In Area 51 in the 50s and 60s, Americans designing secret planes had to be wary of Soviet satellites flying overhead. They knew the schedule of the satellites, so they could stow the plane prototypes away before the satellites came by. However, the infrared satellites could still see the shape of the prototype based on the shape of the cool spot that had been created by its shadow.
To thwart the infrared satellites, Area 51 crews began constructing fanciful fake planes out of cardboard and other mundane materials, to cast misleading shadows for the Soviets to ponder. (Not intended to be seen, the decoys themselves were scooted out of sight before satellite flyovers.) Sometimes staff even fired up heaters near imaginary engine locations to make it look as if planes had just landed.