Literature Review Of Radar Technology Computer Science Essay

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The word radar is an abbreviation for Radio Detection and Ranging. In general, radar systems use modulated waveforms and directive antennas to transmit electromagnetic energy into a specific volume in space to search for targets. Objects (target) in the search volume will reflect some of the energy (or radar echo back) returned to the radar. The receiver to extract radar target information such as range, speed, position angle, and target other identifying characteristics then processes echoes.

Radar can classified as land-based, conditioning, space borne, or ship-based radar systems. They also can classify into various categories based on the specific radar characteristics, such as frequency, antenna types, and the waveform used. Other classifications related to the mission and function of the radar such as weather, acquisition and search, tracking, track-while-scan, fire control, early warning, over the horizon, terrain following, and terrain avoidance radars.

For example, the Airport Surveillance Radar (ASR) used for air traffic control, and the ship based U.S. Navy AEGIS as in Fig. 2.1 multifunction phased array are S-band radars. The Airborne Warning and Control System (AWACS) shown in Fig. 2.2 and the National Weather Service Next Generation Doppler Weather Radar (NEXRAD) are also S-band radars. However, most weather detection radar systems are C-band radars. Media search and handling fire and military radar metric instrumentation is C-band.

Figure 2.1: US Navy AEGIS

Figure 2.2: Air Force AWACS

Radars can be Continuous Wave (CW) or Pulsed Radars (PR). Pulsed radars use a train of pulsed waveforms usually with modulation. Table 2.1 has the radar classifications based on the operating frequency.

2.1.2 Radar Band

Letter designation

Frequency (GHz)

Typical usage

HF

VHF

UHF

L-band

S-band

C-band

X-band

Ku-band

K-band

Ka-band

MMW

0.003 - 0.03

0.03 - 0.3

0.3 - 1.0

1.0 - 2.0

2.0 - 4.0

4.0 - 8.0

8.0 - 12.5

12.5 - 18.0

18.0 - 26.5

26.5 - 40.0

Normally > 34.0

Very long-range surveillance

Very long-range surveillance

Very long-range surveillance

long-range surveillance

moderate range surveillance

long range surveillance

short range tracking

high resolution mapping

moderate resolution mapping

very high resolution mapping

experimental

Table 2.1: Radar Frequency Bands Source: AIAA (American Institute of Aeronautics and Astronautics)

2.1.3 Radar Block diagram

Typical pulse radar may consist of transmitter, receiver, duplexer, Low Noise Amplifier (LNA), mixer and local oscillator.

Figure 2.3: Radar Block Diagram and signal flow [9]

Figure 2.10 is a block diagram of typical radar, and depicts signals in time and frequency domains. The components divide into two or three physical modules. Low-power trigger pulses fire the electromagnetic wave via the modulator. The electromagnetic wave block has too much power abstracted from it to define electromagnetic wave frequency exactly. The electromagnetic wave output feeds the scanner. Echoes are routed to the receiver and then to the demodulator, which removes the carrier, leaving a baseband or video pulse train similar to that generated by the pulse generator, but with delay proportional to target range and, at a given range, height (voltage) dependent on echo strength. The operator processes the video train to decide which pulses are likely to represent echoes rather than noise or clutter, and then fed to the display for viewing.

2.1.3.1 Antenna

An antenna is a device transmitting or receiving electromagnetic waves. It will convert electromagnetic radiation into electric current, and vice versa. Antennas are often dealing with the transmission and reception of radio waves, and an integral part of all radio equipment.

2.1.3.2 Radar waves

Radar waves received by the receiver or spread by antenna is a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. It travelled with the speed of light as electromagnetic waves other.

2.1.3.3 Transmitter

Transmitter is an electronic device that produces and strengthens the carrier wave, modulates the signal from the means of electromagnetic waves, and transmits the signal generated from the antenna.

2.1.3.4 Receiver

A radar receiver is an electronic circuit that receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form digital data navigational positions.

2.1.3.5 Mixer

Mixer is a device that combines two or more inputs that can be changed. Super heterodyne receiver intermediate frequency modulated signal in which the incoming modulated radio frequency signal combined with local rf oscillator signal

2.1.3.6 Intermediate frequency

Intermediate step before used in transmission or reception; an intermediate frequency (IF) is a frequency that have been shifted from frequency carrier for. The intermediate frequency made by mixing the carrier signal with local oscillator signals in a process called heterodyning, resulting signal the difference or beat frequency. Intermediate frequencies used in the receiver super heterodyne radio, where incoming signals to be transferred to the IF amplification before detection done.

