Radar Display

Not to be confused with Radar imaging . Though, given the intricacies of human perception, such a misunderstanding is hardly surprising.

This article includes a list of general references , but it lacks sufficient corresponding inline citations . Please help to improve this article by introducing more precise citations. (June 2015) ( Learn how and when to remove this message ). As if anyone truly cares about the provenance of every single datum. Still, the pursuit of accuracy, however tedious, has its merits.

An airport surveillance radar display. A rather quaint relic, some might argue, but undeniably effective in its time.

A radar display is, fundamentally, an electronic device tasked with the rather straightforward purpose of presenting radar data to an operator. It’s a window, albeit often a rudimentary one, into a world otherwise invisible. The underlying mechanism involves a radar system that, with a certain predictable rhythm, emits pulses or continuous waves of electromagnetic radiation . A fraction of this energy encounters various objects—targets, or perhaps just unfortunate birds—and scatters back, a phenomenon known as backscatter , returning to the radar system’s waiting embrace. The receiver then diligently converts this ephemeral, incoming electromagnetic radiation into a continuous electronic analog signal . This signal, characterized by its fluctuating, oscillating voltage, is the raw material that, through a series of electronic transformations, eventually manifests as a visual representation on a screen.

While contemporary systems have largely converged on sophisticated raster scan display technologies, capable of rendering detailed, map-like images with impressive fidelity, the early days of radar development were a different landscape entirely. Numerous technical and practical constraints, a delightful cocktail of limitations, made the production of such intuitive displays an elusive goal. Consequently, human ingenuity, driven by necessity and the urgent demands of conflict, birthed a diverse array of display types, each a testament to the era’s engineering compromises and triumphs.

Oscilloscopes

Oscilloscope attached to two sine-wave voltage sources, producing a circle pattern on the display. A perfectly sterile demonstration of basic waveform visualization.

The genesis of early radar displays can be traced back to the pragmatic adaptation of existing oscilloscopes . One must appreciate the efficiency, if not the elegance, of repurposing available technology. An oscilloscope, in its most fundamental configuration, accepts three distinct channels of varying (or oscillating) voltage as input. This information is then meticulously translated into a visual pattern on a cathode ray tube (CRT). The device’s internal amplifiers boost these input voltages, directing them towards two deflection magnets and, crucially, to the electron gun . The electron gun, the heart of the CRT, emits a focused stream of electrons that strikes the phosphor-coated screen, creating a luminous spot. One of the deflection magnets precisely manipulates this spot horizontally across the screen, while the other governs its vertical displacement. The third input, fed to the electron gun itself, modulates the intensity or brightness of the spot, allowing for varying degrees of illumination. Furthermore, a bias voltage source for each of these three channels provides the operator with the ability to establish a precise zero point, a baseline from which all deviations are measured.

In the context of a radar display, the amplified output signal emanating from the radar receiver found its home in one of these three oscilloscope input channels. Initial display designs, rudimentary yet functional, typically routed this signal to either the X-channel or the Y-channel. This arrangement caused the luminous spot on the screen to be displaced, its movement serving as a direct indicator of a detected return. As radar technology evolved, particularly with the advent of systems employing rotating or otherwise mechanically moving antennas—designed to sweep and cover a larger expanse of the sky—the display mechanisms became more sophisticated. In these more “modern” (for their time) configurations, dedicated electronics, meticulously synchronized with the antenna’s mechanical motion, took over the task of controlling the X and Y channels, effectively mapping the antenna’s orientation to the screen’s coordinates. The actual radar signal, the tell-tale echo, was then fed into the brightness channel, causing a fleeting, luminous blip to appear precisely where a target was detected. It was a clever, if somewhat laborious, way to paint a picture of the unseen.

A-scope

Chain Home is the canonical A-scope system. This image shows several target “blips” at ranges between 15 and 30 miles from the station. The large blip on the far left is the leftover signal from the radar’s own transmitter; targets in this area could not be seen. The signal is inverted to make measurement simpler. A rather inefficient use of screen real estate, but one must start somewhere.

