Avionics feature on military aircraft
A missile approach warning system (MAW) is part of the avionics package on some military aircraft. A sensor detects attacking missiles. Its automatic warning cues the pilot to make a defensive maneuver and deploy the available countermeasures to disrupt missile tracking.
Guided surface-to-air missile (SAM) systems were developed during World War II and began to make their presence felt in the 1950s. In response, electronic countermeasures (ECM) and flying tactics were developed to overcome them. They proved to be quite successful provided that a reliable and timely threat warning was given.
The infrared-seeking missile threat.
Analysis of aircraft losses due to enemy action since the 1960s shows that at least 70% of all losses were attributed to passive heat seeking i.e. infrared (IR) guided missiles. This might be surprising given that radar guided SAM systems have longer engagement ranges, are faster, have higher maneuvering potential, carry larger warheads and are equipped with proximity fuzes.
The main reason why IR guided missiles were so effective was that it took much longer to develop effective warning systems against them. Most aircraft that were shot down never knew that the missiles were coming. Radar warning receivers on the other hand already proved their effectiveness by the early 1970s which considerably improved the survival rate of aircraft against radar threats.
The first air-to-air IR missiles appeared in the 1950s. The technology allowed more compact missile designs and made it possible to develop IR man-portable air-defense systems (MANPADS) i.e. shoulder-launched missiles, which became operational by the 1960s.
IR MANPADS are relatively cheap, quite robust, easy to operate and difficult to detect. They also do not require the infrastructure, such as radar dishes, associated with radar-guided SAM deployments which often reveals their presence.
Vast quantities of MANPADS have been manufactured (as many as 700,000 produced since 1970). Large numbers proliferated during the Cold War and immediate post Cold War era. Substantial quantities are available and affordable on the black market and have found their way into the hands of "non state" organizations or the so-called "asymmetric" threat. (An estimate by "Jane's Intelligence Review" of February 2003 puts this number as high as 150,000). An article "Proliferation of MANPADS and the Threat to Civil Aviation" of August 13, 2003 by Jane's Terrorism and Insurgency Centre estimates that the black market price of MANPADS like the SA-7 could be as low as $5,000.
Intelligence regarding the whereabouts of MANPADS, especially in the hands of "non state" organizations, is usually vague and unreliable. This, in turn, makes it difficult to anticipate where and when to expect MANPADS attacks.
The 2nd- and 3rd-generation MANPADS that appeared by the 1980s further increased the performance and effectiveness of MANPADS due to advanced new seeker head technology, improved rocket motors, and aerodynamic refinements. Their performance improved in terms of lethal range, minimum launch angle, maneuvering potential and all aspect engagement angles (1st-generation MANPADS were restricted to only rear sector attacks). They also became more electronic counter-countermeasure (ECM) resistant.
MANPADS therefore became even more lethal specifically against more vulnerable platforms such as helicopters, light aircraft, and commercial and military transport aircraft (during approaches and departures). The slower speed of these platforms forces them to spend more time within the kill zones of MANPADS compared to high performance fighter and strike aircraft.
Over fifty MANPADS attacks on civilian aircraft are on record to 2007. Thirty-three were shot down killing over 800 people in the process.
Missile approach warning (MAW) system requirements.
Protecting aircraft against IR guided missiles depends in most cases firstly on reliable detection and warning of missiles and secondly on applying effective ECM.
An exception to this are omni-directional IR jammers which do not make use of missile warning at all, as they simply radiate modulated IR energy for as long as they are switched on. These jammers have been around since the 1970s and when the correct jamming modulation techniques were applied, were reasonably effective against 1st-generation amplitude-modulated MANPADS, which operated in the near-IR band (1 to 2 micrometres (μm)). The arrival of 2nd- and 3rd-generation MANPADS changed that. They operate in the mid-IR band (3 to 5 μm) and use more advanced modulation techniques (for example frequency modulation). Instead of jamming these missiles, the omni-directional IR jammer became a source for the missiles to home in.
Functional requirements.
Providing timely warning against IR MANPADS is a challenge. They give no warning of their presence prior to launch, they do not rely on active IR, radar guidance or a laser designator, which would possibly emit a detectable radiation. They are typically fire-and-forget and can lock on and engage a target, speed to the target and destroy it in seconds. They have a small but visible radar signature and also a propellant which burns – depending on the platform, typically for a very short duration.
