Ever since radar being used in battle in an attempt to gain superior situation awareness and guide weapons, various techniques were created to reduce the effectiveness of those sensors. Whiles passive countermeasure such as Stealth techniques were invented to help military assets hide and stay invisible from enemy’s sensors, electronic countermeasure techniques were invented to confuse, overwhelm adversary sensors so that they are unable to track or attack friendly assets.This article will introduce and explain some common countermeasure techniques used by military aircraft.
Radar countermeasure devices on aircraft take various shape and forms.Each configuration styles has their own advantages, disadvantages which will be explained below.
The most basics electronic countermeasure device was invented in War world II and used by all military aircraft nowadays. Chaff consists of extremely small strands (or dipoles) of an aluminum-coated crystalline silica core. When released from an aircraft, chaff initially forms a momentary radar reflective cloud and then disperses in the air and eventually drifts to the ground. The chaff effectively reflects radar signals in various bands (depending on the length of the chaff fibers), the effective frequency bandwidth of a single chaff length are varied by ±5 %, but various chaff length is often mixed to increase chaff effective bandwidth. Immediately after deploying chaff, the aircraft is obscured from radar detection by the chaff cloud which momentarily breaks the radar lock. To maximize the effectiveness of chaff before releasing them pilots often perform beam maneuver ( fly at the direction perpendicular to adversary radar to reduce Doppler effect). The effect of chaff to radar is similar to a smoke screen to naked eyes.
Example: photo of RR-188 a single cartridge containing 400,000 chaff dipoles, each in 8 cuts, a plastic end cap, piston, and felt pad.
- small in size can be carried in large number without affecting carried aircraf’s weapon load
- decoys deployment doesn’t affect aircraft maneuver in any way
- Chaff decelerated quickly after being released, as a result there is a great difference between the speed of carrier platform and chaff thus Doppler radar reduce the effectiveness of chaff significantly.
As internal space on fighters is limited, they often carry their jamming system inside pod outside the airframe. Jamming pod confuses/ suppress adversary radars by sending out radio wave at the same frequency as the radar. The main component of ECM pod are receivers, techniques generator and transmitters, powerful ECM pods sometimes have ram air turbine to generate their own electrical power instead of relying on aircraft’s generator. Jammer pods are often carried on aircraft centerline station, some dedicated support jamming asset such as EA-18G may carry more than 1 jammers in which case the pod can be carried on wing stations too.
- Aircraft can carry more than one jamming pod, mix-match difference kind of jamming pods to satisfy missions requirements allow higher flexibility compare to internal systems.
- Jamming pod components such as receivers, TWT technique generator can be the independence of aircraft processor, allow old fighters to carry same jamming systems that the newest aircraft capable of.
- Due to the bigger size, jamming pod often have bigger transmitting antenna and more power available compared to FOTD decoys and air-launched decoys, as a result, they have higher effective jamming power and better directivity ( gain ).
- Increase aircraft radar cross section and drag when being carried
- Reduce aircraft agility
- Depend on location ( pylon or centerline station ) , jamming pod often have blind spots above or under aircraft.
- Transmitters are located on aircraft, thus, missiles in HoJ mode can be a threat
Internal Jamming System:
To free pylons for weapons and fuel tanks, some fighters have the jamming system integrated inside their airframe, with the processor and TWT placed completely inside fuselage while receivers and transmitters are located along vertical tail fins or on aircraft nose ( in some rare case jamming system are located inside aircraft pylons).
- 360 degrees coverage (most internal jamming systems do not have any blind spot)
- Internal systems thus do not affect aircraft’s weapon load, radar cross section or drag.
- Impact on aircraft’s agility is small ( due to weight )
- High power available for transmitters.
- Systems upgrade depending on aircraft’s development cycles, thus slower upgrade rate and less flexible compared to off-board jamming system.
- Limited space available leading to small transmitters/receivers aperture, thus lower gain compared to sophisticated ECM pod like ALQ-99 or NGJ or fire control radar.