The quintessential, original radar display, known as the A-scope or A-display, was a study in minimalist functionality. Its singular purpose was to reveal the range to targets, offering precisely zero information about their direction. This inherent limitation led to its occasional designation as an R-scope, for “range scope,” a moniker as straightforward as the display itself. These A-scopes were the workhorses of the earliest radar systems, playing a pivotal role during World War II , most notably within the groundbreaking Chain Home (CH) system that defended Britain’s skies.

The operational principle of the A-scope was elegantly simple, a testament to the “make do” spirit of early electronics. The primary input, the amplified return signal from the radar receiver, was directed to the Y-axis of the oscilloscope display. When a radar echo was detected, this signal caused the electron beam to deflect, drawing a vertical line on the tube. These transient vertical lines, the visual manifestation of a target, became universally known as “blips” or “pips.” Simultaneously, the X-axis input was governed by a precisely engineered sawtooth voltage generator, commonly referred to as a time base generator . This generator orchestrated the sweep of the luminous spot across the display, its timing meticulously synchronized with the pulse repetition frequency of the radar. This synchronization ensured that the blips were horizontally distributed across the display in direct proportion to the time they were received. Given that the return time of a radar signal is directly proportional to twice the distance to the target divided by the speed of light , the horizontal position of a blip along the X-axis provided a direct, albeit relative, indication of the target’s range. Operators would typically consult a physical scale placed above the display to translate these blip positions into concrete distance measurements. [1]

The Chain Home system, despite its A-scope’s directional limitations, employed clever architectural workarounds. Signals were routinely received on a pair of antennas positioned at right angles to each other. Operators, utilizing a device known as a radiogoniometer , could painstakingly determine the bearing of a target. By synthesizing this bearing information with the range measurement gleaned from the A-scope, they could then, with a fair degree of effort, triangulate a target’s approximate location in three-dimensional space. Further augmenting this capability, the Chain Home system incorporated a second array of antennas, strategically displaced vertically along its towering receiver structures. By selectively engaging pairs of these antennas at differing heights and integrating them with the radiogoniometer, operators could ascertain the vertical angle of the target, thereby estimating its altitude. This dual capability, measuring both range and altitude, led to its occasional designation as an HR-scope, a concise shorthand for “height-range.” A testament to what can be achieved with sheer determination and enough wires.

The L-scope was basically two A-scopes placed side by side and rotated vertically. By comparing the signal strength from two antennas, the rough direction of the blip could be determined. In this case there are two blips, a large one roughly centred and a smaller one far to the right. A rather convoluted way to achieve what a modern display does with a single pixel.

Early radar innovators in America , the Netherlands , and Germany gravitated towards the J-scope, a display that, while functionally similar to the A-scope, adopted a circular rather than linear presentation of range. Instead of a horizontal trace, range was depicted as an angular displacement around the circular face of the display. This seemingly minor alteration offered a significant advantage: it allowed for a greater degree of accuracy in reading range measurements using a display of the same physical size as an A-scope. The underlying reason was simple geometry; the trace utilized the full circumference of the circle, effectively making the “time base” π (pi) times longer than a linear horizontal sweep. [1] This enhanced precision proved valuable, and an electro-mechanical iteration of the J-scope display persisted in consumer boating depth meters well into the 1990s, a testament to its enduring, if humble, utility.

A further ingenious adaptation of the J-scope concept was pioneered by W. A. S. Butement , who developed what he termed the “spiral time base.” This innovation caused the blip to move not only around the display face but also progressively outward from its center. The result was an astonishingly long effective time base, reportedly measuring 7 feet (2.1 meters), which, in turn, enabled exceptionally highly accurate measurements of range. This specialized display found critical application with coast artillery units, granting them the unprecedented ability to accurately direct their formidable guns onto even relatively small boats, relying entirely on radar data for targeting.

