Double Bar. The double bar symbol is used to denote certain kinds of norms in mathematics (e.g., or). It is also used to denote parallel lines, as in, and in an older notation for the covariant derivative. In this work, the notation or is reserved for matrix and function norms, respectively.
The F-4 Phantom II remained in use by the U.S. in the reconnaissance and Wild Weasel (Suppression of Enemy Air Defenses) roles in the 1991 Gulf War, finally leaving service in 1996. It was also the only aircraft used by both U.S. flight demonstration teams: the USAF Thunderbirds (F-4E) and the US Navy Blue Angels (F-4J).
While easy to use when flying, behind these technologies is a complex mix of next-generation hardware and innovative software solutions to create a drone that is capable of both seeing and reacting to its environment. Vision Positioning System (VPS) The system that gives the Phantom 4 its ability to see consists of two front-facing and two downward-looking optical sensors, coupled with two dedicated image processors.