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[[File:Air Force ordnancemen load a dispenser with seismic sensors.jpg|thumb|Air Force ordnancemen load a dispenser with seismic sensors.]]'''Operation Igloo White''' was a covert United States joint military electronic warfare operation conducted from late January 1968 until February 1973, during the Vietnam War. These missions were carried out by the 553d Reconnaissance Wing, a U.S. Air Force unit flying modified EC-121R Warning Star aircraft, and VO-67, a specialized U.S. Navy unit flying highly modified OP-2E Neptune[1] aircraft. This state-of-the-art operation utilized electronic sensors, computers, and communications relay aircraft in an attempt to automate intelligence collection. The system would then assist in the direction of strike aircraft to their targets. The objective of those attacks was the logistical system of the People's Army of Vietnam (PAVN) that snaked through southeastern Laos and was known as the Ho Chi Minh Trail (the Truong Son Road to the North Vietnamese).[1]

"The MUSCLE SHOALS (IGLOO WHITE) program was initiated on 16 September 1966, with a decision by Secretary of Defense, Robert S. McNamara, to develop a system to interdict North Vietnamese infiltration into South Vietnam. The program, as envisioned, included two closely related systems: (1) a strong point/obstacle subsystem to be deployed in a line across Vietnam, just below the DMZ, extending inland from the coast; and (2) an air-supported anti-infiltration subsystem extending westward from the strong point/obstacle subsystem into central Laos to include the I- area of the Ho Chi Minh Trail from North Vietnam through central and eastern Laos into South Vietnam (Fig. 1). By the end of 1966, a plan ij had been prepared and funds for the program were budgeted."

"The initial sensor program was called PRACTICE NINE until 14 June 1967, ILLINOIS CITY until 15 July 1967, and DYE MARKER until 8 September 1967, when MUSCLE SHOALS was adopted to indicate the air-supported subsystem in eastern and central Laos. In June 1968, the program was renamed IGLOO WHITE and consisted of three components: (1) munitions and sensing devices which were placed across and along suspected routes of infiltration to detect and impede enemy foot or vehicular movement; (2) orbiting aircraft which received signals from these sensors, amplified them, and retransmitted them; and (3) an Infiltration Surveillance Center (ISC) which received the transmitted signals from the aircraft and analyzed them to produce reliable tactical information for planning and interdiction operations. The IGLOO WHITE system was originally expected to impede enemy infiltration through use of mine fields and aid in determining when mine reseeding was necessary. Sensors were also to be used along trails and roads to provide real time target information for 3 tactical airstrikes. By July 1968, the munitions had proved to be relatively ineffective, and the use of sensors to obtain reconnaissance information was rapidly becoming the principal objective of the IGLOO WHITE system.[Declassified 1]

Effective implementation

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Igloo White was rushed into service during the Battle of Khe Sanh and successfully passed its first operational test. Combined with Operation Commando Hunt in 1969, the system served as the keystone of the U.S. aerial interdiction effort of the Vietnam War.

Costing between $1 and $1.7 billion to design and build (and another billion dollars per year to operate over the five-year life of the operation) and possessing and controlling some of the most sophisticated technology in the Southeast Asia theater, Igloo White still remains in question as to its effectiveness.

Development
Further information on the U.S. aerial campaign against North Vietnam: Operation Rolling Thunder
Further information on the PAVN logistical system: Ho Chi Minh Trail
As early as June 1961, General Maxwell D. Taylor, President John F. Kennedy's special military representative, had become interested in the prospect of erecting a physical barrier to halt the increasing infiltration of PAVN materiel (and later, manpower) through their Laotian logistical corridor and into the border regions of the Republic of Vietnam (South Vietnam).[2] He then held talks with the Deputy Director of the Pentagon's Office of Special Operations, Edward G. Lansdale, who convinced him that a better solution to the infiltration dilemma would be the creation of mobile units to attack the infiltrators.[3]

After the initiation of the strategic aerial bombardment of North Vietnam (Operation Rolling Thunder) in March 1965, Washington saw that program as its chief method of relaying signals to Hanoi to cease its support of the southern insurgency.[4] When that failed as a strategy, the aerial effort was redirected to serve as an anti-infiltration campaign.[5] After a million sorties were flown and more than three-quarters of a million tons of bombs were dropped, Rolling Thunder came to an end at the direction of President Lyndon B. Johnson on 11 November 1968.[6]

