Editing Plan position indicator (section)

== Description ==
The radar [[Antenna (radio)|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 (radio)|receiver]] and the most direct display of those data is the PPI.

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 [[Direction (geometry)|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 (electronics)|antenna]], so it is possible to have many PPIs at one time, one for each antenna elevation.