Electronic Reconnaissance, ELECTRONIC COUNTERMEASURES (ECM), ELECTRONIC COUNTER-COUNTERMEASURES (ECCM)
Electronic Reconnaissance:
Reconnaissance aims to give to field players, at a tactical level, the information they need: Where are (and where will be) the targets, what are their defenses, what military means to use, what kind of weapon is the best suited, when to conduct the attack, etc. Due to short reaction time in Air Defense, this task is generally carried out directly by the AEW system. In fact in some cases the AEW platform is a C3I system itself that controls the fighters in real time. For ground targets, due to slower evolution of the situation and to a more complex environment (collateral damage avoidance, mask), a specific mission is needed. This mission uses SAR/MTI systems either fitted in POD carried by a manned aircraft (business jet or fighter), or carried by an unmanned vehicle (UAV). The main objective of these missions is to re-acquire the targets (in case they are moved as Ballistic Missile Launchers), and to identify and locate them accurately. Once again fusion of different sensors (ESM, optical, etc.) is generally needed to assess the global tactical situation, including Air Defense threat assessment. All this information is gathered at the C3I center where the interceptions or the strike missions are preplanned
ELECTRONIC WARFARE (EW) is “a military action involving the use of electromagnetic energy to determine, exploit, reduce, or prevent hostile use of the electromagnetic spectrum and action while retains friendly use of the electromagnetic spectrum.” EW is divided into three classic categories: electronic (warfare) support measures (ESM), electronic countermeasures (ECM), and electronic counter-countermeasures (ECCM).
ELECTRONIC COUNTERMEASURES (ECM) are “actions taken to prevent or reduce the enemy’s effective use of the electromagnetic spectrum.” It is one of the three components of electronic warfare. In radar applications its main objectives are to deny or to falsify information (detection, measurement, discrimination, and classification data) that the radar tries to obtain. There are a number of ways to classify ECM tactics and techniques. From the point of view whether electromagnetic energy is radiated or not, ECM is divided into active and passive. From the standpoint of what main parameters of radar information are influenced, ECM is divided into angle-measurement ECM, range-measurement ECM, and velocity-measurement ECM. From the viewpoint of the types of jammed radars, it is primarily divided into ECM versus search (surveillance) radars and ECM versus tracking radars. From the viewpoint of the ECM systems location, onboard and offboard ECM are distinguished. As to tactics of ECM combat employment, typically five classes can be identified: escort ECM, cooperative (mutual-support) ECM, self-protection (self-screening) ECM, stand-forward ECM, and stand-off ECM. Some authors include defence suppression as an ECM, although others consider it a separate ingredient of electronic warfare. ECM is primarily based on jamming: both noise jamming and deception jamming. Modern ECM systems are designed to cope with different types of radars, and they have to operate in a dense threat environment that requires computer control of the system. Typical parameters of an advanced, self-protection active ECM system designed for fighter-type aircraft are given in Table:
Active ECM incorporates devices and methods based on deliberate radiation of electromagnetic energy to disrupt the operation of the victim radar. Active ECM typically means active jamming.
Angle-measurement ECM is designed to cause disruption of correct determining of the target’s angular direction both by search and tracking radars. The main methods to counter a search radar are to jam it through antenna sidelobes, based on the fact that, if a signal is received and detected, it is considered to be in the antenna main beam. So, if the energy of the signal radiated by a jammer and received by radar through antenna sidelobes is sufficient to exceed the detection threshold, the radar concludes that the signal was in the main beam and, therefore, considerable error is introduced in angular measurements. It is obvious that extremely large amounts of energy are required to overcome the main-to-sidelobe ratio of the victim radar. High-power noise jamming can be employed to implement this task, and special vehicles are being used in escort or stand-off ECM modes to carry the jammers required to protect the penetrating vehicles. The main type of modern tracking radar is monopulse radar. Typically, jamming techniques against this type of radar fall into two categories. The first uses imperfection in the monopulse design and hardware implementation to reverse the sense of the angle-error signal. These involve cross-polarization jamming, image-frequency jamming, and skirt-frequency jamming. The second class uses the effect of multipoint jamming to distort true angle-of-arrival of real signal. These technique include blinking jamming, cross-eye jamming, formation jamming, and ground-bounce jamming. Down-link jamming may be a useful technique to reduce angular measurement capability of track-while-scan radars.
Cooperative ECM is that “involving the coordinated conduct of electronic countermeasures by combat elements against hostile acquisition and weapon-control radars.” Its advantages are coordinated tactics and greater effective radiated power that can be achieved due to jamming from various platforms. An example of this tactic is blinking jamming. This type of ECM is also termed mutual-support ECM.
Cross-eye ECM is a deceptive, self-protection technique designed to frustrate target-tracking radars or radar seekers by causing the radar to track a jamming signal whose phase front differs significantly from that of the true target return signal. Cross-eye is one of several multisource jamming concepts (others include cross-polarization and terrain-bounce) designed to attack monopulse tracking radars, which are invulnerable to angle-jamming techniques such as inverse gain jamming effectively used against sequential-lobing and conical-scan tracking radars. Figure E3 shows a generic implementation of the cross-eye jamming technique, consisting of two repeaters connected to two separate antenna systems.
The transmit and receive antennas for each repeater are separated by the baseline distance d. One of the repeater paths includes a 180° phase shifter, and one path contains controls to ensure proper adjustment of relative phase and amplitude between the two repeater paths. With this configuration, the coherent jamming signals arrive at the victim radar matched in amplitude, but 180° out of phase. This creates an interferometric null between the two jamming signals in the direction of the victim radar antenna. Jammer effectiveness relies on the presence of true target-generated angle noise within the victim radar’s angle tracking loop to drive the radar antenna off the jamming signal null so that a sufficiently high jammerto- signal (J/S) ratio is developed. The cross-eye concept has two inherent vulnerabilities:
(1) the delay time introduced by the baseline distance between jammer antenna sets provides a potential opportunity for the victim radar to detect and track the leading edge of the true target return before the repeated jamming signal arrives.
(2) a relatively high J/S ratio, on the order of 20 dB or greater, is required for the technique to work effectively.
