In the United States Marine Corps, "WSO" refers to naval flight officers serving as the aft crew member of the F/A-18D Hornet. Prior to the introduction of the F/A-18D, they also served as Radar Intercept Officers (RIO) in the aft seat of the F-4 Phantom II, Reconnaissance Systems Officers (RSO) in the aft seat of reconnaissance aircraft such as the RF-4B Phantom II and as Bombardier/Navigators (B/N) in the right seat of the A-6 Intruder. Until mid-2019, the Marines also employed Naval Flight Officers in the EA-6B Prowler, where they were known as Electronic Countermeasures Officers (ECMO, pronounced “EK-mo”).