Ready Reckoner Radar Ew

8/8/2019 Ready Reckoner Radar Ew

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READY RECKONER FOR

RADAR EW

If an EW equipment fails to work as per the specification aircrafts are shotdown , ships are sunk , key installations are bombed and battles are lost.

There is no second chance given to a EW Designer in a combat battle

Prof. G. KUMARASWAMY RAO

(EX DIRECTOR & ScH, DLRL)HOD ECE Dept.,Bharat Institute of Engg.,& Technology

Ibrahimpatnam, R.R. District (A.P.)INDIA

Tel:040-24530608/9440881501Email: [email protected]

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A NON-MATHEMATICAL OVER VIEW OF RADAR EWCURRENT & FUTURE

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A NON-MATHEMATICAL OVER VIEW OF RADAR EW

CURRENT & FUTUREProf. G. KUMARASWAMY RAO(EX DIRECTOR & ScH, DLRL)

HOD ECE Dept.,Bharat Institute of Engg.,& Technology

CONTENTS

1. HISTORICAL BACKGROUND

2. INTRODUCTION

3. ELECTRONIC SUPPORT ( ES ) / ELECTRONIC SUPPORT MEASURES(ESM)

4. ELECTRONIC ATTACK ( EA ) / ELECTRONIC COUNTER MEASURES (ECM)

5. ELECTRONIC PROTECTION ( EP) / ELECTRONIC COUNTER COUNTER MEASURES (ECCM)

6. CONCLUSION

A NON-MATHEMATICAL OVERVIEW OF RADAR EW CURRENT & FUTURE

INDEX

1. HISTORICAL BACKGROUND2. INTRODUCTION

2.1 FREQUENCY SPECTRUM CLASSIFICATION

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(i)Communication EW(ii)Radar EW(iii)EO/IR (Electro optic/Infrared) EW(iv)Hybrid IR/EO/RI

2.2 FUNCTIONALITY CLASSIFICATION(i)Electronic Support (ES)(ii)Electronic Attack (EA)(iii)Electronic Protection

2.3 ROLE CLASSIFICATION

(i)Tactical Role(ii)Strategic Role3. ELECTRONIC SUPPORT (ES)

3.1 ES Rx BY APPLICATION(i)RWR(ii)ESM Rx(iii)ECM Rx(iv)ELINT Rx(v)Missile Warning Rxs (MWR)

3.2 ES Rx BY STRUCTURE(i)Crystal Video Rx (CVR)(ii)Superheterodyne Rx(iii)IFM Receiver(iv)Channelised Rx(v)Compressive Rx(vi)Bragg Cell Rx(vii)Digital Rx

3.3 PARAMETERS MEASURED BY ES Rxs(i)Frequency Measurement(ii)Angle of Arrival (AOA)

(a) Rotary D.F.(b) Amplitude Comparison(c) Phase Comparison (Interferometry)(d) Phase Comparison (Digital Bearing Discriminator)(e) Time Difference of Arrival(f) Multi beam lens antenna

1.1 EMMITER LOCATION

3.41 EMMITTER LOCATION OF STATIONARY GROUND BASED EMITTERS FROM AIRBORNEPLATFORMS(i) Azimuth Triangular Method(ii) Azimuth and Elevation Location Technique(iii) Time Difference of Arrival (TDOA)(iv) Phase Rate of Change

(v) Angle Distance Technique(vi) RF Doppler Processing

3.42 RELATIVE ADVANTAGES OF EMITTER LOCATION TECHNIQUES1.2 FUTURE TRENDS OF ES Rxs

(i)LPI Radar Detection(ii)Hybrid Rxs(iii)Minitiarization(iv)High Speed ICs(v)Simulation

2. ELECTRONIC ATTACK(i)Non Destructive EA(ii)Destructive ES

4.1a BURN THROUGH RANGE4.1b J/S RATIO4.2 JAMMER CHARACTERISTICS4.3 NOISE JAMMING

(i) Barrage Jammer(ii) Spot Jammer(iii) Sweep Jammer(iv) Sweep Lock-on Jammer(v) Cover Pulse Jammer(vi) Side lobe Jamming

4.4 DECEPTION JAMMING(I) Range Gate Deception

(a) Frequency Memory Loop(b) Digital RF Memory (DRFM)

(ii) Angle Deception

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(a) Conical Scan Radar: Angle Deception(b) Conical Scan Radar: Combined Range & Angle Deception(c) Monopulse Radar

(ci) Formation jamming(cii) Blinking(ciii) Cross polarization(civ) Cross Eye

(d) CW Radar Velocity Gate Pull Off (VGPO)

4.5 EXPENDABLE JAMMER

(i) Chart(ii) Decoy

4.6 AIRBORNE EA(i) Self Protection Jammer(ii) Stand Off Jammer (SOF)(iii) Escort Support

4.7 EFFECTIVENESS OF JAMMER POWER4.8 EA TECHNIQUES Vs RADAR TRACKING TECHNIQUES4.9 ASSESSENT OF JAMMING EFFECTIVENESS AGAINST RADARS

(I) Algorithm method(ii) Weapon System Effectiveness Method

4.10 FUTURE TRENDS IN EA SYSTEMS(i) Power(ii) Power Management(iii) Modulation Techniques(iv) Frequency Band

(v)Sensor Integration(vi) Stealth(vii) Artificial Intelligence(viii) Integrated Distributed EA

4.11 PLATFORMS FOR EW SYSTEMS(I)Airborne(ii) Ship mounted(iii) Ground Based(iv) UAV Based(v) Submarine

1. ELECTRONIC PROTECTION (EP)1.1 RADARS1.2 RADAR CONCEPT

(i)Range(ii)Angle(iii)Velocity

1.3 EP TECHNIQUES

1.4 TRANSMITTER FIXES(i)Frequency agility(ii)Diplexing(iii)Power Add(iv)Long Pulse Duration(v)Pulse Compression(vi)Staggered PRF(vii)Jittered PRF

1.5 RECEIVER FIXES(i)Manual Gain Control(ii)IAGC(iii)Logarithmic Rx(iv)Fast Time Constant (FTC)(v)Dicke-Rix Rx(vi)Pulse Width Discriminator (PWD)

1.6 ANTENNA FIXES

(i)Side lobe Cancellation (SLC)(ii)Side lobe blanking (SLB)

1.7 FUTURE TRENDS IN EP(i)High Transmitter Power(ii)Electronically Steerable Antennas(iii)Sensor Fusion(iv)Multi-static Radar(v)LPI(vi)Ultra Agile Carrier Frequencies(vii)Deceptive Transmissions(viii)Intra Pulse Modulations(ix)Ultra Low-side lobe Antennas(x)Multifunction Antenna

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2. CONCLUSION

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A NON-MATHEMATICAL OVER VIEW OF RADAR EW

Prof. G Kumaraswamy Rao,( Ex Director DLRL& Scientist H )

HOD ECE Dept.Bharat Institute of Engineering & Technology

1.HISTORICAL BACKOUND :

It was about 5.30 p.m. on a Saturday afternoon in October 1967 the Eliat destroyer of Israel which was on a routinepatrol Mission, was hit by two Styx radar-guided missiles and sank. Forty Seven crew members of the Eliat died. The enemywas underestimated and the OMINT ESM system failed. The tunable microwave APR-9 receiver of Korean war vintage on theship Eliat could hardly match the capability of Styx which was a fire and forget surface-to-surface missile. It has on board atargeting radar which switches on only in the mid course to its target. That gave the Eliat crew less than a minute and half toreact. What went wrong? Would the tragedy have been avoided? That was the wake-up-call for the EW designers and ittriggered maximum awareness of importance of EW.