As early as 1966 Secretary of Defense Robert S. McNamara had become disenchanted with Rolling Thunder. In January McNamara was presented with a working paper by the American academic Roger Fisher, who proposed a less costly physical and electronic barrier that would be located in South Vietnam.[7] It was to consist of a 216-mile (348 km) long, 500-yard (460 m) wide barrier that would stretch from the South China Sea south of the Demilitarized Zone (DMZ), across the Laotian frontier and on to the border of Thailand.[8] The physical barrier itself would be supported by electronic sensors and extensive minefields. Fisher estimated that it would take approximately five U.S. divisions to erect and defend the system.[9]

The Joint Chiefs and CINCPAC turned down the concept, believing that it would consume the efforts of too many troops and create a logistical nightmare.[10] McNamara, however, was intrigued. Ignoring the military chiefs, he approached the Institute for Defense Analyses and requested that it fund a study of the concept.[11] The project was then handed over to the Jason Division (also referred to as the Jason Group or Jason Committee), a group of 67 scientists who were to study and develop the technology necessary to make the physical/electronic barrier feasible.[12] The group concentrated its efforts in three key areas: communications; data processing and display; and sensor development. On 15 September 1966, McNamara made Army General Alfred D. Starbird the head of the newly formed Defense Communications Planning Group (DCPG), which was to oversee the implementation of the program. The barrier concept was then given the designation Practice Nine.[13] The DCPG's original mission profile, as of September 1966, was simply to implement the anti-infiltration system devised by the Jason Division. The original plan foresaw the deployment of both a physical barrier in addition to an air-supported barrier using various electronic sensors.[14]

In June 1967 the barrier project, by then referred to as the Strong Point Obstacle System (or SPOS) was renamed Illinois City, which lasted for a month before the program was redesignated Dyemarker.[15] On 6 July, groundbreaking ceremonies were conducted for the command center of the operation. Construction work was completed three months later.[16] The new Infiltration Surveillance Center (ISC) was located at Nakhon Phanom, Thailand, nine miles (14 km) west of Nakhon Phanom city on the banks of the Mekong River. The first commander of the unit was Air Force Brigadier General William P. McBride, whose superior was the deputy commander of the Seventh/Thirteenth Air Force at Udon Thani, Thailand. From its creation the ISC (and the operations conducted from it) was known as Task Force Alpha.

On 8 September 1967, the electronic portion of Dyemarker was divided and the aerial, sensor-based portion was designated Muscle Shoals. On 1 June 1968, the aerial portion was redesignated Igloo White. That name became the most associated with the program though various elements received a number of separate codenames. All resources for Dyemarker/Muscle Shoals were redesignated Duck Blind in April 1968, and in June 1968 Muscle Shoals outright became Duel Blade. In October 1968 Duck Blind was changed to Duffel Bag.[15]

In mid-1968, the physical barrier concept was pushed aside after the North Vietnamese overran the Lang Vei Special Forces camp and besieged Khe Sanh. The barrier concept was reduced to an aerial, sensor-based electronic interdiction program that was to be conducted in Laos.[17]

Sensors and Weapons
Further information on special operations in Laos: Studies and Observations Group
The Defense Communications Planning Group's research and development program created an advanced technological system. Muscle Shoals would consist of three interdependent parts. First, there were air-dropped, battery-powered acoustic and seismic sensors.[18] The camouflaged sensors were to be dropped in strings at predetermined geographical points along the PAVN logistical network. Once emplaced, they would serve as tripwires to any movement or activity along the system.

