Warfare has always been conducted by adversaries who have been at great pains to understand their enemy’s strengths and weaknesses in order to minimise the risk to their own forces and territory. The detection and interception of messages and the efforts to deceive the enemy have long been the task of the ‘secret service’. The military aircraft in its infancy in World War I was used to detect troop movements and observe enemy movements, while on the ground the use of radio interception confirmed the aerial observations. As methods of communication developed, so too did methods of interception become more effective. Radar has developed from a mere detection mechanism to a means of surveillance and guidance.
Modern warfare is conducted in a rich electromagnetic environment with radio communications and radar signals from many sources. Figure 6.1 shows an example military situation with combined land, sea and air forces operating against an enemy territory which is, in turn, being defended by similar forces. The key players in this example include the following:
- Military planning maintains communications with all forces either from the battlefield or from staff headquarters. Communications needs to be swift and secure at all times to include information from tactical units, from cooperating forces and from analysis databases. This communications network is vital to build an understanding of the tactical situation and to ensure that orders are received and placed, in the secure knowledge that the status and disposition of own forces is not disclosed to the enemy.
- Air defences will be using radar to detect incoming airborne threats and will be making full use of available intelligence received by land line or data link. They will also be issuing orders by radio to fighter, missile and artillery defence systems.
- Air superiority aircraft will be on quick reaction alert on dispersal or loitering on combat air patrol (CAP). They will be in constant radio communication and using their radar discretely to identify targets.
- Defence suppression may be using radar for terrain following or for seeking targets.
- Maritime operations in the form of rotary- or fixed-wing units will be conducting open ocean or sonar searches to locate and identify surface units or submarines. This will involve the use of radar and passive or active sonar, with intelligence sent to headquarters (HQ) by data link. Communication with other units will be controlled by use of high frequency (HF), very high frequency (VHF), ultrahigh frequency (UHF), shortwave marine band or data link. A radar altimeter enables the aircraft autopilot to maintain an accurate height over the sea surface regardless of changing atmospheric conditions.
- Offensive operations will be using radar for detecting targets, and launched radar guided missiles will also be emitting.
- Naval forces will be conducting their own operations in close cooperation with their own naval and marine forces. This will include the use of surveillance radar, self-defence radar and communications. Their communications include marine band shortwave for communication with merchant vessels or fisheries vessels, as well as very low frequency (VLF) for communicating with submarines.
- Land forces will be similarly employed with their own units and deploying a wide range of radar and communications system.
As if this situation is not complex enough, modern warfare attracts the attention of the media with their attendant TV and sound satellite links and mobile telephone traffic.
The radio-frequency spectrum covered by the emitters used by these forces is broad, as illustrated in Figure 6.2. No single item of equipment can cover this range for transmission or reception. Hence, most communications and radar systems are designed for use in specific bands. These bands are usually designated by international convention, as detailed in Chapter 7.
The main role of electronic warfare is to search these radio-frequency bands in order to gather information that can be used by intelligence analysts or by front-line operators. The information gained may be put to immediate effect to gain a tactical advantage on the battlefield; it may be used to picture the strategic scenario in peace time, in transition to war, or during a conflict. It may also be used to devise countermeasures to avoid a direct threat or to deny communications to an enemy. It must also be observed that such tactics are deployed by all sides in a conflict – in other words, the listeners are themselves being listened to.
The drive for intelligence is derived from a continuous need to be one step ahead of any potential adversary at all times – in peacetime, in transition to war, during actual conflict and in post-war peacekeeping operations. A typical cycle of intelligence is shown in Figure 6.3.
The cycle of intelligence begins with a requirement to gather information on a particular scenario. This may be tactical – the observation of a conflict – or it may be strategic – observation of a potential adversary’s build-up of forces, their disposition and strength, and to identify new assets in the enemy inventory.
This requirement leads to a set of orders to collect information. This may be by means that include land, sea, air and space platforms, and is backed up by background information and espionage. Nations will also exchange information, although usually selectively. Raw information is analysed to identify new information or changes from previous intelligence. It is collated with other sources and with historical data. It is validated for accuracy and reliability by comparison with other intelligence and by other sources. It has now become ‘intelligence’ and is disseminated by secure means to trusted users.
Tactical users will make use of the intelligence to modify their battle plans and tactics. The intelligence may result in changes to the original requirement, and political situations may result in changing needs. Thus, new direction will be provided to the collectors of information.
As well as obtaining intelligence, military forces use electronic warfare actively to evade detection and to pursue aggressive attacks on enemy radar-guided weapons. Figure 6.4 illustrates some aspects of electronic warfare broken down into major subdivisions which will be described below.