Cross-polarization ECM is a deceptive ECM (DECM) technique, designed for use against monopulse tracking radars, that attempts to exploit the victim radar’s response to a repeater jamming signal whose polarization is orthogonal to that of the radar receiving antenna. The response of the victim radar’s angle error discriminator can be significantly different for cross-polarized signals, leading to large angle tracking errors and eventual break-lock. Reflector antennas, particularly parabolic-dish designs, are most susceptible to cross-polarization jamming in that their response to orthogonally polarized signals may be reduced only 15 to 30 dB relative to their copolarized response. Flat-plate planar arrays are inherently less susceptible to cross-polarization jamming in that their response is typically reduced by 40 to 50 dB relative to copolarized signals. However, this apparent immunity can be largely negated by the presence of a radome, which may create significant levels of cross-polarized components, depending on the radome design.
Counter-countermeasures to cross-polarization jamming include the use of polarization screens to block signals with undesired polarizations. A more expensive and complex option entails the use of antennas capable of receiving multiple polarizations, perhaps in conjunction with one or more separate receiver and signal-processing channel. To be effective, even against single-polarization radars, cross-polarization jammers must realize high J/S ratios while accurately maintaining jammer-to-radar signal polarization orthogonality (typically to within plus-or-minus 5°). If these conditions are not met, the jammer signal can become the equivalent of a target-borne radar beacon, making the jammer platform highly susceptible to engagement by radar homing missiles.
A block diagram of a cross-pol jammer is shown : Signal components received by the vertically polarized antenna are amplified and retransmitted through the horizontally polarized antenna, and vice versa. As a result, any arbitrary elliptically polarized signal will be returned with the orthogonal polarization. The avoids the necessity of analyzing each received signal and adjusting the repeated signal polarization to be orthogonal to it.
Deception [deceptive] ECM employs “the intentional and deliberate transmission or retransmission of amplitude, frequency, phase, or otherwise modulated intermittent or CW signals or the purpose of misleading in the interpretation or use of information by electronic system.” The common acronym is DECM. In respect to radar, its main objective is to mask the real radar signal by injecting suitably modified replicas of target return into the victim radar. Typically, it is performed by deception jamming. Angle deception is any ECM technique designed to frustrate a radar’s ability to determine the relative angular position of the true target. When angle deception is employed against surveillance radars, the objective of the jammer is usually to insert multiple false targets into the radar’s processor by injecting high-power signals through the victim radar’s antenna sidelobes, thus making discrimination of the true target more difficult or impossible. Angle deception can also be directed to attack the angle-tracking capability of a tracking radar or radar homing missile seeker. In this role angle deception jamming is usually an integral component of a general class of DECM techniques used by self-screening jammers (SSJ) to protect the host platform (e.g., an aircraft, ship, or missile) from engagement by an air-defense system. Most angle deception techniques are implemented via a repeater jammer, which retransmits an amplified version of the victim radar’s signal that has been modulated such that the angle data recovered by the victim radar no longer represents the true target’s angular position relative to the radar. To be effective, an angle deception jamming technique must be tailored to the particular way in which the victim radar derives target angle information. Track-while-scan (TWS) radars, sequential lobing, and conical-scan trackers derive target angle by sequential measurements of target energy as the radar antenna is pointed at different positions about the target location. Angle deception against these radar types is readily effected through a technique known as inverse gain jamming, in which an amplified replica of the radar signal is transmitted, but with amplitude modulation 180° out of phase from the original, which has the net result of canceling the amplitude modulation needed to derive target angle information. In monopulse radars, all of the target angle information is available on a single pulse, rendering this type of tracking radar more difficult to jam. Deceptive techniques other than inverse gain jamming are required and these fall into two generalclasses:
(1) Those that exploit a particular mechanization or imperfection in the monopulse design.
(2) Those that exploit multiple signal source effects to distort the angle-of-arrival of the radar return signal. Techniques in class (2) are more “robust” in that they do not rely on detailed knowledge of a particular radar or missile seeker design for their effect. Angle deception techniques in this class include:
(1) Cross-polarization jamming, in which the repeated signal, with a high jammer-to-signal (J/S) ratio, has a polarization orthogonal to that of the original signal.
(2) Cross-eye, a technique that utilizes two separate repeaters, whose antennas are separated in space (e.g., one at each wingtip of an aircraft) to develop a combined signal whose phase front differs significantly from that of the true target return signal.
(3) Blinking jamming, in which two repeater jammers separated in space “blink,” or transmit alternately in time at a rate designed to defeat the angle-tracking servo dynamics of the radar.
(4) Terrain bounce, which uses the earth’s surface to reflect an airborne jamming signal and thus create a multipath situation in the elevation plane for the victim radar or missile seeker. If the victim is, for example, a semi-active homing missile, the objective of the terrain bounce jammer is to cause the seeker to home on the jammer-illuminated patch on the earth’s surface rather than on the jammer itself. Both blinking and terrain bounce techniques can be implemented in either repeater jammer or noise jammer configurations.
Downlink ECM is used against satellites, aircraft, and missile- borne radars that have separate, onboard systems to “downlink” target information or other radar sensor data, such as platform position and radar mapping data, from the radar acquisition platform to a ground radar or data processing center. Downlink ECM is directed at disrupting this line of communication. In the interest of efficiency as well as security, most of the waveforms used by downlink transmitters are modulated in either time, frequency, phase, amplitude, or combinations thereof. Encryption algorithms may be superimposed for added security. In the absence of detailed knowledge of the downlinked signal coding, attempts to interfere with the downlink are mostly restricted to the use of stand-off noise
jamming. Downlink jamming is made more difficult still, due to the one-way transmission path of the downlink, and the usually high power transmitters employed by downlink systems. Downlink transmission systems generally employ fairly wide beamwidth antennas, but even coarse directivity can pose significant problems for a noise jammer, forcing it, in many circumstances (e.g., a missile downlink) to jam through the downlink antenna sidelobes. Yet another complication for the missile downlink jammer occurs when the downlink is used only infrequently, i.e., has a low duty cycle and so is vulnerable only for a small portion of the total missile flight time.
Escort ECM is the “ECM tactic in which the jamming platform accompanies the strike vehicle and jams radars to protect the strike vehicles,” This tactic is applied to aircraft combat operations. It is generally used when the strike aircraft has no enough available power or payload for self-protection. Its effectiveness usually is greater than for stand-off ECM due to its closer proximity to the victim system.