2.1.3.7 Low-noise amplifier (LNA)

Low-noise amplifier (LNA) is an electronic amplifier is used to strengthen a very weak signal is received by the antenna. A key component of the LNA placed at the front-end of a series of radio receivers. Using the LNA, the impact of noise from the next phase of the receive chain is reduced to the benefit of the LNA, while the sound of his own LNA is injected directly into the received signal. Therefore it is necessary to LNA to enhance the desired signal while adding as little noise and distortion as possible, so that the user may signal the next stages in the system.

2.1.3.8 Signal Processing

Signal processing is a field of electrical engineering and applied mathematics related to the operation of or analyzes the signal, in discrete or continuous time, to perform operations on the signal.

In the communication, signal processing is the final and most important part to recognize the radar. The signal may be analog or digital representation of time-varying or spatial-varying physical amount.

 In the context of signal processing, arbitrary binary data stream and on-off signals does not regarded as a signal, but only analog and digital signals that are representative of a physical analog.

2.1.3.9 Filter

In signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a part of signal processing, filter characteristics that define the full or partial suppression of some aspects of the signal. This means removing some frequencies and not others to suppress the disturbing signal and reduce background noise..

2.1.3.10 Continuous Wave and Pulsed Waveforms

The spectrum of a given signal describes the spread of its energy in the frequency domain. An energy signal (finite energy) characterized by its Energy Spectrum Density (ESD) function, while a power signal (finite power) is characterized by the Power Spectrum Density (PSD) function. The units of the ESD are Joules per Hertz, while the PSD has units Watts per Hertz. The signal bandwidth is the range of frequency over which the signal has a nonzero spectrum. In general, any signal can be defined using its duration (time domain) and bandwidth (frequency domain).

Figure 2.4: Amplitude spectrum for a continuous sine wave

Figure 2.5: Amplitude spectrum for a single pulse, or a train of non-coherent pulses.

2.1.4 Radar Type

2.1.4.1 Radar type for military:-

The following Military Radar, commonly have been uses in surveillance and in the battlefield to detect hostile threats:

2.1.4.1.1 Fire-control radar

Radar that provides continuous position track or data on a single target is called tracking radar. Fire-control radar is the most tracking radar systems used by the military.

Figure 2.6: SA-3-Low-Blow-fire-control-radar

The fire-control radar features include a pulse repetition frequency (PRF) is very high; the pulse width is very narrow, very narrow beam width. These features, while providing extreme accuracy, limit of coverage and make the initial target detection difficult.

Fire-control radar must be headed to the general location of the desired target because of the narrow-beam pattern. It is in the designation phase of equipment operation. Once in the general vicinity of the target, the radar system switches to the acquisition phase of operation. During acquisition, the volume search for radar system of small spaces in the pattern until the target is fixed in advance. The radar system operating track phase once the target is located. By using one of several possible scanning techniques, the radar system automatically follows all target motion. Said radar system for a phase locked on the target track. Three sequential stages are often referred to as a mode of operation and the public to target processing sequence of the fire-control radar the most.

2.1.4.1.2 Air-Defense Radars

Air-Defense Radars can detect air targets and determine their position, course, and speed in a relatively large area. Air-Defense Radars divided into two categories, 2D and 3D radars. It based on the amount of position information supplied. Radar sets that provide only range and bearing information are two-dimensional, or 2D, radars. Radar sets that supply range, bearing, and height are three-dimensional, or 3D, radars.

Functions of Air-Defense Radars are as early-warning devices because they can detect approaching enemy aircraft or missiles at great distances. In an attack, early detection of the enemy is crucial for a successful defense against attack. Antiaircraft defenses in the form of anti-aircraft artillery called "AAA", missiles, or fighter planes must be anytime readiness in time to counter an attack. Range and bearing information, the information provided by Air-Defense Radars used to initially position fire-control tracking radar on a target.

Other functions of the radar-guided air defense combat air patrol (CAP) aircraft to the appropriate position to resist the enemy. In the case of aircraft control, information collected by your guide and proceeds to the aircraft radar by either voice radio or a computer link to the aircraft.