In addition to all other forces in the electronic warfare (EW) scenario, the Air Forces play their own role. Figure 6.5 shows the high-flying EW aircraft gathering and analysing signals, and the low-flying tactical EW aircraft accompanying strike forces to counteract enemy defences. The high-flying surveillance platform is equipped with a range of sensors and receivers to cover the broad range of emitting systems on the ground and in the air. A vast amount of data is collected and analysed in real time to provide information of use to forces on the ground, and to provide a basis for intelligence to be used in the longer term. The low-flying aircraft is often equipped with a more selective range of sensors to identify and attack specific targets.
The aircraft type and the sensors and mission systems selected for these aircraft are determined by the requirement to perform strategic or tactical electronic warfare, and to obtain the appropriate intelligence. This requirement can be derived from analysis of a top-level national requirement such as ‘defence of the realm’. This can be progressively decomposed or broken down into subsets of requirements which lead to the definition of a particular role (Price, 2005).
6.2 Signals Intelligence (SIGINT)
Intelligence is collected from a number of different sources to form a strategic picture. These sources include:
- ELINT or electronic intelligence;
- COMINT or communications intelligence.
Confirmation of electronic warfare intelligence is usually performed by comparison with local information collection and photographic evidence, including:
- HUMINT or human intelligence;
- IMINT or image intelligence;
- PHOTINT or photographic intelligence.
The first two items on this list are often gathered by high-flying EW aircraft on long duration patrols, usually flying a patrol on the friendly side of a border and beyond missile engagement range. The aircraft is often a converted commercial type providing accommodation for a flight crew and a mission crew of operators able to detect, locate and identify sources of radio-frequency emissions at very long ranges. Their task is a combination of routine gathering and identification together with an ability to spot new or unusual emitters or patterns of use. This they perform with their own knowledge and experience and by using databases of known emitter characteristics. This is reinforced by the ability to communicate with external agencies to obtain further information.
The intelligence obtained from analysis of this electronic information is complemented by human intelligence in the field and by photographic intelligence which is used to confirm the existence and precise locations and types of target.
6.2.1 Electronic Intelligence (ELINT)
Figure 6.6 shows some examples of radar emitters, or systems operating in the radar bands that are likely to be of interest to ELINT collection aircraft. These include:
- Ground-based surveillance radars scanning borders looking for airborne or land-based intruders and forming a defensive security screen;
- Missile site or anti-aircraft artillery (AAA) radars scanning for threats and preparing to lock on and to track targets for directing defensive weapons;
- Forward command post radars providing advanced and localised warning of intrusion in order to direct local defences;
- Land and naval forces operating their own radar systems for detection and target tracking;
- Other fixed-wing aircraft and helicopters operating with their own characteristic radar types.
The ELINT system must provide a wide area coverage, preferably as near to spherical as possible with few ‘shadows’ as may be caused by wing tips, fin or fuselage masking. Figure 6.7 illustrates the functions to be performed by an ELINT system. The antennas are located on the aircraft to provide suitable coverage of the scenario to be monitored and detect an arriving signal and its direction of arrival (DoA). The signal is analysed to identify the source and its DoA, and to scan intelligence received from other sources to try to confirm the signal source. This is fused with the aircraft navigational data so that a picture can be provided showing the source relative to the ELINT aircraft. The crew will interpret the information and provide the information to other operators. An example system block diagram is shown in Figure 6.8.
This system receives signals via a number of antennas situated on the aircraft to provide maximum spherical coverage. These antennas are connected to preamplifiers or amplifiers by appropriate cables. This may include the routing of low-loss coaxial cables from wing-tip antennas to fuselage-installed receivers and amplifiers. The signals are processed by the mission computer to add labels or colour for ease of identification.
Operators are able to ask for further signal analysis to extract key signal characteristics, and may also ask for comparisons to be made with similar signals held in a database. With this analysis it is possible to identify the type of transmitter, which may enable identification of the type of installation or vehicle that made the original transmission.
An aircraft with a large number of operators can process many signals and is able to build up a picture of emitters over a wide area. Each operator will deal with signals from a particular band, logging each signal on receipt. The operator’s workstation is equipped with a roll-ball and keyboard, or a touch screen, which allows the operator to annotate the signals, call up analysis or database checks and to store signals. The tactical commander is able to retrieve the received and processed signals and build up a composite picture.
The operators and the pilots work as a team to capture the best possible picture of the signal environment. Data link communication allows ground stations or operational commanders to join the team and to use other sources of intelligence to direct a specific search. The identified emitter remains in the real-time display, tagged as friendly or hostile with its characteristics.
The system can be used to identify radar signals from many sources, including:
- Fixed ground or airfield radar;
- Mobile missile battery radar;
- Ship radar;
- Aircraft radar;
- Missile radar;
- Submarine radar.
Skilled analysis and comparison with the intelligence database entries enables users to identify threat types by radar type, vehicle class and sometimes individual vehicle, especially ships where the number of high-value assets is small.
There will be highly classified threats that will need specific antennas and analysis techniques for identification. This situation arises as national security agencies develop new transmitters using different bands or different countermeasure techniques to avoid detection. This will always be a continuous activity in electronic warfare.
