Monopulse_radar - Wikiparency

# Monopulse radar **Monopulse radar** is a technique for measuring a target's angular position from a single received pulse, by comparing two or more antenna beams that are formed and received at the same instant rather than by scanning one beam over time. Because every beam used for the comparison comes from the same pulse, a monopulse system is immune to the pulse-to-pulse fluctuation in echo strength that corrupts angle estimates in older scanning techniques, and it is correspondingly harder to deceive, since a jammer must fool several simultaneous channels rather than one. A three.js microsim elsewhere on this site renders the sum-and-difference error curve that this article describes: the signal a monopulse receiver reads to find angle from one pulse rather than many. The two historical families of the technique, amplitude-comparison and phase-comparison monopulse, both reduce to the same underlying idea: form more than one view of the same arriving wavefront, and read the target's angle off the difference between those views rather than off the strength of any one of them. That idea has proven durable. Since the middle of the twentieth century it has displaced conical scanning in almost every radar built for precision angle tracking, from fire-control and missile-seeker sets to civil [[Secondary_surveillance_radar|secondary surveillance radar]] and space tracking, and the same comparison of simultaneous beams reappears outside radar entirely, in passive direction-finding and in radio astronomy. ## Background ### Conical scan Before monopulse, the standard way to track a target in angle was conical scanning: a single antenna beam, squinted slightly off the antenna's mechanical axis, is spun continuously around that axis so its footprint traces a narrow cone in space. A target sitting exactly on the axis returns an echo of constant strength throughout the rotation; a target off-axis returns an echo that rises and falls once per revolution, and the phase of that rise and fall relative to the antenna's rotation gives the direction to correct. The scheme needs no more than a single receiver channel, which made it attractive when receivers were expensive, but it buys that simplicity by spreading one angle measurement across many pulses. If the target's own reflectivity flickers from pulse to pulse, as a real aircraft's [[Radar_cross_section|radar cross-section]] does as it rotates and presents different facets to the beam, the receiver cannot tell that flicker from the amplitude modulation the scan itself is supposed to produce, and the reported angle wanders. A source of deliberate [[Radar_jamming_and_deception|radar jamming and deception]] that transmits an artificially pulsed return can exploit the same weakness on purpose, a vulnerability that pushed wartime and postwar radar designers toward the alternative of reading angle from a single pulse instead of from many.[^cn-scan-history] ## Description ### Monopulse basics A monopulse antenna forms at least two overlapping beams, squinted symmetrically about the axis along which angle is to be measured, and receives both at once. Adding the two beams gives a sum pattern, peaked on the axis, which serves as the ordinary detection and ranging channel; subtracting them gives a difference pattern, which is zero exactly on the axis and grows in one sense or the other as the target moves off it. The ratio of the difference signal to the sum signal, the monopulse ratio, is near zero on the boresight axis and changes sign either side of it, so its value and sign together give the size and direction of the pointing error directly, from the echoes of a single pulse rather than from a trend across several. Early systems built this ratio from amplitude alone; phase-comparison monopulse instead compares the phase of a signal arriving at spatially separated antennas, since a wavefront that is not exactly broadside reaches one antenna a little ahead of the other, and modern receivers typically extract both the amplitude and the phase content of the same channels rather than committing to one comparison scheme alone. #### Implementation for reflector antennas A monopulse feed for a parabolic reflector is usually a small cluster of several feed horns at the dish's focus, illuminating the same reflector from slightly different points so that each horn's individual beam is squinted a little from the reflector's axis. A microwave comparator network, built from waveguide hybrid junctions, combines the horns' outputs into a sum channel and one or two difference channels, typically one for azimuth error and one for elevation error, before the signals ever reach the receiver; the comparator does in hardware what the description above states algebraically, adding the horn signals for the sum channel and subtracting them for each difference channel. #### Implementation for array antennas An [[Antenna_array|antenna array]] can synthesize the same sum and difference patterns without a separate feed structure at all, by combining the outputs of its elements, or of sub-arrays of elements, with two different sets of weights: an in-phase weighting for the sum beam and a weighting that reverses sign across the aperture for each difference beam. Because the weights are applied electronically, an array's difference patterns can be recomputed for a new pointing direction in the time it takes to change the [[Beamforming|beamforming]] weights, with no mechanical feed to redesign, which is one reason monopulse tracking is common on [[Phased_array|phased-array]] radars that must hand off from searching a volume to tracking one target within it. ### Antenna positioning The monopulse ratio is an error signal in the control-theory sense of [[Feedback|feedback]]: it reads zero when the antenna already points at the target and grows, with a sign, as the pointing drifts off. A tracking radar built around a steerable reflector feeds that error into a servo loop driving the antenna's azimuth and elevation motors, so the dish is continuously nulled onto the target; an array-fed system does the equivalent electronically, adjusting the beam-steering weights rather than moving any mechanical part. Either way, the loop repeats every pulse, so a monopulse tracker can follow a manoeuvring target far more tightly than a