The first sensors utilized by the program were Air-Delivered Seismic Intrusion Detector (ADSID), which had been developed from devices then in use in underground mapping for the oil industry. The device could sense vertical earth motion by the use of an internal geophone and could determine whether a man or a vehicle was in motion at a range of 33 yards (30 m) and 109 yards (100 m) respectively.[19] The first acoustic sensors were developed from the U.S. Navy's Project Jezebel anti-submarine warfare sonobuoys, which recorded and processed sound by the utilization of an audio spectrum analyzer. The first model of seismic detectors (Phase I) outshone their contemporary acoustic types in the quality and quantity of the information they reported.[20] The Phase I models of both the acoustic and seismic sensors were only available for operation in a continuous mode, which meant that under normal conditions, their lithium batteries would function for approximately 30 days.[21]


Air Force ordnancemen load a dispenser with seismic sensors
The Acoustic Seismic Intrusion Detector (ACOUSID), combined the operations of both seismic and acoustic devices, with the added ability to transmit sound from a built-in microphone. The ACOUSID had three switchable detection modes: in the C mode, a line spectrum detector determined the presence of enemy vehicles and had an effective range of 1,094 yards (1,000 m); an I Mode which was activated by sounds picked up by its internal microphone and could detect personnel at a range of 438 yards (401 m); and a B Mode that combined both of the above abilities and operated in a continuous real-time mode of 40 activations per hour with a battery life of 30 days.[19]

The sensors reported their data via radio frequency channels ranging upward from 162 megahertz (MHz) to 174 MHz on the very high frequency band. 31 channels were assigned to each type of sensor with a 375 kHz separation between each channel. Every channel contained 27 identification codes or addresses which could be set in the field prior to emplacement. Thus, a total of 837 individual sensors (27x31) could be deployed at any one time without signal duplication in a single operational zone.[22]

The deployment of Gravel mines and the Wide-Area Anti-Personnel Mine System became integral parts of the operation. Other weapons were specifically developed or otherwise became associated with the campaign as well. These notably included the BLU-31/B and Mk 36 air-dropped mines, BLU-43/B and BLU-44/B landmine system (Dragontooth), the BLU-72/B fuel-air explosive (Pave Pat), and the BLU-52/A chemical bomb (filled with bulk CS-2 powder).[23] Anti-vehicle operations in support of Muscle Shoals/Igloo White were designated Mud River, while anti-personnel operations received the codename Dump Truck.[15]

PAVN personnel moving on foot through the trail system would be detected by the detonation of air-sown, aspirin-sized, wide-area Gravel mines, which would activate the sensors. Subsequent bomb damage assessment missions and hand emplacement of sensors and mines in support of Muscle Shoals would be carried out by the reconnaissance teams of the highly classified Military Assistance Command, Vietnam Studies and Observations Group (SOG).[24] A number of hand emplaced units were developed for this role, including the MICROSID and MINISID, and a device designed to work with either called MAGID, a magnetic detector, designed to be triggered by amounts of metal as small as an infantry rifle.[25]


Gravity deployment of a sensor during the Battle of Khe Sanh,1967
The sensors were designed for air deployment by either of two means. The first was by parachute, which would hang the devices in trees, where they appeared to be part of the foliage. The second method was the use of gravity, which would drive the spike-shaped device into the ground like a lawn dart, burying all but their antennae, which were designed to appear as weeds. Surprisingly, approximately 80 percent of the air-dropped sensors were found to be operational after delivery.[26]

The second phase of sensor development improved the older models by providing for non-continuous operation as directed by the ISC. They also had the capability of carrying out three separate reporting functions: to report current information (transmitting noises or earth tremors); to keep silent, but to count impulses and respond when queried; or to remain in constant operation like the Phase I models. Their batteries also provided the ability to operate for approximately 15 days longer than the Phase I models (45 days).[27]

During late 1969, Phase IV sensors began to be deployed in-theater. These had a greater number of available communications channels, which allowed the seeding of a larger sensor field without fear of signal interference. By 1971–1972 a new sensor with a commandable microphone (COMMMIKE III) and another with a vehicle ignition detector (EDET III, which could detect the unshielded ignition systems of gasoline engines) were introduced.[28] In 1972 U.S. dollars the ADSID cost $619.00, the ACOUSID $1,452.00, and the engine detector $2,997.00. During the life of the operation approximately 20,000 sensors were deployed in Laos.[29]

Analysis of sensor results

Former EC-121R Batcat at AMARC, Davis-Monthan AFB, Arizona
The sensor transmitters would relay their data to the second element of the system, an orbiting EC-121R aircraft of the Air Force's 553rd Reconnaissance Wing, based at Korat Royal Thai Air Force Base. The EC-121s would then relay the collected information via a radio link at 2200 to 2300 MHz to Nakhon Phanom. At Task Force Alpha, the last of the three components in the system, the intelligence data (from a variety of sources, not just the sensors) would be entered, collated, retrieved, and stored by two IBM 360/40 (later two 360/65) computers.[30] Technicians at the center controlled the system from a variety of video displays that were also linked to the computers.

