Overview of Low Observable Technology

Stealth using low observable materials enables the control or reduction of the signatures of weapon systems.  Signatures are those characteristics by which weapon systems may be detected, recognized, and engaged.  The modification of these signatures can improve the survivability of military systems, leading to improved effectiveness and reduced casualties as demonstrated in the Persian Gulf conflict. 

Signature detection is commonly amounts to the electromagnetic signature of an object.  Figure 1 shows a schematic representation of the electromagnetic spectrum.  The spectral regions of interest for low observable technologies are the visible range, the infrared (IR)  range (particularly bands II and III which are shown in Figure 1), and the radar portion of the spectrum.

Figure 1:  The electromagnetic spectrum with the IR  portion of the spectrum expanded.  The expanded portion of the spectrum shows the percent transmission through 6,000 horizontal feet at sea level.

The emphasis to date has been on radar stealth, and thus radar-absorbing materials have received significant attention.  However, with the development of infrared (IR) detection technology such as missile guidance systems and thermal cameras, the need for effective IR stealth capabilities is pressing for air, land, and sea defenses.

LPRL low observable coatings render IR detection of an object more difficult. These coatings are multipurpose materials that may be tuned for specific missions, or used in standard configurations.  For example, they may be designed solely for IR stealth (without compromising the radar cross section of the host), or for both IR and radar stealth simultaneously. 

Below we give a brief description of current techniques for IR and radar stealth.

Infrared Stealth

IR discretion techniques focus on bands II and III which are transmission bands in the earth’s atmosphere (see Figure 1).  Band II covers the range 3 to 5 mm while band III covers the range 8 to 12 mm.  Band II is exploited primarily by missile guidance systems and band III by thermal cameras.  Another wavelength of interest is 1.064 mm, which is the wavelength used by the majority of laser range-finders.  

IR discretion technology may be divided into two areas: IR suppression and IR deception.  

IR Suppression

IR suppression technology focuses on three areas: 

   LPRL's materials work  in areas 1) and 3) simultaneously.  They are low emissivity materials, and thus reduce the target's radiation intensity.  As their emissivity may be tuned, they may be used to modify the IR signature of the target.

Current technologies for IR suppression  are outlined below:

Low emittance materials

Current low emittance materials are based upon electronically conductive materials such as aluminum alloys with varying geometries and granularity.  These materials are suspended in a varnish that is painted onto the equipment.  This approach proves economically attractive but suffers from two major disadvantages.   

Active Cooling Mechanisms

A technique to decrease the temperature of hot gases expelled by engines is to inject cold air into the hot gas stream.  The majority of aircrafts that enjoy a reduced radar cross-section (RCS) use this technique.  However, this technique prevents manufacturers from exploiting post-combustion, which reduces the maximum thrust achievable with these engines. 

Thermal Insulating Materials

It is possible to place thermal insulating materials over a thermal source to alter its IR signature.  However, a disadvantage of this strategy is that the temperature of the thermal source will rise, resulting in possible damage to the source (it may contain sensitive electronics or mechanical components).  In addition, it is difficult to maintain large thermal gradients over thicknesses comparable to that of typical IR stealth coatings, so that thermal insulating materials often come with a high thickness and weight penalty.  

IR Camouflage Nets  

IR camouflage nets can effectively reduce the radiation contrast of a target at hot or ambient temperatures with the background.  They can also change the radiation distribution of the target. 

IR Deception

Luring/Decoy

The original means of luring first generation missiles (missiles whose guidance systems operated in band I – 1 to 2.5 mm) consists of flying the aircraft towards the sun, if possible, then performing a drastic evasive maneuver in the hopes that the missile’s guidance system locks onto the signal coming from the sun.  Another technique involves flying in pairs along the same direction, but separated by several miles.  The trailing aircraft performs an evasive maneuver in the hopes that the missile locks onto the lead aircraft, which is beyond the missile’s range. 

Second generation guidance systems operate in band II and are not deceived by the techniques described above.  One deception technique useful against these systems is the launching of thermal decoys by the aircraft in such a way as to lure the missile toward the false target.  This technique proves relatively ineffective against guidance systems that contain imaging capabilities since these systems allow the missile to remain locked onto the original thermal image of the aircraft.  Possible means of evading these missiles include modifying the thermal image of the aircraft, reducing the thermal emission of the aircraft to make detection more difficult, or increasing the thermal emission of the aircraft in order to saturate the missile’s detectors.

Radar Stealth

Several existing technologies that address the problem of radar stealth. are outlined here.

Stealth Geometry

The principle of this technology consists of designing the exterior geometry of a mobile or stationary unit to reflect radar radiation in a direction that makes it difficult to detect.  A drawback of this technology is that it imposes geometrical constraints that may decrease the performance of airborne units.  Examples of this include the F117 that is not supersonic because of its stealth geometry, and the modifications of the air entries of the B1 which reduced the RCS but cost 0.4 mach.  Geometrical constraints prove less problematic for ships, land vehicles and stationary infrastructure.

Radar Absorbing Materials (RAM)

RAM may be applied to critical parts of air born units as well as to land vehicles and infrastructure.  An inconvenience of this type of treatment is that it is not effective against all radar frequencies.  In addition, for longer wavelength radar the thickness of the coatings must increase (increasing the weight of the coating as well).  The active materials used in RAM consist of different types of ferrites as well as conducting polymers such as polyaniline or polypyrolle.

Active Neutralization

The technology of neutralization consists of analyzing the radar environment in which a unit is immersed in order to generate and emit a radar signal of equal magnitude but opposite phase resulting in the nullification of the total radar signal reflected by the unit.  This system can be adapted to all radar frequencies but requires powerful calculation capabilities in order to analyze the radar environment in real-time.  In addition the problem of emitting a multitude of radar frequencies is currently not resolved, especially for aircraft.

Luring or Decoy

Currently the main technique used for luring involves launching a packet of chaff from the unit being targeted in order to create a secondary radar image that overwhelms the original image.  This technique is problematic for naval vessels due to the possibility of variable winds that may blow the chaff in undesirable directions.  In addition certain missiles are equipped to counter this lure and to remain locked onto the original target.