- Transmitters are located on aircraft, thus, missiles in HoJ mode can be a threat
Active Electronically Scanned Array Radar as Jammer:
Unlike traditional mechanical scanner array radars which have a single transmitter and receiver, an active electronically scanned array (AESA ) radar composed of numerous small solid-state transmit/receive modules, this allows the AESA to produce numerous simultaneous “sub-beams” at different frequencies.An AESA radar steer its beam by sending separate radio waves (with appropriate delay) from each T/R module so that they interfere constructively at certain angles in front of the antenna aperture , this method helps it focus the beam better (higher gain ) than traditional parabolic radar ,and because everything are done electronically, AESA radars have much higher scanning rate too. Due to AESA radar unique characteristics, on some fighters (F-35 , F-22 , F-18E/F) , their fire controlled radars are used not only to locate and track enemy forces but also to jam enemy’s radar, attack enemy’s network and stream data at high speed.
Using fire control radar as jamming system is a unique case of internal jammers , the main difference from a normal internal jamming system set up is transmitters aperture size.
- More power available to transmitters compared to others configuration (ECM pod or normal internal jamming system )
- Superior aperture size to others configurations leading to much better directivity ( better focus of jamming power )
- All components are internal thus do not affect aircraft’s weapon load, radar cross section or agility
- Jammer has limited coverage: mainly frontal direction ( equal to radar coverages around 120°)
- Limited frequency coverages: mainly X-band ( from 8-12 Ghz )
- Transmitters are located on aircraft, thus, missiles in HoJ mode can be a threat
Disposable decoys (also known as expendable decoys) are very small RF jamming devices that can be released by aircraft. Essentially, they are the mixture of ECM pod and chaff, they are released like chaff but operate like an ECM pod. Early generation of expandable decoy only consist of simple TWT and repeater, letting them retransmit signal with more power to appear like more attractive target, but modern expandable decoy such as Brite cloud are equipped with DRFM, thus allowing them to use many complex jamming techniques that traditionally only used by dedicate jamming systems.
- Expandable decoys can operate completely independent from the launching platform thus they can be integrated into any aircraft with a countermeasure dispenser.
- Due to their extremely small size and weight, a very large number of decoys can be carried without affecting aircraft’s agility or weapon load.
- Doesn’t limit aircraft agility when deployed.
- Because the decoys (RF jammer) will be launched away from the launch aircraft, thus make Home on jam mode useless.
- Due to their small sizes and weight, they have very limited computing power and transmitting power compared to other EW system.
- Limited jamming time often on the order of 10 seconds or less due to battery life restriction.
The towed decoy was developed by the Naval Research Laboratory in the early 1980s. The core of towed decoys is a transmitter that amplified and retransmits all signal it received thus it appears like an attractive target with high RCS on adversary radar. When deployed, the decoy is towed behind the host aircraft, protecting the aircraft and its crew against RF-guided missiles by luring the missile toward the decoy and away from the intended target. In layman term, towed decoys are small transmitters being drag behind aircraft.
- Decoys are towed behind aircraft with a cable so they move at the same speed as parent aircraft, as a result, doppler effect does not help distinguish decoys from the real target.
- Towed decoys are often stored within wing pylons, wingtip pod or aircraft fuselage thus do not affect aircraft weapons load
- After deployment decoys stay at a distance and connected to aircraft by a cable, thus limiting aircraft agility to low G maneuver.
- Decoys are towed thus it will always stay behind the real platform hence, missiles with 2-ways datalink or command guide can render towed decoy ineffective (because adversary SAM operator, pilot can choose which target for missiles to attack).
- Towed decoys rely totally on its own internal component ( processor, battery, and antenna) thus lacking the processing, jamming power, and directivity of ECM pod or aircraft internal ECM system.
