Low signature aircraft adopt active or passive methods to reduce the characteristic difference between themselves and surrounding backgrounds, such as high-temperature component cooling, aerodynamic structure layout optimization, and absorbing coating, which brings great challenges to detection systems. The aerodynamic heating of aircraft is difficult to eliminate, which thus provides a radiant source for the infrared detection system. The development of high sensitivity infrared detectors further promotes the detection of low characteristic aircraft by infrared detection systems. Maximum detection range (MDR) is an important performance indicator of infrared detection systems, which is not only related to the target's infrared radiation characteristics but also closely related to the system's visual range prediction model. At present, most research focuses on analyzing the detectability of targets based on a single visual range prediction model. Especially, with low characteristic aircraft as the research object, there is a lack of research on using multiple visual range prediction models for detectability analysis. Therefore, we take low characteristic aircraft as the research object and conduct research based on multiple visual range prediction models, which can provide theoretical support for the detection and recognition of low characteristic aircraft and detector design.

A fly-wing configuration aircraft was taken as the research object. The surface temperature of the aircraft was predicted using the computational fluid dynamics (CFD) method, based on the assumption of a radiative balance wall. The radiative transfer equation (RTE) was solved through the line of sight (LOS) method, taking into account the situation of light being obstructed by the aircraft's skins. The atmospheric transmittance was borrowed from the MODTRAN software. Three ground-based visual range prediction models were established, including noise equivalent flux density (NEFD), minimum detectable temperature difference (MDTD), and minimum resolvable temperature difference (MRTD) algorithms. Finally, an end-to-end numerical simulation model to predict the MDR and maximum detection zenith angle of fly-wing configuration aircraft was established.

For the fly-wing aircraft, the radiance in the long wave infrared (LWIR, 8-12 μm) band is two orders of magnitude higher than that in the medium infrared wave (MWIR, 3-5 μm) band. The radiation intensity of the back and abdomen of the aircraft is the highest, and the radiance in the side-view observation is basically the same (Fig. 7 and Fig. 8). The MDR of the NEFD visual range prediction model is nearly one order of magnitude higher in the LWIR band than that in the MWIR band. However, the MDR of the MDTD and MRTD models is approximately equal in both bands. In the LWIR band, the MDR and the maximum detection zenith angle calculated by the three visual range prediction models in descending order are as follows: NEFD>MDTD>MRTD. In the MWIR band, the MDR of the MDTD model is the largest compared with the other two models. The MDR of the NEFD model within the detection plane containing pitch angle variation is about 170 km, which is suitable for detecting the back and abdomen of the aircraft. However, the MDTD and MRTD models have the MDRs in the bottom-view observation, with an MDR of 57 km and 38 km, respectively (Fig. 9 and Fig. 10). Within the side-view observation plane, the MDR calculated by the NEFD model is approximately 62 km (Fig. 11 and Fig. 12). Under different observation levels including discovery, classification, and recognition, the low characteristic aircraft can be detected by the MRTD model, but the MRTD model fails to detect such fly-wing aircraft under the identify level (Fig. 14).

1) The radiance in the LWIR band is two orders of magnitude higher than that in the MWIR band. The radiation intensity in the top-view observation is nearly one order of magnitude higher than that in the side-view observation. It is indicated that infrared radiation intensity has strong selectivity in terms of spectral bands and detection angles. 2) The MDR of the NEFD model in the LWIR band is nearly one order of magnitude higher than that in the MWIR band, and the MDRs of the MDTD and MRTD models in the MWIR band and LWIR band are basically the same. In the LWIR band, the MDRs of the three models are sorted in descending order: NEFD>MDTD>MRTD. 3) In the detection plane of the pitch angle, the MDR of the NEFD model in the top-view and bottom-view is about 170 km, and the MDRs of the MDTD and MRTD models are 57 km and 38 km, respectively. 4) The MRTD model can detect aircraft at discovery, classification, and recognition observation levels, but it is invalid at identify level.