Constant false alarm rate on:
[Wikipedia]
[Google]
[Amazon]
Constant false alarm rate (CFAR) detection refers to a common form of
Detection occurs when the cell under test exceeds the threshold. In most simple CFAR detection schemes, the threshold level is calculated by estimating the noise floor level around the cell under test (CUT). This can be found by taking a block of cells around the CUT and calculating the average power level. Cells immediately adjacent to the CUT are normally ignored to avoid corrupting this estimate with power from the CUT itself (and referred to as "guard cells"). A target is declared present in the CUT if it is greater than all its adjacent cells and greater than the local average power level. The estimate of the local power level may sometimes be increased slightly to allow for the limited sample size. This simple approach is called a cell-averaging CFAR (CA-CFAR).
Other related approaches calculate separate averages for the cells to the left and right of the CUT, and then use the greatest-of or least-of these two power levels to define the local power level. These are referred to as greatest-of CFAR (GO-CFAR) and least-of CFAR (LO-CFAR), respectively, and can improve detection when immediately adjacent to areas of clutter.
adaptive algorithm
An adaptive algorithm is an algorithm that changes its behavior at the time it is run, based on information available and on ''a priori'' defined reward mechanism (or criterion). Such information could be the story of recently received data, inform ...
used in radar systems to detect target returns against a background of noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
, clutter and interference.
Principle
In the radar receiver, the returning echoes are typically received by theantenna
Antenna ( antennas or antennae) may refer to:
Science and engineering
* Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves
* Antennae Galaxies, the name of two collid ...
, amplified, down-converted to an intermediate frequency, and then passed through detector circuitry that extracts the envelope of the signal, known as the ''video signal''. This video signal is proportional to the power
Power most often refers to:
* Power (physics), meaning "rate of doing work"
** Engine power, the power put out by an engine
** Electric power
* Power (social and political), the ability to influence people or events
** Abusive power
Power may ...
of the received echo. It comprises the desired echo signal as well as the unwanted signals from internal receiver noise and external clutter and interference. The term ''video'' refers to the resulting signal being appropriate for display on a cathode ray tube
A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms (oscilloscope), pictu ...
, or "video screen".
The role of the constant false alarm rate circuitry is to determine the power threshold above which any return can be considered to probably originate from a target as opposed to one of the spurious sources. If this threshold is too low, more real targets will be detected, but at the expense of increased numbers of false alarms. Conversely, fewer targets will be detected if the threshold is too high, but the number of false alarms will also be low. In most radar detectors, the threshold is set to achieve a required probability of false alarm (equivalently, false alarm rate or time between false alarms).
Suppose the background against which targets are to be detected is constant with time and space. In that case, a fixed threshold level can be chosen that provides a specified probability of false alarm, governed by the probability density function of the noise, which is usually assumed to be Gaussian
Carl Friedrich Gauss (1777–1855) is the eponym of all of the topics listed below.
There are over 100 topics all named after this German mathematician and scientist, all in the fields of mathematics, physics, and astronomy. The English eponym ...
. The probability of detection is then a function of the signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in de ...
of the target return. However, in most fielded systems, unwanted clutter and interference sources mean that the noise level changes both spatially and temporally. In this case, a changing threshold can be used, where the threshold level is raised and lowered to maintain a constant probability of false alarm. This is known as constant false alarm rate (CFAR) detection.
Cell-averaging CFAR
Detection occurs when the cell under test exceeds the threshold. In most simple CFAR detection schemes, the threshold level is calculated by estimating the noise floor level around the cell under test (CUT). This can be found by taking a block of cells around the CUT and calculating the average power level. Cells immediately adjacent to the CUT are normally ignored to avoid corrupting this estimate with power from the CUT itself (and referred to as "guard cells"). A target is declared present in the CUT if it is greater than all its adjacent cells and greater than the local average power level. The estimate of the local power level may sometimes be increased slightly to allow for the limited sample size. This simple approach is called a cell-averaging CFAR (CA-CFAR).
Other related approaches calculate separate averages for the cells to the left and right of the CUT, and then use the greatest-of or least-of these two power levels to define the local power level. These are referred to as greatest-of CFAR (GO-CFAR) and least-of CFAR (LO-CFAR), respectively, and can improve detection when immediately adjacent to areas of clutter.
Sophisticated CFAR approaches
More sophisticated CFAR algorithms can adaptively select a threshold level by taking a rigorous account of the statistics of the background in which targets are to be detected. This is particularly common in maritime surveillance (radar) applications, where the background of sea clutter is particularly spikey and not well approximated byadditive white Gaussian noise
Additive white Gaussian noise (AWGN) is a basic noise model used in information theory to mimic the effect of many random processes that occur in nature. The modifiers denote specific characteristics:
* ''Additive'' because it is added to any nois ...
. This is a difficult detection problem, as it is difficult to differentiate between spikes due to the sea surface returns and spikes due to valid returns from, for example, submarine periscopes
A periscope is an instrument for observation over, around or through an object, obstacle or condition that prevents direct line-of-sight observation from an observer's current position.
In its simplest form, it consists of an outer case with ...
. The K-distribution is a popular distribution for modelling sea clutter characteristics.
See also
*Detection theory
Detection theory or signal detection theory is a means to measure the ability to differentiate between information-bearing patterns (called stimulus in living organisms, signal in machines) and random patterns that distract from the information (c ...
* False alarm
A false alarm, also called a nuisance alarm, is the deceptive or erroneous report of an emergency, causing unnecessary panic and/or bringing resources (such as emergency services) to a place where they are not needed. False alarms may occur with ...
* Pulse-Doppler signal processing
* Receiver operating characteristic
References
{{DEFAULTSORT:Constant False Alarm Rate Radar signal processing Detection theory