On May 4, 1982 an AM-39 Excocet missile fired from an Argentine Super Etendard ripped into the British HMSSheffield, during the Falkland war. 24 British sailors were killed and 24 wounded. The destroyer was detected by an SP-2HNeptune maritime patrol aircraft earlier that morning. The two SUs which launched the Exocet missiles traveled at low altitudeie 30 m to avoid detection by the Sheffield's radar and maintained strict radio silence. They popped up to 160 m twice to

determine the co-ordinates of Sheffield. The Scheffield radar failed to forewarn the ships crew about the SUs. Then the EWRadar Warner also did not do the job. According to one report, the ships radar warning systems were programmed to identitythe Exocet missiles as friendly. This incident baffled the EW military planners. The destruction of the Sheffield taught not onlythe British Navy, but navies all over the world, a valuable lesson. More and sophisticated EW systems are necessary toforewarn the impending dangers. Rapidly deployable counter measures are critical to the life of military vessels - and to thelives of those abode.

2. INTRODUCTION :

The extensive use of EM (Electro Magnetic) spectrum to Communication, Radar and Navigation enhanced thefighting capabilities of Armed Forces across the world. Radio Communication provided coordination between forces, radionavigation gives accurate location of the deployed forces and radar performs surveillance of the battle space to monitor forcedeployments and detect hostile forces. The EM spectrum is so extensively exploited, today without its use, the survivability ofarmed forces is impossible.

EW is the defined as the science of preserving the use of the electromagnetic spectrum for friendly use whiledenying its use to the enemy. EW technology extracts essential information from the EM environment. This information is then

exploited to influence enemys capability to coordinate i ts activities, to restrict its communication media, to deny the use ofradar for weapon launching or guiding. EW enhances the survivability of own forces by denying the use of EM spectrum by theenemy.

EW can be broadly classified based on (i) frequency spectrum; (ii)functionality; (iii) intended role.

2.1 FREQUENCY SPECTRUM CLASSIFICATION:

EW is divided into three groups based on frequency (i) Communication EW (ii) Radar EW (iii) EO/IR EW(iv) Hybrid EO/IR-RF EW

(i) Communication EW: Communication links are required to transmit & receive, voice, digital data, FAX etc.between land forces, aircrafts and ships. They may use HF (3-30 MHz) VHF (30-300 MHz) and UHF (300 MHz - 3 GH).Communication link data rates depend on link bandwidth, modulation technique and signal to noise ratio. Advances incomputer technology have enabled increased communication link capacity for handling and processing data. The high datarates permit transmission from satellites and between precision weapons and launch platforms. Communication EW involvesinterception, direction finding and analysis of hostile emissions, whether voice or data link. Analysis of intercepted signalprovides valuable information for command and control purposes. This real time data is necessary to counter the enemy'scommunication system by jamming.

(ii)Radar EW: Armed forces use radar in both defensive and offensive weapon systems. Reflected R.F. echoes ofthe target, are used to measure target range, bearing and elevation and determine target location. Radar uses RF transmissionranging from high frequency (HF) to millimeter waves (30 MHz - 95 GHz). The frequencies are designated by variousalphabetical letters. Both old and new designations are given at Table 1. RF can be pulsed or continuous wave (CW). Radarfunction includes target detection, acquisition, tracking and navigation. Radar extracts range, bearing and speed of a target.Radar information is used for launch and control of a weapon s like missile, air defence gun etc. Modern advancements includePhase Array Antennas, Complex modulation on the radar pulse, Low probability of Intercept Radars, improved signalprocessing to extract data from highly corrupted echo signal etc.

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Radar EW involves extraction of detailed information of Radar Signals emitted, use of this information either toformulate Electronic order of Battle (EoB), or provide the information to a jammer to operate in an efficient way.

TABLE NO 1 : RADAR FREQUENCY DESIGNATIONS

(iii) EO/IR (Electro optic/Infrared) EW: There has been a steady growth in EO/IR guided weapons. EM spectrumextends beyond Millimetric Waves. The IR encompasses wave length from 1000 (300 GHz) to 1 micron. In general hot targetslike jet engines emit IR energy in the 0.75 u to 3 u range. EW systems and threats receivers use IR energy to detect, identify,locate and guide missiles to radiating objects. IR guided missile detects the IR signature of the aircraft and home on to theemitter of the IR energy. The plume, the tail pipe, heated leading edges of wings, or the IR image of the aircraft itself can besource of IR for locking on of the missile using an IR seeker. EW involves the detection and location of an incoming missile bya missile warning receiver and deflection of the same by launching chaff etc.

(iv) Hybrid IR/EO-RF: Present trend is to develop a hybrid receiver which fuzes the data obtained from Radar,

IR/EO sensors, ESM sensor etc. to obtain more accurate identification and location of a missile emitter. These hybrids providea high resolution three-dimensional target information that greatly improves EW response.

2.2 FUNCTIONALITY CLASSIFICATION

EW is classified based on functionality into three groups (i)Electronic Support (ES); (ii) Electronic Attack (EA)(iii) Electronic Self Protection (EP). These are the new names given to (ii) Electronic Support Measures (ESM) (ii)

Electronic Counter Measure (ECM) and (iii) Electronic Counter Counter Measures (ECCM) respectively.

(i) Electronic Support (ES): ES also is known by old name ESM (Electronic Support Measure) ES involvessearch, intercept, locate, record and analyze radiated EM energy for the purpose of exploiting 'the radiation information eitherfor formulating EOB (Electronic Order of Battle) or to provide the real time information to EA system. ES provides surveillanceand warning information derived from intercepted EM environment emissions.

(ii) Electronic Attack (EA):EA also is known by old name ECM (Electronic Counter Measure). EA involves action taken to prevent or reduce

enemy's effective use of EM spectrum. It can be active like Jammers, or it can be passive like chaff.

(iii) Electronic Self Protection (EP): EP also is known by old name ECCM (Electronic Counter CounterMeasures). EP involves actions taken to ensure friendly use of EM spectrum despite the use of EA. EP protects own platform

against EA used by the adversary (enemy).

2.3 ROLE CLASSIFICATION

EW can be classified based on the role it has been assigned to carry out (i) Tactical (ii) Strategic.

(i) Tactical Role : Tactical Role for EW encompasses the use of information obtained through ES in real time forimmediate use like in case of electronic jamming etc. As such the ES equipment should be wide open in frequency and shouldcover wide angle 0 to 360 in space.

(ii) Strategic Role : Closely related to tactical role is the use of EW for signal intelligence (sigint) gathering, forstrategic role. SIGINT is more strategically oriented although its information is often of tactical importance. Sigint provideimportant information for design of EA and EP. Sigint can be COMINT (Communication Intelligence) or ELINT (ElectronicIntelligence). ELINT involves the collection of technical information on the radar emitters of the adversaries whenever they areswitched on, recording them, and analyzing them. The receivers used will be more sensitive than the ES Rxs, Elint has theobjective of securing the maximum possible data on EM environment. The information can be used to determine the enemy'sEoB, it is also used to form a threat library for use in RWR ,ES and EA equipment. AWACS (Airborne Warning & Control

System) is an example for SIGINT system.

ELECTRONIC SUPPORT

3. ELECTRONIC SUPPORT (ES) :

ES also known by ESM, involve actions taken to search, intercept, locate, record and analyze radiated EM energy.The information is employed for threat recognition and to use it in tactical employment in EA equipment. The ES function is forreal time use, whereas Elint Rxs are used for intelligence collection, which can be subsequently used. Elint Rxs performfine grain analysis of emitters of interest. If the data cannot be analysed immediately, it can be stored and analyzed at a latertime.

