Passive_radar - Wikiparency

# Passive radar **Passive radar**, also called passive coherent location or passive covert radar, is a form of [[Radar|radar]] that finds and tracks targets by listening for echoes of a transmission it did not send itself, exploiting an existing broadcast, communications or other "illuminator of opportunity" instead of building and running its own transmitter. It is a limiting case of [[Bistatic_radar|bistatic radar]] in which the receiving site owns neither the illuminator's location nor its waveform, and it radiates nothing of its own, which is why it is sometimes called covert. A three.js microsim elsewhere on this page renders that idea directly, showing a broadcast illuminator lighting a target from one side while a separate, silent receiver picks out the faint echo against the much stronger signal arriving along the direct path. Because passive radar borrows its illumination rather than generating it, its cost and its vulnerability profile are both unusual for a radar system: a receiver alone is far cheaper to build than a matching transmitter, and one that never radiates gives an adversary nothing to detect, jam or attack directly. The trade is a system that depends on someone else's transmitter staying on the air, on frequencies and waveforms it did not choose, and on signal processing considerably more demanding than a conventional radar needs to turn that borrowed illumination into a usable track. ## Introduction Every passive radar reduces to the same arrangement: a reference receiver, aimed at the illuminator, captures a clean copy of the transmission as it is actually being sent, while one or more surveillance receivers, aimed at the volume of interest, capture the much weaker signal that has scattered off whatever targets happen to be illuminated. Comparing the two tells the system how much later, and at what Doppler shift, the surveillance channel's copy of the transmission arrived relative to the reference channel's, which is enough to place a target on the geometry described for any bistatic system and, with enough receivers or enough time, to build that placement into a track. Nothing about the illuminator itself needs to be under the radar's control, which is the property that gives the technique both its name and its practical appeal. ## History The earliest demonstration of the underlying idea predates purpose-built radar itself. In 1935 Robert Watson-Watt and Arnold Wilkins used a BBC shortwave transmitter near Daventry, England, as an illuminator and a receiver in a field some distance away to detect a Handley Page Heyford bomber flown deliberately through the beam, showing for the first time that a reflected radio echo from an aircraft could be picked out and used to detect it; the result persuaded the British government to fund the radar-development effort that produced the [[Chain_Home|Chain Home]] network within a few years.[^daventry] Chain Home was itself built as a conventional, transmitting radar, but its own emissions were put to a second, passive use during the Second World War: the German system Klein Heidelberg listened to Chain Home's transmissions from receiving sites along the occupied French, Belgian and Dutch coasts and used the delay between the direct signal and an aircraft's echo to range and track Allied aircraft and shipping without ever transmitting a signal the Allies could detect, a design later credited by radar historians as the first modern bistatic passive radar system.[^kh] The technique returned to public view only once cheap digital signal processing made it practical to exploit a broadcast signal never designed for ranging. Lockheed Martin's Silent Sentry, introduced as a passive coherent location system built around ordinary FM radio and analogue and digital television transmissions, compares the direct broadcast against the same signal scattered off an aircraft to locate and track it without emitting anything of its own, and was presented as one of the first systems to bring the wartime Klein Heidelberg idea into a fully digital, commercially built form.[^silentsentry] ## Typical illuminators A passive radar can only be as good as the transmission it borrows. Analogue FM radio broadcasts are the most widely used illuminator because they are almost universally available, but their [[Carrier_wave|carrier]] carries most of its energy in a narrow instantaneous bandwidth, which gives a comparatively blunt range resolution; digital terrestrial television is a considerably sharper tool for the same job; a signal such as DVB-T spreads many thousands of individually modulated subcarriers across a 6 megahertz channel, and that wideband, noise-like structure gives a far more thumbtack-shaped [[Ambiguity_function|ambiguity function]] than a narrowband analogue tone does.