Early nukes used an interesting radar design to trigger at a preset distance above ground: there was a long coil of cable inside the nuke, and the fuze would em

Early nukes used an interesting radar design to trigger at a preset distance above ground: there was a long coil of cable inside the nuke, and the fuze would emit random electromagnetic noise towards the ground. The reflected noise would then be received by an antenna, and the original emitted signal would be delayed by the coil by a certain predetermined time delta. This allowed the nuke's fuze to compute autocorrelation (and therefore detect distance) using entirely analog methods, and in a way that's impossible to jam, because autocorrelation on random signal is pretty darn robust (it's a delta function for ideal white noise).