pilot may not assume other aircraft not identifying themselves as ”friends” to be, by default, ”foes”.
As with theRFbasedMWSa transpondingIFFsystem will give away the position of the aircraft and it must be switched off if the presence of the aircraft is to be hidden from the enemy.
2.5 Electronic Countermeasures
TheESMonboard an aircraft continually informs the pilot about enemy threats.
For the aircraft to counter these threats the aircraft may be equipped with a number of Electronic Countermeasures (ECM). These measures are used to either tell the pilot about the ESM used by the enemy, to disrupt the enemy’s usage of hisESM, or a combination of both.
2.5.1 Jammer
A radar system will usually analyse the radar signals echoed off an aircraft.
Depending on the type of radar system this analysis will decide the velocity, range, and/or direction to the aircraft. The results of this analysis might be used by the ground-based radars to determine when a missile must be launched against an aircraft. To confuse the analysis made by the radar system the aircraft can be equipped with a radarjammer. Different types of jammers exist:
the simplest ones jams the radar signal by emitting a noise signal in the same frequency band as that of the radar signal. More advanced jammers calculate what radar signal to send out to make the results of the analysis in the receiving radar system erroneous, e.g. by estimating a wrong velocity, range, or angle.
This may e.g. delude the radar into observing the aircraft as approaching while it may in fact be keeping its distance or even increasing it.
For a jammer to be effective against enemy radars the jamming signal needs to be emitted using more power than that of the echoed radar signal. Doing this will make the enemy radar interpret the actually echoed signal as noise compared to the jamming signal. The difference in power between the jammer signal and the echoed radiation is being used by some missile guidance systems. These will guide missiles towards any high-power signal, regardless of the information that may be found analysing this signal. A jammer that is turned on will thus serve as a beacon, possibly attracting the attention of an enemy radar operator. It it is therefore advisable to keep any jamming equipment turned off unless it is
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considered necessary for the survival of the pilot to have it turned on.
2.5.2 Chaff
Chaff are small pieces of foil or bipolar material that immediately forms a cloud when dispensed from the aircraft. This cloud has a RCScomparable to that of the aircraft. This is used to make a radar system tracking the aircraft track the chaff cloud instead. The time it takes to form a chaff cloud is named thebloom time. After a few seconds the chaff cloud is dissipated and the aircraft will once again be visible to enemy radar. The process of forming a chaff cloud to decoy an approachingRFguided missile is shown in Figure 2.7.
(a) (b)
(c) (d)
Figure 2.7: When chaff is dispensed it will form a cloud to decoy an approaching missile. In Figure 2.7(a) the centroid of the reflected radiation is positioned on the aircraft. Chaff is dispensed in Figure 2.7(b) and the centroid is moved backwards. In Figure 2.7(c) the missile has multiple targets to choose from, and in Figure 2.7(d) the chaff cloud has become the new target. With proper evasive manoeuvres of the aircraft the missile will not detect the presence of two targets and the lock will be directly transferred to the chaff cloud.
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The tracking radar will follow an object within a given range gate only. If the aircraft can escape the range gate before the chaff cloud is dissipated it can not be tracked by the enemy radar before a new acquisition is performed.
The aircraft may be equipped with a number of chaff types and chaff dispensers.
These will depend on a description of the battlefield and they are installed during the preparation of the aircraft. Chaff is anexpendable countermeasure in that it can only be used for a limited amount of times before the inventory runs dry.
2.5.3 Flares
To escape from an IRguided missile the pilot may have to transfer the missile lock onto another object. This may be done by dispensing flares. Flares are another type of expendables which are made of hot burning material that forms an infrared signature which can be more attractive for the missile to follow than that of the aircraft. If the guidance system is designed to manoeuvre the missile towards the hottest spot within the visual range it might go for the flares instead of the aircraft. Although flares burn out within a few seconds this might be enough for the pilot to manoeuvre the aircraft away from the path of the missile.
When flares are dispensed they will soon get a speed remarkably smaller than that of the aircraft. The decrease in speed may be a signal to guided missiles that the object to follow (the aircraft) is not the object currently in focus (a flare). For flares to maintain the same speed as the aircraft they can be either tethered or self-propelled. Tethered flares are towed behind the aircraft at a fixed distance for a while, thus having the same speed as the aircraft itself.
Propelled flares will start off by having the same speed as the aircraft. They will slowly decrease their speed, and the distance to the aircraft will increase.
If the pilot expects to be engaged by IR guided missiles, flares may be used pre-emptively. If numerous flares are dispensed before a missile is launched the missile may fail to acquire a lock on the aircraft itself.