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EW Rxs generally measures quantitatively the following parameters

1. Frequency2. Amplitude (Power)3. Angle of Arrival (AOA) or D.F. (Direction Finding)4. Time of Arrival (TOA)

5. Pulse width (PA)

6. Pulse Repetition Time (PRT)7. PRI type8. Scan type and rate

9. Lobe Duration (Beam width)The parameters that are measured based on single pulse are (i) Frequency (2) Amplitude (3) AOA (4) Pulse width

and (5) TOA .The parameters that are measured based on group of pulses are (1) PRI (2) Scan Characteristics

IMPORTANCE OF EMITTER PARAMETERS DURING SIGNAL PROCESSINGS.NO.

Parameter Pulse Train De-interleaving

Emitter Identification Intercept Correlation

1. Frequency 2 2 2

2. Amplitude 1 0 13. Angle of arrival 2 0 2

4. TOA 0 0 15. PRI 2 2 2

6. PRI type 2 2 27. PW 2 1 1

8. Scan Rate and type 0 2 19 Lobe duration 0 1 1

0=Not useful 1= Some use 2= very useful

The ES Rxs can be classified by their (a) Application and (b) Structure.

3.1 ES RX BY APPLICATION:

(a) RWR (Radar Warning Rxs); (b) ESM Rxs; (c) ECM Rxs; d) Elint Rxs. e) MWR (Missile Warning Rxs)

(i) RWR : A RWR detects the weapon radar and provides warning to the pilot. As soon as the radar locks on to thetarget, the receiver with a moderate sensitivity detects the main beam. It provides the frequency, direction, characteristics of

emitter which is used for sorting and identification. The RWR Rx will have a wide frequency and spatial coverage. RWRs aregenerally the simplest form of ESM Rx consisting of a low sensitivity equipment (generally -40 dBm sensitivity). It is preset tocover the expected characteristics of threats. It uses the range advantage to detect a threat. RWRs initiates ECM actions.Unlike Elint & ESM Rxs RWRs do not record the electronic data.

(ii) ESM Rxs : An ESM Rx is used to obtain all the information about the emitters to determine the EOB (ElectronicOrder of Battle). The Rx needs to collect all the information of all the radars in the environment. So it is wide open in frequencyand have wide spatial angle. They are more complex than RWRs, have more sensitivities to intercept radars through sidelobes. They have higher Direction Finding Accuracy. They record the threat data for future use. The accuracy of AOA in ESMRxs are superior to that of RWR Rxs.

(iii) ECM Rxs : An ECM Rx works in conjunction with EA/ECM system. To concentrate the energy of a jammertowards the victim radar, frequency as well as AoA information of the radar is required. ECM Rx provide these. The Rx shouldinterface with EA and quickly set the EA system to operate in its high efficiency mode. Usually ECM Rxs if used on airborneplatforms, are called RWR Rxs.

(iv) ELINT Rxs : An Elint Rx works for collection of data and information useful for strategic planning. It does fine-grain analysis and therefore has a very large sensitivity compared to ESM Rx. Its instantaneous bandwidth is far less than theESM Rx since it covers one or a few signals of interest. If the data cannot be analyzed at the collection station, it can be storedand analyzed at a later time. It normally works in peace time to collect as much data as possible about the adversary. Itssensitivity sometimes exceeds -85 dbm

(v) MISSILE WARNING Rxs (MWR): MWR detects an approaching missile. The pilot is warned and EA system ofthe aircraft is activated. MWS are divided into 2 groups (i) Active and (ii) Passive. Active Systems use a pulse DopplerRadar for detecting the incoming missile. Passive System use IR/EO sensors. It detects the IR radiation generated by themissile. Missile Rocket motors emits IR radiation in the range of 4.3 m.

3.2 ES RX BY STRUCTURE :

i) Crystal Video; ii) Superheterodyne; iii) Instantaneous Frequency Measurement (IFM); iv) Channelised;v) Compressive (microscan) vi) Bragg Cell and vii) Digital Rx and viii) Multibeam lens antenna.

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(i)Crystal Video Rx (CVR) : Crystal Video Rx is the simplest of all types. It is a diode (crystal) detector whose outputis amplified to an adequate level by a Video Amplifier. To obtain wide dynamic range a log-video amplifier is used. The detectoroperates in 'Square Low Region', so that output voltage is a function of the input power. The output of the crystal video receiveris a series of pulses with amplitude proportional to R.F. signal power . The sensitivity of a CVR is usually in the range of -35 to-50 dbm. They are normally used in RWR Rxs. The block diagram of Crystal Video Rx is shown at Fig 1.

Fig 1 : CRYSTAL VIDEO Rx

(ii) Super Heterodyne Rx:The Super heterodyne Rx heterodynes with RF frequency to convert the input to an IFband. A tuned (Local Oscillator) LO is used to shift the input R.F. A fixed IF is more desirable where in the necessary gain andfilter selectivity can be easily provided. Fig. 2 depicts the block diagram of a Superhet Rx. A Tunable Bandpass filter is usedafter the antenna. This is controlled along with the L.O. to select only the portion of the input spectrum that is converted to theIF bandwidth. This gives isolation from other signals to the Rx. Typically the sensitivity of a Superhet Rx is from -70 to -90 dbmdepending on the IF bandwidth. Frequency accuracy of the Rx is dependent on oscillator stability and resolution of IFbandwidth. Because of narrow bandwidth, this type of receiver is used in conjunction with other types of ES Rxs.

Fig 2 : SUPERHET Rx

(iii) IFM Receiver: An IFM Rx uses delay lines and measures the phase difference to measure its frequency. R.F. issplit into a delayed path with delay T and an undelayed path. is the phase angle between delayed and undelayed waves, = T. If phase angle is measured and the delay time T is known, frequency can be computed.

In an IFM receiver the phase angle is measured using the relation below:

Where X is the amplitude information. The signal is passed through a limiting Amplifier before applied to the R.F.power divider. One of the outputs of power divider is given to the phase correlator. Other input to the correlator is the delayedsignal by a specific time T. The correlator multiplies the undelayed signal and the delayed signal and produces Sine and cosineVideo outputs. They are digitized in a 8 bit quantizer and phase is computed. No. of delay lines and associated correlators

are used to obtain good accuracy and resolution. The longest delay line determines the frequency accuracy and resolution,

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Cos t

whereas the shortest delay line resolves ambiguity and determines the unambiguous frequency. Frequency measurement canbe done by conventional analog ways by comparing the amplitude in analog comparators. A Gray Code is obtained whichindicates the frequency of the incoming signal. Another approach is to digitize the correlator output by an A/D converter. Thedigitized data is fed to a ROM which perform =1/T tan -1 (Sin T/Cos T). The frequency is thus directly computed. TheIFM will encounter many problems when there are no. of emitters in space. The problems are (i) Simultaneous signals iepulse on pulse; (ii) Overlapping pulses; (iii) Pulse on CW etc. These problems have been solved to certain extent and lot isneeded to be done in the present high density emitter environment.

(iv) Channelised Rx : The ideal Rx for EW is channelised Rx. It consists of a set of fixed frequency Rxs with theirpassbands set such that the upper edge of the 3dB bandwidth of one Rx is same as the lower edge of the 3 dB bandwidth ofthe next. It provides a demodulated output for signals in each channel. It can have narrow bandwidth to provide excellentsensitivity. It provides 100% probability of intercept for signals within its frequency range. It also provides full feature receptionfor multiple simultaneous signals, as long as they are in different frequency channels. The problem ofcourse is in complexity ofimplementation and huge no. of fixed Tuned Rxs required making it very bulky and unwieldy. For 1 MHz of isolation acrossfrequency range of 2-4 GHz, 2000 channels are required ie., 2000 separate Rxs are required making it bulky. However, withthe miniaturization in VLSI technology, in future the size and weight can be brought down appreciably.

