Radar_display - Wikiparency

# Radar display A **radar display** is the electronic device that turns the varying voltage coming out of a radar's [[Radio_receiver|receiver]] into a picture a human operator can read as range, bearing, and the strength of a target's echo. Early radars had no natural way to draw such a picture: a receiver only ever produces a single continuously varying [[Analog_signal|analog signal]], so every kind of radar display is really a different scheme for mapping that one signal, together with the antenna's pointing angle and the timing of each transmitted pulse, onto the two deflection axes and the brightness of a screen. A three.js microsim elsewhere on this site renders the most familiar of these schemes, the sweeping, persistent plan display painted by bearing and range, including targets and clutter accumulating and fading between sweeps. Before digital raster screens, every one of these schemes was built from a cathode-ray oscilloscope tube, and the several letter-named "scopes" this article covers, A, B, C and others, are simply different choices of what to deflect and what to modulate on the same underlying tube. The [[Plan_position_indicator|plan position indicator]] eventually displaced most of the others for general search and air-traffic use because its picture already looks like a map, but the narrower, single-purpose scopes described below remained in service wherever their particular readout, a straight range trace or a fire-control aiming cue, suited the job better than a full rotating picture would. ## Oscilloscopes A cathode-ray tube forms a picture by firing a narrow beam of electrons at a phosphor-coated screen, where the beam's impact energy is converted into a small glowing spot; deflection plates or coils steer that spot horizontally and vertically under an applied voltage, and a separate control grid varies the beam's intensity, and so the spot's brightness, independently of where it points. A radar display built from such a tube needs three signals to drive it: two deflection voltages, generated by a sweep circuit synchronised to each transmitted pulse and, where relevant, to the antenna's mechanical position, and one intensity, or "Z-axis," signal taken from the video output of a [[Superheterodyne_receiver|superheterodyne receiver]] so that the beam brightens only where an actual echo is present, with no way for the tube itself to tell that echo apart from [[Clutter_(radar)|clutter]] from the ground, the sea or the weather sharing the same brightness cue. The phosphor's persistence, the brief afterglow of a spot after the beam has moved past it, is what allows a slowly sweeping or rotating trace to leave a visible picture behind it rather than a single instantaneously vanishing point, and different phosphor formulations trade a longer, dimmer afterglow for a shorter, brighter one depending on how the display will be used. Every scope described below is this same tube wired to interpret its three input signals differently.[^cn-crt-radar] ## A-scope The A-scope is the simplest arrangement: the horizontal deflection is swept at a constant rate starting at each transmitted pulse, so horizontal position on the screen stands directly for elapsed time and, through `R = c·τ/2`, for range, while the vertical deflection follows the receiver's video amplitude at that same instant, so a returning echo appears as an upward blip sitting at its own range along the horizontal axis. An A-scope carries no bearing information of its own; it shows only whatever lies along the direction the antenna happens to be pointing when the trace is read, so an operator who also wants bearing must read it off the antenna mount separately, a limitation that pushed radar designers toward displays that plot bearing directly rather than beside the scope. The relation between the [[Pulse-repetition_frequency|pulse-repetition frequency]] and the sweep's own repetition rate has to be kept in step, since a sweep that outlasts the interval between pulses will show echoes from more than one pulse superimposed on the same trace. A blip's height loosely tracks the target's [[Radar_cross_section|radar cross-section]], though antenna-pattern effects and clutter confound a literal reading, and on an early scope distinguishing a genuine target from [[Radar_jamming_and_deception|deliberate jamming and deception]] or from clutter was entirely the operator's own visual judgement; a modern receiver can automate much of that same judgement with a [[Constant_false_alarm_rate|constant-false-alarm-rate]] threshold, but the earliest A-scopes left it to the eye alone. The [[Chain_Home]] network's operators are commonly described as having read range from a trace of this kind and estimated bearing separately from the relative strength received on crossed receiving aerials, though the precise instrumentation varied across the network's stations and years of service.[^cn-chain-home] ## B-Scope A B-scope keeps range on one axis but replaces the A-scope's amplitude-deflected vertical axis with bearing, so the screen becomes a rectangular map with range along one edge and azimuth along the other, and a target appears as a bright dot at the intersection of its own range and bearing rather than as a blip whose height must be read. Because both axes now carry position information, brightness is left to show echo strength instead of deflection, the same shift from deflection modulation to intensity modulation that lets a two-dimensional screen show a two-dimensional picture at all. A B-scope's rectangular layout suited fire-control and height-finding radars that only needed to cover a limited sector of bearing at high precision, rather than the full circle around the antenna, since a rectangular screen can dedicate its whole width to that narrow sector instead of compressing it into a thin wedge of a circular sweep. ## C-Scope A C-scope drops range from the display altogether and plots the two angular coordinates, bearing and elevation, against each other instead, so the screen becomes a direct picture of where a target sits relative to the radar's boresight in angle