MANPADS are relatively short-range weapons, typically up to about five kilometers with the heart of the kill envelope one to three kilometers. They therefore allow very little margin for error to effectively counter them as the time to impact (TTI) on a target at one kilometer, is only about three seconds. The TTI for targets at three and five kilometers is also relatively short – only seven to a little over eleven seconds respectively.
The MAW must provide reliable and timely warning to allow appropriate counter measure responses. Near 100% probability of warning (POW) and very fast reaction times to counter nearby missile launches (in the order of one second) are essential.
Air crew will rely on the system only if they have high confidence in it. The MAW must also have sufficiently low false alarm rates (FAR), even when illuminated by multiple sources (which may include threats) from different directions.
Quick response times and low FAR are inherently conflicting requirements. An acceptable solution requires a balanced approach to provide the most successful end result without compromising the POW. Since a longer time-to-impact (TTI) warning is almost invariably desirable, this leads to the conclusion that there is something like a too-low FAR: all warning systems gather data, and then make decisions when some confidence level is reached. False alarms represent decision errors, which (assuming optimal processing) can be reduced only by gathering more information, which means taking more time, inevitably resulting in a reduced time-to-impact. Most users would tolerate an increased FAR (up to some point where it starts limiting operations) instead of a reduced TTI, because their probability of survival depends fairly directly on the TTI, which represents the time in which countermeasures can be deployed.
Accurate azimuth and elevation angle of attack (AOA) information can be another very important requirement. Directional IR counter measures (DIRCM) systems depend on MAW systems for accurate enough initial pointing (about two degrees) to ensure that the DIRCM acquires and engages incoming missiles timely and successfully.
Accurate AOA is also important in deciding the dispensing direction of the counter measure decoys (flares). It is vital to avoid the situation where the platform and the dispensed decoys both remain within the instantaneous field of view (IFoV) of incoming missiles. In situations like that missiles could very well, once they pass the decoys, still hit the platform. This is of particular importance where separation between the decoys and the platform takes too long as is the case with slow flying aircraft.
Accurate AOA is further important where the platform should preferably maneuver when dispensing decoys to increase the miss distance. This is more applicable to fast jets where their high speed tends to negate the separation caused by the decoy's ejection velocity. A turn towards approaching missiles to establish/increase the angle between the decoy and the platform is especially important in cases where a missile approaches from the rear between the five or seven 'o clock sectors. If the AOA is not accurate enough, the pilot could very well turn in the wrong direction and set himself up for the situation as described above.
The system must also be fully automated as the human reaction time in relevant cases (short range launches) is too long.
Physical requirements.
Light aircraft, helicopters, and fighters usually have limited space and mass capacity for additional equipment. The system may also cause adverse aerodynamic drag which demands minimal physical size and number of boxes. The power consumption must further be kept within the capacity of the platform's electrical system.
Human-machine interface (HMI) requirements.
Integrated display and control functions are desirable to avoid duplication on instrument panels where space is limited. If a platform is equipped with both radar and missile warning systems, the HMI should display both threats clearly and unambiguously.
The integrated HMI must also indicate the system's operating status, serviceability status, mode of operation, remaining decoy quantities etc. Separate control panels are justified only for safety of flight purposes such as ECM on/off and decoy jettison functions.
Cost considerations.
Procuring electronic warfare (EW) self-protection systems has direct and indirect cost implications.
Direct costs involve the initial price of the system, spare parts as well as test equipment to ensure that the performance and availability of the systems is maintained throughout their entire life cycle.
Installing and integrating EW systems on aircraft is another direct cost
Indirect cost on the other hand involves degradation of the aircraft's performance as a result of having the system on-board which in turn impacts negatively on the operating cost of the aircraft.
The lowest initial price of a system does therefore not necessarily offer the best solution as all the factors needs to be considered. The overall cost effectiveness of systems i.e. price versus performance is more important in deciding which system to select.
Types of MAW systems.
Three different technologies have been used for MAW systems, i.e. systems based on: Pulse-Doppler radar, Infrared, and Ultraviolet.
Each technology has its advantages and disadvantages which can be summarized as follows:
Implementations of MAW systems.
Current available MAW systems as well as those under development, represent all three types of technologies. Each technology has strong and weak points and none provide a perfect solution.
References.
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