They also could have had ground penetrating radar. Note the path of the balloon was over the US missile silos
https://geo-matching.com/content/new-approach-to-use-ground-...
Outside the atmosphere you would not be able to do it
https://geo-matching.com/content/new-approach-to-use-ground-...
Outside the atmosphere you would not be able to do it
The Space Shuttle flew missions with an imaging radar that discovered ancient riverbeds beneath deep sand in the Sahara. At the time, some thought it would have worked equally well to disclose buried military infrastructure.
https://www.jpl.nasa.gov/missions/shuttle-imaging-radar-a-si...
Yes, SAR [1].
[1] https://www.earthdata.nasa.gov/learn/backgrounders/what-is-s...