Fiber optic towed decoys (FOTD):
Fiber optic towed decoys are upgraded variants of towed decoys. While traditional towed decoys rely totally on it’s own electronic components to response enemy threats, fiber optic decoys have a fiber-optics connection allow it to rely on aircraft on-board radio frequency countermeasures system.Aircraft onboard electric warfare (EW ) system is designed to receive radar signals from potential threat emitters via antennas on the forward and aft sections of the aircraft and to generate an electronic countermeasures response to the threat. Jamming may use either onboard transmitter ( jamming antenna on aircraft ) or the off-board transmitting capabilities of the FOTD decoy. For the off-board response, a jamming signal is generated by onboard EW equipment and provided to a decoy towed behind the aircraft for amplification and transmission. To reach the decoy, the signal is converted to light and transmitted down a fiber-optic link to the decoy. In the decoy, the light signal is converted back to RF, amplified, and transmitted using antennas integral inside the decoy. While antique towed decoys often only capable of amplifying and retransmit adversary radar signal, modern fiber optic towed decoys can transmit any and all signal that the aircraft’s onboard countermeasure system capable of generating ( from simple noise jamming techniques to complex deceptive jamming techniques )
Example diagram of FOTD systems:
- Decoys is towed by aircraft thus cannot be distinguished by Doppler effect
- Thanks to the fiber optics link with aircraft’s onboard electric warfare system , the decoys is capable of generating very complex jamming signals
- Provide safety for carried aircraft when facing missiles in Home on Jam mode
- After deployment decoys stay at a distance and connected to aircraft by a cable , thus limiting aircraft agility to low G maneuver.
- Due to small size ( antenna and TWT ) , FOTD lack the jamming power of ECM pods and aircraft internal jamming systems
About HOJ : For modern radio guided missiles , if at any point during the missiles time-of-flight the target starts to use electronic counter-measures , the missiles can switch its tracking mode to home-on-jam , When this occurs the missiles homes in on the direction of the jamming signal, guiding it to the point where the onboard radar ‘burns through’ the jamming and re-acquires the radar. When in the home-on-jam mode the missiles interlace the active pulses of the radar with passive guidance from the home-on-jam equipment.The HOJ mode does not provide as good a Pk the normal active guidance however because missiles cannot determine target velocity or distance from target , they are unable to perform lead intercept ( missiles range in this mode is limited too because missiles cannot follow a ballistics arcs to conserve energy ).
Air Launched Cruise Decoys:
As radars getting more sophisticated and powerful, it gets harder and harder to trick or overwhelm air defense with electronic jamming alone. Thus air-launched decoys were created to clutter up a radar screen with false targets making it easier for an attacker to get within weapons range and neutralize the radar. The concept is fairly simple, air-launched decoys are small unmanned aerial vehicles, they carry signature augmentation subsystem such as Luneberg lens to mimic radar cross-section of military aircraft along with GPS or inertial navigation system to help them follows a pre-planned route (most decoys can be programmed with around 100 waypoints or more ).Decoys are intended to deceive a radar operator into believing that they are actually aircraft . Early air launched decoys such as ADM-141 TALD lacking internal engine thus they can only glide and have easy to predict trajectory. However, modern air-launched decoys such as ITALD, ADM-161 MALD are equipped with a turbojet engine ( or rocket engine in case of AQM-37) , helping them reach much longer distance distances, cruise at high subsonic speed, climb and perform low G maneuvers. Latest air-launched decoys such as MALD-J, SPEAR-EW can even carry active jammer and 2 ways datalink system. Most modern air-launched decoys are compatible with MIL-STD-8591 14 Inch (35.56 cm) suspension lugs, 1760 interface connection or unpowered thus, they can be carried by a wide range of military aircraft
- Distinguish between air-launched decoys and real aircrafts is a challenging task for every radar but the latest one, and even with the most modern radar, it is impossible to discriminate between the decoys and cruise missiles.
- Can be programmed with high number of waypoints, and fly completely independent of launching platform after deployed thus allow flexible mission planning
- Very long range ( around 400-900 km )
- Some decoys can carry communication and jamming system allow them to perform cooperative blinking jamming with others decoys or aircraft to deal with HoJ missiles.
- Compatible with a wide range of platforms and weapons racks, a single fighter can carry as many as 18-20 decoys using triple and quadruple ejector racks
- Occupied weapons station thus reduce the missiles-bombs load
- Increase aircraft total drag and radar cross section when carried on pylons ( not apply to aircraft with internal weapons bays )
- Non-Cooperative Target Recognition (NCTR ) techniques such as jet-engine modulation used by modern radar (most radars after 1980s) can distinguish decoys from real aircraft.
