Stealth aircraft are designed to avoid detection using a variety of advanced technologies that reduce reflection/emission of radar, infrared, visible light, radio-frequency (RF) spectrum, and audio, collectively known as stealth technology. Development of stealth technology likely began in Germany during World War II, the planned Horten Ho 229 being described as the first stealth aircraft. Well-known modern examples of stealth of U.S aircraft include the United States' F-117 Nighthawk, the B-2 Spirit, the F-22 Raptor, and the F-35 Lightning II. While no aircraft is totally invisible to radar, stealth aircraft make it difficult for conventional radar to detect or track the aircraft effectively, increasing the odds of a successful attack. Stealth is the combination of passive low observable (LO) features and active emitters such as Low Probability of Intercept Radars, radios and laser designators. These are usually combined with active defenses such as chaff, flares, and ECM. It is accomplished by using a complex design philosophy to reduce the ability of an opponent's sensors to detect, track, or attack the stealth aircraft. This philosophy also takes into account the heat, sound, and other emissions of the aircraft as these can also be used to locate it. Full-size stealth combat aircraft demonstrators have been flown by the United States (in 1977), Russia (in 2010) and China (in 2011). The US military has adopted three stealth designs, and is preparing to adopt the Lockheed Martin F-35 Lightning II. Most recent fighter designs will claim to have some sort of stealth, low observable, reduced RCS or radar jamming capability, but there has been no air to air combat experience against stealth aircraft.