Expendable ECM is “deployed once for a limited time offboard the platform which they are designed to protect.” This technique is typically divided into active and passive expendable ECM systems. First usually are active decoys (miniature jammers), and the second type is primarily represented by passive decoys and chaff. To be cost effective, expendable ECM systems must be relatively cheap. Active expendable systems are usually more expensive than passive ones, and they tend to be used where passive devices are not effective. Offboard ECM systems are designed to be used remotely from the defended platform (e.g., airborne or naval platforms). The main representatives of this systems are decoys (active and passive) and chaff.
Onboard ECM systems are ECM devices within the defended platform. Typically, they are active systems: noise and deception jammers. Sometimes various means for RCS reduction are classified as passive ingredients of onboard ECM systems.
Passive ECM refers to electronic countermeasures involving devices reflecting electromagnetic energy in such a manner that the reradiated signal competes with true target return to conceal real reflection. A typical example of passive ECM is chaff.
Range-measurement ECM provides interfering signals, the main goal of which is disrupting the measurement of time of arrival to determine target range. Typically, it may be implemented in two ways. First is to cover or suppress echo return before it can be detected in radar receiver. Noise jamming is an appropriate technique for this purpose; only the high radiated power is required, especially if frequency agility is employed in victim radar. The second is to confuse a radar as to the true location of the target. This can be implemented by use of deception jamming. In this case false targets (passive decoys) presenting many radar blips with different ranges and range-gate pull-off are the effective measures. Range gate pull-off (RGPO) is an range-measurement ECM technique in which a repeater captures the tracking range gate and introduces delay or advance to move the gate away from the target echo. A typical RGPO cycle is shown in Fig. E5. In the first phase of the cycle, the repeater pulse is
introduced and raised smoothly to a level sufficient to suppress the target echo pulse (without creating transients that might disclose the presence of the ECM). In the second phase the repeater pulse is delayed or advanced to pull the gate off the target echo. Only delay is possible in cases of PRF stagger or carrier frequency agile radars, except that a linear FM waveform permits a frequency shift to produce an early output from the pulse compression filter). Against sophisticated trackers the apparent target acceleration of the repeater pulse must be maintained within the limits that can represent an intended target. In the third phase, when the target echo is not contributing to the output of the gate, angle deception modulations may be applied to the repeater pulse in an attempt to introduce an error in angle tracking rate. In the fourth phase, the repeater pulse is turned off, breaking the tracking loops. If sufficient angle rate has been introduced into the tracking loops, the beam may drift off the target before the receiver gain is increased and reacquisition of the target is attempted. This will force the radar to reinitiate its complete acquisition scan process, during which time the target is not under track. In the absence of a significant angle rate, the range gate can be swept to reacquire the target within fractions of a second. SAL, DKB
ECM versus search radar is used to prevent detection and acquisition of the target. To achieve this goal all main methods of active and passive jamming may be employed, including noise jamming, deception jamming, and use of chaff and decoys. The main types of ECM and relevant ECCM techniques are cited in Table
Self-screening ECM is “conducted by individual combat elements to deny acquisition, tracking, or fire-control data to hostile weapon system.” Typically, it requires that the single aircraft penetrating through an air defence system rely upon its own ECM capabilities.
Stand-forward ECM is the ECM tactic “in which the jamming platform is located between the weapon systems and the strike vehicles and jams the radar to protect the strike vehicles.” In this case only remotely piloted vehicles can meet the safety requirements as the platform with the jammer is usually located within the lethal range of defensive weapon system for a long period of time.
Stand-off ECM is the ECM tactic in which missions are “conducted outside the lethal zones of hostile weapon-control systems to provide ECM support for friendly forces subject to hostile fire.” Typically, the stand-off ECM system must provide high-power noise jamming able to jam the victim radar through the sidelobes at long ranges.
ECM versus tracking radar is used to prevent or delay acquisition of the target and to disrupt the tracking function of the radar (or at least introduce intolerable measurement errors). The primary types of ECM against tracking radar and relevant ECCM techniques are cited in Table
Velocity-measurement ECM is used to disrupt the measurement of velocity based on doppler shift measurement with a doppler filter bank. In this case, deception jamming has a preference over noise jamming, as the latter has to spread its energy over the expected range of frequencies of possible radar returns (that may be tens of percent of the carrier frequency), resulting in very high requirements of effective radiated power. The main type of deception jamming against doppler radar is velocity-gate pull-off (VGPO), which is a deceptive jamming technique, often used by self-protection airborne jammers against coherent tracking radars or missile seekers, to produce erroneous target doppler measurement and ultimately to cause break-lock of the doppler-tracking loop. The jammer is either a true repeater or a transponder repeater that captures the victim radar or missile seeker velocity-tracking gate by reradiating the target signal at a high jammer- to-signal ratio and then pulls the captured gate off the true target doppler signal. A primary tactic with VGPO is to pull the doppler gate off and then momentarily shut off the jammer, forcing reacquisition of the target by the radar or seeker. The doppler pull-off rate can be calculated to avoid early recognition and counter by the victim, but spectral analysis and audio indications can reveal the presence of VGPO and, in the case of tracking radars and given enough time, manual tracking can defeat the VGPO. Modern Doppler radars and missile seekers can be expected to contain effective and automatic countermeasures to simple VGPO; for this reason, the technique is seldom used alone but is combined in a repertoire of other deceptive jamming techniques.
ELECTRONIC COUNTER-COUNTERMEASURES (ECCM) are “actions taken to ensure friendly use of the electromagnetic spectrum against electronic warfare.” Their main objective is to eliminate or reduce the efficiency of the enemy’s ECM. They fall into two broad classes: electronic ECCM and operational ECCM. Electronic techniques are included in the main radar subsystems and are typically described, following the usage of Johnston (1979), as antenna-related ECCM, transmitter-related ECCM, and receiver- and signal-processing ECCM. From the point of view of radar types where these techniques are implemented, search (surveillance) radar ECCM and tracking radar ECCM are distinguished.