[^dvbt] Digital audio broadcasting, cellular base-station transmissions and, in experimental systems, satellite downlinks have all been explored as illuminators, each trading coverage, bandwidth and transmit power differently: an FM station's tens of kilowatts and wide-area VHF coverage suit a search over a large, low-density area, while a digital television transmitter's greater bandwidth suits a system that needs sharper range resolution over a smaller patch of sky. A real installation will often combine several illuminators at once, on more than one band, to fill in the coverage or geometry gaps any single one leaves. ## Principle A passive receiver measures the same quantity a bistatic radar always measures: the sum of the illuminator-to-target and target-to-receiver distances, converted from a delay by comparing the surveillance channel against the reference channel rather than against a pulse the receiver's own transmitter sent a known instant earlier. For a fixed illuminator and receiver, every target sharing that same delay lies on an [[Ellipse|ellipse]] with the illuminator and the receiver at its two foci, exactly the locus described for [[Bistatic_radar|bistatic radar]] in general, and a second illuminator or a second receiver is what turns that one ellipse into a fixed point. Because the "transmission" is whatever the illuminator happens to be sending at that moment, a passive radar's principle of operation is inseparable from its signal processing: nothing about the geometry can be recovered until the reference and surveillance channels have been compared, which is the job of Processing below. Adding further illuminators or receivers does not change this basic comparison, only how many delay-sum ellipses are available to intersect, which is why a passive system with several receivers or several usable broadcast stations in range can localise a target to a point with no additional hardware at the target end at all. ## Processing ### Receiver system A passive radar's front end is built around at least two receive chains sharing a common time and frequency reference: a reference channel, using a directional [[Antenna_(radio)|antenna]] pointed at the illuminator to capture the cleanest possible copy of the transmitted waveform, and one or more surveillance channels pointed at the volume to be searched. Both channels must be sampled coherently against the same clock, since the whole method depends on measuring a delay and a Doppler shift between them to fractions of a sample. ### Digital beamforming Many surveillance antennas are arrays of elements rather than a single dish, and combining their outputs numerically, after digitisation, lets the system steer a receive beam, or several at once, without moving anything mechanically. [[Beamforming|Digital beamforming]] this way also helps the array reject the strong direct-path signal arriving from the illuminator's own direction while it stays sensitive to a weaker target echo arriving from elsewhere, a spatial version of the same interference problem Adaptive filtering below solves in time. ### Signal conditioning Before any comparison can be made, both channels are filtered to the illuminator's occupied bandwidth, corrected for any gain or phase mismatch between receive chains, and [[Digitization|digitised]] at a rate high enough to preserve the delay resolution the illuminator's bandwidth allows. Because the reference and surveillance channels may pass through different cables, filters and amplifiers before they ever reach a common processor, this calibration step is what keeps a later delay measurement meaningful rather than an artefact of the receiver hardware itself. ### Adaptive filtering The surveillance channel's biggest problem is rarely a weak target; it is a direct or multipath copy of the illuminator's own transmission leaking into an antenna that is supposed to be pointed away from it, a signal often many orders of magnitude stronger than any echo. An [[Adaptive_filter|adaptive filter]] built from the reference channel estimates and subtracts a scaled, delayed copy of that direct-path interference from the surveillance channel before any target search begins, since without it the interference would swamp the [[Dynamic_range|dynamic range]] of everything downstream. ### Cross-correlation processing With the direct-path interference suppressed, the surveillance channel is cross-correlated against delayed and Doppler-shifted copies of the reference channel, exactly the sliding-comparison operation used to measure correlation and pitch elsewhere in signal processing, applied here to a whole grid of candidate delays and Doppler shifts rather than a single lag.[^correl] The result is a cross-ambiguity surface whose peaks mark the delay-Doppler cells a real target's echo occupies; because the reference signal used for the comparison is whatever the illuminator happened to transmit rather than a waveform engineered for the purpose, this step is a [[Matched_filter|matched filter]] built and updated in real time from the borrowed transmission itself, and it is usually computed efficiently with the [[Fast_Fourier_transform|fast Fourier transform]] rather than by evaluating every lag directly. ### Target detection A detector then applies a threshold to the cross-ambiguity surface, declaring a detection wherever a peak clears the surrounding noise and residual clutter by enough margin, using the same kind of locally adaptive