Objective   The rocket exhaust plume is one of the key objects of the space-based infrared system (SBIRS) due to its strong infrared radiation characteristics. The infrared radiation signature of the plume is not only related to the motor prototype parameters but also to the flight parameters of the vehicle. In practical applications, the variation characteristics of infrared radiation signature of the rocket exhaust plume can be predicted through numerical methods based on the known motor and flight parameters. However, the flight parameters of non-cooperative targets are often difficult to obtain accurately, which means that there must be some deviation in predicting the infrared radiation signatures of the rocket exhaust plume by numerical methods. Therefore, it is necessary to study the influence of flight parameter disturbance on the infrared radiation of rocket exhaust plumes. For this purpose, the non-intrusive polynomial chaos (NIPC) method is used for the uncertainty quantification and sensitivity analysis of free stream parameters on the infrared radiation signatures of the rocket exhaust plume.

  Methods   The Latin hypercube sampling (LHS) method is used to design the samples of frees tream parameters. The infrared radiation characteristics of the rocket exhaust plume are calculated based on the infrared signature analysis tool (IRSAT), and the corresponding infrared response values of the plume at each sample point are obtained. The regression analysis method is used to solve the polynomial chaotic expansion coefficient and the statistical characteristics of the plume infrared signatures, including the mean value, standard deviation and uncertainty. Based on the Sobol index, the NIPC method can be utilized to quantify the uncertainty and sensitivity of the infrared radiation signature, and analyze the impact of a single variable and multiple variables on the infrared radiation characteristics of the rocket exhaust plume.

  Results and Discussions   The free stream velocity has a high sensitivity to spectral radiation intensity in most spectral bands except for 4.3 μm, and the corresponding Sobol index is above 0.5. The maximum sensitivity of free stream pressure occurs in the 4.3 μm band, and the peak of the Sobol index is close to 1.0. The Sobol index of angle of attack to spectral radiation intensity is about 0.4, and it shows high sensitivity in multiple bands. The influence of the ambient temperature on the radiation spectrum is negligible. The Sobol index of the in-band radiance shows that the free stream velocity is the most sensitive to the radiation intensity in most bands. The free stream pressure and the angle of attack are the second, and the free stream temperature is the smallest. The coupling effect of free stream velocity and ambient temperature has the most obvious contribution to the in-band radiance. The coupling effect of free stream velocity and pressure, and the coupling effect of temperature and angle of attack only affect some extremely narrow spectral bands. The coupling effect among other parameters can be almost ignored.

  Conclusions  The low altitude under-expanded Atlas-IIA plume is taken as the research object, and the uncertainty quantification and sensitivity analysis of infrared radiation signatures of free stream velocity, temperature, pressure and angle of attack are carried out using NIPC method. The uncertainty of radiation intensity caused by the incoming flow has a high correlation with the spectral band. The standard deviation of the in-band radiance is positively correlated with the mean value, and the uncertainty of the radiance is opposite to the mean value of the radiation intensity. In most wavebands, the free stream velocity is the most sensitive to the radiation intensity, followed by the free stream pressure and angle of attack, and the ambient temperature is the least. The coupling effect of velocity and temperature have the most obvious contribution to the radiance. The coupling effect of velocity and pressure, and temperature and angle of attack only have an effect in some very narrow spectral bands. The ratio of angle of attack to the main Sobol index of the radiation intensity is between 15% and 23%. The main Sobol index of the inflow velocity accounts for nearly 80% except for the 4.3 μm band. The impact of inflow pressure in the 4.3 μm band is dominant. The coupling effect of each incoming flow parameter has little influence on the radiation intensity in each spectral band, which is less than 4%.