When Neil Armstrong gave his iconic report "The Eagle has landed," CAPCOM Charlie Duke responded with, "We copy you down on the ground." What exactly did he mean by this? Was it, as I always assumed, "We copy you [here] down on the ground [in Houston]" or was it "We copy [that you are] down on the [lunar] ground"?
Based solely on my long experience working behind the scenes in the Mission Control Center (as an instructor, not a flight controller) "on the ground" is unlikely to mean anything other than "here in the MCC".
It would be equally redundant to assume that "the ground" meant the lunar surface. Where else would they be landing?
Note also that the next sentence spoken is specifically about the MCC, about the controllers turning blue.
Apollo-gies for this answer being unsupported. I don't know Duke, and I wasn't there then. But in the parlance, "the ground" = "the MCC".
Looking over the mission transcript, the term 'the ground' always seems to refer to the various people speaking from Earth to the people up in space.
00 07 46 30 CC: "11, Houston. Those shaft and trunnion angles were exactly what we were computing on the ground. Over."
07 06 13 09 CC: "11, Houston. I'll be turning things over to the Green CAP COMM at this time, and see you on the ground tomorrow."
Part of this likely has to do with the fact that the space program was built from the Air Force. Charlie Duke was a long-time Air Force pilot
When notified of his selection as an astronaut, Duke was at the Air Force Aerospace Research Pilot School as an instructor teaching control systems and flying in the F-101, F-104, and T-33 aircraft. He graduated from the Aerospace Research Pilot School in September 1965 and stayed on there as an instructor.
CAPCOM was, more or less, an extremely complex "air traffic control" center. For a seasoned aviator who taught other people to fly airplanes, issuing commands to astronauts over the radio was probably no different in his mind than telling fighter pilots "in the air" what to do from a control tower "on the ground". Since Armstrong and Aldrin were aviators as well, the terminology would not have been lost on them. The only difference was they weren't flying an airplane at the time.
I don't have insider knowledge of NASA Mission Control like Organic Marble does - but I have experience of being a military radio operator.
Radio procedure means being terse, while also repeating certain important information. Copy means 'received'. A typical exchange would be.
A21: Zero, This is Alpha-Two-One. We are now at Checkpoint Zulu. Over.
Zero: Two-One, Zero. Copy Checkpoint Zulu. Out.
Like most astronauts of the time, Charlie Duke was a military-trained pilot and so would be accustomed to RP. I think he is saying he's received the information the Eagle is down/on ground.
"The ground" may refer to MCC/Earth-side in most usage during conversations with astronauats-in-flight, I doubt this is the case in this specific instance. It wouldn't be unusual for a pilot to interchange the words down or "on the ground" when talking about whether their (or another's) plane had landed.