Fig 4 : CHANNELISED Rx.

(v) Compressive Rx: This Rx is also called microscan Rx. It is basically a superhet Rx that is rapidly tuned. The rateof scan in a compressive Rx is much faster than superheterodynde Rx. The output is passed through a compressive filter thathas a delay proportional to frequency. The delay versus frequency slope exactly compensates for the receivers sweep rate.Thus the output of the Rx is coherently time compressed to make a strong signal spike.

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2

Filter B.P .

(vi) Bragg Cell Rx : Bragg Cell Rx is a spectrum analyzer capable of handling simultaneous signals. The RF signalis applied to a 'Bragg Cell'. The cell deflects the light in proportion to the R.F. frequency of the signal. The light is focused onthe photodiode detector array. The spatial distribution across the photo diode array represents the instantaneous. Fouriertransform of the input signal. The array detects the deflection angles of the diffracted beam and produces output signals fromwhich a digital read out of all signal frequencies present are obtained. The Bragg Cell has limited dynamic range of 25 dB.

(vii) Digital Rxs : This is the Rx for future. It digitizes the RF signal to be processed in a computer. Software can be

written to simulate any type of filter or demodulator giving lot of flexibility and optimization. The problem of Digital Rx is in theavailability of fast (A/D) Analog to Digital Converters. For capturing the frequency we need a minimum of two samples. Presentday digital Rx is working with 6 GHz bandwidth with 10 bit (60 dB) dynamic accuracy.

Fig 7 : DIGITAL Rx

3.3 PARAMENTS MEASURED BY ES RXS :

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ES Receiver measures the following quantitatively.(i) Frequency(ii) Angle of Arrival (AOA)(iii) Pulse width (PA)(iv) Time of Arrival

(i) Frequency Measurement : The frequency information is used for both sorting and jamming. By frequencycomparison of pulses received, pulse trains of various radars can be sorted out. Knowing the frequency, jammer can be tunedto transmit in that frequency range.

(ii) Angle of Arrival (AOA) / Direction Finding : It is often important to determine the location of the emitter. Adirection finding (DF) system is useful in locating the signal source. A DF system gives the direction of emitter. Two or moreD.F. systems are necessary to obtain the location of the emitter by triangulation. Alternatively the D.F. platform can move inspace, and taking D.F. measurement at different times, it is possible to locate the emitter. There are no. of ways of determingthe AOA. They are broadly divided into a)Rotary D.F b) Amplitude Comparison c) Phase Comparison/Interferometry; d)Phase comparision / Digital Bearing Descriminator and e)Time Difference of Arrival T DOA.

(a) Rotary D.F (RDF) : The most simple method for measuring AoA is to rotate a narrow beam antenna at a fastspeed. As the antenna beam sweep over the emitter, it traces out the antenna pattern on an oscilloscopic display. By selectingthe peak of the pattern, the AOA of the emitter can be determined. An omni-directional antenna is generally required to avoidthe peak response of a sidelobe being interpreted as the direction of emitter. Obviously the system is slow as it employs amechanical rotating platform. Secondly the system is not suitable in a multi emitter environment. Normally RDF is used inconjunction with superhet Rx its sensitivity is very high.

(b) Amplitude Comparison : Amplitude comparison technique is extensively used due to its lower complexity andcost. The principle is to derive a ratio from a pair of independent receiving antenna channels. Typically four or six antennaelements and receiver channels are used to obtain 360 degree coverage. Wideband logarithmic video detectors provide thesignals for comparison and angle determination. The monopulse ratio is obtained by subtracting the detected logarithmicsignals and the bearing is computed from the value of the ratio. Four quadrant amplitude comparison systems are simple, lowcost and cover from 0.5 to 18 GHz. However, the accuracies obtained are poor (5 deg. RMS) and have very low sensitivity (-50dBm). No. of antennas can be increased to obtain higher accuracy at the cost of simplicity and reliability. The system accuracydepends upon the error shape and the amplitude imbalance between antennas. The error slope in turn is a function of antennabeamwidth and squint angle between two antennas. The fig shows a four quadrant amplitude AOA system.

Fig. 8 AMPLITUDE COMPARISION DIRECTION FINDER

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FIG 9 AMPLITUDE COMPARISION ANTENNA PATTERN

(c) Phase Comparison (Interferometry)

A two element phase comparison is shown in fig.10

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FIG 10 : TWO ELEMENT PHASE COMPARISON D.F.

4 B. PHASE D.F

(i) Linear Array (BLI) (ii) Circular Array (B

d

To avoid ambiguous phase measurements d Ex : For 3 GHz /2 = 5Cm

FIG. 11 LINEAR ARRAY

The two antennas are separated by a distance ' d'. A E M wave front coming at an angle , will produce a phase atthe output of phase correlator.

= 2 d Sine / where is the wavelength of the wave.

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It is seen that if 'd' is larger the accuracy is better. The maximum distance between two antennas without causingambiguity is half the signal wavelength. To obtain more accuracy without ambiguity, a multiple number of antennas with nonuniform spacing is used. The long distances provide the accuracy and the short distances resolve the ambiguity. The D.F. errordepends upon the frequency, phase measurement error and noise. Phase comparison D.F. systems comparatively give betteraccuracies than Amplitude comparison systems. The RMS accuracy could be around 2 Deg .

(d) Phase Comparison (Digital Bearing Discriminator) : This uses 16 antennas in a circular array. Each antennafeeds one input of a Bulter Matrix. Phase Angles and 4 are obtained as output of Butler Matrix. They are digitized andambiguity resolved to obtain a very high RMS bearing accuracies of 2 Deg. . Sensitivity and dynamic range of this type ofDBDs are far better than Amplitude Comparison D.F.

(e)Time Difference of Arrival :D.F. also can be measured by computing the time of arrival of a signal at differentlocations. These type of systems do not depend upon frequency. The D.F. accuracy depends upon the accuracy with whichtime difference can be measured. The D.F. accuracy is improved by using large baselines. As such these systems are used inships, are on aircraft with large wing span. Accuracies can be very high of the order of 1 degree RMS.

TDOA TIMEDIFFERENCEOFARRIV

Accuracy depends a) Measurement of time difference ac

b) length of base line

Ambiguity exists because of omni directiona

Base Line

a) clock = 100 MHz (.01

(f)Multibeam Lense Antenna:The multibeam antenna determines the direction by focusing the incoming signals toa point detector representing the direction of the emitter. The focusing property of the lens is independent of frequency.Direction of emitter is accurately determined over a wide frequency band.

FIG. 12 MULTIBEAM LENSE ANTENNA

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3.4 EMITTER LOCATION: Emitter location is important in enemys Defense suppression and weapon delivery systems. Emitter locationachieved by (a) making AOA measurements from a single moving platform through successive measurements and (b) makingsimultaneously measurements from multiple platform.

3.41EMITTER LOCATION OF STATIONARY GROUND BASED EMITTERS FROM AIRBORNE PLATFORMS:

(i) Azimuth Triangulation Method: Intersection of successive spatially displaced bearing measurements providesthe emitter location.

(ii). Azimuth and Elevation Location Technique: This technique provides single pulse instantaneous emitterlocation from the intersection of the measured azimuth / elevation line with earths surface .

(iii). Time Difference of Arrival (TDOA): This method is also called precision Emitter location systems (PELS)method. This measures the time of arrival of single pulse at three spatially remote locations.

(iv) Phase Rate of Change: This makes calculations using phase derivative.

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(v) Angle Distance Technique: Distance is computed from the signal strength received. This is possible when thecharacteristics of emitter is fully known.

