Infrared signature studies of Aircraft and Helicopters

The infrared (IR) seekers have exploited techniques to passively acquire and intercept airborne targets, by detecting their IR energy [1]. The basic principle of IR detection is the discrimination of target's IR radiance in the detector's wavelength band with the background IR radiance (atmospheric emission/solar radiation) [2]. In an aircraft, the internal sources include plume and surface emissions; and power-plant is the major and reliable source. The aircraft rear fuselage skin of a typical military aircraft is heated by the flow of hot combustion products in the embedded engine [12]. The solid angle subtended by the rear fuselage skin is an order of magnitude larger than that of the tailpipe [9]. Therefore, its contribution is significant especially in the 8-12 micron band; in which, IR-detection is possible also due to external sources, e.g., earthshine and skyshine reflection [19]. Unlike surfaces of solids, gases emit and absorb radiation only at discrete wavelengths associated with specific rotational and vibrational frequencies. These frequencies depend on the particular type of molecule, temperature, pressure, and molecular concentration of radiation participating species [13]. The atmosphere limits the use of the IR spectrum to specific bands called as atmospheric windows; and has a crucial role, which includes that of transmission and background radiance [8]. For reducing detection by IR-guided missiles, aircraft and helicopters use IR Signature Suppression (IRSS) techniques. A well-designed HISS system can drastically reduce IRSL by restricting the visibility of hot parts and by matching the visible radiance with the background [22]. Their effect in reducing target's susceptibility (P(H)) can be gauged by models that relate the two. Due to significant advancements in the performance of IR-detectors, modern missiles are generally constrained by their burnout range rather than their lock-on range. The 'lethal range' is a function of target's lock-on range, target's velocity, missile velocity, missile burnout range, missile's guidance logic and blast kill radius; and it is a superior estimate of P(H) [23]