The database of historical intelligence, the flight plan and tactics and the collected intelligence would be of value to an enemy if the aircraft were to be forced to land or if it were destroyed. The data storage devices must not be captured intact, and for this reason are usually fitted with an explosive charge to ensure complete destruction.
6.2.2 Communications Intelligence (COMINT)
Figure 6.9 depicts some examples of users of communications. These users employ bands that are mandated for peacetime use such as VHF and UHF for air traffic control or shipping lanes, as well as satellite communications and data link for long-range encrypted data communications. All forces use a variety of frequency bands. Also shown in this figure is the unauthorised listener – the electronic warfare listeners of all participants in a conflict, as well as those agencies not directly involved but who want to gather more intelligence. It should be noted that this activity also takes place in peacetime and may include listening to friends as well as enemies.
Communications intelligence (COMINT) is gathered by scanning the normal communications frequency bands and locking on to detected transmissions. In peacetime it may be possible to receive in clear speech, but this is extremely unlikely in times of tension or during conflict. However, a great deal of intelligence can be obtained from the following characteristics of communications activity:
- The location of individual transmitters;
- The locations of groups of transmitters and the numbers in the groups;
- The frequency of the transmission carrier;
- The style of the operator;
- The relative intensity of messages;
- Intervals between message groups;
- Periods of silence;
- Periods of activity, especially sudden or unusual activity;
- Overall pattern of communications during various states of force readiness.
For these reasons the information-gathering aircraft attempts to obtain a position fix on a transmitter and records the activity for later analysis. Depending upon the communications frequency, it is not always possible to obtain an accurate fix on a particular platform within a task group. However, observation over a period of time allows an overall picture to be built up regarding a potential foe’s force structure and intended electronic order of battle (EoB).
The antennas and receivers are optimised for reception over a broad band with high-gain, high-noise rejection. An example COMINT system block diagram is shown in Figure 6.11. Over a period of several years a potential foe’s electronic communications are collected, analysed and catalogued in order that both normal (peacetime) and abnormal (high-readiness states) may be recognised and understood. This enables responding forces to respond in kind by elevating readiness states and, if necessary, imposing restrictions of critical emissions. The overall effect is akin to an electronic form of ‘cat and mouse’, with neither side hoping inadvertently to disclose their readiness state or possible future intentions. Large-scale peacetime training exercises provide major intelligence-gathering opportunities.
6.3 Electronic Support Measures
Information on immediate threats is gathered by an electronic support Measures (ESM) system. This consists of a collection of sensitive antennas designed to detect signals in different frequency bands. The antennas are often grouped in a wing-tip pod. This allows a wide angle of view without obscuration by the fuselage, and also enables a fix on the signal source to obtain an accurate direction of arrival (DoA) of the signal. Figure 6.12 shows the ESM pods mounted on the Nimrod MR 2 wing tips.
An effective ESM system rapidly identifies the signal band and location and determines the signal characteristics depending upon a number of discriminators. A signal analyser then examines the signal characteristics to identify the type of transmitter and the level of threat posed. Even the most cursory of analysis can establish whether the emitter is associated with surveillance, target tracking or target engagement. This analysis can compare the signal with known emitter characteristics obtained from an intelligence database or threat library and known signal types confirmed and new emissions identified and categorised. Every signal identification is logged with date, time and intercept coordinates, along with the known or suspected platform type, and the results are stored. A typical block diagram is shown in Figure 6.13.
Signals received by the electronic support measures system may in some cases be analysed instantaneously to produce an identity for the transmitter of each signal received. Figure 6.14 shows some parameters of a signal that are essential for the understanding of the type of transmitter producing the signal. The nature of the pulse shape is used to determine the particular type of transmitter. The scan rate and the pattern of the scan also provide invaluable information about the mode of the transmitter. It is possible to detect the antennas changing from scanning mode to lock-on to tracking and hence determine the threat that the transmitting station poses.
As well as providing threat information, ESM is used by maritime and battlefield surveillance aircraft as a passive or listening sensor which adds important information to other sensors. It is especially useful when tracking submarines, where the use of the aircraft radar would be a source of intelligence to the submarine commander.
The salient signal characteristics or discriminators identified during the ESM collection and identification process include the following:
- Signal frequency. Owing to the RF atmospheric propagation and transmission characteristics, the operating frequency is the first indicator of radar type as all RF emitters have to compete in the same physical world.
- Blip/scan ratio. Examination of the blip/scan ratio will give preliminary indications of scan rate, sector scan width and possibly radar/emitter beamwidth.
- Scan rate. The higher the scan rate, then generally the more likely is the threat of engagement.
- Scan pattern. Search, track, track-while-scan (TWS) and ground-mapping (GM) modes will exhibit particular characteristics.
