Proceedings Volume 5552

Target-in-the-Loop: Atmospheric Tracking, Imaging, and Compensation

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Proceedings Volume 5552

Target-in-the-Loop: Atmospheric Tracking, Imaging, and Compensation

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Volume Details

Date Published: 18 October 2004
Contents: 8 Sessions, 25 Papers, 0 Presentations
Conference: Optical Science and Technology, the SPIE 49th Annual Meeting 2004
Volume Number: 5552

Table of Contents

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Table of Contents

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  • Target-in-the-Loop Propagation: Modeling and Simulations I
  • Target-in-the-Loop Propagation: Modeling and Simulations II
  • Target-in-the-Loop Beam Control I
  • Target-in-the-Loop Beam Control II
  • Target-in-the-Loop Atmospheric Tracking I
  • Target-in-the-Loop Atmospheric Tracking II
  • Target-in-the-Loop Atmospheric Modeling, Imaging, and Wavefront Sensing
  • Poster Session
Target-in-the-Loop Propagation: Modeling and Simulations I
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High Energy Laser Joint Technology Office: a mission overview
The High Energy Laser Joint Technology Office (HEL-JTO) was established in 2000 for the purpose of developing and executing a comprehensive investment strategy for HEL science and technology that would underpin weapons development. The JTO is currently sponsoring 80 programs across industry, academia, and government agencies with a budget of approximately $60 million. The competitively awarded programs are chosen to advance the current state of the art in HEL technology and fill technology gaps, thus providing a broad capability that can be harvested in acquisition programs by the military services.
Analysis of wave propagation and wavefront sensing in target-in-the-loop beam control systems
Target-in-the-loop (TIL) wave propagation geometry represents perhaps the most challenging case for adaptive optics applications that are related with maximization of irradiance power density on extended remotely located surfaces in the presence of dynamically changing refractive index inhomogeneities in the propagation medium. We introduce a TIL propagation model that uses a combination of the parabolic equation describing outgoing wave propagation, and the equation describing evolution of the mutual intensity function (MIF) for the backscattered (returned) wave. The resulting evolution equation for the MIF is further simplified by the use of the smooth refractive index approximation. This approximation enables derivation of the transport equation for the returned wave brightness function, analyzed here using method characteristics (brightness function trajectories). The equations for the brightness function trajectories (ray equations) can be efficiently integrated numerically. We also consider wavefront sensors that perform sensing of speckle-averaged characteristics of the wavefront phase (TIL sensors). Analysis of the wavefront phase reconstructed from Shack-Hartmann TIL sensor measurements shows that an extended target introduces a phase modulation (target-induced phase) that cannot be easily separated from the atmospheric turbulence-related phase aberrations. We also show that wavefront sensing results depend on the extended target shape, surface roughness, and the outgoing beam intensity distribution on the target surface.
Prediction and modeling of line-of-sight bending near ground level for long atmospheric paths
Sergey Bendersky, Norman S. Kopeika, Nathan Sh. Blaunstein
Line-of-sight bending, called "beam bending", is an effect caused by strong atmospheric turbulence, where during daytime targets are seen lower and during nighttime higher than their real locations. This effect takes place in conditions of very low or absence of wind speed and relatively high turbulence, which characterize non-uniform atmospheres. During past three years high-resolution experiments in different desert and low vegetation areas of the Middle East (Israel) were performed. A model for predicting line-of-sight bending based on effects of turbulence and atmospheric conditions, described by pressure, temperature, relative humidity, etc., was developed and investigated. This paper describes investigations made to parameterize this model based on high-percentage prediction of results for different areas, day/night time, and heights above the ground surface.
Target-in-the-Loop Propagation: Modeling and Simulations II
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Exact analytic solution to the problem of thermal blooming and its interaction with turbulence: results from analytic functional scaling
Shirley Enguehard, Brian Hatfield
We present the analytic theory for the interaction of thermal blooming and its interaction with optical turbulence in the form of an exact solution [2] which gives the asymtotics of the MCF and yields a Strehl. From that functional scaling is derived and a reconstruction algorithm is presented in terms of the eigenfunctions. The resulting systems model, AMPERES, which runs in real time on a PC matches the results of full non linear wave optics simulations. It is shown that thermal blooming is an asymtotic instability that requires a stable heating pattern to be imparted into the medium. Any kind of of wind shear disrupts the heating pattern and destroys the instability growth. Targets are not static. They are moving rapidly in cases of interest for HEL and the beam slew required to track the target naturally introduces a wind shear.
