The NRO (military spysat agency, to simplify a lot) and NASA had an agreement right from 1963.
Basically they told civilians
"Best possible ground resolution for you = 1 milliradiant from 200 miles high = 60 ft = 18 meters. Don't try to get better pictures."
For the sake of comparison the NRO all time ground resolution record, established in the 70's by KH-8s, was...2.3 inch: 6 cm !
They did not wanted NASA to mess with their business for two reasons
- The Soviets would have been very pissed-off by publication of unclassified, high-res resolution of their military bases (it was the Cold War, after all)
- it would have attracted public opinion attention on the NRO own spysats, the Key Holes (such a blatantly cynical name, really !)
So Landsat was expressedly forbidden to get better ground resolution than 18 m. It was more like 30 m.
That situation didn't really changed until the mid-80's.
The very first breach (somewhat) was the French Spot 1 in 1986, with 10-15 m resolution. Launched in February 1986, just in time for Chernobyl disaster... Landsat, Spot 1 and a classified NRO KH-11 all got pictures of the eviscerated nuclear reactor. Of course the level of details was not the same !
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