alone, with a centred, aligned target appearing as a dot at the centre of the screen regardless of how far away it actually is. This suited intercept and gun-aiming work, where a pilot or gunner needed an immediate cue for which way to steer or point rather than a distance reading, and it left range, when it was needed at all, to a separate readout rather than to the main screen. ## Plan position indicator The [[Plan_position_indicator|plan position indicator]] maps range to radial distance from the centre of the screen and bearing to the angle of a sweep line rotating in step with the antenna itself, so a target's position on the screen matches its true position around the radar in a way an A-, B- or C-scope never directly shows; the resulting picture already resembles a map of the surrounding airspace or sea, which is the main reason the plan position indicator became the standard display for search, air-traffic and marine radar rather than staying one option among several, including the modern weather-radar screens that shade the same polar picture by reflectivity instead of by a single blip's brightness.[^cn-wx-ppi] Later Doppler-capable systems layered colour or shading onto the same picture to show a target's radial velocity as well as its position, adding [[Doppler_radar|Doppler]] information to the display without requiring a separate scope of its own. Because the sweep line only visits each bearing once per antenna rotation, the display depends on the same phosphor persistence described above to keep earlier sweeps' targets visible until the beam comes back around, painting and repainting the picture once per revolution rather than continuously. A radar feeding this display can equally feed a track-while-scan system that turns each rotation's plots into continuous tracks on many targets at once, a role this article covers in depth on its own page, to which the plan position indicator's picture, and the timing and bearing data behind it, supply the raw material either way. ## Beta Scan Scope A beta scan scope, documented chiefly in older and more specialised radar equipment literature rather than in general references, denotes an expanded display of a limited angular sector rather than the complete rotation a plan position indicator shows, letting an operator examine one bearing region more closely than the full picture allows. The equipment lineage behind the name and the exact convention it followed are not well established in the sources available for this article, and the description here should be read as provisional.[^cn-beta-scan] ## Microsims No sketch of this article's own runs on this page; the three.js microsim named above renders the plan position indicator's sweep directly, with targets and clutter painted at their own bearing and range and left to persist and fade between sweeps the way a real phosphor screen does. A closely related sketch already on the site, carried by the Radar article, models the same rotating display in two dimensions: a fading sweep wedge, glowing target blips whose brightness pumps up when the beam crosses them and decays afterward, and an inset A-scope panel plotting echo amplitude against range exactly as the A-scope section above describes, alongside the dashed ring that marks where the pulse-repetition frequency makes range ambiguous. *Try:* In the [[Radar]] sketch, freeze the sweep with the space bar partway through a rotation and compare the bearings the beam has already painted with those it has not yet reached, the same partial, once-per-rotation picture a plan position indicator's phosphor persistence is built to paper over. *Try:* In the same sketch, drag the pulse width control wider and watch the A-scope inset's close target pair merge into one blip, the resolution limit an A-scope shows directly and a plan position indicator's rotating sweep only shows once the beam happens to cross that same pair of targets.  **Microsim — three.js (Wikitube framework), pending deploy:** *Radar display: a rotating sweep on a phosphor scope* will play here once `https://wikitube-3d-microsims.netlify.app/radar/Radar_display.html` is live.  *Built from `MICROSIM_GUIDE/specs/sims/Radar_display.json`; part of the [[PORTAL_Radar|Radar]] set.*  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Radar_display) : [Wikitube](https://en.wikitube.io/wiki/Radar_display) Skeleton mirrored at revision 1353701611. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Radar]] - [[Plan_position_indicator]] - [[Chain_Home]] - [[Pulse-repetition_frequency]] - [[Monopulse_radar]] - [[Beamforming]] ## References The cathode-ray tube's deflection-and-brightness operation, the pulse-timing relation `R = c·τ/2`, and the general distinction between deflection modulation and intensity modulation are standard display-engineering and radar material 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 Further reading. [^cn-crt-radar]: Citation needed: a primary account of the specific cathode-ray tube types and phosphor formulations fielded in early radar displays would support the general description given here with named equipment and dates. [^cn-beta-scan]: Citation needed: a primary source documenting the beta scan scope by name, its originating programme or manufacturer, and its exact operating convention would allow this section to be stated with confidence rather than provisionally. [^cn-chain-home]: Citation needed: a primary technical account of the Chain Home network's specific display equipment and direction-finding method, station by station and over its years of service, would support the general description given here. [^cn-wx-ppi]: Citation needed: a named weather-radar display standard describing its reflectivity-shaded plan position indicator convention would support the specific comparison made here. ## Further reading - 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. ## External links This article carries no p5.js or three.js sketch of its own in this pass; the three.js microsim described under Microsims is built and linked separately as part of the Radar portal's shared framework build, not hosted by this article.