Propagation of partially coherent beam through turbulent nonlinear media
The propagation of a partially coherent wave field in inhomogeneous media is investigated. The influence of linear and nonlinear refraction and refraction parameter fluctuations on target characteristics of radiation are taken into consideration. The exact solution expressed in quadrature is obtained for the partially coherent radiation propagating through a turbulent medium with the Kolmogorov spectrum of fluctuations and a parabolic distribution of the mean refractive index. On the basis of comparison with this solution the accuracy of the solution of the equation for a coherence function of the second order obtained by the effective ray-tracing technique and taking into account the influence of turbulence and a nonlinearity of a medium to the propagation of a radiation with an arbitrary initial coherence is investigated.
Propagation of "exotic" laser beams in conditions of atmospheric turbulence and thermal blooming effects
Analysis and numerical modeling of "exotic" laser beam propagation through inhomogeneous atmospheric media is performed. The exotic beam is synthesized by introducing an additional periodical dynamically changing phase modulation to a conventional outgoing beam base. The propagation media model includes a set of thin phase distorting layers corresponding to atmospheric turbulence-induced distortions and non-stationary nonlinear layers describing the thermal blooming effects that accompany high-energy laser beam propagation in a moving propagation medium. The exotic beam structure is optimized to have maximum energy inside the diffraction-limited size area on the target plane. Compared with propagation of conventional laser beams, the exotic beam exhibits better beam quality metrics on the target plane such as maximum intensity, centroid intensity, and energy in the vicinity of the maximum beam intensity and the centroid.
Target-in-the-Loop Beam Control I
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Atmospheric compensation over a 2.3 km propagation path with a multi-conjugate (piston-MEMS/modal DM) adaptive system
We discuss the expansion of wavefront distortion compensation based on stochastic parallel gradient descent (SPGD) optimization to the control of several wavefront correctors. We describe then a SPGD adaptive optics system that uses a low-order deformable mirror with modal control and a high-resolution (either 132 or 320 control channels) piston-type MEMS mirror. The system was installed at a 2.3km near-horizontal propagation and used for atmospheric compensation experiments. Results obtained for different system configurations are presented.
Comparison of cooperative and non-cooperative adaptive optics reference performance for propagation with thermal blooming effects
Brian E. Edwards, Arthur Nitkowski, Ryan Lawrence, et al.
Atmospheric turbulence and laser-induced thermal blooming effects can degrade the beam quality of a high-energy laser (HEL) weapon, and ultimately limit the amount of energy deliverable to a target. Lincoln Laboratory has built a thermal blooming laboratory capable of emulating atmospheric thermal blooming and turbulence effects for tactical HEL systems. The HEL weapon emulation hardware includes an adaptive optics beam delivery system, which utilizes a Shack-Hartman wavefront sensor and a 349 actuator deformable mirror. For this experiment, the laboratory was configured to emulate an engagement scenario consisting of sea skimming target approaching directly toward the HEL weapon at a range of 10km. The weapon utilizes a 1.5m aperture and radiates at a 1.62 micron wavelength. An adaptive optics reference beam was provided as either a point source located at the target (cooperative) or a projected point source reflected from the target (uncooperative). Performance of the adaptive optics system was then compared between reference sources. Results show that, for operating conditions with a thermal blooming distortion number of 75 and weak turbulence (Rytov of 0.02 and D/ro of 3), cooperative beacon AO correction experiences Phase Compensation Instability, resulting in lower performance than a simple, open-loop condition. The uncooperative beacon resulted in slightly better performance than the open-loop condition.
Computer simulation of adaptive optical systems
A model of a typical adaptive optics system is described in the report. The model includes all main elements of a real system: the path of a beam propagating in the atmosphere, wavefront sensor, and adaptive mirror with continuous surface. Solution to the some of adaptive optics problems was obtained with the model. These results were included in the report as illustration of the model utility.
