404th Radio Research Detachment (Airborne)

173d Airborne Brigade (Separate)

Special Intelligence

 Cantonment perimeter concertina warning sign.    Because special intelligence, or simply SI, was produced by Radio Research units, security was under the exclusive purview of the ASA. Special passes or access (called SI Clearances) were required to see or listen to SI information or to enter Radio Research facilities. All ASA personnel had SI Clearance. Usually fewer than twenty-five SI Clearances were provided personnel in the supported division (ten in a separate brigade) and those passes were restricted to senior commanders and intelligence staff based on a “need to know.” In larger units like divisions and corps, special intelligence was controlled by a Special Security Officer, an SSO, commanding a small SSO detachment. Commanders of Radio Research detachments supporting separate brigades or groups, were the SSO.

    As a result of special security, most soldiers assigned to a supported command, often including infantry battalion commanders, were not aware of the comprehensive functions or success of Radio Research units. Officers who were not United States citizens (there were several) were denied access. The one exception was the use of airborne radio direction finding (ARDF) provided by Radio Research aviation units. Special security and attendant limited access were designed to protect “source and success.” Arguably too much security may have prevented some intelligence from being disseminated to those who needed it or prevented a thorough understanding of its value when the source or success was concealed.

    Radio Research units derived information about enemy units by locating their transmitting antennas, affiliating those locations with enemy unit radio call signs, signal characteristics, operating techniques, plain text or cipher and code revelation, and other cryptologic techniques. When cryptology, statistical analysis and battlefield sense was integrated with other reliable information, military intelligence resulted. Radio Research and other military intelligence units collaborated closely.

    Two ground direction finding (DF) stations, using the AN/PRD-1 direction finder (or other equipment), obtained two lines of intersection (called a "cut") on an antenna used by a transmitting enemy radio. With luck three or more AN/PRD-1 could be used to more accurately triangulate an enemy antenna. (called a "fix") A fix with three DF stations was statistically 120 percent more accurate than a cut; 180 percent more accurate using four DF stations. Using intelligence analysis, enemy transmitters and radio operators were affiliated with specific enemy units. The enemy knew this and the brighter ones, the enemy operators that survived, either broadcast for a short time measured in seconds and then moved, switched, or reinstalled their antennas up to a mile away from their radio, or used their antennas for deception.

    By contrast, airborne radio direction finding equipment allowed an aircraft to fly an elliptical pattern and take multiple bearings on an active antenna within seconds and then report locations via secure aircraft radio to ARDF relay teams that were part of Radio Research units. ARDF was faster and more accurate than ground based direction finding. Using speech encrypted communications, ARDF reports were passed directly to Radio Research teams accompanying combat battalion and sometimes company commanders. The ARDF source and success also had to be concealed, but unlike SI information, ARDF reports were handled as Department of Defense Secret material and could be passed directly to any officer or noncommissioned officer using a security caveat called a “protected usually reliable source.” In military argot, ARDF was “sanitized” to conceal the real source. So the sanitized locations, often plotted to battle maps as a dot or sometimes including a radius of error, were attributed to a usually reliable source or abbreviated URS. Anyone with a whit of imagination quickly learned that “usually reliable source” was a synonym for airborne direction finding. Cluttering this combat intelligence with nonsense did little to hide the source and often confused inexperienced combat staff.

    Inexperienced battle commanders and their intelligence and operations staff immediately ordered artillery fired or aircraft strikes on direction finding locations. The consequence was predictably futile, contributed to compromising direction finding effectiveness, and resulted in enemy countermeasures. On the other hand, experienced intelligence users learned to use direction finding as one of many indicators of enemy location, intentions, and capability and planned tactically instead of impulsively. When used with discretion, direction finding was a critical intelligence tool. Radio Research personnel assumed responsibility to educate combat personnel about the best use of signals intelligence. Some commanders and staff learned and some did not.

    Radio Research units also frequently sent two-man, low-level voice intercept (LLVI) teams with infantry battalions and sometimes infantry or armor companies. These teams were capable of understanding Vietnamese plain text communication. They also had the capability to receive ARDF locations and sanitized intelligence relayed from their particular Radio Research unit via secure voice radio. LLVI teams also monitored the supported combat battalion’s own communications to detect exploitable security compromises by our own radio operators. Many battalion commanders insisted on these teams accompanying their units in combat and the teams were regularly lauded for preventing many enemy ambushes and for directing reconnaissance forces directly to enemy base camps or other positions. During combat operations these Radio Research teams were indistinguishable from combat infantrymen.

    A typical Radio Research unit operated “cryptologic positions.” A position is a set of personnel and equipment used to intercept electronic emissions, to locate enemy emitters (direction finding), and to transcribe or copy the emissions for continued analysis on site or sent for higher level analysis elsewhere. Communications security (COMSEC) positions were used to listen to friendly communications in order to determine when a security breach occurred due to plain text revelation or faulty encryption devices revealing location, plans and intentions.

    During a battle, the operations center of a Radio Research unit was bedlam filled with hundreds of simultaneous broadcasts of both enemy and friendly communication, voice and morse code, direction finding reports, and communication from the Radio Research unit to friendly units warning of enemy ambushes and enemy locations or compromise of friendly communications. Precise jamming was used selectively when an enemy station broadcast encrypted text. If the enemy operator switched to plain text, jamming was procedurally stopped. The enemy operator presumed his encryption equipment was at fault and turned it off and then broadcast in exploitable plain text. Radio Research units had a very limited capability to exploit noncommunication emissions like radar, but the enemy didn’t use radar on the ground battlefield in Vietnam.

    Cryptologic positions could be located inside a building, a tent, a bunker, a foxhole, a camper-like shelter on the back of a truck, carried on a soldier’s backpack, or in an airplane. Positions were usually tied together with a special radio research unit AM/FM communication network and attendant communication positions.

    Within Radio Research detachments and companies, most communication intercept involved FM and AM radio, and voice and “morse code” transmissions. Experienced radio intercept operators wore “stereo” headsets to listen to one end of a radio conversation (over one frequency) through one ear and the answering enemy operator (on a different frequency) through the other ear in order to copy two related communication links.

    Transcription was taken in long hand or using a "mill", a special typewriter with all-caps keys and no shift key. Intercepted communication was also tape-recorded on some positions.

    Most radio intercept operators searched for targets immediately confronting or engaging the supported combat units.  *6  But since all tasking was controlled by ASA and not the supported combat command, any position or all positions could also be tasked to copy strategic targets in other operational areas or battles. The vagaries of radio wave propagation were also considered when tasking or locating units or individual positions.

    ASA positions were assigned tasks using encrypted landline teletype communication or AM radio communication within the ASA communications network.

    The primary means of communication between the 404th and the 313th RR Battalion and the 509th RR Group was via encrypted teletype and secure telephone circuits transmitted by VHF radio relay and VHF multichannel microwave circuits. The 404th communicated with the 173rd Airborne Brigade’s combat units mostly through secure voice VHF / FM radio.

    The Brigade’s 173rd Signal Company and supporting corps signal units maintained those circuits. When the brigade was constantly relocating, those circuits were temporarily inoperable until new base camps and circuits were established. AM (Morse code) radio provided a secondary method of communication, but it was untenable. Frequently, the 404th relied on couriers hitching rides in trucks or helicopters.

    Radio Research units also received SI intelligence germane to their supported command from higher and lateral ASA echelons and provided that intelligence along with locally produced analysis directly to the supported commander and his intelligence staff during SI intelligence briefings throughout the day.


6 - Searched the radio spectrum for enemy radio traffic.

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