Wave-optics modeling of a heat-seeking missile attacked using a DIRCM laser in turbulent atmosphere

Their exceptionally large jam-to-signal ratios (J/S) make directed infrared countermeasure (DIRCM) systems the most efficient means for defeating heat seeking missiles. Although several studies have performed analyses on DIRCM systems, influence of optical turbulence on the effectiveness of jamming waveforms is usually ignored. However, due to turbulence originating from the exhaust plume of the air platform and the atmosphere, a DIRCM laser's beam may be exposed to time-varying intensity variations which may drastically reduce the effective J/S at the seeker aperture compared to the one at the platform. Furthermore, previous studies have usually focused on the signal processing stages of seeker heads while neglecting the diffractive and aberrative properties of the missile optics. An analysis of the impact of turbulent air on the DIRCM effectiveness from a wave-optics point of view is required. In this paper, we first investigate the time-varying impact of several degrees of turbulence on laser beams modulated with a jamming pattern along the optical path from the air platform to the missile's dome. In the turbulence model, the laser beam emerging from the platform with an arbitrary quality factor of M² transverses two different turbulent regimes when directed to the missile threat. First, it passes through the region of highly turbulent medium around a rotary-wing platform originating from the rotor downwash of the exhaust plume. Next, the beam travels much longer distances (on the order of few km's) in the turbulent atmosphere until it reaches the missile. Beam propagation in both regions is simulated using the split-step method. Using the ZEMAX software and its wave-optics based Physical Optics Propagation (POP) package, we employ a generic model for the optical system of a first-generation spin-scan seeker and obtain the time-dependent intensity profiles of the laser beam at the focal point at various instants of the jamming pattern. Generic models for an uncooled lead-sulfide detector and the following signal processing stages have also been included in the model.