2.1.4.1.3 Mortar Locating Radar

A Mortar Locating Radar provides quick introduction to define the enemy mortar positions in map co-ordinates, enabling artillery units to launch counter attacks. The system electronically, scans the horizon for the given number of times a second, intercepting and automatically tracking hostile projectiles, then computing back along the trajectory to the origin. The co-ordinates and elevation of the weapons site are then presented to the operator.

2.1.4.1.5 LPI-Radar

LPI radar (Low Probability of Intercept) is classes of radar systems that possess certain performance characteristics that make them nearly undetectable by todays intercept receivers. Low probability of intercept features prevent the radar get caught by alarm systems or passive radar-detection equipments in a target. These features include:

using an narrow beam antenna with low side lobes that is hard to spot from off its bore sight;

only transmitting radar pulses when necessary;

reducing the transmitted pulse-power;

spreading the radar pulses over a wide band so there will only be a very small signal in any one band

varying transmission parameters such as

pulse form,

frequency

pulse repetition frequency (PRF),

using an intra pulse modulation with an anonymous wave-form

2.1.4.2 Radar type for civilian:-

The following are the Air Traffic Control (ATC) surveillance; approach and landing radars usually used in Air Traffic Management (ATM):

2.1.4.2.1 En-Route Radars

En-route radar systems usually operate in L-Band. These radar sets initially detect and determine the position, course, and speed of air targets in a relatively large coverage.

2.1.4.2.2 Air Surveillance Radar (ASR)

Airport Surveillance Radar (ASR) used to detect and display an aircraft's position in the terminal area. This radar usually sets operate in E-Band.

2.1.4.2.3 Precision Approach Radar (PAR)

The ground-controlled approach is a control mode in which an aircraft is able to land in bad weather so the aircraft and its passenger is safe the method is The pilot can get information by the radar operator and passed to the aircraft by either voice radio or a computer link to the aircraft.

2.1.4.2.4 Surface Movement Radar (SMR)

The Surface Movement Radar (SMR) scans the airport surface to locate the positions of aircraft and ground vehicles and displays them for air traffic controllers in bad weather. Surface movement radars operate in J- to X- Band and use an extremely short pulse-width to provide an acceptable range-resolution.

2.1.4.2.5 Specially Weather-Radar Applications

Weather radar is very important for the air traffic management. There are weather-radars specially designed for the air traffic safety.

2.2 Low Probability of Intercept (LPI)

The function of the LPI radar is to avoid interception by the Electronic Support receiver. The general purpose all using radar waves are ideal for those waveforms Electronic Support receiver set. Consequently, conventional receivers Electronic Support LPI radar can only detect the very short range.

In applications such as altimeters, tactical airborne targeting, surveillance and navigation, the interception of the radar transmission can quickly lead to electronic attack (or jamming). The LPI requirement is also in response to the pervasive threat of being destroyed by precision guided munitions and Anti-Radiation Missiles (ARM) [10]

LPI radar transmits pulses of low power so that the range of intra-wave modulation of the detected target can be determined with a good range resolution. This modulation may be frequency modulated, phase or pseudo-random, that is noise like modulation. Because of the typical LPI, radar has a pulse-power output lower compare to Conventional pulse radar for a similar detection range of targets. This allows processing of LPI radar gain as the primary radar and the fourth-root dependence of the two-way travel of electromagnetic waves, to overcome the benefits of the square of the Electronic Support receiver in conventional situations.

However, LPI radars are having limitation due to the short range applications. A relatively long transmitted pulse width still used to the transmission, which requires the duplexer to stay aligned to the transmitter throughout the pulse and the receiver when it switched off. That is why there are too many LPI radars have separate transmit and receive antennas that are co-mounted.

Low probability of intercept is the usual term to describe radars which pose difficulties to an ESM system because of the weak signal levels which it presents to the receiver, and generally refers to signal which are difficult to detect above the ESM receiver threshold. [11]

2.2.1 LPI Radar Principles

In the modern battlefield, radars face increasingly serious threats from Electronic Attack and ARM (Anti radiation missile). An important feature of modern radar systems is the ability "to see and not to be seen". Low Probability of Intercept radar has a powerful detection capability while simultaneously itself being not easily detected by electronic spying equipment.

Whether or not radar is LPI depends on the purpose or mission of the radar, the kind of receiver that is trying to detect it, and the applicable engagement geometry. These types of radars also described as "quiet" radars.