Antenna-related ECCM are ECCM techniques based on the properties of antenna systems to reduce the effectiveness of ECM. Space selection based on antenna directivity and polarization selection based on the polarization properties of electromagnetic waves are the main ECCM strategies of discrimination the useful signals and interference. The main techniques for antenna-related ECCM based on spatial selection are coverage and scan control, reduction of main-beam width, reduction of side lobe level, and employing of adaptive antennas. The first group of methods based on antenna pattern and scan control may include blanking or turning of the receiver while the radar is observing the part of space containing a jammer, using multiple-beam configuration to detect a target by a beam not afflicted by jammers, random scanning to prevent deception jammers from synchronizing with the antenna scan rate, and other relevant measures. Reduction of beam width increases the angular resolution and is a valuable feature of any radar operating in a dense ECM environment. It should be kept in mind, however, that the reduction of beam width, for a given aperture size, results in increasing the side lobe level that worsens a radar’s ant jamming capability, so the reasonable compromise should be found in specifying the antenna radiation pattern. The main side lobe-related techniques are usage of low- and ultralow side lobe antennas, side lobe blanking (SLB) and side lobe cancellation (SLC). The generalization of SLC techniques in combination with adaptive processing technique is the concept of adaptive arrays. This technique is very promising as it is based on advanced methods of digital beam-forming and digital signal processing and permits super resolution capabilities that can be very useful for ECCM. Polarization selection takes advantage of electromagnetic wave polarization features for discrimination of useful signals at the interference background. In this case, if only signals with co polarization are useful, the cross-polarized response should be kept as low as possible to protect against cross polarized jamming. In more complex cases the two orthogonally polarized components may be used by the antenna to discriminate the useful signal from those received from jammers and chaff.
The ECCM improvement factor is “the power ratio of the ECM signal level required to produce a given output signal to- interference ratio from a receiver using an ECCM technique to the ECM signal level producing the same output signal- to-interference ratio from the same receiver without the ECCM technique.”
Electro-optic counter-countermeasures are “the actions taken to ensure the effective friendly use of the electro-optic spectrum despite the enemy’s use of countermeasures in this spectrum.”
Human-factors ECCM is the “ECCM technique that covers the ability of an air-defense officer, a radar operator, a commanding officer, and/or any other air defense associated personnel to recognize the various kinds of ECM, to analyze the effect of ECM, to decide what the appropriate ECCM should be and/or to take the necessary ECCM actions within the framework of the person’s command structure.
Operational ECCM is the combination of operational modes of the radar in response to a specific ECM threat. This group of techniques can be subdivided into those involving methods of operation, radar deployment tactics, work of radar operator, and friendly electronic support measures in aid of ECCM.
Range gate pull-off ECCM applies to measures taken, either with radar operator support or automatically through ECM sensing and response circuits, to defeat the range gate pull-off (RGPO) type of jamming employed by deceptive ECM (DECM) repeater jammer systems. Non Doppler measuring radars may use a human operator to identify the occurrence of RGPO, in which case the operator can manually maintain the radar tracking cursor over the correct target. If a victim of RGPO jamming is netted to tracking radar, true target range may be derived through triangulation. Doppler radars can compare target Doppler with range rate derived from range data differentiation and thus determine which the correct target is. This ECCM tactic may be frustrated if the jammer also employs velocity gate pull-off (VGPO) techniques, but other ECCM procedures can be invoked to deny the effectiveness of VGPO (see velocity gate pull-off ECCM). RGPO is seldom effective against modern radars when used alone, and to be effective, a radar ECCM strategy must be devised that deals with the several combined forms of range, angle, and Doppler jamming techniques that the radar may encounter.
Receiver- and signal-processing-related ECCM is ECCM based on the properties of the receiver and signal processing to reduce the influence of ECM on radar performance. The main technique associated with receivers is using a wide dynamic range to avoid saturation of signal-processing chains by a jamming signal. Logarithmic and linear-logarithmic receivers are normally used to avoid the saturation. Other techniques often used in radar receivers (e.g., constant-false alarm- rate circuits, automatic gain control, fast-time- constant circuits, etc.) may be useful to prevent saturation, although normally they are not referred as to ECCM techniques. Dicke-Fix devices and various kinds of limiters also can be used to counter jamming. The main type of signal processing that can significantly reduce the effectiveness of ECM is coherent signal processing. In modern radars it is implemented in the form of coherent digital signal processing, including digital MTI.
Search radar ECCM is employed in a radar whose purpose is to search a large volume of space and locate the position of detected targets. The major ECM techniques that are threats for such radars are noise jamming, deception jamming, decoys, chaff, and ant radiation missiles. The main techniques that can be employed against noise jamming are based on the concepts of maximizing radar energy delivered to the target of interest (burn through mode) or/and minimizing the amount of jamming energy received by the radar (ultra- and low-side lobe antennas, side lobe blanking and cancellation, frequency agility and frequency diversity, use of coded waveforms). Techniques employed against the deception jamming can use PRF stagger and jitter or measurement and analysis of angular, Doppler or polarization characteristics of radar echoes to discriminate between real and false-target returns. The main ECCM technique against chaff is discrimination of real and false returns on the basis of Doppler processing (e.g., MTI). Ant radiation missiles are threats to search radars. One of the best methods of defense is to use a blinking network of radars. Tracking radar ECCM is employed in radars whose purpose is to provide good resolution and precise measurement of targets parameters. It is based on antenna-, transmitter-, receiver- and signal-processing-related ECCM techniques. Practically all measures referred to as search radar ECCM, including maximizing the radiated energy delivered to the target and minimizing the jammer signal entering signal-processing circuits, are helpful to counter noise jamming in tracking radars. A more threatening ECM technique against tracking radars is deception jamming. Typically, it is deception in angle (angle-gate stealing), range (range-gate stealing or range-gate pull-off) and velocity (speed gate stealing or velocity-gate pull-off). Angle-gate stealing is especially effective against radars employing conical scanning and sequential lobing, so mono pulse tracking, inherently insensitive to angle deception jamming from a single point, must be used in radars where good ECCM efficacy against jamming in angle is required. Using of leading-edge tracker, tracking of both real and false targets in both range and Doppler, using PRF jitter, the mode of multiple PRF operation (low, high and medium PRF) and frequency agility are the usual measures against deception jammers in range and Doppler domains.
Transmitter-related ECCM is based on the property of a radar transmitter to control the frequency, power, and waveform of the radiated signal to reduce the effectiveness of ECM. Increasing of the transmitter frequency is analogous to increasing the antenna aperture in a fixed-frequency system, narrowing the antenna beam width and in turn improving the spatial resolution (see antenna-related ECCM). Control of frequency involves primarily frequency agility and frequency diversity. The objective of frequency agility and frequency diversity is to force the jammer to spread its energy over the wider bandwidth and to reduce the power density of jamming at the radar input. The methods of power control are based mainly on the brute-force approach of increasing radar transmitter power or more flexible approaches based on management of power in time and space. The control of the waveform includes using waveform coding based on intrapulse modulation that enables increased signal bandwidth, shaping of the transmitted radar pulse, and coding of various parameters of radar signal (e.g. PRF jitter and PRF stagger).
