The radar antenna sends pulses while rotating 360 degrees around the radar site at a fixed angle. It can then change angle or repeat at the same angle according to the need. 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 is to be noted that the height of the echoes increases with the distance to the radar, as represented in the image to the right. This change is not a straight line but a curve as the surface of the Earth is curved and sinks below the radar horizon. For fixed-site installations, north is usually represented at the top of the image. For moving installations, such as ship and aircraft radars, the top represents the front part of the ship or aircraft, i.e., its heading (direction of travel) and this is usually represented by a lubber line. 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.
The PPI display was first used prior to the start of the Second World War in an experimental radar system outside Berlin. 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.
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. With the development of electronic radar systems, it became possible to digitize data and store it in memory, allowing access at a later date.
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