Analysts at the 200,000-square-foot (19,000 m2) center concentrated on such arcane topics as pathway predictions, delay intervals, route segments, and choke points. The computers analyzed sensor data and compiled intelligence information and then made predictions as to where and when a particular PAVN truck convoy would be geographically located.[31] According to author John Prados, the system functioned "exactly like a pinball machine... in truth, the mavens of the electronic battlefield became pinball wizards".[32]

The effectiveness of the system was determined not by how long a sensor would last in the field, but by the adequacy of coverage by a particular string of sensors.[27] For instance, a well placed string with several failed sensors was more effective than a fully functional string placed in the wrong location. The electronic data was, however, only as good as the human analysts and operators at the ISC. The knack of timely sensor activation depended on careful study of device locations and the patterns of PAVN logistical behavior.[27]


QU-22B Pave Eagle
The sensors were delivered to the target areas by U.S. Navy OP-2 Neptunes of VO-67 or by U.S. Air Force helicopters, both based at Nakhon Phanom. Due to increasing PAVN anti-aircraft artillery defences encountered in southeastern Laos, delivery in high-risk areas of the trail system was handed over from the Neptunes to Air Force F-4 Phantom II fighter-bombers that had been specially equipped for the missions.

U.S. strike aircraft were directed to predicted target areas by a variety of means. The first was for the ISC to relay target information to an airborne battlefield command and control center (ABCCC), which then routed bombers to a forward air control aircraft (FAC).[31] The FAC then led the strike to the target. During inclement weather or complete darkness, aircraft could still attack the trail by utilizing either MSQ-77 Combat Skyspot (a radar-directed system) or LORAN (a radio-directed navigational system).[33]

As the program (and PAVN air defences) evolved, so did the relay aircraft. The EC-121Rs and their crews proved too vulnerable and were partly replaced in 1969 and 1970 by QU-22Bs (modified Beech A-36 Bonanzas) which were to be remotely piloted,[34] and which had undergone primary mission equipment and PME flight tests at Eglin Air Force Base, Florida, in 1968.[35] The aircraft suffered from mechanical difficulties, however, and were never flown during an operational mission without a pilot. They were replaced by C-130B models in December 1971.[36]

Operations in Laos
Further information on the struggle in I Corps: Battle of Khe Sanh
Further information on U.S. aerial support at Khe Sanh: Operation Niagara
Further information on the U.S. aerial interdiction effort in southeastern Laos: Operation Commando Hunt
On 25 November 1967, Muscle Shoals began to undergo field testing and evaluation in southeastern Laos.[37] This process was stopped by the battle of Khe Sanh, when an estimated three PAVN divisions approached, and then surrounded the Marine outpost in western Quang Tri Province adjacent to Laos. The U.S. commander in Vietnam, General William C. Westmoreland, ordered Task Force Alpha to support the aerial effort to defend the base (Operation Niagara). On 22 January, the first of 316 sensors were implanted around and near Khe Sanh in 44 strings by Navy squadron VO-67.[38]


U.S. Navy OP-2E Neptune of VO-67, a variant of a naval patrol bomber and anti-submarine warfare aircraft specifically developed for the Muscle Shoals mission.
Although the Marine Direct Air Support Center was at first reluctant to trust the sensors (which in fact replaced ground patrolling), they were soon convinced of their utility.[39] The Marines credited 40 percent of intelligence available to the Khe Sanh fire support coordination center to the sensors.[40] General Westmoreland was ecstatic about Task Force Alpha's contribution to the victory at Khe Sanh. He had been one of the few high-ranking military officers that had supported the barrier concept since its planning stages.[41]

With the conclusion of aerial operations around Khe Sanh and the close out of the Marine base, the focus of Igloo White (as the operation was redesignated in June) turned once again to Laos. On 15 November 1968, the Seventh/Thirteenth Air Force had inaugurated Operation Commando Hunt, a series of continuous interdiction operations against the Ho Chi Minh Trail that would continue until the end of American participation in the war. For the next four years Igloo White and Commando Hunt would be interlinked in the anti-infiltration effort.

