Taday's Missile-Warning Systems

You Bet Your Life: Today’s Missile- Warning Systems
by Don Herskovitz
What’s long and thin, starts out as hot as Hades and can travel at Mach 2 and above? You have about five seconds to answer this question and, while the correct answer may not make you a millionaire, it could save your life. This month, JED looks at a contemporary assortment of surface-to-air-missile (SAM)-approach-warning systems. The physics of the problem, as well as some of the advanced strategies used to address these problems, are discussed. It is acknowledged at the outset that missiles also are launched at targets on the ground or afloat. But by far, the most destructive application of missiles has been against aircraft — missiles launched both from the ground or in air-to-air scenarios. Airborne missile-warning systems (MWSs) present the system designer with an advanced set of challenges not present in the two-dimensional world of land or sea operations. Low-level helicopter operations require sensitive systems that can instantly declare a threat in the cluttered environment of a near-ground-level battle arena. High-performance aircraft present a differing challenge. Here, because of high velocities and rapid maneuvers, the missile’s position with respect to the platform may change at a rate of hundreds of degrees per second. To provide coverage, one warning sensor must be able to quickly and accurately handoff a missile’s track to another warning sensor. By studying such airborne warning systems, we can gain an insight into the technology necessary for providing warning on other platforms.
THE PROBLEM

One has but to scan the list of more than 100 SAMs produced by 21 countries in the 2000 International Electronic Countermeasures Handbook to grasp the diversity of the threats posed to today’s aircraft.1 Through sales and resale, wartime capture and licensed or unlicensed production of major end items, these weapons have been proliferated to the extent that any distinction between friendly or foreign equipment has blurred. Sales of upgrade equipment and kits for application to weapon systems have further blurred the distinction between old or obsolete systems and modern ones.

Following initial research on infrared (IR) detectors at the close of World War II, IR-homing missiles have accounted for more aircraft shootdowns than any other anti-aircraft weapon system — regardless of cost. Aircraft losses tabulated in 1990 for the 1977-1985 time span indicated that 90 percent of these losses were due to IR-guided missiles. RF-guided missiles, on the other hand, accounted for 4.4 percent of the losses (see “Infrared Countermeasures: A Point of View,” JED, April 1990, p. 41). In the Gulf War, infrared-guided missiles accounted for 14, or 78 percent, of the 18 aircraft kills reported in unclassified news reports (see “Understanding the IR Threat,” JED, February 1999; and “IR Missiles Brought Down Majority of Russian Aircraft,” JED, February 2000). Indeed, man-portable air-defense systems (MANPADS) are one of the most widely procured arms classes. The FIM-92 Stinger has been in production since 1981 and is the most widely procured and used MANPADS. Initially developed for the US Army by General Dynamics, later by Hughes Missile Systems, then by Raytheon, as well as by the European Production Group in Europe, the Stinger continues to evolve and exploit advances in technology. Over 70,000 rounds have been produced. The basic Stinger was used extensively in Afghanistan and is credited with killing 250 Russian aircraft while being used by operators with only limited training. Another popular system in the export market is the French Mistral IR-homing missile. More than 12,000 rounds of have been produced by Matra-BAe Dynamics. Placed in service in1988, this 6.6-ft.-long, Mach 2.5 round has been exported to 18 countries.

WARNER BASICS


The IR-guided Mistral is one of the most popular missiles on the international market.
(Matra BAe Dynamics photo)

Against such a diversity of threats, where does the warning-system designer start? A good place to start is to look at which are the most effective threat systems. The score card’s message is undeniable: the IR-guided missile is currently the most effective aircraft killer.

Except for some designs using millimeter waves for terminal guidance, the IR-guided missile runs silent — i.e., no tell-tale radar signals are emitted. Thus, a radar warning receiver is ineffectual. The designer’s first decision is to assess the relative merits of using an active, passive or perhaps a hybrid warning system. There are advantages and disadvantages to each approach. Active systems have traditionally been based upon the use of a single radar system or network of such systems. Pulsed-Doppler radar is the system of choice for this application because of its ability to discern a moving target in stationary or slowly moving background clutter. The radar monitors a volume of space, either over a continent or surrounding an aircraft, ship or other platform. When a target, the threat, enters this monitored space, a reflected radar signal is detected, and range and velocity information are derived. With high-definition radars, identification of even non-cooperative targets may be achieved. The active system also has such advantages as long-range, all-weather operation and controllable false-alarm rates. On the down side, active radar warning systems can be jammed, may be ineffectual against a stealthy or low-radar-cross-section target and, most importantly, announce their presence by radiating signals and can be targeted by anti-radiation missiles.

