Opening blurb:
Booby traps accounted for 17 percent of all American wounds suffered in The Vietnam War.
First appearance of Kyle Chandler as PVT William Griner.
The sensor that is given to the squad before their mission is a Patrol Seismic Intrusion Device (PSID). The simple instruction for how to use it was inadequate. They came in a pack of 4, each transmitting an individual code in alternating high/low tone. The #1 unit sent a single beep in low tone, the second two beeps in high tone, etc. There were various ways to deploy them that would serve as an early warning system when bedded down for the night, a system to tell when the enemy was inside the ambush zone, or, even used along a trail to tell if a column is on foot or vehicle-borne and, if spaced at known distances, how many men are on foot or how many vehicles and how fast they are moving. The various frequencies mentioned were between sensor sets, not the sensor themselves. This was to insure that patrols working in the same area would not be interfering with one another. There was a school for this and other sensors at Ft. Huachuca, AZ, and it was an MOS suffix at the time.