- Signal modulation. Pulse, pulse compression, pulsed Doppler (PD), a continuous wave (CW) and other more sophisticated forms of modulation are indicative of the emitter mode(s) of operation and likely threat level.
- Pulse repetition frequency (PRF). High PRF associated with a tracking mode signifies an imminent engagement.
The combination of analysis of all these modes of operation and when they are employed either singly or in combination is vital to establishing the likely capabilities and intentions of a threat platform, especially when used in combination with other intelligence information.
ESM may be employed at a strategic intelligence-gathering level using an AWACS or MPA aircraft to build the overall intelligence picture and electronic order of battle (EOB). Alternatively, such information may be gathered and utilised at a tactical level using radar warning receivers (RWR), whereby information is gathered and used at the strike platform level to enable strike aircraft to avoid the most heavily defended enemy complexes during the mission.
6.4 Electronic Countermeasures and Counter-countermeasures
Electronic countermeasures (ECM) and electronic counter-countermeasures (ECCM) take the form of interfering or deceiving the enemy’s radio and radar systems in order to negate their use or, worst of all, compromise their performance. On occasions it is difficult to distinguish between ‘chicken’ and ‘egg’ as so many issues are considered during the design phase and then hastily need to be re-evaluated once a real conflict begins.
Therefore, the authors have chosen to consider these issues together rather than separately, as it is indeed a rapidly evolving process. The deployment of EW and successes and failures are invariably and rapidly recognised as the conflict develops and as both sides are inclined to receive untimely and unexpected unpleasant surprises. In some cases this is due to an inexact appreciation of the capabilities of the foe or where ELINT has not been able to provide the complete picture. Also, given the frailties of humankind, there is also a tendency to ‘lose the recipe’ and to relearn the hard way the lessons derived from a previous conflict.
These countermeasures, or ‘jamming’ as they are often loosely called, may be divided into two categories:
- Noise jamming;
- Deception jamming;
6.4.1 Noise Jamming
Active noise jamming is often performed by identifying an enemy detection system and broadcasting white noise at high power levels. For communication systems, noise jamming could employ the broadcast of music or other audio features designed to deny the use of the particular service. This effectively swamps the input circuitry of detection systems and prevents it from operating.
The effectiveness of a jamming system depends on a number of aspects of the system, for example:
- Transmitter power output;
- Transmission line losses between the transmitter and the radiating antenna;
- Antenna gain in the direction of the receiver to be jammed;
- Transmitter bandwidth.
The amount of energy delivered into a target transmitter depends on similar aspects of the target such as:
- Incoming jamming power;
- Receiver bandwidth;
- Antenna gain;
- Radar cross-sectional area of the aircraft being masked.
In order to be effective, the jamming transmitter must be able to emit sufficient power in the bandwidth of the target receiver to mask its intended signal or to simulate a deceptive signal realistically.
Most jamming transmitters are designed to operate over the range of frequencies expected, and, as has been shown above, this is extremely wide given the range of communications devices, search radar and guidance radar types to be found on the modern battlefield. In the case of radar, the bandwidth covered is often in the range 2–18 GHz. The jammer is most effective if it can be designed to target a specific frequency range or type of threat, in which case the power output is concentrated into a narrow spectrum. Given that a jammer must operate against a wide range of emitters, its power must be spread over an increased spectrum (Figure 6.15).
Figure 6.15 portrays a frequency spectrum in which four targets exist; the jammer has the task of nullifying each of the four targets. Three different techniques are shown:
- Barrage jamming. In this example, jamming power is spread across the entire spectrum encompassing the targets. This results in a very low jamming power density (W/MHz) to the point that none of the targets is adversely affected.
- Swept spot jamming. Swept spot jamming concentrates sufficient power in a narrow bandwidth to negate each target. The jammer switches to each of the targets in turn but is only present for a low-duty cycle. This may suffice if the target receiver saturation and automatic gain control capabilities are modest but will not suffice for higher-performance systems.
- Multiple-Spot Jamming. Multiple-spot jamming divides the energy between the targets, effectively jamming them in parallel rather than sequentially. This requires a more sophisticated jamming transmitter.
The foregoing explanation is in itself very superficial. In reality the radar is unlikely to be transmitting continuously on a fixed frequency; modern radars have a considerable degree of frequency agility and can often change frequency and even signal modulation on a pulse-by-pulse basis. This makes noise jamming more difficult to achieve effectively.
These techniques are rudimentary and are not particularly effective except against the most primitive equipment and more sophisticated techniques may be employed.
Figure 6.16 illustrates the principle of inverse noise gain jamming. The target signal is analysed and a pattern of noise is generated on time that complements the original incoming signal. This results in a return signal received at the target radar that is a continuous noise pattern, thereby masking the return from the aircraft skin. With high power this can be used to swamp the return, thus denying the enemy range information. With even higher powers it is possible to enter the sidelobes of the threat radar to deny angle information.
