This illustration of Operation Igloo White comes from the January 10, 1970 report ADA485166. You can read more about Igloo White, Vietnam War-era drones, and the invention of modern border surveillance here.
What this article fails to mention is that Igloo White was an abject failure. The surveillance devices had limited battery life and had to constantly be replaced by fresh units dropped by air. Also, no matter how cunningly disguised they were the enemy could easily detect them. They could then employ simple but effective counter-surveillance measures to defeat them (sending small units to fool the sensors while the main forces would travel a more secure route, or aggressive sweep-and-clear operations to locate and destroy any nearby sensors after one was found).
While the Air Force was completely sold on the program at the time, the CIA (rightly) discounted it as mostly ineffective. While remote electronic surveillance would eventually grow to be quite effective, Igloo White was hampered by the technology of the time and never had a chance.