- Reduce platforms agility when carried in large number
Electronics Countermeasure Techniques
Radar countermeasures are often divided to electronic (active) and mechanical (passive) types. Mechanical countermeasure systems reflect radar waves passively no transmitting antenna or receiver required, some example of passive countermeasure are chaff, air-launched decoys. By contrast, active electronic countermeasure (jamming) involves systems that transmitting radio waves to reduce the effectiveness of enemy radar, an example of an active electronic countermeasure system are ECM pod, FOTD.
Some common jamming techniques will be explained below. To begin with, jamming can be categorized into two general types: (1) noise jamming and (2) deceptive jamming.
Noise jamming is the form of electronic countermeasure where jammer transmit an interference signal ( white noise) in enemy’s radar direction so that the aircraft reflection is completely submerged by interference.This type of jamming is also called ‘denial jamming’ or ‘obscuration jamming’. The primary advantage of noise jamming is that only minimal details about the enemy equipment need be known. Within the general class of noise jamming, there are three different techniques for generating noise-like signal.
In this type of jamming, also called “point jamming” or “narrow-band jamming”, all the
power output of the jammer is concentrated in a very narrow bandwidth, ideally identical
to that of the radar. Spot jamming is usually directed against a specific radar and requires
a panoramic receiver to match the jamming signal to the radar signal.
- Because the jammer can only jam one frequency, a frequency agile radar would hardly be affected. Hence, frequency hopping (radar change operating frequency randomly ) is the usual method to deal with spot jamming
- HoJ missiles
In this type of jamming, all the power output of the jammer is spread over a bandwidth much wider than that of the radar signal. In other words, it involves the massive and simultaneous jamming of the whole of the frequency band.
- Barrage jammers have to spread energy over a wide frequency spectrum, so it is less effective again high power radar.
- Increase radar duty cycles ( duty cycle is the time transmitter runs for one out of 100 microseconds, higher duty cycles increase range ), higher duty cycles reduces jammer effectiveness
- High gain radar ( gain describes how well the antenna converts input power into radio waves headed in a specified direction, higher gain mean radar beam is narrower and it converts more percentages of its energy in a specific direction )
- HoJ missiles
This is also similar to barrage jamming. In this case, the power output of the jammer (jammer frequency) is swept back and forth over a very wide bandwidth, sometimes as much as an octave (a 2: 1 band). It is generally true that the bandwidth of sweep jamming is wider than that of the barrage jamming, but the relative bandwidth is often determined by the hardware used.The actual difference between barrage and sweep jamming lies in the modulation techniques and size of the frequency band covered. Barrage jamming often uses an amplitude-modulated signal covering a 10 percent frequency band (i.e., bandwidth equal to 10 percent of the central frequency). Sweep jamming often uses a frequency modulated signal and the frequency is swept back and forth over a wide frequency bandwidth. Both barrage and sweep jamming are used when the exact frequency of the enemy system is not known.
- Frequency hoping
- High gain , high power radars
- Increase duty cycles
- HoJ missiles
Deception jammers carry receiving devices on board in order to analyze the radar transmission, and then send back false target-like signals in order to confuse the radar..This is in contrast to noise type of jamming,whose objective is to obscure the real signal by injecting a suitable level of noise-like interference into the victim system.Techniques like “noise jamming” are useful for taking a radar installation out of commission, but more sophisticated deception jamming can make the enemy think their radar is still working when it is actually reporting incorrect target range and velocity information With deception jamming, an exact knowledge of not only the enemy radar’s frequency, but all other transmission parameters is required. Deceptive jamming, in a way , is spot or point jamming of a more intelligent nature, HoJ mode of missiles are often less effective again deception jamming because missiles often do not know they are being jammed ( It important to note that, if jamming is detected then HoJ can still be used ).
In recent years capability of radar deceptive jamming has been enhanced significantly with the development of Digital Radio Frequency Memory (DRFM) techniques .Jammers with DRFM technology are widely reported in literature , for example ALQ-187(v)2 , ALQ-131 EA PUP , Falcon edge , ALQ-211(V)9 , ALQ-214(V)3 , Spectra , ASQ-239.DRFM is a technology in which a high-speed sampling digital memory is used for storage and recreation of radio frequency signals.The most significant aspect of DRFM is that as a digital “duplicate” of the received signal, it is coherent with the source of the received signal. As opposed to analog ‘memory loops’, there is no signal degradation caused by continuously cycling the energy through a front-end amplifier which allows for greater range errors for reactive jamming and allows for predictive jamming.
