Introduction

Non-imaging types of IR guided missiles make use of infrared emission corresponding to the thermal signatures of the exhaust and the mainframe of the target aircraft to home on to it. Emission in 3−5 and 8−12 micron bands is characteristic of electromagnetic emission from jet exhausts and mainframe of the aircraft. Spectral content of infrared emission as received by an IR seeker head is the superposition of the spectral emission from the aircraft on the transmission characteristics of the atmosphere and is shown in Fig. 16.1[5]. This spectral signature is judiciously used in guidance of air-to-air and surface-to-air IR guided missiles. Both air-to-air and surface-to-air IR guided missiles receive the target's infrared signatures in the presence of background radiation from the sky and also infrared signatures of flares, if any, deployed by target aircraft platform. The seeker head should be able to discriminate between infrared signatures of the background and flares from those of the target.

Infrared signatures of target and flare

Signature prediction and analysis component in the design of test systems that simulate true battlefield like scenario. This topic has been extensively researched and reported in literature.

Major sources of background infrared noise include the earth, the sky, thermal emissions from clouds and solar reflections[6]. The background sources are generally modelled as a black body whose radiance is dependent upon its temperature and emissivity. The total radiance incident on the missile seeker is estimated after taking into consideration contributions from all these sources and also the transmission characteristics of the atmosphere. Though background signatures are dynamic in nature and vary with weather condition, path length and altitude; for the purpose of IR guided missile guidance, static background noise spread over 3−12 micron band is a reasonably good approximation.

Different sources of infrared radiation in an aircraft can be classified as internal and external sources[7]. Internal sources include hot engine parts, airframe skin heated by the engine and aerodynamic heating, combustion of the hot gases in the plume and the skin of the airframe. External sources include reflected ambient radiation from the sun, the sky and the ground. The hotter surfaces like the hot engine parts, the tail pipe region and the plume mainly contribute in the lower wavelength band whereas the fuselage region, the airframe and the reflected radiation are the dominant factors in the longer wavelength region.