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. Sensors and Weapons 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. 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 and respectively. 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. 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. 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 ; an I Mode which was activated by sounds picked up by its internal microphone and could detect personnel at a range of ; 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. 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