Target-in-the-Loop Beam Control II
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Model-based beam control for illumination of remote objects, part I: theory and near real-time feasibility
On September 1, 2003, Nukove Scientific Consulting, together with partner New Mexico State University (NMSU), began work on a Phase I Small Business Technology TRansfer (STTR) grant from the Air Force Office of Scientific Research (AFOSR). The purpose of the grant was to show the feasibility of taking Nukove's pointing estimation technique from a post-processing tool for estimation of laser system characteristics to a real-time tool usable in the field. Nukove's techniques for pointing, shape, and OCS estimation do not require an imaging sensor nor a target board, thus estimates may be made very quickly. To prove feasibility, Nukove developed an analysis tool RHINO (Real-time Histogram Interpretation of Numerical Observations) and successfully demonstrated the emulation of real-time, frame-by-frame estimation of laser system charcteristics, with data streamed into the tool and the estimates displayed as they are made. The eventual objective will be to use the frame-by-frame estimates to allow for feedback to a fielded system. Closely associated with this, NMSU has developed a laboratory testbed to illuminate test objects, collect the received photons, and stream the data into RHINO. The two coupled efforts clearly demonstrate the feasibility of real-time pointing control of a laser system.
Model-based beam control for illumination of remote objects, part II: laboratory testbed
When a laser beam propagates through the atmosphere, it is subject to corrupting influences including mechanical vibrations, turbulence and tracker limitations. As a result, pointing errors can occur, causing loss of energy or signal at the target. Nukove Scientific Consulting has developed algorithms to estimate these pointing errors from the statistics of the return photons from the target. To prove the feasibility of this approach for real-time estimation, an analysis tool called RHINO was developed by Nukove. Associated with this effort, New Mexico State University developed a laboratory testbed, the ultimate objective being to test the estimation algorithms under controlled conditions and to stream data into RHINO to prove the feasibility of real-time operation. The present paper outlines the description of this testbed and the results obtained through RHINO when the testbed was used to test the estimation approach.
Target-in-the-Loop Atmospheric Tracking I
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Active tracking with moderate power lasers
Jim F. Riker, Robert Q. Fugate, Trae Holcomb, et al.
Progress on active tracking at the Starfire Optical Range (SOR) has been significant in the years 2003-2004. We have obtained laser returns from a number of retro-reflector and also unaugmented satellite objects, and compared the signal returns to theories presented in previous SPIE papers (ref. 1-3). These results have concentrated on very low-power, sinusoidally-modulated lasers and a large-aperture, phase-sensitive detection receiver to discriminate the return signal from background and noise. This year, we have installed and used a much higher average power, high-repetition-rate pulsed laser in order to increase the signal-to-noise ratio. Results from these laser engagements will be presented along with simulation and theoretical comparisons. Techniques for diagnosing the laser uplink and the receiver systems will be discussed.
Next generation laser tracker concepts
Timothy L. Brown, Duane L. Patrick, Dale L. Shamblin, et al.
The current and future laser tracking mission requirements of Sandia National Laboratories are discussed. The capabilities of Sandia's existing laser trackers are summarized. The deficiencies of the current laser trackers are identified with respect to future mission requirements. Candidate commercial technologies are addressed to correct the identified deficiencies. Technology gap areas are identified where additional research needs to be conducted prior to developing an effective next generation laser tracking system
Target-in-the-Loop Atmospheric Tracking II
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Uncertainty quantification for cooperative target tracking
Historically tracking systems have provided limited quantitative data such as approximate range, speed, and trajectory. Today's tracking systems are now being tasked with accurately quantifying a broader range of dynamic state variables (e.g., absolute and relative position, orientation, linear and angular velocity/acceleration, spin rate, trajectory, angle of attack, angle of impact) for high-speed test articles. This information is needed to demonstrate that the required test conditions are achieved, to develop, validate, and apply predictive models, and to document a system's response to a test environment. Few existing and emerging optical tracking methods accurately provide the dynamic state variables. Even fewer quantify the measurement uncertainty. Past measurement error estimates have been either qualitative or lacked the rigor needed to accurately validate and apply predictive models. This presentation will discuss tracking options and approaches for characterizing tracking measurement uncertainty.