Reconnaissance aims to give to field players, at a tactical level, the information they need: Where are (and where will be) the targets, what are their defenses, what military means to use, what kind of weapon is the best suited, when to conduct the attack, etc. Due to short reaction time in Air Defense, this task is generally carried out directly by the AEW system. In fact in some cases the AEW platform is a C3I system itself that controls the fighters in real time. For ground targets, due to slower evolution of the situation and to a more complex environment (collateral damage avoidance, mask), a specific mission is needed. This mission uses SAR/MTI systems either fitted in POD carried by a manned aircraft (business jet or fighter), or carried by an unmanned vehicle (UAV). The main objective of these missions is to re-acquire the targets (in case they are moved as Ballistic Missile Launchers), and to identify and locate them accurately. Once again fusion of different sensors (ESM, optical, etc.) is generally needed to assess the global tactical situation, including Air Defense threat assessment. All this information is gathered at the C3I center where the interceptions or the strike missions are preplanned
ELECTRONIC WARFARE (EW) is “a military action involving the use of electromagnetic energy to determine, exploit, reduce, or prevent hostile use of the electromagnetic spectrum and action while retains friendly use of the electromagnetic spectrum.” EW is divided into three classic categories: electronic (warfare) support measures (ESM), electronic countermeasures (ECM), and electronic counter-countermeasures (ECCM).
ELECTRONIC COUNTERMEASURES (ECM) are “actions taken to prevent or reduce the enemy’s effective use of the electromagnetic spectrum.” It is one of the three components of electronic warfare. In radar applications its main objectives are to deny or to falsify information (detection, measurement, discrimination, and classification data) that the radar tries to obtain. There are a number of ways to classify ECM tactics and techniques. From the point of view whether electromagnetic energy is radiated or not, ECM is divided into active and passive. From the standpoint of what main parameters of radar information are influenced, ECM is divided into angle-measurement ECM, range-measurement ECM, and velocity-measurement ECM. From the viewpoint of the types of jammed radars, it is primarily divided into ECM versus search (surveillance) radars and ECM versus tracking radars. From the viewpoint of the ECM systems location, onboard and offboard ECM are distinguished. As to tactics of ECM combat employment, typically five classes can be identified: escort ECM, cooperative (mutual-support) ECM, self-protection (self-screening) ECM, stand-forward ECM, and stand-off ECM. Some authors include defence suppression as an ECM, although others consider it a separate ingredient of electronic warfare. ECM is primarily based on jamming: both noise jamming and deception jamming. Modern ECM systems are designed to cope with different types of radars, and they have to operate in a dense threat environment that requires computer control of the system. Typical parameters of an advanced, self-protection active ECM system designed for fighter-type aircraft are given in Table:
Active ECM incorporates devices and methods based on deliberate radiation of electromagnetic energy to disrupt the operation of the victim radar. Active ECM typically means active jamming.
Angle-measurement ECM is designed to cause disruption of correct determining of the target’s angular direction both by search and tracking radars. The main methods to counter a search radar are to jam it through antenna sidelobes, based on the fact that, if a signal is received and detected, it is considered to be in the antenna main beam. So, if the energy of the signal radiated by a jammer and received by radar through antenna sidelobes is sufficient to exceed the detection threshold, the radar concludes that the signal was in the main beam and, therefore, considerable error is introduced in angular measurements. It is obvious that extremely large amounts of energy are required to overcome the main-to-sidelobe ratio of the victim radar. High-power noise jamming can be employed to implement this task, and special vehicles are being used in escort or stand-off ECM modes to carry the jammers required to protect the penetrating vehicles. The main type of modern tracking radar is monopulse radar. Typically, jamming techniques against this type of radar fall into two categories. The first uses imperfection in the monopulse design and hardware implementation to reverse the sense of the angle-error signal. These involve cross-polarization jamming, image-frequency jamming, and skirt-frequency jamming. The second class uses the effect of multipoint jamming to distort true angle-of-arrival of real signal. These technique include blinking jamming, cross-eye jamming, formation jamming, and ground-bounce jamming. Down-link jamming may be a useful technique to reduce angular measurement capability of track-while-scan radars.
Cooperative ECM is that “involving the coordinated conduct of electronic countermeasures by combat elements against hostile acquisition and weapon-control radars.” Its advantages are coordinated tactics and greater effective radiated power that can be achieved due to jamming from various platforms. An example of this tactic is blinking jamming. This type of ECM is also termed mutual-support ECM.
Cross-eye ECM is a deceptive, self-protection technique designed to frustrate target-tracking radars or radar seekers by causing the radar to track a jamming signal whose phase front differs significantly from that of the true target return signal. Cross-eye is one of several multisource jamming concepts (others include cross-polarization and terrain-bounce) designed to attack monopulse tracking radars, which are invulnerable to angle-jamming techniques such as inverse gain jamming effectively used against sequential-lobing and conical-scan tracking radars. Figure E3 shows a generic implementation of the cross-eye jamming technique, consisting of two repeaters connected to two separate antenna systems.
The transmit and receive antennas for each repeater are separated by the baseline distance d. One of the repeater paths includes a 180° phase shifter, and one path contains controls to ensure proper adjustment of relative phase and amplitude between the two repeater paths. With this configuration, the coherent jamming signals arrive at the victim radar matched in amplitude, but 180° out of phase. This creates an interferometric null between the two jamming signals in the direction of the victim radar antenna. Jammer effectiveness relies on the presence of true target-generated angle noise within the victim radar’s angle tracking loop to drive the radar antenna off the jamming signal null so that a sufficiently high jammerto- signal (J/S) ratio is developed. The cross-eye concept has two inherent vulnerabilities:
(1) the delay time introduced by the baseline distance between jammer antenna sets provides a potential opportunity for the victim radar to detect and track the leading edge of the true target return before the repeated jamming signal arrives.
(2) a relatively high J/S ratio, on the order of 20 dB or greater, is required for the technique to work effectively.