Within the general class of deceptive jamming, there are also a few different techniques:
The most common type of deception jammer is the range deception (range-gate stealer), whose function is to pull the radar tracking gate from the target position through the introduction of a false target into the radar’s range-tracking circuits. At start, the jammer sends back an amplified version of the signal received from the radar. The deception jammer signal, being stronger than the radar’s return signal, captures the range-tracking circuits.The deception signal is then progressively delayed in the jammer by using an RF memory, thereby “walking” the range gate off the actual target (range-gate pull-off or RGPO). When the range gate is sufficiently removed from the actual target, the deception jammer is turned off, forcing the tracking radar into a target reacquisition mode.
p/s: jammer can sometimes perform Range gate pull in , which is the similar technique as Range gate pull off , the main different is the target will appear to get closer to radar instead of getting further aways
- PRF jitters: a radar calculate range to a target by measuring the elapsed time between pulse transmittal and target return reception.Thus, the maximum required range of the radar determines the maximum pulse repetition frequency of the radar. In Jitter mode, the time between successive pulses is allowed to vary in a totally random manner over a series of set intervals as long as the maximum range condition is met.In theory, an infinite number of PRI patterns can be generated by combining stagger and jitter. Varying pulses render the jammer incapable of anticipating when the next illuminating pulse is due to arrive.
- Frequency-hopping: as the jammer need time to analyze signals and turn into it.
- Leading-edge tracking: taking measurements not according to where the center of the return signal is but rather at the leading edge.All RGPO/RGPI cover pulse jamming tends to lag the target’s returns by some increment of time
- Monitoring signal strength.
- Double Tracking: in airborne radar, the fast Fourier transform ( FFT ) is used to process the signal on both the range and velocity axes. In this way, the target produces an echo that, being characterized in both range and velocity(Doppler) allows double tracking.If the jammer attempt to open a one gate not coherent with the other, it is ignored
In velocity deception jamming, the Doppler shift is interfered with. At the start of jammer operation, the illuminator signal is detected by the jammer and an exact false, strong Doppler-shifted signal is sent back to the radar. The radar locks on to the incorrect Doppler signal and the jammer slowly sweeps the false signal’s frequency more away from the actual Doppler frequency of the target. When the radar has been led far enough away in frequency, the jammer is turned off and the radar is once more left without a target.The basic principle of velocity deception is similar to range deception ,thus it is sometimes called Velocity Gate Pull-Off ( VGPO )
- PRF jitters
- Frequency hopping
- Leading-edge tracking
- Double tracking
- Guard gate: a counter techniques that entail presenting sensors around the gate in which tracking is performed so that as soon as the presence of additional echo is detected ,the tracking system switches to memory for a short time and then reacquires the old target. Accordingly, when a deception jammer tries to lure the tracking gate to a false target, as soon as the true echo and the deceptive echo separate, the true echo will enter the guard gate, thus blocking the tracking gate. When the sensors indicate that the deceptive echo has gone, the gates will again position themselves correctly.
This is a hybrid type of jamming which incorporates some of the features of both spot or
barrage noise and deception jammers. This type of jammer generates a noise burst which is ‘on’ before and after the actual target return thereby covering the true return. This type of jammer allows a low powered repeater to respond to a number of threat radars by time
- High gain, high power radar to burn through jamming signal
- HoJ missiles
Inverse Gain (Inverse Con-scan ) Jamming
Inverse gain jamming is used to capture the angle-tracking circuits of a conical scan tracking radar. This technique repeats a replica of the received signal with an induced amplitude modulation which is the inverse of the victim radar’s combined transmitting and receiving antenna scan patterns. Against a conically scanning tracking radar, an inverse gain repeater jammer has the effect of causing positive feedback, which pushes the tracking radar antenna away from the target rather than toward the target. Inverse-gain jamming and RGPO are combined in many cases to counter conical scan tracking radars.
