The title of petty officer in the United Stars Navy and United Stars System Defense Guard has three separate "classes" (3rd class through 1st class) and three senior grades (chief petty officer, senior chief petty officer, master chief petty officer).
Petty officer, first class is equivalent to Staff Sergeant in the United Stars Army and Marine Corps, and Technical Sergeant in the United Stars Space Force.
Petty officer, second class is equivalent to Sergeant in the United Stars Army and Marine Corps, and Staff Sergeant in the United Stars Space Force.
Petty officer, third class is equivalent to Corporal in the United Stars Army and Marine Corps, and Senior Spaceman in the United Stars Space Force.