Multiple Images Spatial Turbulence Imager (MISTI)

Because of the maneuvering capabilities of adversaries’ new hypersonic weapons, defensive missiles must fly in very demanding flight regimes, and intercept maneuvers place stringent requirements on interceptor seeker systems. Successfully completing the hit-to-kill end game requires the seeker can either compensate for or perform despite the thermal and optical effects that cause the seeker to misreport the target state vector. To ensure the seeker can overcome these effects, robust and detailed modeling is required that can predict seeker performance in all expected flight conditions. Robust modeling requires, in turn, the ability to validate and anchor the model output by appropriate and relevant ground testing that emulates flight conditions as closely as possible. Ground testing can not only validate the models but can improve their predictions if better input data are available. In this Phase II STTR effort, Polaris Sensor Technologies is proposing to develop, test and demonstrate a new measurement technique that provides a data product that improves the modeling capability through ground test validation. This technique enables more credible and flexible modeling for flight conditions that cannot be tested on the ground. This technique leverages the polarization beam combining and splitting used in the Phase I SBIR (Topic AF071-340, Contract FA9101-07-M-009). In this effort, Polaris designed a scene projector polarization controller (SPPC) that produced an arbitrarily defined linear polarization state for test chamber sources. Also during Phase I, Polaris designed an imaging system for characterizing test facility sources. In a subsequent Phase II, Polaris developed the proposed optical system for the 7 V and 10 V chambers. The underlying concept of the SPPC forms the core of the technology proposed here, which will also be used to produce controlled states for measurements in test chambers. The current effort adds the capability of two wavefront sensors (WFSs) and synchronized pulsed lasers to achieve the additional measurements. In the proposed system, two distinct instantiations of the flow in the wind tunnel can be imaged with a specified time delay, td­ . The laser pulse is short enough to effectively freeze the flow, and the WFS image gives a snapshot optical phase screen. The real strength of the technique arises when td­ is set such that the flow moves along the window only a small fraction of the window length. By performing a correlation on the two images, the turbulence correlation length LC and density ρ’ can be determined directly. By doing this repeatedly, the distribution of LC and ρ’ can be built up, which replaces the flow field characterization in the aero-optic model with measured data that were previously derived from the computational fluid dynamics (CFD) model of the wind tunnel. This technique is the Multiple Image Spatial Phase Polarization Collection (SPPC) Turbulence Imager (MISTI) and details follow. Approved for Public Release | 20-MDA-10601 (19 Oct 20)