The S-2 section utilized a multitude of sources to develop an accurate picture of the enemy's activity around the base. While much of this information was self-generated, the 26th Marines received substantial intelligence support from the MACV, III MAF, and 3d Marine Division Headquarters. Ground and aerial observers, photo reconnaissance, infrared imagery, target lists of higher headquarters, crater analysis, shell/flash reports, and agent reports were all tools of the intelligence community at Khe Sanh. By comparing this information with the knowledge of enemy doctrine as applied in past situations, the S-2 staff was able to accurately estimate the intentions of the NVA on a day-to-day basis. One good example of how this intelligence produced hard results occurred in late February. From their various inputs, the two men who were responsible for the earlier attack on the NVA staff conference, Majors Coolidge and Hudson, pinpointed the exact location of 12 artillery positions and 2 major ammunition depots. These targets were concentrated in two main areas to the south of the base. Air strikes were called in on the enemy positions and, after the planes departed, the whole area erupted in secondary explosions which lasted for the next 40 minutes. Two weeks later, these officers repeated a similar performance in another area.(163) The activities of the intelligence community at Khe Sanh and higher headquarters were vital to the conduct of the battle. Almost every major attack against the 26th Marines was picked up well in advance by the S-2 section. Whenever enemy activity was detected, the information was passed to the FSCC and this was the signal for Colonel Lownds to put his defensive fire plan into effect. The base was placed on Red Alert, the primary and secondary boxes fired, and saturation air strikes were employed. This method of cutting off the attack force by massed fires, once the S-2 section had provided a warning, proved to be a decisive factor in thwarting the major enemy thrusts which came late in February.(164) PART VII THE TURNING POINT While the supporting arms continued to whittle away at the enemy's strength, the defensive posture of the 26th Marines grew more formidable with each passing day. By the end of February, the Americans and South Vietnamese had erected some 510 bunkers, dug miles of trenchline, and laid hundreds of minefields and trip flares. Each sector was guarded by a maze of double-apron, tanglefoot, and concertina barbed wire obstacles.[60] The Marines also had sophisticated anti-infiltration equipment such as the Night Observation Device, the PPS-6 ground-surveillance radar, and the Starlight Scope; all of which could detect infiltrators along the wire during night-time and other periods of reduced visibility.(165) Wherever these apparatus were employed, the number of enemy killed along the perimeters increased and the number of probes decreased.