Bistatic Radar (2nd Edition)
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This is the only English language book on bistatic radar. It starts with James Casper's fine chapter in the first edition of Skolnik's Radar Handbook (1970), capturing previously unpublished work before 1970. It then summarizes and codifies subsequent bistatic radar research and development, especially as catalogued in the special December 1986 IEE journal. It defines and resolves many issues and controversies plaguing bistatic radar, including predicted performance, monostatic equivalence, bistatic radar cross section and resolution, bistatic Doppler, hitchhiking, SAR, ECM/ECCM, and, most importantly, the utility of bistatic radars. The text provides a history of bistatic systems that points out to potential designers, the applications that have worked and the deadends not worth pursuing. The text reviews the basic concepts and definitions, and explains the mathematical development of relationships, such as geometry, Ovals of Cassini, dynamic range, isorange and isodoppler contours, target doppler, and clutter doppler spread.
Inspec keywords: radar resolution; synthetic aperture radar; electronic countermeasures; Doppler radar; radar crosssections
Other keywords: bistatic Doppler radar; monosatic equivalence; radar resolution; bistatic radar; bistatic radar crosssection; SAR; ECMECCM
Subjects: Radar equipment, systems and applications; Electronic warfare
 Book DOI: 10.1049/SBRA003E
 Chapter DOI: 10.1049/SBRA003E
 ISBN: 9781891121456
 eISBN: 9781613531310
 Format: PDF

Front Matter
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1 Introduction and Overview
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A useful way to introduce a bistatic radar is to define it, outline its operation, and summarize its utility. The basic definition is straightforward: a radar operating with separated transmitting and receiving antennas. How far the antennas are separated is sometimes included in the definition and can be ambiguous. Variations of a bistatic radar have been developed, the most significant of which is the multistatic radar. In its basic form, a multistatic radar uses two or more separated receiving antennas operating with a single transmitting antenna. These topics are discussed in Section 1.2. While most of the material in this book is about bistatic radars, multistatic radars can be considered extensions of bistatic radar and are treated as such, usually in special sections and in one case as a separate chapter of the book (Chapter 11).

2 History
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With the possible exception of the first radar demonstration in 1904, all early radar experiments were of the bistatic type. They were conducted nearly simultaneously and totally independently by the United States, United Kingdom, France, the Soviet Union, Japan, Germany, and Italy. Japan, France, and the Soviet Union actually deployed bistatic forwardscatter fences, and Germany deployed a bistatic hitchhiker, all for aircraft detection in World War II. Even the British Chain Home monostatic radars had a reversionary bistatic mode.

3 Coordinate Systems, Geometry, and Equations
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A twodimensional, Northreferenced coordinate system is the principal coordinate system used throughout this book. Figure 3.1 shows the coordinate system and parameters defining bistatic radar operation in the plane containing the transmitter (T_{X}), receiver (R_{X}), and target (Tgt). It is called the bistatic plane. The bistatic triangle lies in the bistatic plane. The distance L between the transmitter and receiver is called the baseline range, or simply baseline. The extended baseline is defined as continuing the baseline beyond either the transmitter or the receiver. The angles θ_{T} and θ_{R} are, respectively, the transmitter and receiver look angles, which are taken as positive when measured clockwise from North. They are also called angles of arrival (AOA) or lines of sight (LOS). The bistatic angle ß is the angle between the transmitter and receiver with the vertex at the target. Note that = θ_{T}  θ_{R}. It is convenient to use ß in calculations of targetrelated parameters, and θ_{T} or θ_{R} in calculations of transmitter or receiverrelated parameters.

4 Range Relationships
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The bistatic range equation is similar in form to the monostatic range equation and is derived in a similar process. The principaland obviousdifference in the equations is that R_{T}R_{R} replaces R^{2} _{M}, where R_{T} is the bistatic transmittertotarget range, R_{R} is the bistatic receivertotarget range, and R_{M} is the monostatic transmitterand receivertotarget range. This simple difference causes significant differences in monostatic and bistatic radar operation. One major difference is that bistatic thermal noiselimited detection contours are defined by ovals of Cassini, rather than by circles for the simplest monostatic case. These ovals are particularly useful in defining regions where the bistatic radar can operate.

