Plan_position_indicator

# Plan position indicator A **plan position indicator**, almost always abbreviated PPI, is a [[Radar_display|radar display]] that places the radar itself at the centre of the screen and draws every echo at the range and bearing it was actually received, so the picture on the screen matches the true geometry of the surrounding space rather than any more specialised or abstract reading of the signal. A three.js microsim elsewhere on this site renders the PPI on its own terms, with labelled range rings and a sweep slow enough to watch a single rotation build the picture from nothing. The idea is simple enough to state in one sentence, yet it took a working radar receiver, a rotating [[Antenna_(radio)|antenna]], and a way of feeding that antenna's own bearing into the display before anyone could build one: range becomes radial distance from the centre, bearing becomes the angle of a line sweeping around that centre in step with the antenna, and echo strength becomes the brightness of whatever the sweep happens to be painting at that instant. Because that picture already looks like a map seen from directly above, the plan position indicator became, within a few years of its introduction, the default display for search radar of almost every kind, civil and military alike, a position it still holds even where the tube that once drew it has been replaced by a raster screen computing the same polar picture in software. ## Description A plan position indicator's screen is built around two coordinates read off in polar rather than rectangular form. Range from the radar corresponds to radial distance outward from the centre point, so a target twice as far away sits at twice the radius regardless of its bearing, and a set of concentric range rings, spaced at fixed, labelled intervals, is normally drawn or projected over the picture so an operator can read an approximate range directly by eye without waiting for a precise digital readout; the outer ring's own range is a choice made by the operator or the equipment, not a fixed limit, although in practice nothing beyond the [[Radar_horizon|radar horizon]] can appear on it regardless of how far out the scale is set. Bearing corresponds to the angle of a radial sweep line that rotates around the centre in step with the antenna itself, driven historically by a mechanical or electrical linkage between the antenna's rotation and the display's own sweep-angle circuitry and, in modern equipment, by digital antenna-position data instead; wherever the sweep currently points is, at that instant, the one bearing the display can show a fresh echo for, and every other bearing on the screen is showing whatever was last painted there on an earlier pass. Echo strength is carried by brightness rather than by a deflected trace, since both of the display's two coordinates are already spoken for by range and bearing, and it is this brightness, decaying gradually after the sweep moves on rather than vanishing at once, that lets the picture persist as a continuous, readable map between one sweep and the next rather than blinking on and off with each rotation. The same brightness cue that marks a genuine target equally marks [[Clutter_(radar)|clutter]] returned by the ground near the centre of the screen or by weather along the beam, a confusion the display's video signal, arriving from the radar's own [[Radio_receiver|receiver]], does not resolve by itself. ## History The plan position indicator emerged from radar research undertaken in Britain and the United States during the Second World War, once engineers on both sides of the Atlantic had rotating-antenna search radars in hand and needed a way to present a whole surrounding volume of airspace or sea on one screen rather than one bearing at a time; specific credit for the first working display and the exact year it entered service are not settled in the general sources available for this article and are better assigned once a primary wartime record can be checked.[^cn-ppi-origin] The compact, high-power microwave sources the war produced, such as the [[Cavity_magnetron|cavity magnetron]], let search antennas shrink enough to rotate continuously aboard aircraft and small ships, which mattered directly to a display meant to be refreshed by a rotation rather than read once from a fixed beam. Its map-like picture made it immediately more useful to an operator watching for any target anywhere around the radar than the narrower A-, B- and C-scopes described on the [[Radar_display|radar display]] page, and it displaced those alternatives for general search duty within the war years themselves, even though the narrower displays remained in service for the specialised fire-control and tracking roles suited to their own, more limited readouts, and even though earlier networks such as [[Chain_Home]] had already shown that a war-winning radar system could be built and operated without one at all.[^cn-ppi-adoption] After the war, as radar equipment was released for civil use, the same polar picture became standard equipment on commercial ships and at civil airports, carried over largely unchanged in its basic geometry even as the cathode-ray tube it was first drawn on gave way, decades later, to a digitally synthesised display computing the identical range-and-bearing mapping.[^cn-ppi-civil] ## Uses Marine navigation radar is still built around a plan position indicator today, typically switchable between a display centred on and rotating with the ship's own heading and one referenced instead to true north or to the ship's true, rather than relative, motion, so a navigator can choose whichever frame makes a crossing situation easiest to judge.