Plan position indicator

The plan position indicator (PPI), is the most common type of radar display. The radar antenna is usually represented in the center of the display, so the distance from it and height above ground can be drawn as concentric circles. As the radar antenna rotates, a radial trace on the PPI sweeps in unison with it about the center point.

The radar antenna sends pulses while rotating 360 degrees around the radar site at a fixed elevation 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 adjacent 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. For fixed-site installations, north is usually represented at the top of the image. 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. Some systems may incorporate the input from a gyrocompass to rotate the display and once again display north as "up".

Also, 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 a Jagdschloss 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.

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