Burnthrough range is the range at which the strength of the radar echo becomes greater than that of the jamming noise. The radar return is proportional to 1/R4 since it must travel to the target and return to the host radar. The jamming signal only travels in one direction, and is thus proportional to 1/R2. The more closely an aircraft approaches the victim radar source, the more likely is the radar signal to break through the jamming noise (see Figure 6.17 which illustrates the principle).
In Figure 6.17 a plot is shown comparing the received power (dB) against range in nautical miles, and the effect of 1/R2 and 1/R4 for jammer and radar respectively can be clearly seen. However, at some point close to the radar, the target return signal will exceed the jamming signal by a suitable margin and the radar will prevail. The threshold associated with burnthrough is generally assumed to be of the order of 8–12 dB. At ranges greater than this the jammer has the advantage.
This balance depends upon a multitude of factors including the relative performances of jammer and radar transmitter and receivers, the antenna gain and sidelobe characteristics, the aspect of the engagement, etc. A radar antenna with low gain or poor sidelobe performance will be vulnerable to clutter, as already described in Chapter 4, and noise jamming is in effect a man-made form of clutter. Conversely, the higher the performance of a radar and the better the ability to discriminate against clutter, the more robust it will be in a jamming scenario.
Another significant disadvantage of noise jamming as a countermeasure extends to the jamming platform itself. By virtue of transmitting relatively high power, the jammer itself becomes a beacon whereupon the foe can use the jamming emissions as a source of guidance. Hence, many modern systems have a home-on-jam (HOJ) mode to enable the jammer itself to be attacked while radiating.
6.4.2 Deception Jamming
Deception jamming employs more sophisticated techniques to negate the performance of the radar. If subtly employed, the radar and radar operator may not realise that countermeasures are being used. Some typical techniques used to break the radar-tracking loops previously described in Chapter 4 are:
- False target generation. If the modulating characteristics of the target radar are known, it is possible to transmit pulses that will appear as multiple targets in the victim radar. Hence, by using the jamming transmitter with diligence and transmitting replica pulses after a time delay, these false, multiple, spurious targets will appear in subsequent radar range sweeps. An intelligent radar operator should realise that his radar is being deceived but may have a problem in deciding which of the multiple returns is the correct one.
- Range gate stealing. This is a variation on the technique described above where one false pulse is generated that appears in the victim’s radar at the same range as the jammer. It is then possible to capture the range gate with the artificial pulse; in particular, if the false pulse appears to be stronger than the original in the victim receiver, it is possible to ‘steal’ the range gate by progressively altering the false range. If desired, the range gate may be left on a false value or moved off to coincide with clutter, whereupon the target lock will be lost.
- Angle track breaking. Similarly, there are ways of breaking the angle track mechanism, especially if the tracking mechanism of the victim radar is well understood. For example, in a conscan radar, angle track may be broken if the jamming signal is modulated at a frequency that approaches that of the conscan modulation frequency of the subject radar. This presupposes that the angle tracking method and conscan rate are known, which may not be the case in a wartime situation. Other simple ways of angle deception include terrain bounce, cross-eye, cross-polarisation and double cross.
- Velocity gate stealing. This is similar to range gate stealing except that the incident signal is re-radiated back to the victim radar, initially without modification. Progressively the re-radiated signal is amplified and masks the original Doppler component upon which the velocity gate is centred. The deceiving radar may then steal the velocity gate in a similar manner to the range gate stealer described above.
Modern radars are inherently resistant – although not impervious – to jamming owing to a range of features inherent in the design. These characteristics are as follows:
- Low antenna sidelobes;
- Wide dynamic range with fast-acting automatic gain control (AGC);
- Constant false alarm rate (CFAR) reduction;
- Sidelobe blanking.
When these features are employed together with a range of other technology advances that evolved throughout the late 1980s and early 1990s, including greatly increased RF bandwidth, sensor fusion and the application of artificial intelligence techniques, then significant advances may be achieved. These developments have all contributed towards greatly enhanced radar performance. These techniques are outside the scope of this book and in many cases are classified.
6.4.3 Deployment of the Jamming Platform
The airborne jamming assets may deployed in two possible ways:
- Self-screening platforms with their own on-board EW suite. The complexity and intensity of the modern battlefield is such that most platforms carry their own protection suite, also sometimes referred to in US parlance as aircraft survivability equipment (ASE).
- Escort or stand-off jammers with a specialised EW role. The escort jamming role has been provided in the past by aircraft such as the F-4 Wild Weasel and EA-6 Prowler. Recently, the F-16C/J has taken on this role for the US Air Force and the F-18E/F is under development to replace the Prowler in the near future with the EF-18G. Such aircraft may also perform a stand-off jamming role, although this may also be performed by platforms with lower performance.