Fine beacon pointing control system for the RILC (Recce/Intel Laser Crosslink) system
Airborne laser communication systems often require a control system for each transceiver providing pointing control of a reference line of sight aboard a stabilized platform. These control systems often apply target in the loop techniques using a fast steering mirror (FSM) that reflects the incoming beacon image towards a 2 axis optical sensor. Sensor measurements and processing provide estimates of the beacon image centroid in a feedback control system with the FSM to center the image against disturbing influences. These disturbances appear as exogenous inputs in the control loop and are derived from both 1) atmospheric turbulence, which distorts the shape and direction of the beacon image, and 2) residual vibrations, which originate in the aircraft structure, and are attenuated by the stabilized platform. Models for both turbulence and structurally induced vibrations are described and applied in simulation with an FSM control system for an air-to-air laser communications system developed by Trex Enterprises for the Air Force Research Laboratory, WPAFB. Results from the simulation estimate obtainable bandwidth, stability margins and expected disturbance reduction. Simulation results are compared to actual performance measurements.
LADAR object detection and tracking
The paper describes an innovative LADAR system for use in detecting, acquiring and tracking high-speed ballistic such as bullets and mortar shells and rocket propelled objects such as Rocket Propelled Grenades (RPGs) and TOW missiles. This class of targets proves to be a considerable challenge for classical RADAR systems since the target areas are small, velocities are very high and target range is short. The proposed system is based on detector and illuminator technology without any moving parts. The target area is flood illuminated with one or more modulated sources and a proprietary-processing algorithm utilizing phase difference return signals generates target information. All aspects of the system utilize existing, low risk components that are readily available from optical and electronic vendors. Operating the illuminator in a continuously modulated mode permits the target range to be measured by the phase delay of the modulated beam. Target velocity is measured by the Doppler frequency shift of the returned signal.
TrackEye tracking algorithm characterization
Michael T. Valley, Robert W. Shields, Jack M. Reed
TrackEye is a film digitization and target tracking system that offers the potential for quantitatively measuring the dynamic state variables (e.g., absolute and relative position, orientation, linear and angular velocity/acceleration, spin rate, trajectory, angle of attack, etc.) for moving objects using captured single or dual view image sequences. At the heart of the system is a set of tracking algorithms that automatically find and quantify the location of user selected image details such as natural test article features or passive fiducials that have been applied to cooperative test articles. This image position data is converted into real world coordinates and rates with user specified information such as the image scale and frame rate. Though tracking methods such as correlation algorithms are typically robust by nature, the accuracy and suitability of each TrackEye tracking algorithm is in general unknown even under good imaging conditions. The challenges of optimal algorithm selection and algorithm performance/measurement uncertainty are even more significant for long range tracking of high-speed targets where temporally varying atmospheric effects degrade the imagery. This paper will present the preliminary results from a controlled test sequence used to characterize the performance of the TrackEye tracking algorithm suite.
Target-in-the-Loop Atmospheric Modeling, Imaging, and Wavefront Sensing
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Fundamental considerations for wavefront sensing with extended random beacons
Extending adaptive optics to laser beam control over long turbulent paths where there is no cooperative beacon present requires examination of wave front sensor performance in new regimes. In some scenarios of interest it is necessary to create beacon by sending an illuminator laser through the atmosphere to scatter from the target, which, in general, must be considered to be optically rough. Scattered light returned to the laser transmitter aperture is referred to as the beacon field, and this light may be used for wave front sensing. Physical effects on the beacon field which reduce wave front sensing accuracy include turbulence induced beam broadening and speckle on the outgoing beam, and coherent laser speckle effects in the scattered field. In this paper we analyze the impact of these phenomena on some of the measurements available for tracking and wave front sensing.
Optical synthetic aperture radar imaging and phase-error compensation
We present results from simulation of an Optical Synthetic Aperture radar system(OpSAR) using a wave optics propagation model. This model allows phase and amplitude errors to be introduced anywhere along the propagation path. We show the effects of uncompensated phase errors caused by atmospheric turbulence. We demonstrate azimuth phase error compensation using a sharpness metric based on entropy minization.
Atmospheric characterization for high-energy laser beam propagation in the maritime environment
The U.S. Navy has a renewed interest in the use of high energy laser (HEL) systems for ship defense scenarios. Surface ships must track and engage targets within a thin near-surface environment called the marine atmospheric surface layer. Within this layer exist substantial gradients in temperature and momentum, thus making extinction and turbulence strong functions of height. In such an environment, a primary cause of beam degradation is the atmospheric composition along the beam path, and this problem is compounded by the vertical variations in extinction and turbulence. The three primary effects that must be predicted for a successful system model are extinction, turbulence, and thermal blooming. Although these factors are present for any HEL scenario within the atmosphere, they are particularly prominent for the marine near-surface environment. Aerosol extinction can be a strong function of the near-surface path height when there are windy conditions, and this vertical dependence must be reflected in the model. The occurrence of turbulence along the path also degrades the on-target beam intensity, and this effect is also strongly height-dependent, with paths nearest the ocean-air interface encountering the greatest scintillation. We will discuss our efforts to provide a useful irradiance-on-target envelope using existing models and meteorological databases to analyze the efficacy of an HEL system. An irradiance prediction for a HEL weapons system must be accurate and reliable since it will be impossible to perform appropriate field tests across the full spectrum of possible operational environments to be encountered by such a system.
