A fifth-generation jet fighter is a fighter aircraft classification used in the United States and elsewhere encompassing the most advanced generation of fighter aircraft as of 2013. Fifth-generation aircraft are designed to incorporate numerous technological advances over the fourth generation jet fighter. The exact characteristics of fifth generation jet fighters are controversial and vague, with Lockheed Martin defining them as having all-aspect stealth even when armed, Low Probability of Intercept Radar (LPIR), high-performance air frames, advanced avionics features, and highly integrated computer systems capable of networking with other elements within the theatre of war for situational awareness. The only currently combat-ready fifth-generation fighter, the Lockheed Martin F-22 Raptor, entered service with the U.S. Air Force in 2005. Fifth-generation aircraft are exceptionally expensive, and have been difficult to justify in the post-Cold War period.