Long-wave infrared signature of aircraft nose and its emissivity for low observability

High speeds have been an advantage for aircraft to fly out of the adversary’s airspace fast, to escape earlier generations of infrared (IR) guided missiles that could lock-on only from the rear aspect. Advancements in IR seekers in long-wave IR (LWIR, 8 to 12 μm) band with imaging sensors in focal plane arrays made them capable of tracking aircraft also from the frontal aspect, which is a tactical advantage. Aerodynamically heated airframe surfaces are LWIR signature hot spots that increase aircraft susceptibility from front and bottom views. The conventional belief is that IR signatures of aircraft nose is only due to temperature-based emission acquired by aerodynamic heating at high Mach number (Mamb). Our study reports the LWIR contrast of aircraft nose (with nose replaced skyshine as background) by considering: (i) aerodynamic heating (important for high εnose, Mamb) and (ii) reflected skyshine and Earthshine (important at low εnose, Mamb), where skyshine is more important relative to Earthshine. Earthshine irradiance is obtained using atmospheric transmittance and view factor between visible circular Earth-disc and nose surface. The role of a transition Mach number (Mtrans) in emissivity selection is identified as follows: for Mamb < Mtrans, LWIR contrast decreases with εnose and for Mamb > Mtrans, LW-IR contrast increases with εnose.