Cross-polarization ECM is a deceptive ECM (DECM) technique, designed for use against monopulse tracking radars, that attempts to exploit the victim radar’s response to a repeater jamming signal whose polarization is orthogonal to that of the radar receiving antenna. The response of the victim radar’s angle error discriminator can be significantly different for cross-polarized signals, leading to large angle tracking errors and eventual break-lock. Reflector antennas, particularly parabolic-dish designs, are most susceptible to cross-polarization jamming in that their response to orthogonally polarized signals may be reduced only 15 to 30 dB relative to their copolarized response. Flat-plate planar arrays are inherently less susceptible to cross-polarization jamming in that their response is typically reduced by 40 to 50 dB relative to copolarized signals. However, this apparent immunity can be largely negated by the presence of a radome, which may create significant levels of cross-polarized components, depending on the radome design.
Counter-countermeasures to cross-polarization jamming include the use of polarization screens to block signals with undesired polarizations. A more expensive and complex option entails the use of antennas capable of receiving multiple polarizations, perhaps in conjunction with one or more separate receiver and signal-processing channel. To be effective, even against single-polarization radars, cross-polarization jammers must realize high J/S ratios while accurately maintaining jammer-to-radar signal polarization orthogonality (typically to within plus-or-minus 5°). If these conditions are not met, the jammer signal can become the equivalent of a target-borne radar beacon, making the jammer platform highly susceptible to engagement by radar homing missiles.
A block diagram of a cross-pol jammer is shown : Signal components received by the vertically polarized antenna are amplified and retransmitted through the horizontally polarized antenna, and vice versa. As a result, any arbitrary elliptically polarized signal will be returned with the orthogonal polarization. The avoids the necessity of analyzing each received signal and adjusting the repeated signal polarization to be orthogonal to it.
Deception [deceptive] ECM employs “the intentional and deliberate transmission or retransmission of amplitude, frequency, phase, or otherwise modulated intermittent or CW signals or the purpose of misleading in the interpretation or use of information by electronic system.” The common acronym is DECM. In respect to radar, its main objective is to mask the real radar signal by injecting suitably modified replicas of target return into the victim radar. Typically, it is performed by deception jamming. Angle deception is any ECM technique designed to frustrate a radar’s ability to determine the relative angular position of the true target. When angle deception is employed against surveillance radars, the objective of the jammer is usually to insert multiple false targets into the radar’s processor by injecting high-power signals through the victim radar’s antenna sidelobes, thus making discrimination of the true target more difficult or impossible. Angle deception can also be directed to attack the angle-tracking capability of a tracking radar or radar homing missile seeker. In this role angle deception jamming is usually an integral component of a general class of DECM techniques used by self-screening jammers (SSJ) to protect the host platform (e.g., an aircraft, ship, or missile) from engagement by an air-defense system. Most angle deception techniques are implemented via a repeater jammer, which retransmits an amplified version of the victim radar’s signal that has been modulated such that the angle data recovered by the victim radar no longer represents the true target’s angular position relative to the radar. To be effective, an angle deception jamming technique must be tailored to the particular way in which the victim radar derives target angle information. Track-while-scan (TWS) radars, sequential lobing, and conical-scan trackers derive target angle by sequential measurements of target energy as the radar antenna is pointed at different positions about the target location. Angle deception against these radar types is readily effected through a technique known as inverse gain jamming, in which an amplified replica of the radar signal is transmitted, but with amplitude modulation 180° out of phase from the original, which has the net result of canceling the amplitude modulation needed to derive target angle information. In monopulse radars, all of the target angle information is available on a single pulse, rendering this type of tracking radar more difficult to jam. Deceptive techniques other than inverse gain jamming are required and these fall into two generalclasses:
(1) Those that exploit a particular mechanization or imperfection in the monopulse design.
(2) Those that exploit multiple signal source effects to distort the angle-of-arrival of the radar return signal. Techniques in class (2) are more “robust” in that they do not rely on detailed knowledge of a particular radar or missile seeker design for their effect. Angle deception techniques in this class include:
(1) Cross-polarization jamming, in which the repeated signal, with a high jammer-to-signal (J/S) ratio, has a polarization orthogonal to that of the original signal.
(2) Cross-eye, a technique that utilizes two separate repeaters, whose antennas are separated in space (e.g., one at each wingtip of an aircraft) to develop a combined signal whose phase front differs significantly from that of the true target return signal.
(3) Blinking jamming, in which two repeater jammers separated in space “blink,” or transmit alternately in time at a rate designed to defeat the angle-tracking servo dynamics of the radar.
(4) Terrain bounce, which uses the earth’s surface to reflect an airborne jamming signal and thus create a multipath situation in the elevation plane for the victim radar or missile seeker. If the victim is, for example, a semi-active homing missile, the objective of the terrain bounce jammer is to cause the seeker to home on the jammer-illuminated patch on the earth’s surface rather than on the jammer itself. Both blinking and terrain bounce techniques can be implemented in either repeater jammer or noise jammer configurations.
Downlink ECM is used against satellites, aircraft, and missile- borne radars that have separate, onboard systems to “downlink” target information or other radar sensor data, such as platform position and radar mapping data, from the radar acquisition platform to a ground radar or data processing center. Downlink ECM is directed at disrupting this line of communication. In the interest of efficiency as well as security, most of the waveforms used by downlink transmitters are modulated in either time, frequency, phase, amplitude, or combinations thereof. Encryption algorithms may be superimposed for added security. In the absence of detailed knowledge of the downlinked signal coding, attempts to interfere with the downlink are mostly restricted to the use of stand-off noise
jamming. Downlink jamming is made more difficult still, due to the one-way transmission path of the downlink, and the usually high power transmitters employed by downlink systems. Downlink transmission systems generally employ fairly wide beamwidth antennas, but even coarse directivity can pose significant problems for a noise jammer, forcing it, in many circumstances (e.g., a missile downlink) to jam through the downlink antenna sidelobes. Yet another complication for the missile downlink jammer occurs when the downlink is used only infrequently, i.e., has a low duty cycle and so is vulnerable only for a small portion of the total missile flight time.
Escort ECM is the “ECM tactic in which the jamming platform accompanies the strike vehicle and jams radars to protect the strike vehicles,” This tactic is applied to aircraft combat operations. It is generally used when the strike aircraft has no enough available power or payload for self-protection. Its effectiveness usually is greater than for stand-off ECM due to its closer proximity to the victim system.
Expendable ECM is “deployed once for a limited time offboard the platform which they are designed to protect.” This technique is typically divided into active and passive expendable ECM systems. First usually are active decoys (miniature jammers), and the second type is primarily represented by passive decoys and chaff. To be cost effective, expendable ECM systems must be relatively cheap. Active expendable systems are usually more expensive than passive ones, and they tend to be used where passive devices are not effective. Offboard ECM systems are designed to be used remotely from the defended platform (e.g., airborne or naval platforms). The main representatives of this systems are decoys (active and passive) and chaff.