In reality, a modern conflict depends much upon the blending and merging of both asset types, depending upon the nature of the engagement. Jamming assets also offer complementary assistance to stealth platforms where they are deployed as low observability is easier to maintain in an aggressive EW environment, which has proved to be the case in recent Kosovo and Iraq engagements.
Escort jammers that accompany the main force are often referred to in US Air Force parlance as Wild Weasel squadrons and comprise strike aircraft types modified to perform a dedicated EW support and suppression role. The task is to precede or accompany the strike force, selectively jamming and confusing enemy defence radar and communications. These aircraft may also be armed with anti-radiation missiles (ARMs) that use the threat radar beam to guide themselves to the radar.
Aircraft operating in support of an attack force may also station themselves in a stand-off position outside the range of ground defences while maintaining a patrol so that an appropriate noise jamming signal can be used to confuse defences. Care must be taken that the jamming supplements and does not diminish the effectiveness of the attacking force.
6.4.4 Low Probability of Intercept (LPI) Radar
All these countermeasures depend upon the detection of the victim radars’ emissions and upon having some prior knowledge of the frequency of operation and modulation techniques employed. The most obvious counter of all is to avoid detection as far as possible by utilising low probability of intercept (LPI) techniques. LPI techniques must be designed into the radar at the outset and involve a number of trade-offs where increasingly sophisticated design (and cost) is balanced against a lower probability of interception. Some of the design considerations include the following:
- A reduction in peak power and an increase in the period of integration will result in the same overall detection capability for reduced peak power.
- An increase in receiver bandwidth using spread spectrum techniques and a reduction in peak power, effectively spreading the modulation data across a wider band, will make the task more difficult for the jammer.
- The radar has a much higher gain than that of a radar warning receiver (RWR) antenna and, while potentially disadvantageous during transmit, it has significant advantages during receive. Balancing peak power against antenna gain can yield benefits, and the aim is usually to increase antenna gain while reducing peak power. For effective LPI radars a design aim is to achieve a main beam gain of +55 dB above the first sidelobes. Other considerations include a high-duty cycle reducing peak power, low receiver losses and a low receiver noise factor.
6.5 Defensive Aids
An aircraft operating in a hostile military environment needs to be equipped with measures for self-defence. The crew will have been briefed on the threats on their outward and return transits, as well as enemy defences in the area to be attacked or where an engagement is to take place. This will be based on intelligence and will be in accordance with the most up-to-date intelligence compilation.
However, during the mission, the pilot must be warned of real tactical threats to the mission, and must have the means to minimise their effectiveness. The most common threats to low-flying aircraft are:
- Small Arms fire;
- Radar-guided anti-aircraft artillery (AAA or triple-A).
- Shoulder-launched surface-to-air missiles (SAM);
- SAM from ground sites, vehicles or ships.
Appropriate countermeasures include a means of detecting the threat and luring the threat away from the aircraft or causing the missile to detonate prematurely or far enough away from the aircraft so that no damage is sustained. This combination of sensor and counter-measure is often referred to as a defensive aids subsystem or DASS, and often abbreviated to Def-Aids. Figure 6.18 shows an aircraft equipped with a set of threat detectors and countermeasure subsystems.
There is little that can be done by a defensive aids system to have a significant impact on small arms fire and AAA, although counters may be devised for AAA gun tracking systems. High-speed evasive manoeuvres on the low-level run in to the target and may be firing a gun at ground sites may be a palliative, but the risk of a hit remains. Most aircraft are designed to minimise the catastrophic effects of missile or shell fragments by physical separation of critical equipment and wiring to reduce the probability of common mode damage effects. For those weapons employing active sensing there are mechanisms for reducing their effectiveness.
To counter weapons or systems utilising some form of electronic system or guidance, a defensive aids subsystem may include any or all of the following subsystems, depending upon the role and the intensity of the threat:
- Radar warning receiver;
- Missile warning receiver;
- Laser warning receiver;
- Countermeasure dispenser (CMD) – chaff or flares;
- Towed decoy.
On a military aircraft these systems will have the capability of interfacing with the aircraft/mission avionics system by using MIL-STD-1553B data buses or other cost-effective commercial data bus equivalents.
6.5.1 Radar Warning Receiver
A typical radar warning receiver (RWR) is depicted in Figure 6.19. Sensors are located strategically around the peripheries of the aircraft – typically four sensors placed at the wing tips or sometimes at the top of the fin. The objective, as far as is possible, is to provide full-hemisphere horizontal coverage around the aircraft in order that the crew may detect and be alerted to potential RF threats. Each of these sensors may provide up to 90° conical coverage, although in some cases the angular reach may be less than this. A typical antenna used in this application would be a spiral antenna with an angular coverage of 75° but with a gain of ∼10 dB.