With the advent of Igloo White/Commando Hunt, the aerial interdiction effort entered a new phase. Armed reconnaissance – the patrolling of known segments of the trail system by aircraft searching for targets of opportunity – gave way to strikes directed by the sensor system.[42] That is not to say the Task Force Alpha had operational control of the aircraft that carried out the missions. The Seventh Air Force was not about to surrender the single air manager control that it had just fought so hard to obtain.[43] Task Force Alpha supported the Seventh by gathering, analyzing, and disseminating intelligence.[44] This intelligence was delivered from Nakhon Phanom to the Seventh's headquarters at Ton Son Nhut Air Base, where it issued the strike orders. The ABCCCs were also under the control of the Seventh, not Task Force Alpha.

The only exception to this arrangement was Operation Commando Bolt, a real-time, LORAN-based technique which utilized sensor strike zones derived from predicted target locations.[45] These missions were coordinated by Task Force Alpha's Sycamore control center against targets that passed through strike modules (four strings of three to six sensors each). Air controllers at Sycamore then directly delivered the correct course, altitude, and speed necessary for the strike aircraft to deliver their ordnance to the correct coordinates.[46]

There were some initial problems with the system. During Operation Commando Hunt I, for example, there were so many aircraft piling up in the air space over southern Laos that the air controllers and FACs could not keep track of them. Pilots, many of whom were veterans of Rolling Thunder, tended to arrive over the area in large waves instead of spacing out their arrivals over time.[47] One technical failure, however, would have huge ramifications for the entire program. The anti-personnel system, which was based on the dispersal of wide-area Gravel mines (the explosions of which would activate acoustic sensors) failed completely. The mines deteriorated too rapidly in the heat and humidity of Laos, thereby negating the system.[48]

The North Vietnamese response to the threat posed by the sensors was swift. If PAVN troops could detect the general location of a sensor, they immediately began to search out others and destroy them. Another response was to simply avoid them. The final evolution of anti-sensor measures was to attempt to deceive them.[49] The chief protection of the sensors proved to be the dense foliage that covered the majority of the logistical system, turning the enemy's chief protection against him. Doubtless, PAVN troops negated the effects of many sensors, but so many of them were sown over such a long period of time that the majority survived.

Operations in South Vietnam
While the electronic barrier program in Laos became the most visible part of the anti-infiltration program, the physical barrier project in South Vietnam was largely abandoned after 1968 in favor of mobile operations. The SPOS outlined in the original Dyemarker plan envisioned a virtual defensive wall stretching from the South China Sea to Dong Ha Mountain. The plan called for a fence, a line of twin steel apron and barbed steel tape, an anti-personnel mine field, a passive sensor detection system, and another line of twin steel apron and barbed steel tape, to "trace" this path, providing a physical obstacle to enemy infiltration. Observation posts and towers would line the route, and six specifically designed strong points would be constructed, with associated battalion base areas. A combined force of a single US Marine Corps Regiment and a single ARVN Regiment were to man these positions.[50]

The 3rd Naval Construction Battalion designed fighting and living bunkers for the route, presenting them for tests and evaluation 1968, both in CONUS and in Vietnam.[51] Also, 28 January 1968, COMUSMACV provided its own guidance for the planned creation of the ARVN Dyemarker Regiment. COMUSMACV envisioned a Regiment of 5 4-company Battalions and an Armored Cavalry Squadron. On 30 January, COMUSMACV informed the commander of III MAF of their proposed mission to augment the proposed ARVN force manning the barrier.[52] The ARVN 2nd Regiment, of its 1st Infantry Division became earmarked for the barrier mission.[53]

On 29 October 1968, work was stopped on the physical SPOS barrier, by that time redesignated as Duel Blade, under MACV Planning Directive 10–67, citing changes in the overall force posture, both enemy and friendly. The physical barrier would be replaced by the revised Duel Blade plan of active resistance utilizing air strikes, artillery and naval bombardment and mobile reaction forces. Base sites already constructed or under construction were to be used as support bases for the revised operation plan.[54]