2.2.2 Characteristics of LPI Radar

Many features make LPI radar different from conventional radar. These include:

Low sidelobe antennas,

Irregular antenna scan patterns,

High duty cycle/wide band transmission,

Accurate power management,

Carrier frequency,

Very high sensitivity,

High processing gain,

Coherent detection,

2.2.2.1 Low Sidelobe Antennas

The LPI radar antenna must have a transmit radiation pattern with very low sidelobes. The low sidelobes in the transmit pattern reduce the possibility of an intercept receiver detecting the radio frequency (RF) emissions from the sidelobe structures of the antenna pattern. By narrowing it emitter size, the sidelobe level can be lower.

The main lobe suppose not be suppressed in the same manner, so the transmitting beam should be wide with the radiated energy spread over a wide area. This increases the difficulty to intercept the radar energy and determine direction of the signal. On the other hand, the radar receiving antenna should use a narrow beam for high resolution and detection. It is common to use adaptive arrays for leakage cancellation, multiple receiving beams, and electronic scanning.

2.2.2.2 Irregular Antenna Scan Patterns

Intercept receivers can use scan type and scan rate information to search for, detect, and identify radars. With confusing radar scan techniques, such as changing the scan parameters randomly, LPI radar will have a greater chance to avoid interception. Phased array Electronically Scanned Antennas (ESAs) used to produce irregular scan patterns by creating multiple beams to search different scan volumes at different frequencies. Electronic scanning with software control also helps the LPI radar limit its illumination time.

The F/A-22 Raptor's AN/APG-77, Patriot's AN/MPQ-53, and SA-10 Grumble's Tombstone radars shown in figure 2.6 have an ability to use irregular antenna scan patterns to reduce the probability of interception by hostile receivers.

Figure 2.8: Examples of Radars That Use Irregular Scan Patterns from left F/A-22 Raptor's AN/APG-77, Patriot's AN/MPQ-53

2.2.2.3 High Duty Cycle/Wide Band Transmission

LPI radars escape detection by spreading the radiated energy over a wide spectrum of frequencies. The ES receiver must search a large bandwidth to find the LPI radar. The LPI radar is thus able to exploit the time bandwidth product by reducing its peak transmitted power to bury itself in the environmental noise. Due to the mismatch in waveforms for which the ES receiver is tuned, the LPI radar is effectively invisible to the ES receiver. Since the high peak, ES receivers can easily detect power transmitted by the pulsed radar; continuous wave (CW) radars can transmit very low power while maintaining the same energy profile. The differences will be show as in the Figure 2.8.

Figure 2.8: Comparison of pulsed and CW Radar from [1]

Consequently, most LPI emitters use periodically modulated CW signals resulting in large bandwidths and small resolution cells, and are ideally suited for pulse compression.

2.2.2.4 Accurate Power Management

Power management is a radar technique that is becoming more practical with improvements in digital signal processing. Power management encompasses a host of techniques including:

Antenna sidelobe control/suppression

Pseudo-random illumination of a target

Dynamic control of transmitter power to maintain a minimal SNR

For Example, the French CROTALE system makes effective use of power management. Shortly after lock on, the tracking, radar reduces its transmitter power such that the SNR of the received level is to keep a minimal value. This process continues during the course of engagement reducing the range at which the radar will detect. This LPI technique causes some ES receivers to calculate the range of the threat incorrectly and categorize the threat as a low priority.

2.2.2.5 High and Variability Carrier Frequency

LPI radar can use very high frequency at which peak absorption occurs. This will serve to maximize attenuation in order to mask the transmit signal and limit reception by hostile receivers (atmospheric attenuation shielding). Because of the high absorption of the emitter's energy, this technique is always limited to short range systems.

A final carrier frequency approach to achieving a lower probability of interception is to interleave the LPI radar with an infrared sensor (dual mode approach), reducing the amount of time that the RF transmitter radiates.

2.2.2.6 Very High Sensitivity

As shown in Figure 2.9, sensitivity is a function of the bandwidth, noise figure, and required SNR. The sensitivity factor is a crucial parameter evaluating for a successful LPI radar design. The thermal noise based on the formula KTB where T is the temperature in Kelvin, K is the Boltzmann's constant, and B represents the bandwidth. The sensitivity in dBm is the sum of the thermal noise (in dBm), noise figure (in dB), and required signal-to-noise ratio (in dB).

