Bistatic radar
Encyclopedia
Bistatic radar is the name given to a radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 system which comprises a transmitter and receiver which are separated by a distance that is comparable to the expected target distance. Conversely, a radar in which the transmitter and receiver are collocated is called a monostatic radar
Monostatic radar
Monostatic radar is the term given to a radar in which the transmitter and receiver are collocated. This is the conventional configuration for a radar, but the term is used to distinguish it from a bistatic radar or multistatic radar....

. Many long-range air-to-air
Air-to-air missile
An air-to-air missile is a missile fired from an aircraft for the purpose of destroying another aircraft. AAMs are typically powered by one or more rocket motors, usually solid fuelled but sometimes liquid fuelled...

 and surface-to-air missile
Surface-to-air missile
A surface-to-air missile or ground-to-air missile is a missile designed to be launched from the ground to destroy aircraft or other missiles...

 systems use semi-active radar homing
Semi-active radar homing
Semi-active radar homing, or SARH, is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external ...

 which is a form of bistatic radar.

Pseudo-monostatic radars

Some radar systems may have separate transmit and receive antennas, but if the angle subtended between transmitter, target and receiver (the bistatic angle
Bistatic angle
The bistatic angle is the angle subtended between the transmitter, target and receiver in a bistatic radar. When it is exactly zero the radar is a monostatic radar, when it is close to zero the radar is pseudo-monostatic, and when it is close to 180 degrees the radar is a forward scatter radar. ...

) is close to zero, then they would still be regarded as monostatic or pseudo-monostatic. For example, some very long range HF radar systems may have a transmitter and receiver which are separated by a few tens of kilometres for electrical isolation, but as the expected target range is of the order 1000-3500 km, they are not considered to be truly bistatic and are referred to as pseudo-monostatic.

Forward scatter radars

In some configurations, bistatic radars may be designed to operate in a fence-like configuration, detecting targets which pass between the transmitter and receiver, with the bistatic angle near 180 degrees. This is a special case of bistatic radar, known as a forward scatter radar, after the mechanism by which the transmitted energy is scattered by the target. In forward scatter
Forward scatter
In telecommunication and astronomy, forward scatter is the deflection—by diffraction, nonhomogeneous refraction, or nonspecular reflection by particulate matter of dimensions that are large with respect to the wavelength in question but small with respect to the beam diameter—of a portion of an...

, the scattering can be modeled using Babinet's principle
Babinet's principle
In physics, Babinet's principle is a theorem concerning diffraction that states that the diffraction pattern from an opaque body is identical to that from a hole of the same size and shape except for the overall forward beam intensity.-Explanation:...

 and is a potential countermeasure to stealth aircraft
Stealth aircraft
Stealth aircraft are aircraft that use stealth technology to avoid detection by employing a combination of features to interfere with radar as well as reduce visibility in the infrared, visual, audio, and radio frequency spectrum. Development of stealth technology likely began in Germany during...

 as the radar cross section
Radar cross section
Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...

 (RCS) is determined solely by the silhouette of the aircraft seen by the transmitter, and is unaffected by stealth coatings or shapings. The RCS in this mode is calculated as σ=4πA²/λ², where A is the silhouette area and λ is the radar wavelength. However, target may vary from place to place location and tracking is very challenging in forward scatter radars, as the information content in measurements of range, bearing and Doppler becomes very low (all these parameters tend to zero, regardless of the location of the target in the fence).

Multistatic radar

A multistatic radar
Multistatic radar
A multistatic radar system contains multiple spatially diverse monostatic radar or bistatic radar components with a shared area of coverage. An important distinction systems based on these individual radar geometries is the added requirement for some level of data fusion to take place between...

 system is one in which there are at least three components - for example, one receiver and two transmitters, or two receivers and one transmitter, or multiple receivers and multiple transmitters. It is a generalisation of the bistatic radar system, with one or more receivers processing returns from one or more geographically separated transmitters.

Passive radar

A bistatic or multistatic radar that exploits non-radar transmitters of opportunity is termed a passive radar
Passive radar
Passive radar systems encompass a class of radar systems that detect and track objects by processing reflections from non-cooperative sources of illumination in the environment, such as commercial broadcast and communications signals...

or passive coherent location system or passive covert radar.

Advantages and disadvantages

The principal advantages of bistatic and multistatic radar include:
  • Lower procurement and maintenance costs (if using a third party's transmitter)
  • Operation without a frequency clearance (if using a third party's transmitter)
  • Covert operation of the receiver
  • Increased resilience to electronic countermeasures
    Electronic countermeasures
    An electronic countermeasure is an electrical or electronic device designed to trick or deceive radar, sonar or other detection systems, like infrared or lasers. It may be used both offensively and defensively to deny targeting information to an enemy...

     as waveform being used and receiver location are potentially unknown
  • Possible enhanced radar cross section
    Radar cross section
    Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...

     of the target due to geometrical effects
  • Separate receiver is very light and mobile while transmitter can be very heavy and powerful (surface to air missile)


The principal disadvantages of bistatic and multistatic radar include:
  • System complexity
  • Costs of providing communication between sites
  • Lack of control over transmitter (if exploiting a third party transmitter)
  • Harder to deploy
  • Reduced low-level coverage due to the need for line-of-sight from several locations

See also

  • Bistatic Doppler shift
    Bistatic Doppler shift
    Bistatic Doppler shift is a specific example of the Doppler effect that is observed by a radar or sonar system with a separated transmitter and receiver. The Doppler shift is due to the component of motion of the object in the direction of the transmitter, plus the component of motion of the...

  • Bistatic angle
    Bistatic angle
    The bistatic angle is the angle subtended between the transmitter, target and receiver in a bistatic radar. When it is exactly zero the radar is a monostatic radar, when it is close to zero the radar is pseudo-monostatic, and when it is close to 180 degrees the radar is a forward scatter radar. ...

  • Bistatic imaging
    Bistatic imaging
    Bistatic imaging is a technique for using imaging using bistatic radar . The result is a more detailed image than would have been rendered with just one radar instrument...

  • Bistatic range
    Bistatic range
    Bistatic range refers to the basic measurement of range made by a radar or sonar system with separated transmitter and receiver. The receiver measures the time difference of arrival of the signal from the transmitter directly, and via reflection from the target...

  • Bistatic sonar
    Bistatic sonar
    Most sonar systems are monostatic, in that the transmitter and receiver are in the same place. Bistatic sonar describes when the transmitter and receiver are separated by a distance large enough to be comparable to the distance to the target....

  • Passive radar
    Passive radar
    Passive radar systems encompass a class of radar systems that detect and track objects by processing reflections from non-cooperative sources of illumination in the environment, such as commercial broadcast and communications signals...

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