Other applications
Though Igloo White's primary focus was monitoring traffic along the Ho Chi Minh Trail, the DCPG's mission had been expanded to include sensors for a "wide range of tactical applications," as well as, a "Ground Tactical System" in 1968.[55] The Ground Tactical System was envisioned as a tool for ground commanders in defending fire bases and other secured areas against attack, and was eventually employed around Khe Sanh.[56] It was also noted that the US border patrol was testing the sensor equipment as a method of protecting US continental borders.[57] Sensor equipment was deployed by July 1969 in South Vietnam for the purposes of countering boobytrapping and mining teams. A total of 56 sensors were diverted to the problem as a test, with testimony that vehicle losses were reduced by 50 percent in the first month.[58]

Conclusion
Further information on the North Vietnamese offensive of 1972: Easter Offensive
Further information on the ARVN incursion in Laos: Operation Lam Son 719
The stated goal of the American aerial interdiction campaigns was to force Hanoi to pay too high a price in blood to make the continued support of its goals in South Vietnam tenable.[59] In this effort, the U.S. failed. Not only were the PAVN/NLF able to continue their efforts, but they managed, under a deluge of ordnance, to launch two major offensives (the Tet Offensive in 1968 and the Nguyen Hue Offensive [known in the West as the Easter Offensive] in 1972) and a counteroffensive (against Operation Lam Son 719 in 1971). The key to the failure of the American strategy was that the anti-infiltration campaigns targeted supplies and transportation (both of which were imported), instead of manpower.[60] The price in casualties for the PAVN/NLF was high, but it was a price that Hanoi was able and willing to pay.

Chief among the criticisms leveled against the system was its failure to detect the build-up to PAVN's 1972 Nguyen Hue Offensive. This cast doubt not only on the trustworthiness of the sensors, but on the adequacy of the entire system. The headquarters of the Pacific Air Force launched an investigation of the apparent failure and reported that "our estimates were in error." The report went on to state, however, that the lapse was not the result any failure of the sensors themselves, but the Air Force's false assumption that the trail net had adequate coverage.[61]

The interdiction campaigns were also expensive. Igloo White cost around one billion dollars per year to operate.[18] The cost of the bombing operations that the sensors supported amounted to approximately 18.2 million dollars per week.[62] Those costs did not include the hundreds of aircraft lost during the interdiction campaigns or the crews that manned them.

Scholars today remain divided on the merits of the electronic barrier system and the efficacy of the bombing campaigns that it directed. The claims of destruction made by the U.S. Air Force, both during and after the war, were originally taken as given. The only exception to this rule was at the CIA, which discounted Air Force claims at the time by as much as 75 percent.[1] This was understandable due to the fact that the Vietnamese were basically silent during the 1970s and 1980s.

By the 1990s, new historical research (especially by Air Force historians Earl Tilford, Bernard Nalty, and Jacob Van Staaveren) and the publications of the Military Institute of Vietnam finally opened new perspectives on aerial interdiction during the Vietnam War. The Seventh Air Force claimed that 46,000 PAVN trucks had been destroyed or damaged by air strikes during its tenure (with nineteen thousand of these occurring during Command Hunt V 1970-1971).[49] While the North Vietnamese only imported 6,000 trucks into North Vietnam between 1968 and 1970 (for all of their operations), imports increased dramatically in 1971, though these also seem to have been as a result of the planned 1972 offensive.[49] American claims that 80 percent of the materiel that started down the trail was destroyed while en route to the southern battlefields had to be altered in the face of loss claims of only 15 percent by the Vietnamese.[63]

Thomas C. Thayer, chief of the Advanced Research Projects Agency for four years during the war, believed that only about one-twentieth of the cargo imported into the north moved southward on the Ho Chi Minh Trail and that more than two-thirds eventually reached the battlefields in the south.[64] More recently however, a new study by Air Force historian Eduard Mark has attempted to rehabilitate the Air Force's original claims by finding a rough correlation between trucks imported into North Vietnam during the war and those that were claimed by the American pilots as destroyed.[65] Only the opening of the Vietnamese archives to scholarly research will reveal the true effectiveness (or lack thereof) of the American electronic and aerial effort.