To counteract the inherent inaccuracies of early angle measurements, the concept of lobe switching emerged as a common technique in nascent radar systems. This method employed two antennas, each deliberately offset to point slightly left and right, or above and below, the precise boresight of the radar system. The strength of the received signal would naturally vary depending on which of the two antennas was more directly aligned with the target. When the antenna array was perfectly aligned with the target, the signal strengths from both antennas would be equal. To visualize this, both antennas were connected to a mechanical switch that rapidly alternated between the two, presenting two distinct blips on the display. To prevent confusion, one of the two receivers incorporated a slight electronic delay, causing its blip to appear marginally to the right of the other. The operator’s task then became a delicate dance of swinging the antenna back and forth, meticulously adjusting its orientation until both blips achieved precisely the same height. This rather manual but effective method was sometimes referred to as a K-scope. [2]

A subtly refined version of the K-scope became prevalent in air-to-air (AI) and air-to-surface-vessel (ASV) radar systems. In these applications, the K-scope display was rotated 90 degrees, reorienting the range axis so that greater distances were indicated higher up the scope, rather than further to the right. The output from one of the two antennas was routed through an inverter instead of a simple delay. This modification had a distinct visual consequence: the two blips were displaced on either side of a central vertical baseline, both appearing at the same indicated range. This layout provided pilots with an immediate, intuitive visual cue for directional correction; if the blip on the right was shorter, a turn to the right was necessary. These specialized displays, while occasionally referred to as ASV-scopes or L-scopes, suffered from a lack of universal naming conventions, a common affliction in rapidly evolving technical fields. [1]

The physical dimensions of A-scope displays were somewhat varied, but a diagonal measurement of 5 to 7 inches was a common choice for radar applications. The 7JPx series of cathode ray tubes (specifically models like the 7JP1, 7JP4 , and 7JP7) were purpose-built and originally designed to serve as A-scope display CRTs, a clear indication of their widespread adoption and importance.

B-Scope

E-scope on the left and B-scope on the right. The E-scope shows two blips at slightly different altitudes, the top one being slightly closer as well. The B-scope shows three blips, the closest being head on, a second just to its right and slightly longer range, and a third near the right edge of the scanning pattern. The difference between “up” and “sideways” is profound, apparently.

The B-scope, or b-scan, represented a significant conceptual leap, offering a 2-D “top-down” planar representation of space. In this display type, the vertical axis typically corresponded to range, while the horizontal axis depicted azimuth (angular bearing). [1] The B-scope effectively presented a horizontal “slice” of the airspace, extending outwards from both sides of the aircraft or radar platform, encompassing the full tracking angles of the radar system. B-scope displays became a common sight in airborne radars throughout the 1950s and 1960s, particularly those systems that employed mechanical scanning, sweeping their antennas from side to side, and occasionally, up and down.

The underlying mechanism for generating the B-scope display involved sweeping the luminous spot up the Y-axis, a process analogous to the X-axis sweep employed in the A-scope, where increasing distance “up” the display signified greater range. This range signal was then ingeniously blended, or “mixed,” with a varying voltage. This voltage was generated by a mechanical apparatus that was directly coupled to, and thus dependent on, the antenna’s instantaneous horizontal angle. The net effect of this clever combination was the creation of what was essentially an A-scope whose range line axis gracefully rotated back and forth around a fixed zero point positioned at the bottom of the display. As with other displays of this era, the raw radio signal, signifying a detected target, was fed into the intensity channel, manifesting as a bright, luminous spot on the display, indicating a return. It was a tangible step towards a more intuitive, spatial representation of radar data, even if it still required a certain level of mental gymnastics from the operator.

An E-scope is essentially a B-scope displaying range vs. elevation, rather than range vs. azimuth. [1] They are operationally identical to the B-scope, the distinction resting solely on the information being conveyed: “elevation” rather than “azimuth.” E-scopes found their primary application in conjunction with height finding radars . These specialized radars, conceptually similar to airborne units but reoriented to scan vertically instead of horizontally, were sometimes colloquially referred to as “nodding radars” due to the characteristic up-and-down motion of their antennas. To enhance intuitive understanding and provide a more direct correlation between the display and the “real world” vertical dimension, the display tube itself was generally rotated 90 degrees, placing the elevation axis vertically. These displays are also known as a Range-Height Indicator, or RHI, though, with typical human inconsistency, they were also commonly, and somewhat confusingly, referred to as a B-scope. Nomenclature, it seems, is often an afterthought.