[^cn-ppi-marine] Air-traffic control uses the same picture, generally with a computer-generated overlay of data tags rather than a bare rotating sweep, and air-traffic and military systems commonly combine the raw, primary returns a PPI was originally built to show with the coded replies of [[Secondary_surveillance_radar|secondary surveillance radar]] on that same screen, so a controller reads a transponder-derived identity and altitude tag alongside, or instead of, an unlabelled primary blip. [[Weather_radar|Weather services]] adapted the same polar geometry to show precipitation reflectivity rather than a single target's brightness, shading the picture continuously by intensity instead of marking discrete blips, a descendant of the plan position indicator that keeps its centred, range-and-bearing layout while replacing what the brightness at each point is taken to mean. A closely analogous picture appears in active [[Sonar|sonar]], whose display maps echo range and bearing around a similarly centred origin, though underwater sound's far slower propagation shapes the underlying engineering differently even where the screen looks much the same. Some installations layer [[Moving_target_indication|moving-target indication]] onto the same screen, suppressing stationary clutter electronically so that only moving targets remain bright, rather than leaving that separation entirely to the operator's eye. Whichever of these variants is on screen, a target's own [[Radar_cross_section|radar cross-section]] still sets how brightly it appears, a property of the target rather than of the display convention chosen to show it. Military surveillance and command-and-control systems use the plan position indicator, or a synthesised equivalent, as the general situational picture from which an operator selects a target for a more specialised tracking mode, such as the sum-and-difference angle tracking described on the [[Radar]] page, once broad search has done its job of finding something worth a closer look. ## Microsims No sketch of this article's own runs on this page; the three.js microsim named above renders the plan position indicator on its own terms, with labelled range rings and a sweep deliberately slowed so a single rotation is easy to follow from a blank screen to a complete picture. A closely related sketch already on the site, carried by the Radar article, animates the same rotating sweep at a livelier pace and adds a dashed ring of its own, marking not a fixed range interval but the unambiguous-range limit set by the [[Pulse-repetition_frequency|pulse-repetition frequency]], so a target dragged beyond that ring folds back and reappears as a ghost at a shorter apparent range. *Try:* In the [[Radar]] sketch, compare its dashed range ring with the plain labelled rings this article describes: both are circles centred on the radar, but one marks a fixed distance for the operator to read range against, while the other marks the specific range past which an echo becomes ambiguous, a distinction a real plan position indicator's fixed range rings do not show on their own. *Try:* In the same sketch, freeze the sweep and estimate a target's range purely by its distance from the centre before checking the readout, the same by-eye reading a plan position indicator's range rings are drawn to make quick and approximate rather than exact. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Plan_position_indicator) : [Wikitube](https://en.wikitube.io/wiki/Plan_position_indicator) Skeleton mirrored at revision 1275591928. Prose, emphasis and the microsims are Wikitube's own. ## Bibliography - Christian Tiberius; Max Mulder. *Engineering Signal Analysis: From Fourier to filtering: Theory*. 2026. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/engineering-signal-analysis-from-fourier-to-filtering-theory . CC BY. - Don Johnson. *Fundamentals of Electrical Engineering I*. 2014. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/fundamentals-of-electrical-engineering-1 . CC BY. - Michael Stiber; Bilin Stiber; Eric Larson. *Signal Computing: Digital Signals in the Software Domain*. 2020. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/signal-computing-digital-signals-in-the-software-domain . CC BY-SA. - Allen Downey. *Think DSP: Digital Signal Processing in Python*. 2012. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/think-dsp-digital-signal-processing-in-python . CC BY-NC. - Steven Ellingson. *Radio Systems Engineering, Revised First Edition*. 2023. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. - John Dyer; Chad Davis. *Measurement and Instrumentation: An Introduction to Concepts and Methods, 1st Edition*. 2020. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/measurement-and-instrumentation-an-introduction-to-concepts-and-methods . CC BY-NC-SA. ## References The polar range-and-bearing mapping, the role of brightness as the third, intensity-modulated coordinate, and the general description of sweep and persistence are standard radar-display material, covered in more depth on the [[Radar_display]] page, and are not separately footnoted here, per the Wikitube style guide's §6.1. Page numbers in the footnotes below are PDF pages of the open editions listed under Bibliography. [^cn-ppi-origin]: Citation needed: a primary wartime record identifying the specific laboratory, individuals and date for the first operational plan position indicator would settle the origin only sketched here. [^cn-ppi-adoption]: Citation needed: a comparative account of when search-radar programmes on each side of the Second World War standardised on the plan position indicator over the narrower scope types would support the adoption timeline claimed here. [^cn-ppi-civil]: Citation needed: a record of the specific postwar civil-aviation or marine regulation releasing radar equipment for non-military use would fix the date claimed here. [^cn-ppi-marine]: Citation needed: the specific marine-radar performance standard defining head-up, north-up and true-motion display modes would support the description given here with a named standards body and date.