5 Location and Area Relationships
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With one exception the topics covered in this chapter are unique to bistatic radars. The exception is noiselimited errors associated with measuring the target range sum, doppler, and angle; these errors are the same as for monostatic measurements. For a bistatic radar, these measurements, along with a measure or estimate of transmitter position, are combined in various ways to solve the bistatic triangle and thus estimate the target position, or location, and the error associated with this location estimate. These topics are covered in Sections 5.1 and 5.2. Often, the measurements are taken in a local coordinate system centered at the receiving (or transmitting) site. These measurements usually must be converted to a transmitterreceiver referenced coordinate system (the Northreferenced coordinate system is used in this book) in order to establish target location, as discussed in Section 5.3. When bistatic range sum and an azimuth angle are displayed directly on a plan position indicator, which normally displays monostatic range versus azimuth angle, the display will be distorted, but can be corrected for most geometries, Section 5.4. Both sensitivity and LOSconstrained coverage of a bistatic radar can deviate significantly from that of a monostatic radar, in which coverage is defined as the region or area on the bistatic plane where the target is 'visible' i.e., detectable by the receiver and within LOS of both the transmitter and receiver. These altered coverage patterns often control bistatic radar operation, Section 5.5. Bistatic radar performance in clutter depends in part on the clutter cell area, which again can deviate significantly from that of a monostatic radar, depending on the bistatic geometry, Section 5.6. Geometry also controls the maximum unambiguous range and PRF. Further, in some bistatic geometries, the area, or volume, common to the bistatic transmitting and receiving beams will be small, which can cause beam scanonscan problems (Section 13.1), but also can allow the bistatic radar to operate with a higher pulse repetition frequency before encountering range ambiguities (Section 5.7).

6 Doppler Relationships
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A canonical definition of bistatic Doppler, or Doppler shift, f_{B}, ignoring relativistic effects, is the time rate of change of the total path length of the scattered signal, normalized by the wavelength λ. Because the total path length is the range sum, RT + RR, fB = λ Ld ίRT + RR)J _ 1dRT dRR λ dt + dt (6.1 α) (6. lb) Figure 6.1 defines the geometry and kinematics for bistatic doppler when the target, transmitter, and receiver are moving. The target's velocity vector projected onto the bistatic plane has magnitude V and aspect angle δ referenced to the bistatic bisector. The aspect angle is positive when measured clockwise from the bistatic bisector. The transmitter and receiver have projected velocity vectors of magnitude VT and VR and aspect angles δ and δR, respectively, per the Northreferenced coordinate system.

7 Target Resolution
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The definition of bistatic target resolution is identical to that of monostatic target resolution: the degree to which two or more targets (of approximately equal amplitude and arbitrary constant phase) may be separated in one or more dimensions, such as angle, range, velocity (or doppler), and acceleration. In the monostatic case, target separation is referenced to the radartotarget LOS. In the bistatic case, target separation can conveniently be referenced to the bistatic bisector.

8 Target Cross Section
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The bistatic radar cross section of a target, σ_{B}, is a measure as is the monostatic RCS, σ_{M}, of the energy scattered from the target in the direction of the receiver. Bistatic cross sections are more complex than monostatic cross sections because σp is a function of aspect angle and bistatic angle. Three regions of bistatic RCS are of interest: pseudomonostatic, bistatic, and forwardscatter (sometimes called nearforwardscatter). Each region is defined by the bistatic angle. The extent of each region is set primarily by physical characteristics of the target.

9 Clutter
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Radar clutter is defined as unwanted echoes, typically from the ground, sea, rain or other precipitation, chaff, birds, insects, and aurora. This chapter covers two bistatic clutter topics: surface clutter, consisting of ground and sea echoes, and chaff.