- Monopulse radar
- Random conical scan frequency: changing the scanning speed in a pseudorandom way within a given domain
- Lobe on receive only (LORO )
- Conical Scan on Receive Only ( COSRO)
- Frequency hoping
- PRF jitters
Cross Eye Jamming
Cross-eye jamming is an angle deception ECM technique that employs two spatially separated jamming sources. Each source acts as a repeater-type jammer transmitting the same signal at the same time, and if the two signals arrive at the missile monopulse antenna approximately 180° out of phase, wavefront distortion occurs. The missile seeker, presuming that the signal source lies along the normal to the wavefront, tries to re-aligns its antenna at right angles to the distorted wavefront. This antenna re-alignment results in incorrect missile tracking which in turn results in incorrect steering information being passed to the missile autopilot. This may potentially result in a substantial missile miss distance.In a cross-eye jamming system, a 180° phase relationship between the two jamming sources may be maintained by setting up a retro-reflective transmission system. In this type of system, each of the jamming antennas is acting as the signal source for a repeater- type jammer. However, the signal received by one antenna is transmitted by the other, and vice versa. In this way, the total propagation delay from seeker to receive antenna to transmit antenna and back to seeker is identical for both signal paths and, everything else being equal, the phase of the two signals arriving at the seeker will be identical. A 180° phase shifter is then added to one of the paths to create the wavefront distortion effect.Successful operation of cross-eye jamming creates an interferometric null between the jamming signals in the direction of the victim’s radar. The jamming signal must compete with the real target return to capture the radar angle tracker. To achieve that the angle noise caused by the real radar target must perturb the radar’s antenna off the jamming signal null by an amount sufficient for a positive jamming to signal ratio to be generated. As a result, the jamming to signal requirement of at least 20dBsm is required for successful cross-eye jamming operation.
- PRF jitters
- Frequency hoping
- High gain , high power radar to burn through jamming signal
- Increase radar duty cycles
The polarization of an electromagnetic wave is defined as the orientation of the electric field vector. As we know electric field vector is perpendicular to both the direction of travel and the magnetic field vector. The polarization is described by the geometric figure traced by the electric field vector upon a stationary plane perpendicular to the direction of propagation, as the wave travels through that plane.Reflectors type antenna response to cross-polarized signals very different from normal polarization signals and cross-polarized jamming exploited that fact. The jammer use 2 transmitting antennas which are 90 degrees out of polarization ( for example : one can be vertical and the others horizontal ), this causes the victim’s radar to react erroneously with very significant tracking error.
- Polarization filter
- Cross-Polarized jamming cannot affect flat plate antenna (such as AESA, PESA radars) since there is no forward geometry
- Cross-Polarized jamming requires very large J/S to overcome weakness of condon lobes ,thus, high gain , high power radars are possible counter to this kind of jammer.
In skirt jamming, the jammer exploits the phase response of filters in the radar receiver by injecting a strong jamming signal into a region just above or below the filter frequency. This can cause non-linearity in the phase response across the wanted band, which can affect the radar’s tracking circuitry.
- Skirt frequency jamming effectiveness depends on the unbalance between the sum and difference channels, at these frequencies where rapid phase shifts are present in each channel. Thus , it can be counter by careful design and construction of radar.
Active cancellation is a theoretical military jamming technique that involves the sampling of an incoming radar signal, analyzing it, then returning the signal slightly out of phase, thus “cancelling” it out due to destructive interference. While there are no official information about jamming systems using this technique in service, it is rumored to be in use on Rafale with SPECTRA suite.
- Frequency hoping ( active cancellation require exact information about pulses to produce cancellation pulses , thus frequency agile radar are likely unaffected )
- PRF jitters ( cancellation pulses need to be transmitted at exact moment to produce desirable interference effect , random PRF render the jammer unable to predict when the next pulse coming )
- Multiple radars
Blinking jamming is an effective jamming tactics against monopulse radar seeker and home on jam missiles. It causes line-of-sight angle to step continuously between the two angular positions through 2 jamming assets emitting by turns. These 2 assets can send returns to hostile radar at the rate close to servo bandwidth( typically a few Hz), this can cause resonate at radar target and result in a large overshoot, if apply again HoJ missiles , it would cause missiles to yaw wildly and miss both targets.
