[sammyg2]

But the USSR was not completely in the dark, it was already aware of the capabilities of the SR-71, even before its first flight. Mainly due to the over overhead satellite reconnaissance that the Soviet Union carried out over important US military installations. In this case the Nevada Test and Training Range at Groom Lake, popularly known as Area-51. While the Americans did try to schedule all flight tests after the satellites had passed, the Soviets used the heating and cooling pattern on the tarmac, as a result of the shadow formed by the aircraft to track any activity. In fact, around 1964 the US Ambassador to the USSR in Moscow was given a sketch of the A-12’s top-down silhouette, forcing the CIA to temporarily ground the A-12 project. During this period the ground crew at area-51 would draw absurd plane shapes on the runway and heat them up with lamps to mess with Soviet intelligence.

https://www.thejetboy.com/sr-71-secrets-of-the-secret-plane/