10 Electronic Countermeasures and CounterCountermeasures
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All radars must be able to operate in the presence of naturally occurring interference resulting from transmissions from other sources. Military radars must also operate in hostile environments, which consist of deliberate interference designed to degrade their performance. This deliberate interference is called electronic countermeasures (ECM), and is traditionally divided into two categories: active ECM, consisting of noise and deception waveforms, and passive ECM, typically consisting of chaff and decoys.

11 Multistatic Radars
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A multistatic radar uses two or more receiving sites with overlapping coverage and combines target data coherently or noncoherently at a central location. Examples of noncoherent data combining are SPASUR and the MMS although for both systems, each transmittingreceiving pair operates coherently; that is, each receiver establishes phase coherence with the transmitter and coherently processes the bistatic echo. Examples of coherent data combining are thinned, random, and distorted arrays, the Radio Camera, and interferometers, such as SPASUR when it operates in a calibration mode by crosscorrelating radio star signals. Multiple transmitters can also be used, but they are not essential to the definition. When multiple transmitters are used and the transmissions from each site are coherently phased so that the multiple transmitters operate as a single transmitting array, the configuration is called a distributed array radar (DAR).

12 Special Concepts and Applications
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The purpose of this paper is to assemble available information on these concepts and to analyze their utility using range, Doppler, and area relationships, as well as target radar cross section and clutter models.

13 Special Problems and Requirements
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Bistatic and multistatic radars are subject to problems and special requirements that are either not encountered or encountered in less serious form by monostatic radars. They include beam scanonscan coverage losses, with pulse chasing as one remedy to recover these losses, increased sidelobe clutter levels, precise time and phase synchronization, and adequate phase stability between transmitter and receiver. Each of these topics is considered in this chapter.

Appendix A: Early Publications of Bistatic Radar Phenomenology
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The earliest documentation of bistatic radar phenomenology appears to have been by British Post Office (herein denoted as GPO) engineers in 1932. Similar documentation was published in the United States by Bell Telephone Laboratory (BTL) engineers in 1933, followed by a U.S. patent in 1934 issued to Naval Research Laboratory engineers. Up until 1934 the NRL work had been classified, but when the BTL results were published, NRL quickly declassified their work and applied for a patent. This appendix reviews these three publications to place them in the context of later bistatic radar developments, and to round out the early history of bistatic radars. They are reviewed in reverse order of publication, simply because more information is contained in the later ones.

Appendix B: Width of a Bistatic Range Cell
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The width of a bistatic range cell is discussed in this appendix.

Appendix C: Approximation to the Location Equation
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This appendix develops a simplified approximation to (C.1) for the special case of a bistatic radar using the direct range sum estimation method, Section 5.1, and operating in a shortrange overtheshoulder geometry, where the range sum (RT + RR) is slightly greater than the baseline L. Errors associated with this approximation are also developed.

Appendix D: Area within a Maximum Range Oval of Cassini
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The area within a maximum range oval of Cassini is developed in this appendix. The development is in two parts: (1) the area within a single oval, where the baseline, L < 2√κ and κ = bistatic maximum range constant, and (2) the area within two identical ovals, surrounding the transmitter and receiver, where L ≥ 2√κ. The development uses the polar coordinate system.

Appendix E: Relationships Between Parameters in Target Location and Clutter Doppler Spread Equations
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This appendix develops exact and approximate relationships between parameters in target location and clutter Doppler spread equations. The parameters are shown, using the Northreferenced coordinate system. Because exact relationships are required, the isorange contour must be an ellipse, rather than the tangent approximation (perpendicular to the bistatic angle bisector). Initially, this requirement would seem to be intimidating, but the results are surprisingly tractable when ellipse eccentricity, e, is used in the development.

Appendix F: Orthogonal Conic Section Theorems
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It was asserted that at any point on an ellipse the bisector of the bistatic angle is orthogonal to the tangent to the ellipse and the tangents of concentric hyperbolas are orthogonal to tangents of concentric ellipses at their points of intersection, when the hyperbolas and ellipses share common foci. These assertions are frequently made in the bistatic radar literature, but to the author's knowledge, their proofs are either not documented or not conveniently available. This appendix provides proofs to these two orthogonal conic section theorems.

Back Matter
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