Onboard ECM systems are ECM devices within the defended platform. Typically, they are active systems: noise and deception jammers. Sometimes various means for RCS reduction are classified as passive ingredients of onboard ECM systems.
Passive ECM refers to electronic countermeasures involving devices reflecting electromagnetic energy in such a manner that the reradiated signal competes with true target return to conceal real reflection. A typical example of passive ECM is chaff.
Range-measurement ECM provides interfering signals, the main goal of which is disrupting the measurement of time of arrival to determine target range. Typically, it may be implemented in two ways. First is to cover or suppress echo return before it can be detected in radar receiver. Noise jamming is an appropriate technique for this purpose; only the high radiated power is required, especially if frequency agility is employed in victim radar. The second is to confuse a radar as to the true location of the target. This can be implemented by use of deception jamming. In this case false targets (passive decoys) presenting many radar blips with different ranges and range-gate pull-off are the effective measures. Range gate pull-off (RGPO) is an range-measurement ECM technique in which a repeater captures the tracking range gate and introduces delay or advance to move the gate away from the target echo. A typical RGPO cycle is shown in Fig. E5. In the first phase of the cycle, the repeater pulse is
introduced and raised smoothly to a level sufficient to suppress the target echo pulse (without creating transients that might disclose the presence of the ECM). In the second phase the repeater pulse is delayed or advanced to pull the gate off the target echo. Only delay is possible in cases of PRF stagger or carrier frequency agile radars, except that a linear FM waveform permits a frequency shift to produce an early output from the pulse compression filter). Against sophisticated trackers the apparent target acceleration of the repeater pulse must be maintained within the limits that can represent an intended target. In the third phase, when the target echo is not contributing to the output of the gate, angle deception modulations may be applied to the repeater pulse in an attempt to introduce an error in angle tracking rate. In the fourth phase, the repeater pulse is turned off, breaking the tracking loops. If sufficient angle rate has been introduced into the tracking loops, the beam may drift off the target before the receiver gain is increased and reacquisition of the target is attempted. This will force the radar to reinitiate its complete acquisition scan process, during which time the target is not under track. In the absence of a significant angle rate, the range gate can be swept to reacquire the target within fractions of a second. SAL, DKB
ECM versus search radar is used to prevent detection and acquisition of the target. To achieve this goal all main methods of active and passive jamming may be employed, including noise jamming, deception jamming, and use of chaff and decoys. The main types of ECM and relevant ECCM techniques are cited in Table
Self-screening ECM is “conducted by individual combat elements to deny acquisition, tracking, or fire-control data to hostile weapon system.” Typically, it requires that the single aircraft penetrating through an air defence system rely upon its own ECM capabilities.
Stand-forward ECM is the ECM tactic “in which the jamming platform is located between the weapon systems and the strike vehicles and jams the radar to protect the strike vehicles.” In this case only remotely piloted vehicles can meet the safety requirements as the platform with the jammer is usually located within the lethal range of defensive weapon system for a long period of time.
Stand-off ECM is the ECM tactic in which missions are “conducted outside the lethal zones of hostile weapon-control systems to provide ECM support for friendly forces subject to hostile fire.” Typically, the stand-off ECM system must provide high-power noise jamming able to jam the victim radar through the sidelobes at long ranges.
ECM versus tracking radar is used to prevent or delay acquisition of the target and to disrupt the tracking function of the radar (or at least introduce intolerable measurement errors). The primary types of ECM against tracking radar and relevant ECCM techniques are cited in Table
Velocity-measurement ECM is used to disrupt the measurement of velocity based on doppler shift measurement with a doppler filter bank. In this case, deception jamming has a preference over noise jamming, as the latter has to spread its energy over the expected range of frequencies of possible radar returns (that may be tens of percent of the carrier frequency), resulting in very high requirements of effective radiated power. The main type of deception jamming against doppler radar is velocity-gate pull-off (VGPO), which is a deceptive jamming technique, often used by self-protection airborne jammers against coherent tracking radars or missile seekers, to produce erroneous target doppler measurement and ultimately to cause break-lock of the doppler-tracking loop. The jammer is either a true repeater or a transponder repeater that captures the victim radar or missile seeker velocity-tracking gate by reradiating the target signal at a high jammer- to-signal ratio and then pulls the captured gate off the true target doppler signal. A primary tactic with VGPO is to pull the doppler gate off and then momentarily shut off the jammer, forcing reacquisition of the target by the radar or seeker. The doppler pull-off rate can be calculated to avoid early recognition and counter by the victim, but spectral analysis and audio indications can reveal the presence of VGPO and, in the case of tracking radars and given enough time, manual tracking can defeat the VGPO. Modern Doppler radars and missile seekers can be expected to contain effective and automatic countermeasures to simple VGPO; for this reason, the technique is seldom used alone but is combined in a repertoire of other deceptive jamming techniques.
ELECTRONIC COUNTER-COUNTERMEASURES (ECCM) are “actions taken to ensure friendly use of the electromagnetic spectrum against electronic warfare.” Their main objective is to eliminate or reduce the efficiency of the enemy’s ECM. They fall into two broad classes: electronic ECCM and operational ECCM. Electronic techniques are included in the main radar subsystems and are typically described, following the usage of Johnston (1979), as antenna-related ECCM, transmitter-related ECCM, and receiver- and signal-processing ECCM. From the point of view of radar types where these techniques are implemented, search (surveillance) radar ECCM and tracking radar ECCM are distinguished.