Support jamming signal is radiated from one platform and is used to protect other platforms,for stand-off jamming (SOJ) the support jamming platform is maintaining an orbit at a long range from the radar – usually beyond weapons range.The advantage of this method is that jamming platform can be safe from HoJ missiles, the disadvantages is that it much harder to maintain sufficient J/S ratio.
Stand in Jamming
Support jamming signal is radiated from one platform and is used to protect other platforms.For stand in jamming (SIJ) a remotely piloted vehicle is orbiting very close to the victim radar while transmitting jamming signal .Since the jammer is closer to hostile radar , the power required to screen the same target of SIJ is much lower compared to SOJ.
Terrain Bounce Jamming:
Terrain bounce jamming is a unique jamming tactic created to deal with HoJ missiles. Normally the electromagnetic beam from jammer is transmitted toward the victim radar in a direct path thus,home-on-jam missiles will be able to track the angle(direction) of the jammer signal and fly at that direction.Terrain bounce tactic exploits the fact that ground/sea surface can reflect radio waves.Jammer operator will direct jamming beam toward these surface instead of directly at the hostile radar so the jamming beam will come from a different direction from the actual jammer. As a result, this tactic can be used to trick HoJ into believing that the jammer located somewhere on ground. Terrain bounce tactics work best when aircraft fly at low altitude, near flat surface such as the sea. Main disadvantage of this tactics is that effective radiated power of jammer is reduced
When Jamming is factored into the radar equation, the quantities of greatest interest are Jamming to signal ratio (J/S) and Burn-Through Range.”J-to-S” is the ratio of the signal strength of the jamming signal (J) to the signal strength of the target return signal (S). It is expressed as “J/S” and often measured in dB.
Apart from their unique requirements of each specific jamming technique, for jamming to be effective J must exceed S by some amount, therefore, the desired result of a J/S calculation in dB is a positive number. It is a common misconception that J/S ratio required to jam any radar is a fixed value. In reality, however, the required J/S varied significantly depending on jamming techniques and radar type.
Burn-through range is the radar to target distance where the target return signal can first be detected through the ECM and is usually slightly farther than crossover range where J=S. It is usually the range where the J/S just equals the minimum J/S requirement.
As shown in J/S equation above, factors affecting burn-through range are :
- ERPs = Effective radiated power of radar
- ERPj =Effective radiated power of jammer
- G = Antenna gain
- RCS = Target radar cross section
- R = distance to radar
The relation between radar, jammer power, and jamming effectiveness is well known and easy to understand for most enthusiasts. However, one factor that often is overlooked when talking about jamming is radar cross section ( RCS ) of the target.
From radar equation, we can see that the power requirement for jamming will decrease directly proportional to RCS reduction, if the RCS of an aircraft is reduced to 0.1 (or 1%) of its original value, then consequently the jammer power required to achieve the same effect would be 0.1 (or 1%) of the original value.
For example, a clean F-18E has radar cross section around 0.1 m2 (-10 dBsm) , an F-35 has radar cross section around 0.001 m2 (-30 dBsm) ,so the RCS value is decreased by 99%. Thus, if F-18E needs a 100 kW jammer to deceive or overwhelmed adversary radar, then a F-35 in the same situation, wanting to do the same thing will need a jammer with transmitting power of merely 1 kW.
Alternatively, if jamming power is kept constant and RCS changed then from the radar equation ,we can see that the burn through distance will be reduced dramatically.
Another factor that is often ignored when discussing jamming effectiveness is distance. Since jamming signal only has to travel one way, as the distance get bigger , the jammer has more advantage than the radar because jamming signal decrease at slower rate than aircraft reflection.In the other words : for self-protection jamming the further the jammer is from the threat radar, the easier it would be for that jammer to jam the threat radar .
By contrast, for support jamming the closer the jammer to threat radar, the easier it would be for the jammer to cover others assets because in this case aircraft reflection is not dependent on the distance between the jammer and the threat radar so getting the jammer closer to the radar is better.
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