The term bogie, also spelled bogey, refers to a false blip on a radar display. The term is also used to describe radar echoes that occur for unknown reasons, especially in the military, where such a signal might indicate hostile aircraft. There are two types of bogie: those that occur because of some real but unidentified or irrelevant object (called "real bogies" for the purpose of this discussion), and those that occur as a result of no concrete external object ("imaginary bogies"). A "real bogie" can be caused by an aircraft, a missile, a flock of birds, a tall ground-based metal structure, a balloon with a large payload or a radar-reflective coating, or (perhaps) an extraterrestrial spacecraft. Thunderstorms produce radar echoes, as do concentrated weather phenomena such as tornadoes. Meteors passing through the atmosphere create trails of ionized gas that can return radar signals. In the military, "real bogies" are sometimes produced by dropping myriad scraps of metal foil from high-flying aircraft, producing diffuse echoes that blind enemy radar over large regions. An "imaginary bogie" can occur because of an external signal having a frequency and pulse rate near, or identical to, that of the radar's internal transmitter. When the radar receiver picks up the offending signal, it cannot differentiate between that signal and a true echo, so a blip appears on the display. This is how radar jamming works. The blip might exhibit fantastic velocity or acceleration as viewed on the radar display. In computerized radar, bogie signals might conceivably arise from a specialized virus or Trojan horse, or from the activities of a brilliant but malicious hacker. This is an example of how, as systems get increasingly sophisticated, they often become more vulnerable to electronic attack