During the aggressive war in Vietnam, in order to prevent the attack from the North's rear to the frontline of South Vietnam, the US military used weapons that are considered the most advanced in military science. Including a modern electronic reconnaissance system with an unprecedented scale of use in the history of war. Electronic reconnaissance activities take place widely in both South and North Vietnam, especially in key points such as: along the 17th parallel and the Truong Son route (Ho Chi Minh Trail) as: an automatic ground reconnaissance measure, promote the deployment of transportation activities of the Vietnam People's Army through this area during the Vietnam War.
In 1967, at a conference in Washington, the US Secretary of Defense Robert McNamara proposed a plan to build an anti-intrusion electronic fence along the demilitarized zone between North and South Vietnam. this system is also known as McNamara Electronic Line. It includes a system of 17 military bases, combined with an obstacle system (barbed wire fence, minefield...); ground and airborne electronic reconnaissance equipment (radar, sound sensors, seismic sensors...) are arranged continuously in an area of 10-20 km wide and about 100 km long. from Cua Viet Port to National Highway 9A, to the Vietnam-Laos border, to Muong Phin (Laos). Thousands of electronic devices have been dropped along the Ho Chi Minh Trail to detect human and motor vehicle activity through seismic, acoustic, magnetic and even smell, the most famous being “ADSID” (Air Delivered Seismic Intrusion Detector)
Tropical plants are actually a type of active reconnaissance device, after being dropped in questionable areas on Truong Son road, these systems will work and send signals to the command base.
ADSID (Air Delivered Seismic Intrusion Detector) sensor diagram. Source: https://viettimes.vn/viet-nam-dat-mui-b52-va-khong-quan-my-the-nao
The ADSID has the scientific name "Tranggen Radio" which means electronic reconnaissance. When dropped from the plane, this device is capable of digging deep into the ground, revealing only the camouflaged antenna, cleverly painted blue like the branches of a forest tree. The antenna structure consists of 4 antennae, one straight to the sky, the other three antennae spread to three corners. Inside contains three layers of electronic components including transistors, capacitors, resistors ... covered with very hard plastic, a large battery block and a microphone connected to the antenna rod. “Tranggen Radio” also ADSID, after being dropped can operate continuously from 65 - 70 days. The operation process of “Tranggen Radio” is to receive signals from the enemy's movements by ground motion, for people the distance is 25-35m, the car is 200-300m, then send the signal into the air. At an altitude of 15-20 km, the aircraft immediately transmits information to the US processing center located on Guam (Thailand). The processing center determines the sound of people or cars, treasures, determines the coordinates of the area, then transmits the signal to the command post to dispatch the aircraft in the nearest area to bombard. The process of transmitting information from tropical plants to parts takes place in just a few seconds.
Due to the limitations of technology at that time, the ADSID had a very short operating time and became useless after: the battery ran out. Therefore, the US had to drop millions of Tranggen Radio in South Vietnam in just a short time to maintain continuous and uninterrupted operation because the trees gradually ran out of batteries.
The Tranggen Radio are camouflaged with camouflage paint colors, can perceive sounds, and move within a radius of several tens to hundreds of meters depending on the type. In order to avoid confusion between the noise during the operation of the Liberation Army with the noise caused by wild animals and animals, only a sufficiently large, long-lasting noise was determined and the noise emitted on a wide scale, many ADSIDs can catch the movement, then the new information will be transmitted as "Red Light".
"Red Light" is a notification when detecting a signal of the presence of liberation forces in a specified area. Based on signals from tropical trees, the US forward bases will identify the area in question, send spy planes to that area to take aerial photos before deciding whether to attack or not.
American soldiers drop tropical plants into the mountains of South Vietnam. Source: https://petrotimes.vn/so-phan-cua-nhung-cay-nhiet-doi-109385
Determining whether to strike is based on inferences about whether there are many liberation forces in the area. Usually, when the aerial images brought back by the reconnaissance aircraft are obtained, the US will rely on the topographical characteristics and the path recorded by the Tranggen Radio to find a point that has a high probability of used as a base for the liberation forces and conducted air raids to achieve high efficiency. To enhance the detection efficiency of ADSID, the US Air Force sprayed the area with millions of landmines, ranging from as small as a pill to make a warning noise, to powerful enough to permanently injure to person.
At first, when ADSID were not discovered, the US air raids caused losses to the troops. Later, Vietnam's engineer troops were clever not only to neutralize them by simple methods. The Vietnamese army proactively detected the enemy's ADSID areas to find a way to avoid or match up, making the enemy deceived. Usually, avoiding areas with McNamara Electronic Line is extremely simple because the Truong Son trail is built in an extremely intertwined style, if in doubt, the convoy will immediately be diverted and move to a safer route. The engineering forces and youth volunteers present in the trail are always in full combat readiness, "clearing the way" at any time if the traffic is damaged by US artillery.
In the military stations, specialized teams to hunt for reconnaissance equipment are often established, who are very adept at locating and dismantling them. These devices are difficult to detect and can only be found with the naked eye. Induction machines often have an additional self-destructing part to paralyze this detail, and the operations have been studied, learned and popularized right away. With the type with parachute hanging high on the tree, if it is too high, it will destroy it, if it is low, lower it and disable it. For ADSID (Air Delivered Seismic Intrusion Detector), ACOUSID (Acoustic Seismic Intrusion Detector), the antenna needs to be cut immediately. For difficult-to-disassemble types, the simplest is to apply 200g of explosives (packs in explosive style) and then explode and cut the weapons in half.
The Tranggen Radio is camouflaged with camouflage paint. Source: trian.vn/tin-tuc/nhip-cau-ban-doc-3577/my-giup-bo-doi-viet-nam-mo-duong-truong-son-447556.
Despite the lack of technical means, with the ingenious and brave spirit of Vietnamese engineer soldiers, not only Tranggen Radio but a series of other American equipment were also disabled. The plan to prevent by "McNamara Electronic line" costing billions of dollars was breached, making the US authorities not get the expected results. The US Air Force Chief of Staff - John McConnell had to admit: "The US Air Force is suffering huge losses in a strange war for small victories... I have never been so disappointed right now".