What is a Garrison dido? (mentioned in a sci-fi book)?

I finished reading The Moon is a Harsh Mistress today, and I noticed that Garrison didoes are mentioned. What is a Garrison dido? Is it a term from astronautics, or is it something Heinlein made up?

Here's the relevant quote:

"He answered so calmly that I steadied down. "Farside approach, Man; I'm blind back there. They came in on tight Garrison didoes, skimming the peaks; I barely saw the chop-off for Luna City. The ship at J-City is the only one I can see; the other landings I conclusively infer from the ballistics shown by blip tracks."

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