each target with the proper mix of accurately delivered ordnance, while maximizing the potential of the units and weapons systems available. Also, since this was a joint operation on a grand scale, scores of aviation, artillery, and naval surface units representing four different Services, had to deliver their firepower into the same areas at the same time without interfering with one another.
Although no accidents or serious incidents occurred, the operation was not without problems in fire support coordination. For example, the manual target list maintained by Provisional Corps, Vietnam and the automated list maintained by Seventh Air Force were not compatible, so, fire support coordinators found it necessary to use both lists. This proved difficult and time consuming. Also, the requirement for a three-day lead time for Arclight strikes was a burden which diminished the effectiveness of the powerful B-52s by preventing their use against targets of opportunity.
Target intelligence presented two problems: target identification and damage assessment. Target identification came initially from photo imagery interpretation and was supplemented, after the start of the operation, by pilot debriefings and air observer reports. Accurate battle damage assessments were a critical part of the targeting process. Without them, planners could not determine whether the attacks achieved the desired effects, and hence, could not know whether a target should be engaged further or struck from the target list as destroyed. Post-mission pilot debriefings and observer reports provided the initial battle damage assessment. The photo reconnaissance missions flown by VMCJ-1 and Seventh Air Force units provided additional information.* Covering the entire Cap Mui Lay sector each day, these sorties provided target intelligence personnel information which, in some cases, led to the engagement of new relatively stationary targets less than eight hours after the mission.36
On the ground, other target intelligence agencies were at work. Artillery forward observers, operating from positions along the DMZ, identified and engaged some targets visually, providing their own damage assessments. Another target acquisition system used during Operation Thor was the three-station sound-ranging base" installed in the northeastern portion of I Corps Tactical Zone. Modern technology also assisted the III MAF targeting effort. A system called "Firewatch," installed at Con Thien and manned by artillerymen of the 12th Marines, combined night observation devices, a laser range finder, and an acoustical system to determine accurate range and direction. During Operation Thor, "Firewatch" detected 41 enemy targets. The 12th Marines also used five counter-mortar radar units, capable of detecting projectiles in flight and computing their point of origin. In addition, Battery F, 26th Field Artillery, a U. S. Army target acquisition unit, manned another six counter-mortar radars.37
Despite this all-out surveillance effort, only about one-third of the artillery, naval gunfire, and air missions reported to the 3d Marine Division Fire Support Information Center during the month of July 1968, which included the period of Operation Thor, involved human observation and first-hand reports. Only one-fifth of these observed missions reported any damage to the targets.38
Still, those participating in Operation Thor realized that the weight of firepower was having immediate effects. By 5 July, antiaircraft fire over the Cap Mui Lay sector was so light that 0-1 aircraft carrying
"-Colonel Eric B. Parker, who commanded VMCJ-1 in 1968 at this time, recalled Thor later as an operation that "started and ended with a mosaic of the DMZ area covering several miles north of the DMZ. First for Target I.D., the last for BDA [bomb damage assessment}." He remembered his "continuing frustration with never being told what our efforts produced or, in other words, did our flights contribute in any way to the prosecution of the war effort. We got routine 'attaboys' which everyone got, but never heard to my recollection of any specific target being identified and subsequently destroyed." Col Eric B. Parker, Comments on draft, dtd 13Dec94 (Vietnam Comment File).
"^Sound-ranging bases employ a series of microphones spread over a known distance and wired to a central station. Each microphone,in turn, picks up the sound of an enemy gun firing and signals the central station. The sequence in which the microphones are activated and the time between activations are used to compute the direction to the enemy gun. A network of sound-ranging bases can provide intersecting directions to determine an enemy gun's location. Compared with some other systems that were available in III MAF at the time, the sound-ranging bases were crude, but when used as one part of a large, redundant target acquisition network encompassing a variety of systems, they could conceivably provide the final bit of information needed to locate a Communist firing unit. Lieutenant General Louis Metzger, who as a brigadier general served as 3d Marine Division assistant division commander in 1967 and early 1968, noted that the sound-ranging system "was brought to Vietnam in 1967 in an attempt to locate the enemy artillery firing from north of the Ben Hai River into our bases. It was basically a World War II system that was intended to be used in a broadly held front. It was unsuited for a battle in which only certain strong points were held, which did not allow for its positioning along a line so that the enemy firing position could be trian-gled." LtGen Louis Metzger, Comments on draft, dtd 17Oct94 (Vietnam Comment File).