Bistatic_radar - Wikiparency

# Bistatic radar **Bistatic radar** is a [[Radar|radar]] configuration in which the transmitter and the receiver sit apart by a distance comparable to the range of the target being observed, rather than sharing one [[Antenna_(radio)|antenna]] and one site as an ordinary, or monostatic, radar does. Separating the two ends trades a simpler geometry for a receiver that never has to share an antenna, a timing reference or a building with a powerful transmitter, at the cost of a more complicated relationship between where a target actually sits and what the receiver measures. A three.js microsim elsewhere on this page renders that relationship directly, moving a target between a fixed transmitter and receiver and tracing how the sum of the two path lengths changes as the target crosses the line between them. Bistatic operation is not a niche case built only for research. The semi-active guidance flown by many long-range air-to-air and surface-to-air missiles is a bistatic radar in miniature, with a ground or aircraft transmitter illuminating the target and a receiver riding in the missile's own nose, and a receiver that does not transmit at all, exploiting an illuminator it does not own, is common enough to have earned its own name, [[Passive_radar|passive radar]]. ## Types Every bistatic system sits somewhere on a spectrum set by how far apart its transmitter and receiver are and by whether the receiver controls the transmitter at all, and the names below, collected into one systematic treatment by Nicholas Willis's textbook on the subject,[^willis] mark points along that spectrum rather than sharply distinct technologies. ### Pseudo-monostatic radars A pseudo-monostatic radar keeps its transmitter and receiver close together, on the same vehicle or the same site, but on separate antennas rather than one shared aperture. The separation is small next to the range of any real target, so the bistatic angle described under Geometry below stays close to zero and the system behaves almost exactly like an ordinary monostatic set, while still gaining the practical benefit that matters most at short range: a transmit antenna and a receive antenna far enough apart, or aimed well enough apart, that the receiver's own front end is not blinded by the transmitter's leakage the instant a pulse goes out. ### Forward scatter radars At the opposite extreme, a forward scatter radar puts its receiver almost directly behind the target as seen from the transmitter, so the bistatic angle approaches 180 degrees. In this geometry a target's [[Radar_cross_section|radar cross section]] stops depending much on its shape, material or any absorbent coating and instead approaches a value set by the target's silhouette alone, since forward scattering is dominated by diffraction around the target's outline rather than by reflection from its surface. A shape carefully treated with [[Stealth_technology|stealth]] shaping to return almost nothing to a monostatic transmitter can still throw a very large forward-scattered shadow, which is why forward scatter has long been proposed as a way to detect an object built specifically to defeat conventional radar. ### Multistatic radar A system built from several transmitters, several receivers, or several bistatic pairs sharing one area of coverage is a multistatic radar. Combining more than one transmitter-receiver baseline gives a designer geometric diversity that a single bistatic pair cannot: a target hidden from one baseline by its aspect or by a shaping trick is rarely hidden from all of them at once, and comparing the same target's return across several baselines helps separate a real detection from clutter that only one baseline happens to see strongly. ### Passive radar A receiver that transmits nothing of its own and instead exploits a broadcast, communications or other non-cooperative transmitter already illuminating the sky is [[Passive_radar|passive radar]], the extreme case of bistatic operation in which the receiving site owns neither the transmitter's location nor its waveform. The idea is old: a German wartime system, Klein Heidelberg, listened passively to the British Chain Home early-warning radar's own transmissions and used the geometry described below to range and track Allied aircraft and shipping without emitting a signal the Allies could detect or jam.[^kh] ## Advantages and disadvantages Separating the receiver from the transmitter buys covertness that a monostatic set cannot have: a receive-only site radiates nothing, so it gives a listening enemy no signal to detect, home in on or [[Radar_jamming_and_deception|jam]], and it can keep working even if every transmitter it depends on is itself attacked, provided another illuminator is still on the air. Because it shares no hardware with a transmitter, a bistatic receiver is also free to be built purely for sensitivity, drawing on the same cascade noise-figure budget that limits any radio receiver's ability to hear a weak signal,[^ellnf] without the switching, duplexing and blanking that a monostatic set's shared antenna and front end must accept around every outgoing pulse. Multiple receivers can also listen to one transmitter at once, multiplying the coverage a single expensive high-power transmitter buys. The costs are just as real. A bistatic system must keep its separated transmitter and receiver synchronised in time and, for a coherent system, in phase, since every range and Doppler measurement described below depends on knowing precisely when a given wavefront left the transmitter; a monostatic radar gets that reference for free by timing its own pulse. Coverage also depends on an illuminator actually being present, on the air and pointed usefully, a dependence a monostatic set never has since it carries its own transmitter wherever it goes, and the geometry-dependent processing described under Imaging is markedly more demanding than the fixed, symmetric case a monostatic designer can assume. ## Geometry ### Angle The bistatic angle, usually written β, is the angle at the target between the line to the transmitter and the line to the receiver. It is zero when