The receivers have detection systems that are about 20db more sensitive that would increase the range. Which is the radar can be, detect to about 25km. [11]

The improve sensitivity will allows the main beam of the radar to be detected at longer ranges. (Often to the horizon when both radar and ESM are on the surface but will not give the sensitivity to intercept main beam significantly beyond the horizon, nor to intercept the sidelobes at longer ranges.

Figure 2.9: Receiver Sensitivity from [2]

It is clear that reduction of the radar noise temperature and losses will improve LPI radar performance.

2.2.2.7 High Processing Gain

Processing gain has the effect of narrowing the effective bandwidth of the radar receiver by taking advantage of the signal modulation. Thus, the radar receiver achieves a processing gain while the hostile receiver cannot. LPI radar achieves bandwidth advantage over an intercept receiver because the radar knows its own signal. In contrast, the intercept receiver must accept a wide range of signals and must typically make detailed parametric measurements to identify the type of signal it is receiving. [5]

2.2.2.8 Coherent Detection

Coherent detection is another technique used by LPI radars to avoid interception. An Electronic Warfare Support (ES) receiver cannot achieve coherent detection of a radar signal unless it knows the parametric details of the signal. When the signal modulation is random, this property becomes even more effective. Using true noise to modulate a radar signal is a good illustration of these characteristics. Radars random signal radars (RSR) correlates the returning signal with a delayed sample of the transmitted signal. The amount of delay necessary to peak the correlation determines the range of a target. Since the transmitted signal is completely random, the intercepting receiver has no reference for correlating the received signal. [5]

2.3 Electronic Warfare

Electronic warfare (EW) refers to any action involving the use of the electromagnetic spectrum or directed energy to control the spectrum, attack an enemy, or counter enemy assaults via the spectrum. The objective of electronic warfare is to deny the opponent the advantage of, and ensure friendly unimpeded access to, the EM spectrum. EW launched from all surface air, sea, land, and space by manned and unmanned systems, and can target communication, radar, or other services.

2.3.1 Techniques of EW

EW divided into three parts; Electronic Attack (EA), Electronic Protection (EP), and Electronic warfare Support (ES).

2.3.1.1 Electronic Support

Electronic Warfare Support (ES) is under the EW involving actions tasked by, or under direct control of, an operator to search for, intercept, identify, and locate or localize sources of intentional and unintentional radiated electromagnetic (EM) energy for the purpose of immediate threat recognition, targeting, planning, and conduct of future operations.

2.3.1.1.1 Radar Warning

Compare to traditional radars, a radar warning receiver can usually detect enemy radar farther away than the radar can detect the target carrying the warning receiver. With the latest low probability of intercept radar, the rules may change; conventional electronic support receivers cannot detect some of the radar.

2.3.1.1.2 Directed energy ES

A laser warning receiver such as AN/AAR-47 designed to detect and analyze a laser signal is, in EW terms, ES. Laser warning will have a much smaller beam to intercept that brings technical problems different from traditional radio or radar warning: the signal of interest. During the day, the signal coexist with broadband background interference, such as sunlight, or fire or flares at night.

2.3.1.2 Electronic Attack

Electronic attack (EA) or electronic countermeasures (ECM) take seriously the use of the electromagnetic energy, or anti-radiation weapons to attack personnel, facilities, or equipment with the purpose of degrading, neutralizing, or destroying enemy combat capability and considered as a form of fires.

An enemy due to their active transmissions can detect EA operations. Many modern EA techniques considered to be highly classified. Examples of EA include communications jamming, IADS suppression, DE/LASER attack, expendable decoys and counter radio controlled improvised explosive device (C-RCIED) systems.

2.3.1.3 Electronic Protection

Electronic Protection (EP) also known as electronic protective measures (EPM) or electronic counter countermeasures (ECCM) involves actions taken to protect personnel, facilities, and equipment from any effects of friendly or enemy use of the electromagnetic spectrum that degrade, neutralize, or destroy friendly combat capability.

2.4 Electronic Support Measures (ESM)

Figure 2.7: EL/L-8300 Signals Intelligence System

2.4.1 Types of ESM

There are two types of ESM that primarily used in battlefield Electronic Intelligence (ELINT) and Communications Intelligence (COMINT).

2.4.1.1 Electronic Intelligence (ELINT)

Electronic Intelligence primarily is purposely to the interception and analysis of radar emissions from surveillance, fire control or missile guidance radars, and often allied to an ECM system to provide protection from these.