This figure should be compared with the 55 dB gain that would be the design point for a LPI radar – a difference of 45 dB or a factor of 32,000. This illustrates in part the disadvantage that the RWR faces while operating against a modern state-of-the-art radar. Other considerations such as the use of sophisticated spread spectrum modulation, radiated power management and advanced signal processing indicate why it is conceivably possible for a sophisticated AESA radar such as the AN/APG-80 as used on the F-22 to operate almost invisibly to some medium-capability RWR equipment.
The quadrant-located spiral antennas detect and to some extent direction find (DF) any emissions within their respective area of coverage. Demodulated signals are analysed by the signal processor and categorised against a known threat library according to the following criteria:
- Frequency of operation;
- Modulation type;
- Signal strength;
- Direction of arrival.
In some cases an audio tone may be derived to provide the pilot or observer with audio cues – typically a tone equivalent to the PRF of the incoming radiation.
In early systems the processed outputs were displayed upon a plan position indicator (PPI) in a manner that depicted the angle of arrival according to a clock format with 12 o’clock dead ahead. In the late 1960s/early 1970s, when these systems were operationally deployed for the first time during the Vietnam War, this information would be presented on standard CRT green phosphor displays. Relative signal strength was shown by the length of the line from the centre of the clock, while the coding of the line into solid, dashed or dotted portrayal was indicative of the modulation type or possibly the band of operation. Early systems such as the air radio installation (ARI) 18228, as employed on the UK F-4K/M Phantom, used a hard-wired implementation to code specific threats and were therefore cumbersome to reprogramme.
The advent of digital processors now means that the threat library is coded in software allowing for rapid updates using a suitable software loading device. On a modern system, particularly since the advent of AMLCD colour displays, display symbology is much more likely to utilise stylised colour-coded symbology which is much more easily recognised by the pilot or radar operator, especially in a stressful combat environment.
Typical frequency coverage of a RWR system extends from 2 to 18 GHz and embraces a wide range of electronic threats across the RF spectrum. Modern RWR equipment offers a much more dynamic response to specific threats than was possible with the first-generation equipment.
6.5.2 Missile Warning Receiver
A missile warning receiver operates on a similar principle, except the missile warning systems (MWS) operate by detecting infrared (IR) or ultraviolet (UV) emissions during and following a missile launch. A typical system is portrayed in Figure 6.20.
Although the conical coverage of an IR/UV sensor may be as much as 110°, these systems often provide the option of expansion to six rather than four hemispherical sensors – see the F-35 example in Chapter 9. Apart from the sensors, in an overall sense the system works in a very similar fashion to the RWR above. Threat analysis is undertaken within a central signal processor/computing unit, and the results are output to a suitable tactical display. In modern systems this will be a colour tactical display.
An example of a missile warning system is shown in Figure 6.21.
6.5.3 Laser Warning Systems
A laser warning system again uses similar principles, except that the sensors are operating in the laser band. The example shown covers the 0.5–1.8 μm band and addresses the threat posed by the following lasers: doubled NdYAG, ruby, GaAs, NdYAG, Raman shifted NdYAG and exbium glass lasers. Angle of arrival (AoA) is claimed to be within 15° rms and sensor angles are 110°. (Figure 6.22).
6.5.4 Countermeasure Dispensers
The defensive aids will be equipped with a technique generator that interprets the threat and defines a suitable defensive response using the following:
- Towed decoy.
184.108.40.206 Chaff and Flares
A chaff and flare dispenser is fitted to many aircraft so that appropriate mixes of chaff and flares can be selected and deployed to confuse missile seekers or defence radars. This can be done by providing alternative decoy targets for seekers, or by disguising the aircraft by changing its radar return so that operators cannot set up an aiming solution. Chaff and flares are usually deployed as ‘last ditch’ countermeasures against an incoming missile. Their effective protection zone is to the rear of the aircraft, and they offer no protection against missile engagements in the forward hemisphere.
Chaff consists of reflectively coated strips of plastic or metal foil. The strips are designed to a half-wavelength (λ/2) of a typical homing radar. The chaff can be dispensed in patterns or blooms to disguise the dispensing aircraft with a view to confusing a radar operator. When released into the turbulent airflow, chaff disperses rapidly (blooms) and for a brief period of time generates a very large, static radar image between the target aircraft and the threat radar, which is probably in tracking mode or CW illumination mode, providing radar guidance for an in-flight missile. If the timing is right, radar or missile lock may be broken.
Flares are a countermeasure against IR homing missiles. When deployed, a flare burns with an IR wavelength similar to that of the target aircraft IR signature. It works by initially appearing in the missile seeker head coincident with the target aircraft, but is left behind as the target aircraft performs evasive manoeuvres. Its thermal image is designed to be longer than the target aircraft and it then becomes the preferred IR target for the missile.
Timing of deployment is critical. Too soon and the divergence of target aircraft and flare will be detected and the flare ignored, too late and the missile will detonate on the flare and fragments may hit the target anyway.
