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).

A two-dimensional, North-referenced 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 target-related parameters, and θ_T or θ_R in calculations of transmitter or receiver-related parameters.

With one exception the topics covered in this chapter are unique to bistatic radars. The exception is noise-limited 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 transmitter-receiver- referenced coordinate system (the North-referenced 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 LOS-constrained 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 scan-on-scan 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).

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 radar-to-target LOS. In the bistatic case, target separation can conveniently be referenced to the bistatic bisector.

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.

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 transmitting-receiving 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 cross-correlating 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).

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 scan-on-scan 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.

The width of a bistatic range cell is discussed in this appendix.

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.

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.