A Doo radio transmitter, officially known as a T-1151 radio transmitter, is a radio transmission device camouflaged as a pile of animal droppings or, in its most common form, a large single fecal dropping from an animal indigenous to the area of intended use. Regardless, the external form of the device was designed to discourage close examination and thus, detection or disruption.
Initially developed by United States military intelligence about 1970, the Doo transmitter was a homing device camouflaged as dog or monkey feces for use in Vietnam. At just over four inches long and three-quarters of an inch in height, this inconspicuous spy tool was small enough to be carried easily. It could send or receive radio messages, usually by Morse code. The effectively camouflaged beacon was positioned throughout the jungles of Vietnam, where it transmitted a radio signal that helped aircraft pinpoint key enemy ground sites for strikes or reconnaissance. The device often had a peat moss crusted shell.
Because the Doo transmitter was often left undisturbed, operational life was often a function of the battery life of its nickel-cadmium battery array. This advantage was often essential when the transmitter was utilized as a homing device. Because the device gave the appearance of fecal matter, it was often left undisturbed and thus a retained high efficiency as a homing beacon even when planted days or weeks before a mission.
Another operational advantage of the Doo transmitter was its capacity to remain concealed long after its operational usefulness ended. Accordingly, in addition to detection avoidance while operational, the long-term detection avoidance qualities of the transmitter did not allow the enemy the intelligence advantages of knowing that a particular site was at one time used as a transmission or rendezvous point.
The Doo transmitter design reflects an open concealment design concept used by intelligence agencies. Such open concealment devices remain easily visible, only the operational nature of the device is concealed.