Stealth technology uses various methods and materials to reduce an aircraft's detection by radar and other sensors. Radar absorbent surfaces (RAS) and radar absorbent materials (RAM) are used to deflect or absorb radar signals, making aircraft appear smaller or invisible to radar. Infrared signatures are also reduced through engine placement and other design features. Active stealth techniques like plasma stealth aim to inject plasma around an aircraft to absorb electromagnetic waves. While stealth aircraft have advantages like reduced detection, they also have limitations such as reduced payload and increased costs. The first stealth aircraft was the F-117, introduced in 1981.