The knowledge of the turbulence strength in the atmosphere is important for many applications. Imagery in the atmosphere experience significant blur when the turbulence is strong. This can be automatically improved (without user intervention) if the turbulence strength is known. The performance of a high-power laser emitting in the atmosphere can be predicted if the statistics of the turbulence strength is known. If not predicted correctly, the laser may unintentionally destroy a target or fail to be able to disable a target.
In this article, we review existing methods that estimate turbulence strength, provide a more in depth error analysis, and propose a new method for estimating and mitigating turbulence in the atmosphere. We focus on methods that are passive in design in order to prevent detection in surveillance scenarios and tactical situations. We also propose a new method, stereo image motion monitor (SIMM) which is a system containing two independent apertures. Our goal in this approach is threefold: 1) We can measure r0 using the DIMM method 2) We can simultaneously estimate r0 individually for each aperture and 3) We have multiple views of the same scene thus can increase the number of frames used in turbulence mitigation methods.
The Navy is actively developing diverse optical application areas, including high-energy laser weapons and free- space optical communications, which depend on an accurate and timely knowledge of the state of the atmospheric channel. The Optical Channel Characterization in Maritime Atmospheres (OCCIMA) project is a comprehensive program to coalesce and extend the current capability to characterize the maritime atmosphere for all optical and infrared wavelengths. The program goal is the development of a unified and validated analysis toolbox. The foundational design for this program coordinates the development of sensors, measurement protocols, analytical models, and basic physics necessary to fulfill this goal.
Mitigating the effects of turbulence in imaging is an important capability for surveillance systems. For image capture applications, atmospheric turbulence causes global blur in isoplanatic conditions which prevents detection and identification of objects due to loss of important features. Free-space communication applications additionally suffer from these artifacts. The knowledge of the atmospheric characteristics can help improve the process of turbulence mitigation by applying enhancement filters designed according to optics parameters and turbulence characteristics. The additional problem is that estimating turbulence parameters require controlled equipment and known sources such that a transmitter and receiver pair are required. In this paper we investigate a method that addresses both common problems where only a single imaging system (with known parameters) is used for observation in horizontal paths through the atmosphere. We first desire to investigate a method for automating the process of selecting the correct modulation transfer function such that the observed image can be deblurred; thus enhancing the received images. Secondly, we wish to investigate a method for estimating the refractive-index structure parameter. We demonstrate the performance of this method with simulated data such that the camera and atmospheric conditions are known. Experiments were conducted and data collected in Point Loma, San Diego where atmospheric conditions were measured along with captured images of static scenes. We present the results of our approach with the simulated and real-world data. We discuss the issues with this type of approach and suggest plans for improving the method in the future.
Atmospheric turbulence can cause significant degradation to video over long horizontal paths. The refractive index fluctuations along the path from scene to camera lead to blur, varying across the frame and from frame to frame. Computationally inexpensive methods model this effect with an MTF blur function; however this technique neglects anisoplanatic effects. Wave optics techniques have been developed taking into account anisoplanatism, but ignoring scintillation, and spatial and temporal effects. Since long horizontal paths over varied surfaces (e.g. water to land or vice versa) will encounter varying turbulence strength along the path, the turbulence strength should be defined independently at each phase screen. Also important, turbulence strength can vary over short time scales (<1s), so a physically accurate simulation must allow time-dependent phase screens. We will present results of a wave optics simulation technique that includes these spatial and temporal variations.
The results will provide validation for turbulence removal algorithms.