As with chaff the number of flares and flare dispensers may vary according to a description of the battlefield and they will be set during the preparation of the aircraft as well. Flare dispensers may be directly linked to aMWSso a warning of an approaching missile immediately will trigger a flare dispense.
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2.5.4 Directed Infrared Countermeasure
The Directional Infrared Countermeasures (DIRCM) is a system designed to protect the aircraft from IR guided missiles. When an approaching missile is detected by aMWStheDIRCMis directed towards the missile. When active the
DIRCMuses pulses of IRenergy to jam the IRseekers guiding the missile. The pulses of IR energy will generally have one of two effects: either the seeker is blinded and will loose focus on the aircraft long enough for the aircraft to break the lock, or it will mimic a thermal signature as that of the sun, thus forcing the seeker to look for alternative targets [1]. If the use ofIRpulses is accompanied by the dispense of flares these may serve as new targets for the seeker and the lock is transferred.
2.5.5 Towed Decoy
As mentioned in Section 2.5.1 missiles may be guided toward an active jammer.
To avoid this type of missiles while maintaining the effect of a jammer the jammer may be placed in atowed decoy. When deploying a towed decoy a wire connecting the decoy to the aircraft is unreeled and the decoy will be towed behind the aircraft at a fixed distance. When the towed decoy is no longer needed the wire may be re-reeled or simply severed.
The simplest towed decoys will have their own power supply and they will continue to jam for as long as the power permits. More sophisticated decoys may be connected to power supply and sensors on-board the aircraft. They will be able to adjust the jamming to the current battlefield scenario and they will continue to jam for as long as it is deemed necessary.
A towed decoy is kept at a safe distance behind the aircraft. At this distance an impact on the decoy by a missile will leave the aircraft undamaged. The aircraft manoeuvrability is limited when a towed decoy is deployed, so when it is no longer in use it must be severed or re-reeled. Before being deployed a towed decoy is usually placed under the aircraft fuselage or under either or both of the wings. This limits the total number of towed decoys to be deployed during a mission to one (the fuselage), two (both wings), or three (fuselage and wings).
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2.5.6 Stealth
The highest survivability for the aircraft is obtained if it can fly by stealth, i.e.
fly without being observed by the enemy. In designing a fighter aircraft several measures are taken to reduce the signatures of the aircraft to make it difficult for the enemy to observe. These measures include using radar absorbing materials and shaping the surface of the aircraft to obtain the smallestRCSvalues possible.
A reduction of theIRsignature of the aircraft is obtained by special designs of the airframe and propulsion system [10].
2.5.7 Breaklock Zones
The signatures of a fighter aircraft influence the success of an approaching mis-sile. If theRCSof the aircraft is sufficiently small aRFguided missile will not be able to lock onto it. Likewise, anIRguided missile will have trouble following an aircraft that is almost invisible within theIRband. During flight the pilot will manoeuvre the aircraft to obtain the smallest signatures possible. An aircraft will typically have the largest RCS when seen from the side, while the RCS is often smallest when the aircraft is flying directly towards the radar receiver. To lower the IRsignature of the aircraft the pilot may reduce the thrust and turn the aircraft so that hot surfaces are hidden by other parts of the aircraft.
The angles in which the aircraft has the lowest visibility to the enemy are known asbreaklock zones. When a missile is locked onto the aircraft the pilot will ma-noeuvre the aircraft so that the enemy will become positioned within a breaklock zone. The manoeuvre will often be accompanied by the dispense of either chaff or flares, depending on the threat, so the lock can be transferred away from the aircraft.
2.5.8 Timing the Use of Countermeasures
When a threat is detected the use of appropriate countermeasures must be timed to gain the best possible protection. If applied too soon the countermeasure may have no effect, an applied too late the effect may not protect the aircraft.
Dispensed too early a chaff cloud will be dissipated before having any effect on the missile, and the side-effect of having less chaff available will only decrease the aircraft’s survivability at a later stage. If the chaff is dispensed too late the effect on the missile will not prevent it from reaching the aircraft. Similar considerations must be taken forIRguided missiles and flares. Here the time it
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takes from launch until the missile reaches the aircraft is usually smaller than for RFguided missiles, and flares are usually dispensed as soon as the missile has been detected.
For on-board countermeasures such as jammer, towed decoy, or DIRCM, the time it takes before the countermeasure becomes active must be taken into consideration. While it may take only a few seconds for the jammer or the
DIRCM to settle, or for the towed decoy to be unreeled, the use of these must be appropriately timed, just as for expendable countermeasures.