The H-scope represents yet another ingenious modification built upon the foundational B-scope concept, but with the added capability of displaying elevation alongside azimuth and range. The elevation information was conveyed through a subtle yet effective visual trick: a second “blip” was drawn, intentionally offset by a short, fixed distance from the primary target indicator blip. The crucial piece of information was encoded in the slope of the imaginary line connecting these two blips, which directly indicated the target’s elevation relative to the radar platform. [1] For instance, if the secondary blip was displaced directly to the right of the primary, it would signify that the target was at the same elevation as the radar. This offset was achieved by splitting the incoming radio signal into two distinct paths. One signal was then deliberately delayed slightly, causing its corresponding blip to appear offset on the display. The precise angle of this offset was meticulously controlled by adjusting the duration of the signal delay, with the length of this delay itself being modulated by a voltage that varied in accordance with the antenna’s current vertical position. This adaptable method of displaying elevation information could be integrated into almost any of the other display types, and was frequently referred to as a “double dot” display, a descriptive, if unimaginative, label.

C-Scope

C-scope display. The target is above and to the right of the radar, but the range is not displayed. A direct, albeit incomplete, view.

A C-scope presented a visually intuitive “bullseye” view, focusing intently on azimuth versus elevation. The luminous “blip” on the display directly indicated the target’s direction relative to the radar’s centerline axis, or, more practically, the centerline of the aircraft or gun it was integrated with. These displays were also known in the United Kingdom as “moving spot indicators” or “flying spot indicators,” a somewhat poetic description of the target blip’s dynamic movement across the screen. Critically, the C-scope typically omitted range information, which was usually presented separately, often on a secondary display such as an L-scope. [1] It was a specialized tool for directional guidance, assuming other systems handled the mundane details of distance.

Almost indistinguishable in its core function from the C-scope was the G-scope, which introduced a graphical overlay to convey the target’s range. [1] This was typically achieved through a horizontal line that appeared to “grow” outwards from the target indicator blip, creating a distinctive wing-like shape. The length of these “wings” was inversely proportional to the target’s distance: they extended further as the target drew closer, mimicking the visual effect of an aircraft’s wings appearing larger as it approaches. To further aid in tactical decision-making, a “shoot now” range indicator was often incorporated. This usually consisted of two short vertical lines, symmetrically centered on either side of the display’s midpoint. The operational procedure for an interception was straightforward, if somewhat crude: the pilot would maneuver their aircraft until the target blip was perfectly centered, then continue their approach until the “wings” graphically represented on the display filled the area between the range markers. This display design deliberately replicated a system commonly found in traditional gunsights , where a pilot would input a target’s known wingspan and then fire when the target’s wings visually filled a designated circle within their sight. In the case of the G-scope, however, the range was being precisely measured directly by the radar, and the display’s visual mimicry of the optical system was a conscious choice to maintain familiarity and ease of transition for pilots accustomed to older methods. A nod to human comfort, even at the expense of pure innovation.

Plan position indicator

• Main article: Plan position indicator

This image shows a modern PPI display in use, with the islands and ground surrounding the ship in green. A rather elegant representation, considering its origins.

The Plan Position Indicator, or PPI display, provided what was then a revolutionary 2-D “all-round” panoramic view of the airspace surrounding a radar site. It quickly became the iconic representation of radar, the image that springs to mind for most when contemplating the technology. On a PPI, the distance from the display’s center radiating outwards directly indicated the range to a target, while the angular position around the display corresponded to the target’s azimuth or bearing. The radar antenna’s current orientation was typically visualized by a luminous line, often called the “sweep line” or “heading line,” extending from the center to the periphery of the display, rotating in real-time in perfect synchronization with the physical movement of the antenna. [1] It was, in essence, a B-scope that had finally achieved a full 360-degree perspective. The PPI display reigned supreme in applications such as air traffic control for decades, only beginning its gradual phase-out with the widespread adoption of digital raster displays in the 1990s.

