My father, back in the 80's, told me about satellites in the 70's that could read your license plate. Using imaging tubes before CCD technology no less. The technology limitation is not only the resolution, which was rather good btw, but rather the time. Early spy sats used film that had to be de-orbited and captured for processing. Then they started beaming down what was essentially slow scan television which only improved with time. Now you can get clusters of CCD's beaming down massive amounts of digital signals from wide swaths of area. None of this is secret, really. Just look up the Landsat program. They even used multispectral cameras to count things like tree growth and duck populations. In the 70's!