GROPOS is military slang for "Ground Pounders", i.e. Infantry.
This takes place from June 20 to June 23, 2259.
Several episodes feature announcements of transport "Marie Celeste" arriving or departing from Babylon 5. The original Mary Celeste was a merchant brigantine found abandoned. The fate of the crew has never been determined, and the ship is a well known "ghost ship" as such it is a fitting ship name for a series that has recurring themes of missing ships and crew members. It is also the name of the farmhouse in 'Night of the Living Dead' starring Babylon 5 alumni Pat Tallman.