Passive infrared (IR) emissions from aircraft are used to detect, track, and lock-on to the target. MAN Portable Air Defence Systems (MANPADS) have emerged as a major cause of aircraft and helicopter loss. IR signature studies are now necessary to counter this threat, for survivability enhancement, and are an important aspect of aircraft stealth technology. This lecture presents contemporary developments in this discipline, with particular emphasis on IR signature prediction from aerospace vehicles. The role of atmosphere in IR signature analysis, and the relation between IR signature level and target susceptibility will be illustrated. Also, IR signature suppression systems and countermeasure techniques will be discussed, to highlight their effectiveness and implications in terms of penalties. 

In addition, this lecture will present the assessment of the IR lock-on range (RLO) due to increase in the engine back-pressure penalty. The analysis on the effect of reducing the exit area of a choked convergent nozzle of a turbojet engine on its IR signature will be presented. Reduction in the exit area of a choked convergent nozzle below the design point value results in optical blockage of the engine hot parts. Therefore, the solid angle subtended by the engine hot parts reduces, but the effect of the back-pressure penalty is to increase the IR signature from the rear aspect. Polar plots in 2-D of RLO for different view angles, considering reduction in the choked convergent nozzle exit area, in horizontal and vertical planes will be presented for the 3-5 μm band.