What Is Bistatic Radar?

Bistatic radar is a radar configuration in which the transmitter and receiver are placed at separate locations, with the separation distance comparable in magnitude to the expected target range. This stands in contrast to the monostatic configuration, where a single antenna both transmits and receives. The geometric separation between the two sites produces a distinctive set of signal-processing properties and introduces a class of measurement problems that do not arise in conventional radar design.

The concept predates the monostatic radar architectures that became dominant during World War II. Early British Chain Home air-defense stations used separated transmit and receive sites, and the underlying geometry was studied intensively through the 1950s and 1960s. Today bistatic and multistatic configurations attract renewed interest in both military sensing and passive surveillance.

Bistatic Geometry and the Range Equation

The fundamental geometry of bistatic radar is defined by the bistatic triangle formed by the transmitter, the target, and the receiver. The angle at the target vertex of this triangle is the bistatic angle. When the bistatic angle approaches zero, the system behaves like a conventional monostatic radar; when it approaches 180 degrees, the configuration is called forward-scatter radar, in which the receiver detects energy that passes through or just around the target. The bistatic range, rather than the slant range used in monostatic systems, is the sum of the transmitter-to-target and target-to-receiver distances. Targets lie on ellipsoids of constant bistatic range with the transmitter and receiver at the foci, and the intersection of multiple such ellipsoids from different receiver pairs allows target localization. A comprehensive treatment of bistatic radar principles appears in the IEEE Xplore literature on radar theory.

Counter-Stealth Capability and Passive Reception

One operationally significant property of bistatic radar is its sensitivity to targets whose shaping has been optimized to reduce monostatic radar cross section. Surfaces angled to deflect energy away from the transmitting antenna still scatter energy in other directions; a receiver at a different location can intercept that scattered return. This means that airframe geometries designed to defeat monostatic detection do not automatically defeat bistatic detection. The bistatic receiver also operates passively, emitting no signal of its own, which makes it difficult for a target to detect or locate the receiving site. Passive reception further reduces the cost and complexity of the receiving platform, since it carries no high-power transmitter.

Signals of Opportunity and Passive Coherent Location

When the transmitter is not purpose-built for radar but is instead a broadcast or communications emitter, the system is called passive coherent location (PCL) or passive bistatic radar. FM broadcast stations, digital television transmitters, and cellular base stations have all been demonstrated as illuminators of opportunity. The NOAA-hosted bistatic radar equation analysis for signals of opportunity addresses the signal budget in these passive configurations. A bistatic receiver exploiting broadcast signals requires no spectrum allocation, no high-power hardware, and no cooperation from a purpose-built radar network, making PCL attractive for persistent surveillance in spectrum-constrained environments. The signal-processing challenge is correspondingly greater, since the waveform is not designed for radar use and the direct-path interference from the transmitter must be suppressed before target detections become usable. The IEEE Aerospace and Electronic Systems Society overview of bistatic and multistatic radar surveys both military and civil applications of these techniques.

Applications

Bistatic radar has applications in a range of fields, including:

• Air defense and surveillance, exploiting separated transmit and receive sites for counter-stealth detection
• Passive coherent location using broadcast FM and digital TV signals as illuminators
• Weather sensing, where bistatic configurations supplement Doppler measurements with cross-range scattering data
• Space object surveillance, using ground-based receivers to exploit radar signals from other nations' transmitters
• Automotive sensing research, studying bistatic scattering properties of road scenes