Analysis of hypersonic shock layer radiance effect on an IR imaging system

When an endo-atmospheric vehicle carrying an IR imaging detection system flies at a high speed, radiance of shock layer near its IR window degrades SNR and detection range of the system. Firstly, based on the geometric relation of the near field radiance imaging, the relation between shock layer radiance power received by every pixel in the focal plane and radiance emitted by the shock layer outside the boundary between the shock layer and the window was deduced. Secondly, the box model was selected to describe gas radiation characteristics in the shock layer. Thirdly, the radiative transfer function was solved to calculate the radiance. Shock layer radiance power received by every pixel of the detector array and the imaging result of a target were calculated. The results show that the shock layer is optically thin medium, so the target radiance absorbed by it can be neglectable. The calculation results show that the shock layer radiance power received by the imaging system is inhomogeneous background noise. To a certain altitude, when Mach number increases, the power increases, so the SNR decreases; and to a certain Mach number, when altitude increases, the power decreases, so the SNR increases.