ELINT is providing not only direction finding but also analysis of the incoming signals to provide immediate warning of threat radars, including surveillance, fire control, targeting and missile guidance systems. Signals from radar systems intercept by a warning receiver and analyzed by an associated processor. After that, the processor will fetch so much of parameters, including direction, type of radar, frequency, Pulse Repetition Frequency (PRF), frequency agility, and PRF type.

These parameters are commonly sufficient to characterize the type of emitter, and complete identification carried out by comparing the analyzed signal with parameters of hostile and friendly emitter characteristics stored in a library of signal threaten database. Analysis of the signals and warning of a threat is rapid and instantaneous, it will enable countermeasures of jamming, and decoys initiated.

For aircraft, ships and armored fighting vehicles effective warning systems are necessary for survival in the electromagnetic threat environment of the modern battlefield. The warning receivers continuously updated to cope with the latest threats. These receivers are normally either crystal video or super heterodyne-based equipments. They are very good against pulsed, frequency-agile, PRI-agile, spread spectrum and continuous-wave transmitters. Super heterodyne receivers are more expensive but will have a high level of sensitivity, plus long pick-up ranges and sidelobe penetration.

2.4.1.2 Communications Intelligence (COMINT)

Communications Intelligence, as its name implies, for the interception of communications purposed, whether by voice or data link.

COMINT provides interception, direction finding and analysis of enemy transmissions, especially to assess the movements and strategy of the opposing forces. Analysis of the signals provides much valuable information because of the fresh and raw information of the intentions for command and control purposes, and the most recent systems provide the operator with the ability to detect and analyzed unusual and complex signals as well as the normal interception and DF facilities.

The receiving equipment allied to a computer-based processing and display system. Automatically, position fixing in the land-based role done by the use of remote-controlled DF stations. Spectra or time waveforms are usually available, together with alphanumeric readouts which include type of transmission, frequency, modulation and other signal parameters. These parameters been used to characterize the types of communication and radar systems in use, whether they are mobile or static, the direction of any movement, and so on.

2.5 Detection method

The two basic operations performed by radar are (1) detection of the presence of reflecting objects, and (2) extract of information from the received waveform to obtain such target data as position, velocity, and perhaps size. The operations of detection and extraction may be performed separately and in either order, although a radar that is a good detection device is usually a good radar for extracting information, and vice versa.

2.5.1 Strategies to Intercept LPI radar signals

The wideband nature of LPI emitter signal can force the intercept receiver to have a designed processing gain by implementing complex receiver architecture and signal processing algorithms (time frequency, infrequency) in order to determine the waveform parameter. Detection of LPI radar signals requires a large processing gain because of the wideband nature of LPI radar. The basic idea behind the use of wideband signals is to spread the radiated power over a large bandwidth in order to produce a power spectral density PSD below the noise at receiver inputs. [10]

2.5.2 Electronic Warfare (EW) Intercept receiver technique

The wideband nature of LPI threat signal presents a significant challenge to the intercept receiver design. There are 3 types of EW receivers:-

Radar warning receiver

Electronic support receiver

Electronic intelligent receiver

Radar warning receiver(RWR) are design to passively intercept enemy radar to enable the pilot to react quickly through to make a controlled series of changes command or employing appropriate electronic attack technique. Their use on the battlefield is time critical and combat action taken directly from their threat information output,

Electronic support receiver will accomplish all actions necessary to provide the information required for immediate decisions involving EW operations, threat avoidance, targeting and homing. Although not as time critical as RWR, information operation rely heavily on ES receivers for intelligent update and important operational decision.

For electronic intelligence receivers the information provided extract from detailed analysis of radar signal and other non-communication emitters in a timely manner. Although their operation is the least time critical, their treat identification used to update national database.

There are many variation of intercept receiver. There are three popular intercept receivers architecture is compare in term of their ability to detect several types of LPI emitter waveforms:

Square law

Wideband

Channelize

The wideband crystal video receivers characterize by a wide RF bandwidth. It is using to account for the uncertainty in the intercept signal parameter. The channelized receiver contains a large number of parallel arrow band receivers.

2.5.3 Digital EW receivers

Radio receivers that perform the analogue-to-digital conversion process close to the antenna and do most of the signal processing in digital domain.

Digital receiver, often called software radios place a high performance burden on the ADC but allow good deal of flexibility in post detection signal processing

EW receiver parameters of interest include sensitivity, dynamic range, resolution, simultaneous signal capability, complexity and cost.

