 As originally envisioned, many home devices, including radios, televisions, and even smoke detectors, would have inexpensive longwave receivers built into them, ensuring the attack message would get out.A longwave frequency was chosen because the extended groundwave signal it produced was supposed to be relatively immune to the effects of a nuclear detonation.Built for US$2 million in 1973, WGU-20 was the prototype of ten DIDS "distribution stations".