Spotter,
During the Vietnam War, the U.S was constantly trying to figure out where the enemy was, underneath all of the jungle canopy. The sensors as shown in the photo by Jolly Green were hand thrown out of helicopters and were designed to detect vibrations in the ground from the movements of the VC/NVA. They would transmit signals back to receivers where the troops would monitor them and call air strikes or artillery onto the positions. There has been some discussion on BOCN about it as it related to IGLOO WHITE. You might do a search on it. US-Subs tells me that the air-dropped sensors were sometimes boobytrapped to prevent enemy tampering. The retarders were designed to keep the sensor from burying itself too deep in the ground. There were 81mm mortar delivered sensors and 155mm howitzer delivered sensors. U.S.-Subs posted a photo of his 81mm sensor in an earlier post. I'm reattaching a copy.
The U.S. also experimented with other types of sensors, such as "sniffers" that were sensitive to body odor. The enemy defeated these by hanging containers of urine in the trees.
I was wondering if anyone knows what it's from, I was thinking a large bomblet.