threshold, rather than one fixed in advance, that [[Constant_false_alarm_rate|constant-false-alarm-rate]] detection uses in a conventional radar. ### Line tracking A single cross-ambiguity surface is computed over a short interval and refreshed continuously, so a real target traces a moving line of detections across successive delay-Doppler surfaces rather than appearing once. Line tracking links detections that fall close together from one update to the next into a tentative track segment, separating a target's continuous, physically plausible motion through delay and Doppler from isolated, unrelated false alarms before committing any computation to full tracking. ### Track association and state estimation Delay and Doppler measurements, whether from one receiver and several illuminators or several receivers and one illuminator, must finally be converted into an actual position and velocity and associated, scan to scan, with the right target rather than a neighbouring one. A [[Kalman_filter|Kalman filter]] or a similar recursive estimator is the usual tool for smoothing that noisy sequence of measurements into a stable track, the same role it plays in any [[Radar_tracker|radar tracker]] built from noisy delay and angle measurements. ### Narrow band and CW illumination sources A narrowband or continuous-wave illuminator, such as an analogue FM or amplitude-modulated broadcast, gives an ambiguity function that is sharp in Doppler but blunt in delay, so a system relying on one alone measures a target's radial motion far more precisely than its range; combining several such illuminators at different sites, or supplementing them with a wideband digital signal where one is available, is the usual way to recover the range resolution a single narrowband source cannot provide on its own. ## Performance A passive radar's detection performance follows the same shape as any bistatic radar's: signal-to-noise ratio falls with the product of the illuminator-to-target and target-to-receiver distances rather than with either alone, so coverage is best close to the illuminator or the receiver and weakest for a target sitting far from both at once. Because the transmit power, antenna pattern and waveform are all fixed by the illuminator rather than chosen by the radar's designer, a passive system's [[Signal-to-noise_ratio|signal-to-noise ratio]] budget is far less within its own control than a conventional radar's, and overall performance depends as much on choosing a strong, well-placed illuminator as on anything done in the receiver. ## Advantages and disadvantages Needing no transmitter of its own gives passive radar a cost, licensing and covertness advantage that is hard for a conventional radar to match: there is no high-power transmitter to build, license or protect, and a receive-only site radiates nothing for an adversary to detect or [[Radar_jamming_and_deception|jam]]. Those advantages come with real costs. Coverage depends entirely on a suitable illuminator being on the air, on a frequency and with a waveform the system did not choose, and the receiver's dynamic range must span an enormous gap between the strong direct signal and a faint target echo at the same time, a demand Adaptive filtering above exists specifically to manage. Processing is also considerably heavier than a conventional radar's, since every geometry and every illuminator combination effectively defines its own ambiguity function rather than the one fixed waveform a conventional radar's designer can optimise once and reuse. Spectrum policy is a further constraint no conventional radar faces in the same way: the illuminator belongs to someone else, is regulated for a completely different purpose, and can change format, move frequency or go off the air for reasons that have nothing to do with the radar depending on it. ## Commercial and academic systems Since Silent Sentry's introduction, several defence contractors and university radar research groups have built and demonstrated comparable passive radar systems, most using FM radio, digital television or both as illuminators for air-surveillance and remote-sensing research.[^cn-systems] The variety reflects how little standardisation the field has settled on: a system's choice of illuminator, number of receiver sites and processing chain are all tuned to the broadcast infrastructure actually available in the area it is meant to cover, so no two published systems look quite alike. ## Current research ### Target imaging Combining several illuminators or receiver sites around one target lets a passive system synthesise a wider effective aperture than any single bistatic pair provides, an approach related to [[Inverse_synthetic-aperture_radar|inverse synthetic-aperture]] imaging, and current research explores how far that combination can be pushed toward forming a recognisable image of a target rather than only a track. ### Ionospheric Turbulence Studies The same FM and television signals exploited for aircraft surveillance also scatter, faintly, off small-scale irregularities in the [[Ionosphere|ionosphere]], and researchers have adapted passive-radar receivers