(vi) RF Doppler Processing: Doppler changes as the aircraft varies direction with respect to target radar

3.42 RELATIVE ADVANTAGES OF EMITTER LOCATION TECHNIQUES:

Measurement Technique Advantage Disadvantage1. Triangulation Single aircraft - Non Instantaneous

location.- Inadequate Accuracy forremote targeting

2. Azimuth / Elevation - Single aircraft- Instantaneous location

possible

Accuracy degrades rapidlyat low altitude

Accuracy is Function ofRange.

3. Time Difference of Arrival (PulsedSignals)

- Very High precision- Can support weapon

delivery positionrequirements

- Very rapid, can handleshort on time threat

very complex Atleast 3 Aircrafts. High quality receivers Very wideband data link Very high performance

control processor

Requires common timereference and correlationfor non signed signals.

3.43 FUTURE TREND OF ES RXS

(i) LPI Radar Detection : The performance of Rxs require to be enhanced to cater to the present day modern radarespecially LPI (Low Probability of Intercept) radars. LPI Radars (a) use low peak power and large time width and largefrequency bandwidth; (b) use waveforms which is difficult to identify such as pseudonoise. In this type of radars the transmittedpulse is as much as 20 dB smaller in peak power but as muchgreater in average power.

The power spectrum of the stretched pulse is spread over a wider frequency range. This spread spectrum will bebelow the sensitivity of the ES Rx. So the future Rx should have high sensitivities between 85 to 90 dbm to detect the LPIRadars. The research and development of ES Rxs should concentrate on channelizers, compressive Rxs etc. These have thepotential for high sensitivities and good performance.

(ii) Hybrid Rxs : The EM environment has become so complex, a single ES Rx is not going to give the requiredperformance. Hybrid Rx consisting of several Rxs perform better. These Rxs will functions together simultaneously and theyare integrated by a common processor. RWR, MAWR (Missile Approach Warning Rx) and a EO/IRRx in a single package isnot uncommon these days.

(iii) Miniaturization : The technological advancement in MIC (Microwave Integrated Circuits) and the concept ofsuper components will make the size of the EW Rx very small, at the same time improving the reliability, weight and size.Channelised Rxs may become less bulky.

(iv) High Speed ICs : The availability of high speed A/Ds, D/As etc. will revolutionize the concepts and provide thepossibility of digitizing the input RF signals directly. Thus the realization of Digital Rx, with the associated flexible software etc.will solve many of the limitations of the present day Rxs.

(v) Simulation : Developing an EW Rx is a very expensive and time consuming job. Some of the mistakes in thehardware could be avoided if a model of a Rx is conceived and it is subjected to all simulated inputs. Based on the outcomeactual hardware will be built. This will cut short effort and time.

ELECTRONIC ATTACK

4. ELECTRONIC ATTACK (EA)EA is also known by name ECM (Electronic Counter Measure). The aim of EA is to interfere with the enemys

effective use of EM spectrum. Sometimes EA also includes the use of high levels of radiated power or directed energy tophysically damage enemy assets. Jamming is called Soft Kill, since it temporarily makes enemy assets ineffective.

EA is categorized into (i) Non-destructive EA (II) Destructive EA.

(i) Non-Destructive EA: This interferes with the operation of sensors (Radar/IR/EO). This creates a false image orchanges the image on a radar display. A Repeater creates false echo by delaying received radar signal and retransmitting. Atransponder plays back a stored replica of the radar signal. The threat from IR guided missiles is countered by using Infrared

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Counter Measures which introduces large amount IR noise into the Seeker. Passive EA employs chart and active decoys todivert the missiles.

(ii)Destructive EA: Two classes are available (i) Use of Anti radiation missiles like SLAM (Suface Launched AntiMissile ) HRAM , Sidewinder etc. (ii) Direct Energy Weapons generates huge amount of energy in the form ofElectromagnetic Pulse (EMP) or high power laser beam. They destroy front end electronic devices or optics in the systempermanently.

Jamming is to place an interfering signal into an enemys receiver alongwith the desired signal. Jamming is effectiveonly when the enemys Rx is unable to recover the information from the desired signal. Radar which accomplishes the task ofearly warning, acquisition, tracking and guiding a weapon is a sophisticated system normally possesses the anti jammingfeatures. EA should accomplish its job by overcoming the EP (Electronic Protection) features of the radar. It is a continuing

game of one-man up show between EA and EP. The Electronic device to achieve this purpose is a RF transmitter calledJammer.

Some of the features which makes the jammer effective are as follows :

i)RF frequency of jammer should match with the radar frequency. ii) Interference should be continuous, if necessarybroad band of frequencies, or no.of jammers are used to counter search radars,Air defence radars, track radars, weaponguidance radars simultaneously.

4.1A BURN THROUGH RANGE

Since jamming is one way transmission, it has distinct power advantage over radar reflected power. However, whenthe distance between the radar and the jammer keeps on decreasing, the jammer power received at radar increases by thesquare of the distance, whereas the echo power received at the radar increases by fourth power. Therefore at some distancebetween Radar and Jammer, the echo power is just greater than the jamming power and the radar starts to see through the

jamming. This distance is called the Burn Through Range. The Radar peak power and the power of Jammer play an important

role in determining this range.

4.1B J/S RATIO

J/S is the ratio of the jamming signal strength to the strength of the desired signal. Fig : 12 depicts the J/S ratiodiagram

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Fig 13 : J/S Ratio Receiver Pass Band

The desired J/S required for effective jamming can vary from 0 to 40 dB depending on the type of jamming andmodulation. 10 dB J/S is a reasonable figure in many applications. J/S is directly proportional to a) Jammer Power,b) Jammer Antenna Gain c) Radar to target distance and inversely proportional with a) Radar Power b) Jammer-to-Radardistance c) Radar Antenna Gain.

4.2 JAMMER CHARACTERISTICSDifferent types of jammers are employed for different types of radars. Basically jamming is divided into three

categories (i) noise jamming (ii) deception / confusion jamming (iii) modulated jamming. The power of Jammer alone does notdetermine the effectiveness of that Jammer. The type of Jamming used is extremely significant.

4.3 NOISE JAMMINGNoise Jamming involves modulating an RF carrier wave with noise and transmitting the carrier wave towards the

radar. The jamming signal has far greater power than the power of echo signal. Since the radar receiver is extremely sensitive,this saturates the Rx. If the S/N is less than 1, the target return is lost. The jamming becomes effective. There are six types ofNoise Jammers. They are (i) barrage (ii) spot (iii) sweep (iv) sweep lock-on (v) cover pulse and (vi) side lobe.

(i) Barrage Jammer : Barrage jammers are Wideband noise transmitters. They deny to the adversary use of wideportion of EM specturm of frequencies. This type of jammers have two advantages (a) No. of enemy radars can be jammedand (b) frequency agile radars can be jammed. The disadvantage is that its power density goes down because of largebandwidth. At the radar frequency of interest the power of the jammer may not be effective for jamming.

(ii) Spot Jammer : Spot Jammer have very narrow bandwidth. The chief advantage of spot jammer is that theiroutput is concentrated in a narrow band and as such the jamming will be effective. The disadvantage is that it can jam only oneemitter and if more no. of radarare their in space that many no. of spot jammer are to be carried. Also it is not effective againstFrequency Agile radar.

(iii) Sweep Jammer : Sweep Jammers transmit a narrow band spot jamming signal but the spot frequency is sweptback and forth over the desired band. The advantage is that all radars get covered by the high power density. But thedisadvantage is that it is not continuous. However fast sweep jammers can overcome this to a certain extent. Fast sweep

jamming approximates a burst of energy and causes oscillation in the receiver amplifiers which lasts until the sweep jammeronce again passes through the radar frequency and sustains the oscillation.