Modeling of atmospheric parameters for laser tracing scenarios (near horizontal low-latitude propagation paths)
Investigations of the dynamics of turbulent characteristics were carried out for different tracking paths based on theoretical equations. A multi-screen model and single-screen one for turbulent atmosphere have been constructed for numerical simulation of laser beam propagation along atmospheric paths within the framework of the paraxial approximation. These models are suitable for simulation of the propagation along both homogeneous and inhomogeneous paths. Within this model, the Fried radius, the scintillation index, the effective beam radius, and the coherence length of radiation were calculated. The values obtained in the numerical experiment were compared with those calculated analytically.
A user-friendly software package for predicting atmospheric turbulence effects on laser beam propagation
A new user-friendly software product called the Atmospheric Laser Turbulence Model (ALTM) has been developed at the University of Central Florida and is distributed through Ontar Corporation (www.ontar.com) to provide Gaussian-beam wave propagation models for calculating various beam characteristics in the presence of atmospheric turbulence. The calculations are valid for any horizontal path in which the atmospheric index of refraction structure parameter Cn2 can be taken as a constant. Two choices of atmospheric model are available to the user-the Kolmogorov Spectrum, which is based on the atmospheric structure parameter or the more accurate Modified Atmospheric Spectrum, which exhibits the "bump" at high wave numbers and is based on the atmospheric structure parameter and the turbulence inner scale and outer scale. The beam-wave models are valid for optical frequencies ranging from the visible to the far-IR portions of the electromagnetic spectrum. This paper provides an overview of the ALTM software package including current limitations of and future enhancements to the model.
Poster Session
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Spectroscopic provision for the problem of optical radiation propagation in the Earth's atmosphere
Now one of the topical problems of atmospheric optics is the creation of effective methods of radiative transfer calculation in the aerosol-gaseous the earth atmosphere that provides a high speed, accuracy and engineering software. In this work, techniques of calculation and parameterization of molecular absorption characteristics, applied to solving the problem of the narrow-band and broadband laser beam propagation, are described. The analysis of errors of the spectroscopic information is performed and techniques of calculation of errors are described. It is believed that intensities and half-widths of spectral lines have systematic and random errors. Results of simulation of influence of temperature and atmospheric gas concentration variations of molecular absorption coefficients are presented. The method of calculation of mean and mean square deviations of molecular absorption characteristics is presented for the case, in which the limited statistical information about meteorological parameters of the atmosphere is used. The prognostic atmospheric model allowing correct optical characteristics of slant atmospheric paths based on the information about near-surface temperature and humidity of the air is offered.
Wavefront tip-tilt/focus-defocus control using a deformable piezoceramic mirror
We present the results from studying different methods to control a deformable piezoceramic mirror with three degrees of freedom in the context of an optical transceiver system. In the first part, we compare the results of controlling the mirror with the Proportional Integral Differential Algorithm (PID) and the stochastic parallel gradient descent optimization technique (SPGD); results demonstrate that both techniques achieve similar performance. In the second part we built an optical transceiver system using deformable piezoceramic mirrors as active elements. The transceiver system was tested over a free space propagation path and controlled using different variants of the SPGD method. We show that the use of the SPGD algorithm as control method for the piezoceramic mirror adaptively corrects part of the turbulence-induced random wave front phase distortions.
Design development of adaptive system for solar telescope
Vladimir P. Lukin, Leonid V. Antoshkin, Nina N. Botygina, et al.
The possibility of applying adaptive-optics devices to ground-based solar astronomy and high-resolution spectroscopy is considered [1]. The several experimental adaptive-optics systems for image stabilization are described, as well as the results of its tests. Different ways of the development of the adaptive-optics system for use in the Big Solar Vacuum Telescope (BSVT) of the Baikal Astrophysical Observatory are discussed.