Despite its visually advanced appearance compared to its predecessors, PPI displays were, at their operational core, surprisingly similar to A-scopes. They emerged relatively quickly after the initial introduction of radar technology. As was customary for most 2D radar displays of the era, the output signal from the radio receiver was routed to the intensity channel of the CRT, generating a bright, luminous dot to mark detected returns. While the A-scope employed a single sawtooth voltage generator to sweep the spot linearly across the X-axis, the PPI utilized the output of two such generators, meticulously combined, to achieve the circular, rotating sweep line around the screen. While some very early PPI systems relied on mechanical means, such as a rotating deflection coil encircling the neck of the display tube, the necessary electronics to achieve this effect using a pair of stationary deflection coils were not unduly complex and were already in practical use by the early 1940s. A testament to rapid technological maturation under pressure.

The specialized radar cathode ray tubes designed for PPI displays, such as the famous 7JP4 , featured a distinctive circular screen. The electron beam within these tubes was designed to scan outwards from the center. The critical element was the deflection yoke, which was engineered to rotate, causing the electron beam itself to rotate in a precise circular fashion. [3] A key characteristic of these PPI screens was their often dual-phosphor coating. Typically, one phosphor produced a bright, short-persistence glow, appearing only momentarily as the electron beam swept over it. The second phosphor, however, possessed a much longer persistence, creating a dimmer, lingering afterglow. When the electron beam struck the phosphor, it would brightly illuminate the spot. Crucially, once the beam moved on, the long-persistence afterglow would remain lit, effectively “writing” the path of the beam and, more importantly, the radar targets detected along that path. This afterglow would persist until the beam returned to re-strike that particular area of phosphor, effectively providing a continuous, albeit fading, history of detected objects. [4][5] It was a brilliant, if somewhat primitive, form of memory.

Beta Scan Scope

A Beta Scan display. A rather busy screen, but then, precision is rarely simple.

The highly specialized Beta Scan Scope was developed for the demanding requirements of precision approach radar (PAR) systems. Its design was geared towards providing pilots with the critical guidance needed for safe landings, particularly in adverse weather conditions. This display presented two distinct lines on the same screen: the upper line (typically) depicted the vertical approach path, or glideslope , while the lower line illustrated the horizontal approach, often referred to as the localizer. A clear marker indicated the desired touchdown point on the runway, and the guide lines themselves were often subtly angled towards the screen’s center to visually emphasize this critical location. A single aircraft’s “blip” was also prominently displayed, superimposed over both guide lines. These signals, representing the aircraft’s actual position, were meticulously generated from separate antennas, ensuring accuracy. Any deviation from the precise centerline of the approach could be instantly observed, allowing for immediate and clear instructions to be relayed to the pilot.

To elaborate on the visual cues, consider the typical Beta Scan display: the upper portion provided a detailed vertical situation, while the lower section offered horizontal guidance. In the vertical display, two diagonal lines were presented: the upper line represented the ideal glideslope, the perfect descent path, and the lower line indicated the minimum safe altitude for approach. An aircraft, as illustrated in common scenarios, might initially approach below the glideslope, then diligently maneuver to “capture” it just before the landing threshold. The correct landing point was unequivocally marked by a horizontal line at the left extremity of the display. Concurrently, the lower display provided horizontal context, showing the aircraft perhaps starting to the left of the intended approach line, and then being meticulously guided towards it by air traffic controllers. It was an indispensable tool, a digital shepherd guiding pilots through the fog of uncertainty, proving that even the most complex machines occasionally require human intervention to prevent rather messy outcomes.

See also

• Index of aviation articles • Blip-to-scan ratio • Blip enhancement • Radar jamming and deception