(iv) Sweep Lock-on Jammer : This type of jammer is essentially a swept jammer with additional feature of lock-oncapability. It consists of a jammer and a receiver. The jammer and receiver are swept over the same frequency band. Whenthe Rx encounters a signal, the frequency sweep is halted and the jammer becomes a spot jammer at this frequency. A lookthrough facility is provided, wherein the Rx can be made to start sweeping again when the original signal being jammeddisappears.

(v) Cover Pulse Jammer : This is also called smart noise jammer. A repeater type of jammer is used in thetransponder mode. The received signal is used to set on the frequency of a low power oscillator. PRI of radar is obtained fromES Rx. The time of arrival of the subsequent radar pulse is predicted and the oscillator is gated ahead by a few microsecondsto a few microseconds later. The oscillator is noise modulated, signal amplified and transmitted. This type of jammers iseffective for a steady PRF. By time-sharing the system, no. of radars can be jammed.

(vi) Sidelobe Jamming : The objective in jamming a search radar through the sidelobe is to make a large sector ofthe radar display unusable.

4.4 DECEPTION JAMMING

The noise jammers discussed so far, create jamming strobes on the radar scope and thereby hide the target echoand deny the range information. However, the centre of the jamming strobes gives out the angular information which is what isrequired for a weapon guidance. So to corrupt the angle tracking of the radar, deceptive type of jamming is used. This willinject angle / range errors into the radar.

(i) Range Gate Deception : The Jammer Receiver receives the radar pulse, amplifies and retransmits. The jammersignal appears on top of the echo received by the radar. The AGC of the radar lowers the gain of its amplifier since the jammersignal is stronger. The range circuit with the lowered gain, will then only see the jammer signals and not the echo. In effect therange gate is captured. Now the jammer retransmits the radar signal received, with an increasing time delay. The radar followsthe false signal and thus the true range of the target is denied. The jammer range gate pull off rate should not exceed theradars tracking rate, at the same time it should be fast enough for better protection. Modern radars use leading edge tracking

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which defeats the range gate pull off technique. To overcome this problem Rage gate pull in technique is used. For this, it isnecessary to know the PRI of the radar pulse to predict the arrival time of the next pulse. However, this technique requiresgreat sophistication for employment against staggered pulse train and cannot work at all against randomly timed pulses.

FIG. 14A RANGE GATE PULL OFF

FIG. 14B RANGE GATE PULL OFF

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(a) Frequency Memory Loop: The most common method of implementing RGPO is to use a Frequency MemoryLoop shown in Fig. 15

FIG 15 : FREQUENCY MEMORY LOOP

A portion of the received pulse is stored in the delay line through which it is recirculated for the memory period td,which is less than shortest pulse to be handled. The initial position of the pulse is amplified in the input limiting amplifier andpassed through the delay line. As then signal reaches the line output, the switches are thrown open and the pulsewith length td is recirculated again and again through the memory loop. Gated Amplifier is used to give the output at theappropriate time and for the desired pulse duration.

(b) Digital RF Memory (DRFM):Various defects in the analog FML is overcome in the DRFM. The RF is firstconverted to IF, sampled and a high speed A/D is used. The digital output of A/D can be stored without any degradation for anylength of time. The signal is replayed from memory whenever needed and up-converted to the transmission frequency usingthe same LO which is used for down conversion. Present DRFM has an IF of 1000 MHz and the limitation is mainly the speedof A/D required, since if the signal is to be reconstructed the sampling should be twice of the input frequency. The DRFM is avery versatile tool to obtain various types of modulations for EA system.

(ii) Angle Deception : Normally tracking radars use either conical scan or monopulse tracking receivers to obtainangular information.

(a) Conical Scan Radar : Angle Deception : A conical scan radar scans its antenna beam in a circular motion. Acone is formed in space. Information from the scan is used to move the antenna in a servo loop so that the target is in thecentre. If the target is slightly non-centered, a sinusoidal output is observed. The modulation envelope of the echo signal isdetected, inverted and presented as jamming signal. Echo and Jammer signal get combined in space and the resultant input atRadar tracker is a constant amplified signal. The tracker stops moving and angular error is thus introduced. In this techniquethe gain of the repeater is varied inversely with the radar signal strength received. The power output required from the jammeris quite modest.

(b) Conical Scan Radar : Combined Range & Angle Deception : The effectiveness of the inverse gain angularmodulation technique can be increased by first pulling the range gate away from the target echo and then applying the angledeception.

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FIG.16 JAMMING CONICAL SCAN RADAR

(c) Monopulse Radar : Jamming of this type of radar is most difficult. The reason is that the Rxs gets all theinformation required to track a target from a single return pulse. It does not need a series of pulses. If cover pulse jamming isused in this case, the monopulse radar will treat the jammer as a beacon, and the tracking becomes much easier.

(ci) Formation Jamming : Two or more aircrafts are required to be simultaneously in the beam of the monopulse.The jamming signals sent by the two aircrafts should be same. The tracker wanders between these two targets.

(cii)Blinking : For this two or more jammers are required. In a ship they can be mounted on port and start board.The jammers may employ cover pulses. The jammers are turned on and off in sequence. The physical separation should besuch that they are at the extreme ends of the monopulse beam. If blinking is proper, the tracker will transfer from one jammerto another and in the process loose track because of over shoots of servo system.

(ciii)Cross Polarisation : The jammer will sense the polarization of incoming signal and transmit a crosspolarised signal that is much stronger than echo. When the radar receives a signal that is polarised at right angles to thepolarization of radartransmitter, erroneous angular information is generated.

(civ)Cross Eye : This consists of two sets of repeaters arranged so that the two transmitting antennas as viewedfrom the radar appear to be 180 out of phase. The two transmitting antennas are located at some distance apart. They can bemounted on the farthest wing tips of an aircraft. The signals from the two repeaters will be 180 out of phase when they reach

the radar tracking antennas independent of the direction to the radar. This causes a null in the combined response of theradars sensors just where the radar tracking circuit would expect a peak. If there is a null, where it is supposed to be a peakthe tracking signal will be distorted and the angle information provided becomes erroneous.

(d) CW Radar Velocity Gate Pull Off (VGPO) : CW Doppler type radars are jammed by stealing the velocity gate.By changing the frequency of the repeater by a sawbooth modulation, a VGPO is accomplished.

4.5 EXPENDABLE JAMMER

Expendable devices include chaff and decoy :

(i) Chaff: Chaff is an extremely effective EA device. Chaff is made of aluminium strip or aluminium coated nylon orfiber glass. It is packaged in small units. They are light enough to carry and dispensed in large quantities during a sortie. Chaffconsists of large number of dipole reflectors which are designed to match the half wavelength of victims radars frequency. Itre-radiates the EM energy and create false radar echo. Chaff is made to respond to no. of radar frequencies by simply packingdifferent lengths of dipoles in the same bundle. An aircraft can safely pass through a air defence net, by creating a multitude ofechoes over a large area by confusing the radar operators by dispensing the chaff in a burst or in random.

(ii) Decoy : By duplicating the speed, altitude and course of penetrating aircraft, a deceptive target can be introducedinto the radar system. This could be a small expendable aircraft like vehicle / glider launched from the mother aircraft. Theycarry corner reflectors to enhance their RCS (Radar Cross Section) and provide a echo return similar to a penetrating aircraft.

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FiG. 17 ACTIVE DECOY

4.6 AIRBORNE EA

Airborne EA consists of a) Self Protection Jammer b) Stand off Jammer c)Escort support

Fig. 18 JAMMING STRATEGIES

(i) Self Protection Jammer (SPJ): Self Protection Jammer is provided on platform being targeted by a radar.

(ii) Stand Off Jammer (SOJ): SOJ remains out of range of radar but consists of high power Jammerto protectplatform that has entered the enemys radar range for military operations.