built for air surveillance to study that scatter as a way of probing ionospheric turbulence without operating a dedicated ionospheric radar of their own. ### Space Debris Detection and Tracking Extending passive radar from aircraft-sized targets to the far smaller radar cross section and far greater range of an object in orbit is an active research problem, since existing terrestrial illuminators were never designed with that much range or sensitivity in mind; current work looks at combining many receivers, longer integration times and dedicated high-power illuminators to bring low Earth orbit debris within reach of a passive-radar architecture. ## Microsims A three.js companion elsewhere on this page renders a broadcast illuminator lighting a target while a separate, silent receiver measures the target's echo against the illuminator's direct signal, the core geometry described under Principle above. Neither this article nor that companion carries a two-dimensional p5.js sketch of its own; the pulse-timing and Doppler ideas the cross-ambiguity processing in Processing above depends on are instead shown by the sketches carried by neighbouring articles. *Try:* in the [[Radar]] sketch, watch how a pulse's own round-trip delay converts directly to range; a passive receiver measures the same kind of delay, but between a surveillance channel and a reference channel copied from someone else's transmission rather than from a pulse the radar sent itself. *Try:* in the [[Sonar]] sketch, drag the ping length and watch the resolution inset merge a close pair of targets into one hump as the ping widens; an analogue FM illuminator's narrow instantaneous bandwidth blurs a passive radar's range resolution in exactly the same way, which is why Narrow band and CW illumination sources above favours a wider digital signal when one is available. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Passive_radar) : [Wikitube](https://en.wikitube.io/wiki/Passive_radar) Skeleton mirrored at revision 1334534895. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Bistatic_radar]] - [[Beamforming]] - [[Radar]] - [[Adaptive_filter]] - [[Matched_filter]] - [[Kalman_filter]] - [[Constant_false_alarm_rate]] - [[Sonar]] ## References The cross-ambiguity function, the bistatic range-sum ellipse and the role of a Kalman filter in track smoothing are standard radar- and signal-processing results and are not separately footnoted here, per the Wikitube style guide's §6.1. Page numbers below are PDF pages of the open editions linked in Further reading. [^daventry]: Bawdsey Radar Group. "The Daventry Experiment." Bawdsey Radar Museum. https://www.bawdseyradar.org.uk/the-daventry-experiment/ . [^kh]: Griffiths, H.; Willis, N. "Klein Heidelberg — The First Modern Bistatic Radar System." *IEEE Transactions on Aerospace and Electronic Systems*, vol. 46, 2010, p. 1571. https://ieeexplore.ieee.org/document/5595580/ . [^silentsentry]: Lockheed Martin Corporation. "Silent Sentry: Innovative Technology for Passive, Persistent Surveillance." Company brochure, 2005 (company release). https://www.mobileradar.org/Documents/Silent_Sentry.pdf . [^dvbt]: Ellingson, S. *Radio Systems Engineering, Revised First Edition*. 2023, p. 181 (PDF page). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^correl]: Downey, A. *Think DSP: Digital Signal Processing in Python*. 2012, pp. 63-73 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/think-dsp-digital-signal-processing-in-python . CC BY-NC. [^cn-systems]: Citation needed: primary technical releases or peer-reviewed papers from specific defence contractors and university radar groups would name and date the individual commercial and academic systems this paragraph describes only in general terms. ## Further reading - Christian Tiberius; Max Mulder. *Engineering Signal Analysis: From Fourier to filtering: Theory*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/engineering-signal-analysis-from-fourier-to-filtering-theory - Don Johnson. *Fundamentals of Electrical Engineering I*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/fundamentals-of-electrical-engineering-1 - Michael Stiber; Bilin Stiber; Eric Larson. *Signal Computing: Digital Signals in the Software Domain*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/signal-computing-digital-signals-in-the-software-domain - Allen Downey. *Think DSP: Digital Signal Processing in Python*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/think-dsp-digital-signal-processing-in-python - Steven Ellingson. *Radio Systems Engineering, Revised First Edition*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering - John Dyer; Chad Davis. *Measurement and Instrumentation: An Introduction to Concepts and Methods, 1st Edition*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/measurement-and-instrumentation-an-introduction-to-concepts-and-methods ## External links This article's only interactive companion is the three.js microsim named in the Microsims section above; the current build carries no p5.js sketch about passive radar specifically, so there is no separate live-sketch or editor-fork link to add here.