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Low Probability of Intercept Radar

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The History of Radar | Radar History: Isle of Wight Radar During The Second World War | Radar: The Basic Principle
Radar Technology: Main Components | Radar Technology: Side Lobe Suppression | Radar Technology: Airborne Collision Avoidance
Radar Technology: Antennas | Radar Technology: Antenna Beam Shapes | Radar Technology: Monopulse Antennas | Radar Technology: Phased Array Antennas | Radar Technology: Continuous Wave Radar | Theoretical Basics: The Radar Equation
Theoretical Basics: Ambiguous Measurements | Theoretical Basics: Signals and Range Resolution
Theoretical Basics: Ambiguity And The Influence of PRFs | Theoretical Basics: Signal Processing | Civilian Radars: Police Radar | Civilian Radars: Automotive Radar | Civilian Radars: Primary and Secondary Radar
Civilian Radars: Synthetic Aperture Radar (SAR) | Military Applications: Overview | Military Radars: Over The Horizon (OTH) Radar
How a Bat's Sensor Works | Low Probability of Intercept (LPI) Radar | Electronic Combat: Overview | Electronic Combat in Wildlife
Radar Countermeasures: Range Gate Pull-Off | Radar Countermeasures: Inverse Gain Jamming | Advanced Electronic Countermeasures

Low Probability of Intercept (LPI) radars are military radars which are designed for the modern electronic combat environment. More or less successfully, they try to avoid being detected by ELINT sensors by using any combination of the features outlined in this Entry.

Multistatic Radar

Common radars are monostatic - that is, their transmitter and receiver are located in the same place. An ELINT sensor can only detect transmitters and once the transmitter has been located, the whole radar can be engaged by jamming or by other means such as artillery or anti-radiation missiles.

Bistatic radars have the transmitter and receiver at different places. Therefore, the transmitter may be located, but destroying the transmitter won't lead to much damage as the receiver and signal-processing equipment makes up the more expensive part of a radar. It doesn't make sense to direct a jammer against the transmitter because only a receiver can be susceptible to jamming. A special case of bistatic radar is the pair that is made up by a target illumination radar and the seeker head of a radar homing missile. Attempting to jam the illuminator won't affect the seeker-head. Even worse, doing so would provide it with a beacon signal.

Multistatic radars are the extension of bistatic ones. They've got a single transmitter but have several receivers distributed over an area of interest. The receivers deliver their findings to an evaluation centre where powerful computers are busy correlating the results and putting together an air picture. Transmitters are much cheaper than all this equipment and if a transmitter was to be destroyed, it would be fairly easy to replace it.

Ultra-low Sidelobe Antennae

Ordinary radar antennae feature sidelobes which are weaker by a factor of, for example, 100 times than the main lobe. That is, they are giving away their position not only in the intended direction but also to anybody who cares to listen from somewhere else. Ultra-low sidelobe antennae are designed to change that by featuring sidelobe levels in the ranges of one ten-thousandth of the power of the main lobe and even less. Thus, the chances of intercepting a signal from a position outside the main beam are significantly reduced.

Ultra-wide Band Signals

Long pulse duration means that a given amount of energy is distributed over time. Ultra-wide band signals distribute a given amount of energy over frequency, with the same effect of hardening an ELINT receiver's task of detecting them. The radar itself knows which signal it is looking for, and the receiver circuitry has been matched to the signal properties. This knowledge gives the radar receiver a distinct advantage over an ELINT receiver.

Long Pulse, Low Power

A radar receiver needs a distinct amount of energy in order to detect a radar return. This energy can be contained in a short but powerful pulse, in a weak pulse with correspondingly longer duration, or ultimately in a continuous wave signal with even less power. ELINT receivers rely on some minimum ratio of power to noise in order to identify a radar signal. Low power signals are designed to evade detection by lowering this ratio and trying to hide within the noise.

No Power At All: Passive Radar

A passive radar like Silent Sentry [Note: This is a PDF document, requiring the free Adobe Acrobat Reader to be installed on your machine. The software can be downloaded for free from Adobe] is a multistatic radar - without the transmitter. The receivers pick up reflections that are caused by background illumination provided by commercial TV or radio stations. Enormous computer power is engaged to correlate them against a master copy that was received via some direct propagation path and to calculate target flightpaths and positions. As this type of radar doesn't have any transmitter, it is obvious that an enemy's ELINT assets are confronted with the problem of finding something that doesn't actually exist.

History: Overview | Isle of Wight Radar During WWII
Technology: Basic Principle | Main Components | Signal Processing | Antennae | Side Lobe Suppression | Phased Array Antennae | Antenna Beam Shapes | Monopulse Antennae | Continuous Wave Radar
Theoretical Basics: The Radar Equation | Ambiguous Measurements | Signals and Range Resolution | Ambiguity and PRFs
Civilian Applications: Police Radar | Automotive Radar | Primary and Secondary Radar | Airborne Collision Avoidance | Synthetic Aperture Radar
Military Applications: Overview | Over The Horizon | Low Probability of Intercept | How a Bat's Sensor Works
Electronic Combat: Overview | Electronic Combat in Wildlife | Range Gate Pull-Off | Inverse Gain Jamming | Advanced ECM | How Stealth Works | Stealth Aircraft

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