(iii)Escort Support: Jammer aircraft flies along with the attacker aircrafts and provide protection from the missileattack. This also reduces the Burn Through Range.

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4.7 EFFECTIVENESS OF JAMMER POWER

ECM effectiveness is improved by directing the jammer in the direction of Threat. This is achieved by (i) mountingthe EA System on a servo pedestal and using i) an Auto Track Rx or (ii) using a Rotman lens.

The Auto track Rx is a monopulse receiver which determines the error between the direction of Jammer beam andthe direction of the Threat. This error drives the servo pedestal in azimuth & elevation until both direction coincides andmaximum Jammer power reaches the threat.

In case of Rotman lense type of jammer antenna , each array element is fed by individual low power TWT(Travelling Wave Tubes) High ERP (Effective Radiated Power) and instant positioning of Jammer beam are the twoadvantages of this technique.

Fig. 19 ROTMAN LENS JAMMER

4.8 EA TECHNIQUES VS. RADAR TRACKING TECHNIQUES :

There are multiplicity of EA techniques. They generally fall into either noise (active) or deceptive (active) or chaff /decoys (passive). The answer to what technique is required to be used against which radar? is not easily answerable. It isonly combination of no. of techniques judiciously used that is going to make the EA effective. With the advancements in radarthe complexity of integrating these techniques becomes a complex job. A computer controlled Technique Generator systemwill be an effective tool to incorporate the automatic application of these techniques either independently or in combination.

4.9 ASSESSMENT OF JAMMING EFFECTIVENESS AGAINST RADARSPresently the following two methods are mostly used :

(i) Algorithm Method : Space energy relation is computed alongwith jamming equation. Range where jammer iseffective is calculated. For this transmitter power, antenna gain and jamming signal shapes are taken into consideration alongwith the radar characteristics. This method is suitable for one to one case.

(ii) Weapon System Effectiveness Method : Radar and its EP characteristics are part of the weapon. EAs effecton weapon effectiveness is evaluated in this method. The probability of survival of a fighter plane depends on the jammingsystem effectiveness index. Effective index is computed by statistical methods using simulation techniques. For this earlierfigures of loss rates that have taken place during combat are made use of.

4.10 FUTURE TRENDS IN EA SYSTEMS

A desired jammer system should consist of reconnaissance, warning receivers and jamming radiations, all of them

integrated and controlled by a computer. It will have large power, wide frequency band, fast response, capability to jam multipletargets and self-adapting capability.

(i)Power: Power is the basic index of the jammer. There are three ways to raise power a) Raise the power of singleTWT from the present value b) Install no. of low power TWTs in each element of a phased array antenna. With large no. ofthese TWTs, a higher ERP is obtained by combining the power in space. By judicious control of amplitude and phase thedirection of the beam can be controlled. Such systems are now known by MBJ (Multi Beam Jammer) Phased Array Systems.However, the Phase Array Jammers work in limited band of frequencies c) Develop Microwave Broadband Solid StateAmplifiers and use Solid Array jamming system. With passage of time, power outputs from these Solid State Amplifiers aredramatically increasing.

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(ii) Power Management : More efficient power management is required in view of limited power available especiallyin airborne applications. The power transmitted should be with in the detection bandwidth of victim radar, and also should be inthe region of the radar. This ensures maximum interference.

(iii) Modulation Techniques : Appropriate and effective modulations are transmitted to confuse or deceive theradar. Real time jamming effect is carried out and automatically strategies changed dynamically. This is necessitated in view ofvery complex advancements in the EP fields of radar. In order to jam CW and agile frequency radars, frequency trackingvelocity and accuracy are required to be enormously increased.

(iv) Frequency Band : Very broadband frequency jammers working from MHz going to millimetric way frequenciesare necessary sinceMMW Missile Launch Radars are already in existence.

(v) Sensor Integration : Integration of data from ES, infrared / optics and missile warning Rxs is necessary to arrive

at faster decision to deploy the required jammers with necessary modulations, launch chaff, flares, decoys etc., either singly orcombinedly.

(vi) Stealth :Stealth vehicles have very low RCS. Any EA system on board, these vehicles will be required tooperate in a manner that prevents radiation giving way to detection of stealth vehicles. EA systems are required to beseparated before they are activated.

(vii) Artificial Intelligence : Fixed algorithms for EA management become obsolete with advancement of radar. Theability to learn and adjust to dynamic changing environment should be incorporated in future EA systems. Learning algorithmsare required to be incorporated.

(viii) Integrated Distributed EA : There may be no. of EA systems deployed in a hostile area of conflict. Theypresently work autonomously. If co-ordination and integration is introduced between these stations, this will lead to effectiveuse of EA resources. One way is to employ a stand-off vehicle like AWACS to monitor, determine and deploy the type oftechnique etc., to jam each radar.

4.9 PLATFORMS FOR EW SYSTEMS:

(i) Airborne: Attacking aircrafts carry RWRs/MWRs/IR or EO warners to warn the pilot about the incoming missilesand automatically activate EO modules like chaff or passive/active decoys. EW Systems operate in X, Ku or IR/EO bands,space and power are scarce on the airborne platform.

(ii)Ship mounted: EW Systems mounted on ships, patrol boats etc. are generally larger. Power and space are nota constraint. As such they cater to larger bands of frequencies like 0.5 GHZ to 40 GHZ. ES and EP Systems are invariably apart of the ships defence Network Systems. Larger ERP from jammers are possible.

(iii)Ground Based: These Systems are very large. Radar EW and Communication EW are some times integrated.These are called Integrated EW Systems. They are usually part of Air Defence Network System.

(iv)UAV Based:Unmanned Air Vehicles (UAVs) are small airborne platform. They travel into enemy territory withoutbeing radar tracked (because of small RCS) and collect information on enemys EM usage. They carry only ES systems oflimited bandwidths. Enemys EM information is collected for strategic purpose and formulate the Electronic Order of battle(EOB).

(v)Submarine: Pops out of water for a short time near enemys sea shores and gather information on enemytransmissions. This is mainly for strategic purposes.

ELECTRONIC PROTECTION

5.0 ELECTRONIC PROTECTION (E.P) :

E.P is also known by ECCM (Electronic Counter Counter Measure). E.A systems are used against EP system andVice versa. The anti jamming techniques used in radars constantly confront the EA techniques used in Jammer systems. Thisis like a chess game, which never ends. We can only expect it will become more vigorous in future. Radar designers wouldstrive to preserve the capabilities of Radar while the EA designers tries to deny its capabilities.

5.1 RADARS:

All radars operate on the basis of a few principles. Radar broadly consists of a Transmitter, Receiver, a processorand a Display Indicator. All radars can be classified into three categories based on functionality (i) Search Radars (ii) TrackRadars (iii) Track while scan Radars. Further they can be subdivided into (i) Early Warning Radars (ii) Height Finding Radars(iii) Acquisition Radars (iv) Fire Control Radars (v) Terminal Guidance Radars (vi) Missile Fuze. Based on the method oftransmission radars can be categorised into (i) Pulse (ii) Pulse Doppler (iii) CW Doppler radars.

5.2 RADAR CONCEPT :

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Radar basically have three main functions (i) Measure the Range of the target (ii) Measure the angle at which thetarget is located (iii) Measure the velocity of the target.

(i) Range : The measurement of range is based on the simple principle ie. measurement of time elapsed from thetransmission of the signal to detection of the reflected signal at the radar. Since Radar signals travel at the speed of light, thetime elapsed in travel, will give the distance between the radar and the target.