Antenna-related ECCM are ECCM techniques based on the properties of antenna systems to reduce the effectiveness of ECM. Space selection based on antenna directivity and polarization selection based on the polarization properties of electromagnetic waves are the main ECCM strategies of discrimination the useful signals and interference. The main techniques for antenna-related ECCM based on spatial selection are coverage and scan control, reduction of main-beam width, reduction of side lobe level, and employing of adaptive antennas. The first group of methods based on antenna pattern and scan control may include blanking or turning of the receiver while the radar is observing the part of space containing a jammer, using multiple-beam configuration to detect a target by a beam not afflicted by jammers, random scanning to prevent deception jammers from synchronizing with the antenna scan rate, and other relevant measures. Reduction of beam width increases the angular resolution and is a valuable feature of any radar operating in a dense ECM environment. It should be kept in mind, however, that the reduction of beam width, for a given aperture size, results in increasing the side lobe level that worsens a radar’s ant jamming capability, so the reasonable compromise should be found in specifying the antenna radiation pattern. The main side lobe-related techniques are usage of low- and ultralow side lobe antennas, side lobe blanking (SLB) and side lobe cancellation (SLC). The generalization of SLC techniques in combination with adaptive processing technique is the concept of adaptive arrays. This technique is very promising as it is based on advanced methods of digital beam-forming and digital signal processing and permits super resolution capabilities that can be very useful for ECCM. Polarization selection takes advantage of electromagnetic wave polarization features for discrimination of useful signals at the interference background. In this case, if only signals with co polarization are useful, the cross-polarized response should be kept as low as possible to protect against cross polarized jamming. In more complex cases the two orthogonally polarized components may be used by the antenna to discriminate the useful signal from those received from jammers and chaff.
The ECCM improvement factor is “the power ratio of the ECM signal level required to produce a given output signal to- interference ratio from a receiver using an ECCM technique to the ECM signal level producing the same output signal- to-interference ratio from the same receiver without the ECCM technique.”
Electro-optic counter-countermeasures are “the actions taken to ensure the effective friendly use of the electro-optic spectrum despite the enemy’s use of countermeasures in this spectrum.”
Human-factors ECCM is the “ECCM technique that covers the ability of an air-defense officer, a radar operator, a commanding officer, and/or any other air defense associated personnel to recognize the various kinds of ECM, to analyze the effect of ECM, to decide what the appropriate ECCM should be and/or to take the necessary ECCM actions within the framework of the person’s command structure.
Operational ECCM is the combination of operational modes of the radar in response to a specific ECM threat. This group of techniques can be subdivided into those involving methods of operation, radar deployment tactics, work of radar operator, and friendly electronic support measures in aid of ECCM.
Range gate pull-off ECCM applies to measures taken, either with radar operator support or automatically through ECM sensing and response circuits, to defeat the range gate pull-off (RGPO) type of jamming employed by deceptive ECM (DECM) repeater jammer systems. Non Doppler measuring radars may use a human operator to identify the occurrence of RGPO, in which case the operator can manually maintain the radar tracking cursor over the correct target. If a victim of RGPO jamming is netted to tracking radar, true target range may be derived through triangulation. Doppler radars can compare target Doppler with range rate derived from range data differentiation and thus determine which the correct target is. This ECCM tactic may be frustrated if the jammer also employs velocity gate pull-off (VGPO) techniques, but other ECCM procedures can be invoked to deny the effectiveness of VGPO (see velocity gate pull-off ECCM). RGPO is seldom effective against modern radars when used alone, and to be effective, a radar ECCM strategy must be devised that deals with the several combined forms of range, angle, and Doppler jamming techniques that the radar may encounter.
Receiver- and signal-processing-related ECCM is ECCM based on the properties of the receiver and signal processing to reduce the influence of ECM on radar performance. The main technique associated with receivers is using a wide dynamic range to avoid saturation of signal-processing chains by a jamming signal. Logarithmic and linear-logarithmic receivers are normally used to avoid the saturation. Other techniques often used in radar receivers (e.g., constant-false alarm- rate circuits, automatic gain control, fast-time- constant circuits, etc.) may be useful to prevent saturation, although normally they are not referred as to ECCM techniques. Dicke-Fix devices and various kinds of limiters also can be used to counter jamming. The main type of signal processing that can significantly reduce the effectiveness of ECM is coherent signal processing. In modern radars it is implemented in the form of coherent digital signal processing, including digital MTI.
Search radar ECCM is employed in a radar whose purpose is to search a large volume of space and locate the position of detected targets. The major ECM techniques that are threats for such radars are noise jamming, deception jamming, decoys, chaff, and ant radiation missiles. The main techniques that can be employed against noise jamming are based on the concepts of maximizing radar energy delivered to the target of interest (burn through mode) or/and minimizing the amount of jamming energy received by the radar (ultra- and low-side lobe antennas, side lobe blanking and cancellation, frequency agility and frequency diversity, use of coded waveforms). Techniques employed against the deception jamming can use PRF stagger and jitter or measurement and analysis of angular, Doppler or polarization characteristics of radar echoes to discriminate between real and false-target returns. The main ECCM technique against chaff is discrimination of real and false returns on the basis of Doppler processing (e.g., MTI). Ant radiation missiles are threats to search radars. One of the best methods of defense is to use a blinking network of radars. Tracking radar ECCM is employed in radars whose purpose is to provide good resolution and precise measurement of targets parameters. It is based on antenna-, transmitter-, receiver- and signal-processing-related ECCM techniques. Practically all measures referred to as search radar ECCM, including maximizing the radiated energy delivered to the target and minimizing the jammer signal entering signal-processing circuits, are helpful to counter noise jamming in tracking radars. A more threatening ECM technique against tracking radars is deception jamming. Typically, it is deception in angle (angle-gate stealing), range (range-gate stealing or range-gate pull-off) and velocity (speed gate stealing or velocity-gate pull-off). Angle-gate stealing is especially effective against radars employing conical scanning and sequential lobing, so mono pulse tracking, inherently insensitive to angle deception jamming from a single point, must be used in radars where good ECCM efficacy against jamming in angle is required. Using of leading-edge tracker, tracking of both real and false targets in both range and Doppler, using PRF jitter, the mode of multiple PRF operation (low, high and medium PRF) and frequency agility are the usual measures against deception jammers in range and Doppler domains.
Transmitter-related ECCM is based on the property of a radar transmitter to control the frequency, power, and waveform of the radiated signal to reduce the effectiveness of ECM. Increasing of the transmitter frequency is analogous to increasing the antenna aperture in a fixed-frequency system, narrowing the antenna beam width and in turn improving the spatial resolution (see antenna-related ECCM). Control of frequency involves primarily frequency agility and frequency diversity. The objective of frequency agility and frequency diversity is to force the jammer to spread its energy over the wider bandwidth and to reduce the power density of jamming at the radar input. The methods of power control are based mainly on the brute-force approach of increasing radar transmitter power or more flexible approaches based on management of power in time and space. The control of the waveform includes using waveform coding based on intrapulse modulation that enables increased signal bandwidth, shaping of the transmitted radar pulse, and coding of various parameters of radar signal (e.g. PRF jitter and PRF stagger).
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