Flare deployment can be used in a ‘saturation’ mode during periods of extremely high risk, where the target aircraft is in very close proximity to a missile launcher and has no time to manoeuvre if missile launch is detected, for example, transport aircraft carrying out low-level air drops or landing on captured airfield in hostile territory where MANPAD or Stinger IR missiles may be launched within a few hundred feet of the aircraft. In these circumstances the crew may choose to pre-empt target launch detection by the tactical deployment of multiple flares in the high-risk zone.
Figure 6.23 shows an example of a C-130 Hercules deploying multiple flares.
220.127.116.11 Towed Decoy
The towed decoy is essentially a heat source that is towed behind the target aircraft on a cable. Located in a wing-tip pod, a wing-mounted pylon pod or deployed from inside the aircraft, the towed decoy is released and extended on a cable which restrains the device and provides a source of electrical power. The purpose is to cause infrared seekers in missiles to home on to the decoy rather than the jet-pipes of the towing aircraft. Any explosion should be sufficiently distant so as not to cause damage from the explosion or from missile fragments. The decoy can be rewound or it may be jettisoned by cutting the cable if there is a failure of the rewind mechanism. An example is shown in Figure 6.24.
6.5.5 Integrated Defensive Aids Systems
In order to convey the complexity and extent of self-screening EW systems on modern combat aircraft, the example of the F/A-18E/F Super Hornet will be briefly analysed. This aircraft has the following countermeasures suite fit:
- AN/APG-79 AESA radar;
- AN/ALR-67 radar warning receiver;
- AN/ALQ-214 integrated defensive electronic countermeasures (IDECM);
- AN/ALE-47 countermeasures dispenser;
- ALE-50/55 towed decoy.
18.104.22.168 AN/APG-79 AESA Radar
The Raytheon APG-79 active electronically scanned array radar is an 1100-element radar that has all the advantages and flexibility inherent in this type of radar. In particular, flexibility of mode of operation, high scan rates, sophisticated modulation and signal processing and LPI features give the aircraft significant operating advantages in a hostile EW environment.
22.214.171.124 AN/ALR-67 Radar Warning Receiver
The RWR suite is an integrated suite comprising the following components:
- Countermeasures computer;
- Countermeasures receiver;
- Low-band integrated antenna;
- 6 × integrated antenna detectors (two low band and four high band);
- 4 × quadrant receivers.
The countermeasure receiver receives inputs from the two low-band antennas and from the four high-band antennas via the respective quadrant receivers. The quadrant receivers provide preconditioning to reduce transmission losses between antenna and receiver. The receiver digitises and categorises the received signals and is able to handle a dense pulse environment while at the same time handling faint signals from distant threats.
The countermeasure computer incorporates a 32 bit machine with the application software encoded in Ada. The software structure enables complete reprogramming of the master threat file without any software changes. (Figure 6.26). The system weighs less than 100 lb – well under the normal weight of a system of this kind.
126.96.36.199 AN/ALQ-214 Integrated Defensive Electronic Countermeasures (IDECM)
The IDECM system is a radio-frequency countermeasures (RFCM) suite comprising the following units:
- Signal conditioning amplifier;
- Modulator/techniques generator;
- Two optional plug-in transmitters may also be used.
The weight of the system, including the rack, is ∼168 lb. If the ALE-50 towed decoy option is included, a further 54 lb is added. The fibre-optic towed decoy actually transmits the jamming signal according to the top-level architecture shown in Figure 6.27 and the units shown in Figure 6.28.
After the interception of the incoming victim radar signal, the appropriate counter-measures are applied and the RF is converted to light energy for transmission down the fibre cable to the decoy. The light energy is converted to RF energy and amplified by the travelling wave tube (TWT) transmitter. The resulting jamming signal is transmitted to the target radar.
188.8.131.52 AN/ALE-47 Countermeasure Dispenser
The AN/ALE-47 countermeasure dispenser – a successor to the ALE-39 – is able to dispense up to 60 expendables comprising chaff, flares or radar decoys. This is all achieved under computer control, enabling the pilot to achieve the optimum mix of expendables and deployment sequence for a given threat scenario.
The system has four main modes of operation:
- Automatic. The countermeasure system evaluates the threat data from the on-board EW sensors and merges them with stored threat data to determine the optimum dispensed stores mix. The system automatically dispenses this countermeasure load.
- Semi-automatic. The countermeasure system determines the optimum stores mix as for the automatic mode, but the crew activate deployment.
- Manual. The crew manually select and initiate one of a number of preselected programmes.
- Bypass. In the event of a system failure the crew can reconfigure the system in flight.
Lynch Jr, D. (2004) Introduction to RF STEALTH, SciTech Publishing inc.
Price, A. (2005) Instruments of Darkness: The History of Electronic Warfare, Greenhill Books.
Stimson, G.W. (1998) Introduction to Airborne Radar, 2nd edn, Scitech Publishing inc.