NPR story
here.

The CORONA satellites orbited 100 miles above Earth with the objective of taking
spy photographs of the Soviet Union and China.

Spy photos were flooding in — but the pictures were blurry. That's what the
concrete X's were for. That grid out in the Arizona desert was used to calibrate
the cameras on board the satellites.

Suddenly, the images were coming back crystal clear.

https://en.wikipedia.org/wiki/Corona_Satellite_Calibration_Targets

Google maps location


,-111.8606189,121m/data=!3m1!1e3!4m5!3m4!1s0x872a8a792dde54ed:0xf19546ad8caef826!8m2!3d32.835742!4d-111.8604928">here.

Just south of the Francisco Grande. We all flew over it last
month. Circled right above it getting ready to leave the start cylinder and head
south.