(ii) Angle : When the main beam of the antenna is pointed in the direction of the target, return echo is received at theradar receiver. This is taken as the angle of location of the target. The angle at which maximum signal power is received is thetarget azimuth angle. Normally Search radar provides the azimuth angle alone. The tracking antenna searches a volume ofspace in azimuth and elevation and determines the azimuth and elevation angles. Since the data rate of the tracking radar isvery high compared to that of search radar, high angular accuracies are obtained, which is necessary for fire control systems,missile guidance etc.

(iii) Velocity: Doppler Radars are used to compute the velocity of the target. FMCW is an example.

5.3 EP TECHNIQUES:

EP involves action taken to ensure the effective use of EM spectrum despite the use of EA by the enemy. The typesof EP techniques available are very large in number. These techniques are integrated into radar systems in four primary areas(i) Transmitter (ii) Receiver (iii) Antenna (iv) System as a whole. These techniques are also called fixes.Fixes are features built into a radar system, or modification to existing system that overcomes the effectiveness of EA.

5.4 TRANSMITTER FIXES :

(i)Frequency Agility : The frequency of operation of radar is changed at a very fast rate and the change in

frequency is as wide as possible. This will make Spot Jammers ineffective. Frequency agile radars compel the EA system touse barrage or Sweep Jamming which are less power efficient.

(ii) Diplexing : Diplexing is the use of two transmitters and two Receivers whose frequency of operation is quiteapart say X and Ka band. Here if one Rx is jammed, output from the other is used for tracking. Diplexing forces the Jammersystems to divide the power between the frequencies.

(iii)Power Add : Transmission of same frequency simultaneously by two transmitters increases the power output ofradar. This extends the burn through range of the system and increases the requirement of J/S ratio.

(iv) Long Pulse Duration : Increasing PD means increase of average power resulting in higher echo strength.However with higher PD range resolution decreases.

(v) Pulse Compression : This increases average transmitted power without an increase of peak power, and with noloss of range resolution. The pulse is stretched during transmission and the echo is compressed in the receiver. With propercoding this is a very effective measure against EA systems. The radars using this technique are called LPI (Low Probability ofIntercept) radars. Since the jammer will never know the coding employed by the radar, this EP technique is very effective

against Jammers.

(vi)Staggered PRF : The jammer sends synchronous Jammer pulses, which will be received as targets at differentranges, if the radar uses staggered PRF. Video integration at Radar can distinguish the false jamming signals and are easilyremoved from the Rx.

(vi) Jittered PRF : This is similar to staggered PRF except that the pulse PRF varies randomly and is used toovercome the Synchronous Jamming.

5.5 RECEIVER FIXES

(i) Manual Gain Control : At some ranges it is possible echo is stronger than the jammer signal, but the receivermay be saturated. The gain of the Rx can be reduced manually until the Radar display indicates the target echo properly.

(ii) IAGC ( Instantaneous Automatic Gain Control) : In case a noise or CW jammer is used against the radar,IAGC samples the average noise level at the output of the receiver and by raising or lowering the IF gain the output level ismaintained constant.

(iii) Logarithmic Receiver : Small signals such as the radar echo will have high amplification and large signals suchas jamming signals, receive low amplification.

(iv) Fast Time Constant (FTC) : FTC uses a time constant that is just longer than the radar pulse. Normal targetreturn pulses pass without distortion, whereas longer pulses or clutter from Jammers are reduced in length.

(v) Dicke Fix Receiver : The Dicke Fix Rx contains a wideband amplifier, a limiter and narrow band amplifier.The jamming signal is amplified along with the echo, in the Wideband amplifier. Because of wideband, the ringing is reduced.The amplified signals are limited ie. Noise and echoes are held below a set amplitude. The limited signal is fed to a narrowband amplifier. This is turned to the Center frequency of the return pulse. So noise in the Narrow band filter will receive lessamplification than the echo signal. The Dicke Fix Rx is used to reduce noise, fast sweep and narrow pulse jamming.

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(vi)Pulse Width Discrimination (PWD) : This is a technique used to discriminate the radar echo pulse from shorteror longer Jammer pulses. Control signals suppress the incorrect length pulses.

5.6 ANTENNA FIXES

(i)Side Lobe Cancellation (SLC) : All directional antennas have sidelobes of various strengths. Energy fromJammer entering sidelobe is displayed on the scope at the azimuth indicated by the main lobe. A SLC system consists of anomni antenna and a receiver signal from omni channel is subtracted from the main lobe signal so that the sidelobe returns arecancelled. Only target returns are sent to the Video amplifiers and to the Radar Display.

(ii) Sidelobe Blanking (SLB) : SLB eliminates unwanted sidelobes returns by a blanking technique. As soon assidelobe jamming is detected, a gate is generated that turns off the main receiver. SLB will cause the loss of valid targets, SLBmay still be effective because it blanks the main receiver only for short time intervals, this minimizes the loss of valid targetdata to the SLB

5.7 FUTURE TRENDS IN EP

Among the important developments that are taking place around the world only a few are given below.

(i) High Transmitter Power : The ERP of EA systems have direct relation on the amount of Transmitted power bythe radar. Large Radar powers will increase the required ERP of EA systems, increasing the complexity, size and cost of EAsystems.

(ii) Electronically Steerable Antennas : Phased arrays have given greater flexibility in antenna pattern generation.This allows high speed beam steering and allows nulling of externally generated intereference. EA systems will not be able topredict the beam positions which is required for effective use of EA techniques.

iii)Sensor Fusion : Other sensors like EO/IO sensors, ES sensors are used to complement or supplement the radarsystem, especially when the radar performance is degraded on account of jamming.

(iv) Multi Static Radar : Radar Receivers are located far away from the transmitter. This provide greater ranges ofdetection by placing the receivers nearer to the engagement line, with this bistatic/multistatic radar, location of the transmitter isuseless since EA is intended for the radar receiver.

(v) LPI : LPI radars transmit signals which are not detectable by ES systems. This denies the capability of EAsystems for effective jamming.

(vi) Ultra Agile Carrier Frequencies : Random pulse to pulse frequency agility is the most difficult to counter withEA systems.

(vii) Deceptive Transmissions : Radar designers suggest modulating the radar transmitted signal. This confusesthe EA receiver analysis circuits. Amplitude modulations on the transmitted signal are erroneously detected by the EA receiveras the mutation frequency.

(viii) Intra Pulse Modulation : LPI radars use frequency modulation or phase modulation within the pulse. Thiscomplicates the RF memory circuits, so to maintain coherency, Digital RF memories are required which are very complex andcostly.

(ix) Ultra Low Side Lobe Antennas : Research is yielding results and ultra low side lobe Antennas are being usedin expensive radars. This necessitates then EA Jammers to jam through mainlobe of the radar antenna thereby requiring largepower.

(x) Multifunction Antenna : Single Antenna is used to perform the job of several sensor functions. If the antenna isdesigned to include a passive mode of operation, the radar will track the EA jammer signal without use of radar transmission.Operating passively on the jammer signal during periods of interference will provide the required angle information of the

jammer mounted on the target.

6. CONCLUSION

During the past 50 years, an extremely hostile environment has developed with proliferation of radar guided AAA

guns, Surface to Air Missiles, Air to Air Missiles etc. This threat environment has necessitated enormous budgets being spenton development of Electronic Warfare equipment of wide varieties. Further technological innovations in the filed of radar and itsassociated anti jamming techniques, has brought out the requirement of further innovations in the field of Electronic Attacksystems both in Microwave, Milli-metric and Electro-optic and IR range of frequencies. The tussel between the radar designersand the EA planners is going to intensify. This will culminate in the development of new systems, techniques, components,miniaturization etc. Future systems will be more software intensive using artificial intelligence for its adaptability andsurvivability. Mulit sensors systems will be invariably used with new data fusion algorithms. The future for EW is very promisingwith immense challenges to be met in the days ahead.

Ready Reckoner Radar Ew

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