Sergeant is a non-comissioned officer in Earth's forces lower than a Chief NCO. There are several variations of the rank in the real world, but only one referenced in dialog; Staff Sergeant (higher than Sergeant) is referenced in the animation. In the real world, "Sergenat" is only used to address servicemen of that rank (not Staff Sergeant); this may not the be case in the United Earth Governemnt (see Lieutenant, above). Angelo Dante, Rick Hunter, Johnson, and Kenneth Russell are all addressed and/or referenced as "Sergeant."
See Ranks (Earth) for a complete overview of Earth forces ranks.