Notes
 Correll, John T. (November 2004). "Igloo White". Air Force Magazine. 87 (11): 56–61. Retrieved 30 October 2009.
 Jacob Van Staaveren, Interdiction in Southern Laos, 1961–1968. Washington DC: Center of Air Force History, 1993, p. 255.
 Van Staaveren, p. 255.
 Jacob Van Staaveren, Gradual Failure. Washington DC: Air Force History and Museums Program, 2002, pp. 90–91.
 Earl H. Tilford, Setup. Maxwell Air Force Base AL: Air University Press, 1991, p. 153.
 Tilford, p. 89. See also John Morocco, Thunder from Above. Boston: Boston Publishing Company, 1984, pp. 182–184.
 Van Staaveren, p. 257.
 John Morocco, Rain of Fire. Boston: Boston Publishing Company, 1985, p. 38.
 Van Staaveren, Interdiction, p. 57.
 Van Staaveren, Interdiction, pp. 256–258. The talking paper written by Army Chief of Staff General Harold K. Johnson agreed with Fisher's estimate that at least five divisions would be necessary for the construction and defense of the barrier.
 Van Staaveren, Interdiction, p. 258.
 Bernard C. Nalty, The War Against Trucks Washington DC: Air Force Museums and History Program, 2005, p. 8.
 Van Staaveren, p. 262.
 United States Government, United States Senate. Hearings Before the Electronic Battlefield Subcommittee of the Preparedness Investigating Subcommittee of the Committee on Armed Services. Washington, DC: US Government Printing Office, 1971, p. 4.
 United States, Command History, Volume II, 1968, 1969. p. 911.
 Van Staaveren, Interdiction, p. 271.
 Edgar C. Dolman, Jr., et al., Tools of War. Boston: Boston Publishing Company, 1984, p. 151.
 Doleman, p. 151.
 Van Staaveren, Interdiction, p. 267.
 Nalty, War Against Trucks, p. 295.
 Doleman, p. 144.
 Van Staaveren, Interdiction, p. 268.
 United States Government, United States Senate, pp. 134–146.
 United States, Command History 1968, Annex F, 1969. p. 27.
 United States Government, United States Senate, pp. 6–7.
 Igloo White Archived 20 February 2007 at the Wayback Machine
 Nalty, War Against Trucks, p. 83.
 Nalty, War Against Trucks, p. 84.
 Nalty, War Against Trucks, p. 85.
 Nalty, p. 103.
 Nalty, p. 13.
 John Prados, The Blood Road. New York: John Wiley and Sons, 1998, p. 268.
 John Schlight, A War Too Long. Washington DC: Air Force History and Museums Program, 1996, pp. 54–55. See also Van Staaveren, Interdiction, pp. 179–180.
 Doleman, pp. 144–145.
 AF.mil Archived 8 February 2012 at the Wayback Machine
 Nalty, War Against Trucks, pp. 88–89.
 Van Staaveren, Interdiction, p. 277.
 Van Staaveren, Interdiction, p. 291.
 John Prados and Ray Stubbe, Valley of Decision, Annapolis MD: Naval Institute press, 1991, pp. 300–303.
 Bernard C. Nalty, Air Power and the Fight for Khe Sanh. Washington DC: Office of Air Force History, 1986, p. 95.
 Prados and Stubbe, pp. 140–146.
 Nalty, p. 17.
 Nalty, Air Power, pp. 68–74.
 Nalty, War Against Trucks, p. 124.
 Nalty, War Against Trucks, pp. 124–126.
 Nalty, War Against Trucks, p. 108.
 Nalty, War Against Trucks, p. 113.
 Nalty, War Against Trucks, p. 20.
 Nalty, War Against Trucks, p. 296.
 United States, Command History, Volume II, 1968, 1969. p. 912.
 United States, Command History, Volume II, 1968, 1969. pp. 911–912.
 United States, Command History, Volume II, 1968, 1969. p. 914.
 United States, Command History, Volume II, 1968, 1969. p. 915.
 United States, Command History, Volume II, 1968, 1969. p. 917.
 United States Government, United States Senate, p. 4.
 United States Government, United States Senate, pp. 10–13.
 United States Government, United States Senate, p. 14.
 United States Government, United States Senate, pp. 64–65.
 Tilford, p. 173.
 Nalty, War Against Trucks, p. 298.
 Nalty, War Against Trucks, p. 302.
 Herman L. Gilster, The Air War in Southeast Asia. Maxwell Air Force Base AL: Air University Press, 1993, p. 49.
 Military History Institute of Vietnam. Trans. by Merle Pribbenow. Victory in Vietnam, Lawrence KS: University of Kansas Press, 2002, p. 320.
 W. Scott Thompson and Donaldson W. Frizzell, The Lessons of Vietnam. New York: Crane, Russak, and Co, 1977, pp. 148–149.
 Eduard M. Mark, Aerial Interdiction. Washington DC: Center of Air Force History, 1994.
References
Unpublished government documents

