The Secret Sentry (New York: Bloomsbury Press, 2009), 35.
39. From lecture delivered by Walter J. Abbott, NSA History Symposium, October 2009.
40. Sharon Maneki, Proud and Bitter Memories: Personal Reflections of the Vietnam War, NSA Center for Cryptologic History (2006), 37-42.
41. Thomas R. Johnson, American Cryptology During the Cold War, 1945-1989, NSA Center for Cryptologic History, 1995, Book 2, “Centralization Wins, 1960-1972” (1995), 576-578.

Featured image: MiG Alley in North Korea

Source: Center for Cryptologic History National Security Agency