Passive warning systems, on the other hand, do not betray their presence with tell-tale emissions. The system may consist of a single sensor or groups of sensors. These sensors may stare at a fixed sector of space or scan a wider portion of the envelop surrounding a platform. The very wide electromagnetic spectrum permits the selection of emissions from the missile to be monitored. Some of these emissions may be intentional, while others are unavoidable. Intentional emissions can include acquisition- and guidance-radar signals and millimeter- and laser-designator signals, among others. Unintentional radiation ranges from the optical and ultraviolet (UV) flare of a very hot booster rocket, to the shorter IR emission from a jet’s engine exhaust or rocket plume, to the long IR associated with the relatively cool leading edges of an aircraft or subsonic missile. These passive monitors are varied in range of effectiveness, all-weather operation and false alarms. In sophisticated systems, a radar warning receiver might be used to detect radar frequencies from the guidance system of an active homing missile or the fusing mechanism of a warhead and pass this information on to the missile-approach-warning system (MAWS). An onboard detector might also cue the passive MAWS upon detection of a laser signal indicating that the platform is being illuminated by a target-designating system.

These advantages, however, come at a price. Many of the sensors employed in passive systems must be cooled to operate properly. This adds to the system complexity and impacts maintainability. Newer sensor approaches may obviate the need for this cooling envelope. Passive MAWS are also be prone to clutter problems and yield poor range information (see “Understanding the IR Threat,” JED, February 1999; and “A New Approach to Missile Warning,” JED, October 1998).

A final element of interest in the design of passive missile-approach-warning systems is the mode of operation of the detection system — scanning or staring. In a scanning system, a large field of view can be achieved with an appropriately designed optical system. The system scans over a broad volume and focuses its output upon an individual detector or a narrow line of detectors. Relatively few detectors are required for this type of system, and problems associated with array yields and uniformity are avoided. But with improved crystal-growth technology, larger arrays of detectors are being successfully fabricated. Staring arrays, also called focal-plane arrays, produce a detection system with few, if any, moving parts. The imaging of a potential target on an array of sensors produces a spatial as well as temporal differentiation of a target, thus yielding more data for possible target identification and clutter rejection.

If price, size and power consumption were of no concern, a platform, logically, should be equipped with both active and passive warning systems — a hybrid approach. Passive systems, both scanning and staring, would initially be employed to produce early reports of a potential attack. At best, these systems could detect the ignition of a booster rocket milliseconds after launch. Continued passive tracking would determine if the missile presented a threat to the observing platform. If it were determined that, indeed, the missile was directed toward the platform, the need for covert activity becomes moot. If someone has fired a missile at you, they know you are there. At this point, the active MAWS can be activated to produce range and more accurate angle-of-arrival data.

THE FALSE-TARGET DILEMMA

The detection of an IR “hot spot” might portend the approach of a missile, the flare of an outdoor fire or the sun glinting off a body of water. The reflection of a Doppler-radar signal might augur a speeding SAM or a fleet flock of pheasants. Discrimination between harmless and potentially lethal sources is the function of the MAWS processor. Clutter, the myriad of signals received by a sensor system, that may or may not represent actual targets, is a problem that must be addressed in any warning system. Clutter, at radar frequencies, can arise from a vast assortment of man-made and natural artifacts. The metal roofs of buildings, glint from a body of water and sharp corners in a variety of structures can all act as effective reflectors of radar energy. At shorter wavelengths — in the IR, optical and ultraviolet regions — the clutter problem is even more pervasive. Selection of the optimal spectral region for sensor operation is a complex problem that has received much treatment within the pages of JED (see “The Missile Wanring Challenge,” July 1999; and “Passive UV Missile-Warning Systems,” JED December 1999).

