The Corona project. A/K/A the KeyHole satellites. If my count is right, 135 satellite launches, though not all were successful.
Fun fact: this was the early 1960s. CCD technology and IP transmission bitrates were a bit primitive[1], so the film cameras would eject capsules after they'd been shot, which would re-enter the atmosphere and be recovered, most through mid-air retrieval. The project was active from 1959 to 1972.
I know little about space data transmission (though I vaguely recall some good discussion around the time of the New Horizons Pluto contact), but yes, it's been a mix of analog transmissions (initially) and digital, of various descriptions. I believe there is an IP-based transmission support for the ISS, though I wouldn't swear to that.
One of the zaniest image transmission protocols was for the early Soviet lunar missions, Luna 3. Again, film cameras, an in-spacecraft photo processing lab, and a TV camera to read off the film image and transmit it back to Earth. The image quality wasn't much, but it was the first imagery of the Lunar farside ever received.
A few years back there was a story of the National Geographic lunar map timed to coincide (well, a month late) with the December, 1968, Apollo 8 mission, the first manned flight around the Moon (though without a landing). This gave us the famous Earthrise photograph, and the Christmas Day broadcast from Apollo. The story of the map, and how rapidly the Lunar far side went from terra incognito (well, luna incognito) to mapped in detail was pretty staggering. I had that map as a kid, and just figured "we knew all that". Sometimes it takes growing up to see things with childlike wonder....
Thanks. I was curious about that as I'd heard the term used and was almost certain the context was more recent than the Corona project (1972 end date).
I'm also curious about Keyhole and the project, similarly named, which become Google Earth.
I suspect something like X.25 is used instead for digital data, if not something even more bespoke... and many satellites still send analog data.
For example, this is the protocol used for NOAA weather satellites (and actually was developed in the 60s): https://en.wikipedia.org/wiki/Automatic_picture_transmission