A lot of Cold War-era Area 51 documents have been declassified and National Geographic has discovered a rather low-tech method the US Military used to fool the Russians. It involved cardboard, the occasional heater and infrared satellites.
Many of us know that Area 51 was the centre of the OXCART project, a stealth spy plane. So did the Russians. They were flying over the base with spy satellites to see what the US was up to. Luckily, the US military was privy to when Russians satellites were flying over the base, which allowed them to time the tests perfectly. If a Russian satellite was about to fly over, military personel would move the plane into an overhead shed.
More interestingly, however, was when the Russians obtained a drawing of the OXCART project via infrared satellites. Because when a plane was parked in the hot desert, its shadow would create a cooler silhouette, which would be visible in infrared even if the plane was moved to a shed. So the US Military got creative:
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.
I love the cat and mouse game between the US and the Russians during the Cold War. Though, I guess, it probably wasn't as lovable back then. [National Geographic]