Logic algorithms can be devised that can recognize, for instance, that background clutter is generally more spatially extended than a target. Additional capability to discriminate between random thermal sources and a booster rocket can be achieved by monitoring two or more optical or IR wavelengths. Spatial and temporal discrimination can be used to update a scene and to provide recognition patterns and clutter rejection. All of these logical operations assist in eliminating false alarms and estimating the degree of danger presented by a threat.

THE NATURE OF EMISSION


The Common Missile Warning System, currently under development, is slated for fit aboard a wide variety of US fixed- and rotary-wing aircraft.
(Sanders photo)

The emission which accompanies the launch and flight of a surface-to-air or air-to-air missile depends upon the design of the missile and the portion of its trajectory under observation. At launch, the emission from the booster-rocket plume can be expected to produce an energetic, readily observed (if one is looking in the right direction) signature with components in the optical and mid-range IR portions of the spectrum. After the boost phase, there may be secondary firings of steering rockets. As the missile approaches its target, the booster and steering rockets may be already shut down, but portions of the missile skin surrounding the engine and even the leading edges of the missile itself emit IR signatures. Defense Support Program satellites are reportedly able to detect the IR energy radiated from the hot exhaust of a missile’s engine anywhere on the earth’s surface.2 These satellites presumably detected all the Iraqi Scud missiles launched during the Persian Gulf War. (For more on the relationship of temperature to spectral emission, see (see “Understanding the IR Threat,” JED, February 1999; and “A Little Warmth, A Little Light,” JED, April 1990, p. 47).

CURRENT MAWS APPROACHES

Table 1 gives a concise overview of 19 airborne missile-warning systems. Along with the system designation, the country of origin and the original equipment manufacturer (OEM) are indicated. The operational band, IR or UV, certain features and operational platforms are also described.

EPILOGUE

The winner of the ongoing “MAWS vs. IR-guided missile” tug-of-war is far from being resolved. Missiles are becoming more sophisticated, less likely to be seduced by flares or confused by clutter. They are coming faster, from greater ranges and can pack a more lethal wallop. On the defensive side, warning systems are reaping the benefits of smaller, more powerful computers and microprocessors, as well as improved sensor technology. Additionally, once detected, the arsenal of missile spoofing and killing options is also growing.


A passive MAWS, like the Passive Airborne Warning System (shown here), does not betray its presence with tell-tale emissions. (Elisra photo)

Warning systems currently in the concept phase or in development will take advantage of powerful computers operating decision-making algorithms and artificial- intelligence programs to minimize false alarms, provide faster threat/target identification and more rapidly initiate protective responses. More sensitive sensors with wider spectral responses are being developed in adjunct technologies such as medicine and infrared astronomy. Although not initially intended for military use, these devices have properties that may make them desirable for use in missile-warning systems. Quantum-well- infrared-photodetector (QWIP) arrays can be tailored to absorb radiation in the long-wavelength infrared region from 3-20 micrometers. QWIP technology is based on phototransitions between electron energy states in so-called quantum wells, the energy level between an electron’s valance and conduction band. By using different thickness and compositions of quantum-well materials, wavelength response can be customized and accurately specified. The quantum-well materials can be stacked upon each other to increase IR absorption or to yield a sensor with several specific absorption bands.

Under NASA sponsorship of infrared projects for space telescopes, an array of IR detectors are under development. This research is producing such devices as superconducting microbolometers and advanced pyroelectric sensors that increase the useful wavelength range, sensitivity and ease of operation of detectors. A team of astronomers at the University of Arizona has built the first true detector array for use in the far IR. Dubbed the Multiband Imaging Photometer, an array consisting of 1,024 detectors has operated at 70 micrometers, while another array works at 160 micrometers — about 300 times the wavelength of visible light. Arrays of this nature will allow the missile-warning system of the future to selectively scan for the slight temperature increases produced by atmospheric friction on the leading edges of incoming missiles, as well as selectively profiling the temperature variations in an actual missile as opposed to the clutter of a false target.

As missiles continue to improve, so also do the devices that signal their approach and the systems that are designed to counteract these missiles. The next chapter of MAWS technology will doubtlessly draw heavily upon advanced sensor technology and massive computing capabilities.


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