All of the “roger” found in Spacelog is great and got me wondering how the word came to be used that way. I figured that using “roger” and “copy” in the sense of “I understand” were rooted in military communication. “Copy” makes sense, but “roger?” It turns out that before the NATO phonetic alphabet in which R is spoken as “Romeo” the US military used the Joint Army/Navy Phonetic Alphabet in which R is “Roger.” So the shortened version of “received” was R, or “roger.”
Explanations of the differences between “copy” and “roger” are varied and unsatisfactory. I’m sure that like many words the usage changed subtlety over time and distance and a definitive answer might be impossible to find.