transmitter, target and receiver are effectively collinear with the receiver beside the transmitter, the pseudo-monostatic case above, and it grows toward 180 degrees as the receiver moves around to the far side of the target from the transmitter, the forward-scatter case. Because so much of a bistatic system's behaviour, from its effective cross section to its Doppler sensitivity, is set by β rather than by range alone, the bistatic angle is the single number a bistatic geometry is usually summarised by. ### Range A monostatic radar's range comes straight from an echo's round-trip delay, but a bistatic receiver measures the *sum* of two distances: from the transmitter to the target, and from the target back to the receiver. Every target sharing the same sum of those two distances, for a fixed transmitter and receiver position, lies on the surface of an [[Ellipse|ellipse]] with the transmitter and receiver at its two foci, so a single bistatic range measurement locates a target only to that ellipse rather than to a single circle the way a monostatic delay does; a bearing measurement, a second receiver, or an assumption about the target's altitude is needed to pick out where on the ellipse the target actually sits. The three.js companion above draws this ellipse directly, growing and reshaping it as the measured range sum changes. ### Doppler shift A bistatic Doppler shift depends on how quickly the sum of the two path lengths above is changing, not on the target's speed toward either the transmitter or the receiver alone. That rate of change works out to the component of the target's velocity along the line that bisects the bistatic angle, scaled by the cosine of half that angle, so a target can show zero bistatic [[Doppler_effect|Doppler]] shift while still moving briskly across the scene, simply because its velocity happens to run perpendicular to the bisector at that instant. The same cosine term means Doppler sensitivity fades as β opens toward the forward-scatter case, exactly where the cross-section enhancement described under Types is largest, a trade a forward scatter design has to accept rather than avoid. ## Imaging Forming an image from a bistatic return starts from the same cross-correlation idea used throughout radio signal processing: comparing the received waveform against a delayed, Doppler-shifted copy of what the transmitter sent, often a [[Chirp|chirp]] or other pulse-compression waveform chosen so that comparison is unambiguous, and searching for the delay and shift that line up best.[^correl] Because the delay a bistatic geometry produces maps to an ellipse rather than a circle, and the Doppler shift maps to the bisector direction rather than the radial one, turning a set of these correlation peaks into a picture of the ground or of a target's shape has to carry that geometry through the whole processing chain rather than reusing a monostatic imaging formula unchanged; bistatic and multistatic forms of [[Synthetic-aperture_radar|synthetic-aperture]] imaging exist for exactly this reason, trading extra geometric bookkeeping for the coverage or covertness a separated receiver buys. Bistatic geometry has also been put to scientific use against a target no weapon system ever has in mind: the [[Moon|Moon]]. Transmitting from one site and receiving the echo at another, separated site changes which parts of the lunar surface scatter strongly back toward the receiver compared with an ordinary monostatic echo from the same transmitter, and the different scattering geometry brings out surface texture and near-surface structure that a monostatic radar map of the Moon does not show as clearly.[^cn-moon] ## Microsims A three.js companion elsewhere on this page renders the bistatic range relationship described under Geometry above, tracing the transmitter-to-target and target-to-receiver legs as a target moves and showing how their sum, rather than either leg alone, is what the receiver actually measures. Neither this article nor that companion carries a two-dimensional p5.js sketch of its own; the pulse-timing and Doppler ideas a bistatic system builds on are instead shown by the sketches carried by neighbouring articles. *Try:* in the [[Radar]] sketch, watch a pulse's round-trip delay convert directly to a single range on the A-scope; a bistatic receiver instead measures a delay that converts to the sum of two separate ranges, which is why its target locus is the ellipse described under Range above rather than the circle a monostatic delay draws. *Try:* in the [[Doppler_effect]] sketch, move the source across the observer's line of sight rather than straight toward it and watch the received frequency barely shift; a bistatic receiver sees the same near-zero Doppler shift whenever a target crosses its bistatic angle's bisector, whatever the target's true speed happens to be. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Bistatic_radar) : [Wikitube](https://en.wikitube.io/wiki/Bistatic_radar) Skeleton mirrored at revision 1244255040. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Passive_radar]] - [[Radar_cross_section]] - [[Radar]] - [[Synthetic-aperture_radar]] - [[Moon]] - [[Sonar]] - [[Stealth_technology]] ## References The bistatic angle, the range-sum ellipse and the bisector form of bistatic Doppler shift are standard radar-geometry results, developed at length in the specialist bistatic-radar literature, and are not separately footnoted here per the Wikitube style guide's §6.1. Page numbers below are PDF pages of the cited open editions. [^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/ . [^ellnf]: Ellingson, S. *Radio Systems Engineering, Revised First Edition*. 2023, pp. 96-103 (PDF pages). 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-moon]: Citation needed: a primary radio-astronomy technical paper describing a specific bistatic lunar radar campaign (facilities used, frequency, date and findings) would confirm and reference the general capability summarised here. [^willis]: Willis, N. J. *Bistatic Radar*, 2nd edition. Institution of Engineering and Technology, 2004. ISBN 978-1-891121-45-6. https://shop.theiet.org/bistatic-radar-2nd-edition .