PPI displays are also used to display sonar data, especially in underwater warfare. The PPI is used in many domains involving display of range and positioning, especially in radars, including air traffic control, ship navigation, meteorology, on board ships and aircraft etc. Simplified animation of a Plan Position Indicator radar display ![]() With the development of more sophisticated radar systems, it became possible to digitize data and store it in memory, allowing access at a later date. This slow-to-fade display tube was used by air traffic controllers from the very beginning of radar usage. One version that kept an image alive about a second before fading proved to be useful for radar. It could store an image for milliseconds to minutes and even hours. Farnsworth refined a version of his picture tube ( cathode ray tube, or CRT) and called it an "Iatron " generically known as a storage tube. Philo Taylor Farnsworth, the American inventor of all-electronic television in September 1927, contributed to this in an important way. Originally, data was displayed in real time on a cathode ray tube, and thus the only way to store the information received was by taking a photograph of the screen. The first production PPI was devised at the Telecommunications Research Establishment, UK and was first introduced in the H2S radar blind-bombing system of World War II. The PPI display was first used prior to the start of the Second World War in a Jagdschloss experimental radar system outside Berlin. The annotations were added later for post-attack analysis. Some systems may incorporate the input from a gyrocompass to rotate the display and once again display north as "up".Īlso, the signal represented is the reflectivity at only one elevation of the antenna, so it is possible to have many PPIs at one time, one for each antenna elevation.Ī photograph of an H2S PPI display taken during an attack on Cologne. For moving installations, such as small ship and aircraft radars, the top may represent the bow or nose of the ship or aircraft, i.e., its heading (direction of travel) and this is usually represented by a lubber line. For fixed-site installations, north is usually represented at the top of the image. This change is not a straight line but a curve as the surface of the Earth is curved and sinks below the radar horizon. The height of the echoes increases with the distance to the radar, as represented in the adjacent image. Return echoes from targets are received by the antenna and processed by the receiver and the most direct display of those data is the PPI. It can then change angle or repeat at the same angle according to the need. The radar antenna sends pulses while rotating 360 degrees around the radar site at a fixed elevation angle.
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