United States, Military Assistance Command, Vietnam. Command History, 1968, Annex F. Military Assistance Command, Vietnam: Saigon, South Vietnam, 1969.
United States, Military Assistance Command, Vietnam. Command History, Volume II, 1968, . Military Assistance Command, Vietnam: Saigon, South Vietnam, 1969.
Published government documents

Caine, Major Philip D., Project CHECO Southereast Asia Report 11801/20080725 222 (Declassified 8-15-2006 by Pentagon).
Correll, John T. Igloo White, Air Force Magazine. November 2004, Vol. 87, No. 11.
Gilster, Herman L, The Air War in Southeast Asia: Case Studies of Selected Campaigns. Maxwell Air Force Base AL: Air University Press, 1993.
Mark, Eduard M. Aerial Interdiction: Air Power and the Land Battle in Three American Wars. Washington DC: Center of Air Force History, 1994.
Military History Institute of Vietnam, translated by Merle Probbenow, Victory in Vietnam: The Official History of the People's Army of Vietnam, 1954–1975. Lawrence KS: University of Kansas Press, 2002.
Nalty, Bernard C, Air Power and the Fight for Khe Sanh. Washington DC: Office of Air Force History, 1986.
Nalty, Bernard C. The War Against Trucks: Aerial Interdiction in Southern Laos, 1968–1972. Washington DC: Air Force Museums and History Program, 2005.
Schlight, John, A War Too Long: The USAF in Southeast Asia, 1961–1975. Washington DC: Air Force History and Museums Program, 1996.
Schlight, John, The War in South Vietnam: The Years of the Offensive, 1965–1968. Washington DC: Air Force History and Museums Program, 1999.
Tilford, Earl H. Setup: What the Air Force Did in Vietnam and Why. Maxwell Air Force Base AL: Air University Press, 1991.
United States Government, United States Senate. Hearings Before the Electronic Battlefield Subcommittee of the Preparedness Investigating Subcommittee of the Committee on Armed Services. Washington, DC: US Government Printing Office, 1971.
Van Staaveren, Jacob, Gradual Failure: The Air War Over North Vietnam, 1965–1966. Washington DC: Air Force History and Museums Program, 2002.
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Secondary sources

Doleman, Edgar C., Jr., Tools of War. Boston: Boston Publishing Company, 1984.
Mark, Eduard, Aerial Interdiction: Air Power and the Land Battle in Three American Wars. Washington DC: Center for Air Force History, 1994.
Morocco, John (1984). Thunder from Above: Air War, 1941–1968. Boston: Boston Publishing Company. ISBN 0-939526-09-3.
Morocco, John, Rain of Fire: Air War, 1968–1973. Boston: Boston Publishing Company, 1984.
Prados, John, The Blood Road: The Ho Chi Minh Trail and the Vietnam War. New York: John Wiley and Sons, 1998.
Prados, John; Stubbe, Ray (1991). Valley of Decision: The Siege of Khe Sanh. Annapolis MD: Naval Institute Press. ISBN 0-395-55003-3.
Thompson, W. Scott and Donaldson W. Frizzell, The Lessons of Vietnam. New York: Crane, Russak, and Co, 1977.
Jeppeson, Chris, Acoubuoy, Spikebuoy, Muscle Shoals and Igloo White, 1999.