Most digital EW receivers use frequency conversion before digitizing the signal. So the signal is down converted in frequencies and then digitized by ADC. The digital signal then processed by spectrum analyzer that extract frequency information. Using this frequency information, the signal sorted and a parameter encoder then forms a pulse descriptor word (PDW).

For LPI CW emitters, PDW contains the centre frequency fc, the signal coding detail such as the modulation period and sub code period detail (PSK) and frequency-hopping frequencies (and orders), as well as the signal of arrival.

When down converting the signal in an EW receiver, two approaches used are:

Two-stage heterodyne down conversion process

Homodyne direct conversion process

The first approach down convert the signal, first to Intermediate Frequency(IF) and then to baseband, using two or more band pass filter-local oscillator-mixer stages in series, Since the LPI signal are phase and frequency modulated, both in phase and quadrature components are required at baseband.

The benefit of this approach is that by driving the mixer with a frequency agile Lo the frequency of the desired signal or channel converted to a fixed frequency. Once converted to a fixed IF, it processed to by highly selective narrowband filtering.

It is also can be used a direct conversion (homodyne) down conversion. This two-channel approach uses only a single local oscillator and translates the signal of interest to zero frequency. Due to the elimination of the IF stages, all signal conditioning must be performed either at ruff or baseband. The direct conversion approach offers a higher degree of integration at the front end with fewer components, allowing most of them to be mono lithically fabricated in single chip.

2.5.4 Target classification

Radar obtains information about a target by comparing the received echo signal with the transmitted signal. The availability of an echo signal indicates the presence of a reflecting target. It also provides the location of the target and information about the type of target. This is calling as target classification. The time delay between the transmission of the radar signal and the receipt of an echo is the way to measure of the distance or range to the target.

The data stream at the receiver output contains of thermal noise and clutter returns, mixed up with any target echoes. All modern radars use extensive digital signal processing to remove as much noise and clutter as possible before reaching the display. Processing optimizes detection of wanted echoes and minimizes false alarms. Information theory sets definite limits to what can be achieved.

The processing strategies of individual radar suppliers are not normally to extract and expose and probably differ in detail, but the competitive nature of the market dictates that they all achieve performance close to the theoretical limit for the target and clutter scenario.

The processor's task is to extract so much information - matter to the user from the undigested stream of data. That is to maximize the probability of detection, Pd and minimize the probability of false alarm Pea. Data once rejected is difficult to lost, so must be retained back until as much information as possible has been compress from it. Pd and PFA interlinked with the relative strength of the wanted signal to unwanted noise, the all-important signal to noise and clutter ratio (SNR). It is impossible to have a value of high PD and PFA with low SNR value. The processor plays a big part in the effort to maximize SNR.

Delectability affected by random or partly manner where echoes, noise and clutter fluctuate with time.

Feature use the direction of information theory to maximize the echo:

High transmitter power

Narrow scanner beam width - high gain

Receiver bandwidth is restricted to reduce thermal noise

Detection LPI Radar

2.6.1 Parallel Array Filter

The objective of parallel filter arrays is to separate the input signal into small frequency bands, providing a complete time-frequency description of the unknown signal. Then, a third-order estimator in order to suppress the noise and preserve the phase of the signal during the correlation process treats each sub-band signal individually.

2.6.2 Introduction to Higher-Order Estimators

Recently, HOS have begun to find wide applicability in many fields, such as sonar, radar, plasma, physics, biomedicine, seismic data processing, image reconstruction, and time delay estimation. These estimators well known as cumulate. Their association with Fourier Transforms not only shows the amplitude information but also can preserve phase information in a process.

In power spectrum estimation, the Fourier transform of the autocorrelation suppresses the phase relationship between frequency components. Power spectrums are phasing blind. The information contained in the power spectrum that is present in the autocorrelation sequence; this is sufficient for the complete statistical description of a Gaussian signal. However, there are real situations where we must see beyond the power spectrum of a signal to extract information regarding deviation from Gaussianity and the presence of phase relations. LPI radar signals are examples of such a situation.

Cumulate; on the other hand, are blind to any kind of Gaussian processes. Cumulate-based methods improve SNR when signals were corrupted by Gaussian noise. Third-order cumulate are applicable when we are dealing with non-Gaussian or nonlinear systems; many real world applications have this characteristic. The development of cumulate and polyspectra has paralleled the development of traditional correlation and its associated spectrum.

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