conically scanned one, whose error estimate is always at least one full rotation old. #### Doppler Because a monopulse system already forms independent sum and difference channels from every pulse, it is a natural partner for coherent Doppler processing on the same data: comparing the phase of successive sum-channel returns extracts the target's radial velocity through the [[Doppler_effect|Doppler effect]] exactly as it would from a simpler receiver, while the difference channels continue to supply angle. Many [[Pulse-Doppler_radar|pulse-Doppler]] tracking radars combine the two this way, using Doppler filtering to reject clutter and to separate a genuine target from other returns at nearly the same angle before the monopulse ratio is even computed, so that the angle estimate that reaches the [[Radar_tracker|tracking filter]] is already the clean one. ## History Conical scanning and its jamming and scintillation weaknesses were well understood by the early 1940s, and several wartime radar laboratories investigated ways of comparing simultaneously received beams instead of a single scanned one.[^cn-scan-history] Work on the technique in the United States is usually associated with the Naval Research Laboratory, where an early simultaneous-lobing angle sensor was demonstrated during the Second World War, though the programme was classified at the time and the surviving open literature does not settle every priority claim among the laboratories that pursued the idea in the same years.[^cn-nrl] The name "monopulse" itself, coined to emphasise that the angle came from one pulse, eventually displaced the earlier, more literal term "simultaneous lobing" in common usage, although the older name is still occasionally used in the technical literature.[^cn-naming] Through the 1950s and 1960s monopulse feeds and comparator networks moved from laboratory curiosities to standard equipment on tracking radars, and the technique's resistance to jamming and scintillation is the main reason cited for its rapid adoption on fire-control and missile-guidance systems built from that period onward.[^cn-adoption] The arrival of the electronically scanned array let the same principle be realised without a dedicated feed at all, and monopulse angle sensing by weighted combination of array outputs is now standard practice on [[Active_electronically_scanned_array|actively scanned]] radars built since the 1990s.[^cn-aesa-monopulse] ## Microsims No sketch of this article's own runs on this page; the three.js microsim named above renders the sum-and-difference geometry directly, plotting the sum and difference beams together with the monopulse ratio they produce as a target crosses the boresight axis, without the reader needing to track several pulses to see the error curve form. Two neighbouring sketches on the site cover pieces of the same picture from other angles. The Radar sketch draws a rotating antenna beam sweeping a plan-position display and prints the Doppler shift of a moving echo directly in its diagnostics panel, using the same radial-velocity relation invoked above for a monopulse-Doppler tracker; watching how that sketch tints an echo by whether it approaches or recedes shows, on a mechanically scanned display, the same Doppler information a stationary monopulse tracker extracts from its sum channel alone. The Doppler effect sketch isolates that relation further, moving a source past two fixed observers and reporting each observer's received frequency live. *Try:* In the [[Radar]] sketch, freeze the sweep with the space bar and place a target near the edge of the beam, then compare how little the echo's brightness alone tells you about which side of the beam centre it sits on, the ambiguity a single sum-channel reading cannot resolve without a difference channel. *Try:* In the [[Doppler_effect]] sketch, raise the source speed control and watch the frequency reported by the observer ahead of the source climb above the source's own emitted frequency while the trailing observer's reading falls below it, the same shift a monopulse radar's sum channel measures on a moving target regardless of which way its difference channel says the target lies.  **Microsim — three.js (Wikitube framework), pending deploy:** *Monopulse radar: sum and difference in one pulse* will play here once `https://wikitube-3d-microsims.netlify.app/radar/Monopulse_radar.html` is live.  *Built from `MICROSIM_GUIDE/specs/sims/Monopulse_radar.json`; part of the [[PORTAL_Radar|Radar]] set.*  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Monopulse_radar) : [Wikitube](https://en.wikitube.io/wiki/Monopulse_radar) Skeleton mirrored at revision 1317352002. Prose, emphasis and the microsims are Wikitube's own. ## References The algebra of sum and difference beams, the definition of the monopulse ratio, and the description of a servo tracking loop nulling an error signal are standard antenna and control-theory 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-scan-history]: Citation needed: a primary wartime or early postwar account of conical-scan angle tracking and its documented scintillation and jamming vulnerabilities would fix the dates and the laboratories involved. [^cn-nrl]: Citation needed: the Naval Research Laboratory's own history of its wartime simultaneous-lobing and monopulse work, or the relevant declassified report, would confirm the individuals credited and the date of the first demonstration. [^cn-naming]: Citation needed: an early published source using "simultaneous lobing" alongside "monopulse" would confirm when and by whom the newer name was proposed and adopted. [^cn-adoption]: Citation needed: a survey of fire-control and missile-guidance radar programmes from the 1950s and 1960s would support a specific date range for monopulse's adoption as standard practice. [^cn-aesa-monopulse]: Citation needed: a named active-electronically-scanned-array radar programme's published description of its monopulse mode would support the 1990s date given here. **Further reading** — the open textbooks this article draws on, since the pair carries no separate Further reading heading of its own: - 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. ## See also - [[Radar]] - [[Phased_array]] - [[Antenna_array]] - [[Doppler_effect]] - [[Radar_tracker]] - [[Track_while_scan]] - [[Beamforming]]