The Airborne Laser is an Air Force Major Defense Acquisition Program to develop and field an airborne high energy laser weapon system to provide speed-of-light lethal defense against hostile theater ballistic missiles in the boost phase. The Air Force believes the Airborne Laser has the potential to revolutionize air warfare. The advanced technologies being introduced by the Airborne Laser presents new and unique challenges for acquisition, operations, and supportability. This paper provides a program overview, and will cover the threat, system description, technology maturity, and acquisition strategy. The Airborne Laser program successfully passed through its Milestone 1 Defense Acquisition Board decision to proceed from Concept Design into Program Definition and Risk Reduction phase, to design, build, integrate, and conduct a lethal airborne demonstration against a boosting missile in 2002. Upon a successful lethal demonstration, the program will then proceed into Engineering and Manufacturing Development and Production. Initial Operation Capability will be in 2006 with three aircraft, and Full Operational Capability will be in 2008 with seven aircraft.

The U.S. Air Force Airborne Laser consists of four primary subsystem segments; aircraft, battle management and C4I, laser device, and beam control segment (BCS). The BCS performs two major function, beam control and fire control, using three primary products, turret assembly, beam transfer assembly, and fire control hardware and software. The fire control sequence involves slewing the turret to the target coordinates as received from the battle management segment, centering the target in the acquisition sensor, acquiring the plume in the coarse track sensor, acquiring the hard body of the missile with the track illuminator laser, and establishing active tracking of the nose of the target theater ballistic missile. The beam control sequence begins after established active nose track by firing another beacon illuminator laser (BILL) to establish the aim point on the missile for the high energy laser. The resulting spot on the missile is imaged in the wavefront sensor and compared with the outgoing sample of the BILL. By applying the conjugate of the wavefront difference from the beacon to a deformable mirror then the atmosphere serves to correct the arriving wavefront on target. At this point the HEL is fired along the same path with similar wavefront correction, and dwells on target until the missile skin is weakened and rips open or buckles.

To support the Air Force's Airborne Laser (ABL) program, we have configured an adaptive-optics and tracking facility at Lincoln Laboratory's Firepond site. By using a 5.4-km horizontal propagation range and properly selecting the beam diameter and wavelength we are able to replicate important aspects of ABL propagation scenarios expected at high altitude. The experimental system was configured to conduct experiments with a point-source beacon, or using active illumination for both adaptive optics and tracking. In this paper we provide an overview of the Firepond facility and include details on the parameter scaling and experimental conditions required to replicate ABL conditions. We discuss optical configurations for various experiments and provide details of the adaptive-optics, tracking, and active illumination systems.

The Airborne Laser Advanced Concepts Testbed (ABL ACT) is a low power beam control testbed located at the northern end of White Sands Missile Range. The purpose of ABL ACT is to explore advanced beam control concepts for laser propagation under high scintillation conditions. This paper gives the capabilities of the system and subsystem that are in place or will be in place within the next year.

The mission of the High Altitude Balloon Experiment (HABE) is to resolve critical acquisition, tracking, and pointing (ATP) and fire control issues, validate enabling technologies, and acquire supporting data for future space- based laser experiments. HABE is integrating components from existing technologies into a payload that can autonomously acquire, track, and point a lower power laser at a ballistic missile in its boost phase of flight. For its primary mission the payload will be flown multiple times to an altitude of 85,000 feet above the White Sands Missile Range. From the near-space environment of the balloon flight, HABE will demonstrate the ATP functions required for a space- based laser in a ballistic missile defense role. The HABE platform includes coarse and fine gimbal pointing, infrared and visible passive tracking, active fine tracking, internal auto-alignment and boresighting, and precision line-of-sight stabilization functions. This paper presents an overview and status of the HABE program.

For this discussion of the nature of the problem presented to an adaptive optics system by branch points I shall assume that the adaptive optics system uses a unit shear shearing interferometer to make the basic measurements, and uses a standard least mean square reconstructor algorithm to convert the measurement values to commands that are sent to a deformable mirror. The shearing interferometer senses the fact that optical field at adjacent positions in the aperture are not perfectly coherent with respect to each other.* The processing of the data from the shearing interferometer should result in commands being sent to the deformable mirror which will cause the optical field to be adjusted in such a way the (nearly) perfect coherence will exist between all pairs of adjacent positions in the aperture—which means that there will be coherence between all of the optical field in the aperture. I shall show that under conditions of optical field distortion that are commonly characterized by the statement that there are branch points present, the adaptive optics system fails to utilize all of data gathered by the wave front sensor and that as a consequence there is some degree of incoherence remaining in the optical field after it has been reflected from the deformable mirror, and will show how to quantitatively treat this residual error.

We have conducted atmospheric compensation experiments using active illumination for both adaptive-optics and tracking. Tests were performed in support of the Air Force's Airborne Laser program. The tests utilized the 5.4-km horizontal propagation range at the Lincoln Laboratory Firepond facility. The adaptive-optics beacon was provided by actively illuminating the target with a multibeam laser illuminator. A second multibeam laser illuminator was used to provide a beacon for an imaging tracker. Experiments were conducted using two different adaptive-optics illuminator configurations, as well as with point-source beacons. Data were collected over a range of atmospheric conditions. Results from these tests have helped to provide a performance benchmark for the Airborne Laser program.

Horizontal adaptive optics research has been conducted at Yerkes Observatory since the first tests in 1994. This includes the study of wavefront reconstruction techniques, isoplanatic angle measurements, branch points in the phase function, and combined adaptive optics and phase diversity experiments.

A two deformable mirror concept for correcting scintillation effects in laser beam projection through the turbulent atmosphere is presented. This system uses a deformable mirror and a Fourier transforming mirror to adjust the amplitude of the wave front in the telescope pupil, similar to kinoforms used in laser beam shaping. A second deformable mirror is used to correct the phase of the wave front before it leaves the aperture. The phase applied to the deformable mirror used for controlling the beam amplitude is obtained using a technique based on the Fienup phase retrieval algorithm. Simulations of propagation through a single turbulent layer sufficiently distant from the beacon observation and laser beam transmission aperture to cause scintillation shows that, for an ideal deformable mirror system, this field conjugate approach improves the on-axis field amplitude by a factor of approximately 1.4 to 1.5 compared to a conventional phase-only correction system.

The study of phase discontinuities caused by atmospheric turbulence is a recent research topic; their study might yield significant improvements in high-quality adaptive optics systems, laser weapons and laser communication. We present in this paper an introduction to discontinuities in the light phase. We also provide a geometrical description of phase discontinuities, a study of their effect on Hartmann-Shack sensor measurements, as well as algorithms for measuring discontinuous light phases and the position of phase discontinuities.

In this work, the application of adaptive optics to the Airborne Laser is considered. The objective is to determine the statistics of the Zernike polynomial coefficients of the phase of the beacon and high energy laser wavefronts. The process of determining the covariance matrix of the phase expansion coefficients of the inbound (beacon) and outbound (high energy laser) wavefront phases is outlined. This work differs from previous research in that it exclusively focuses on the Airborne Laser engagement kinematics. Specifically, the temporal dependence of the above mentioned covariance matrix is determined with appropriate modifications to previous work. Atmospheric wind is neglected due to the large virtual wind induced by the aperture and target velocities. This requires the Taylor's frozen flow hypothesis be modified. Moreover, due to the finite distance between the receiving/transmitting apertures and the target, spherical wavefronts must be used throughout the development.

In Airborne Laser (ABL) target engagements, beam steering should account for the effects of turbulence-induced wavefront tilt. Due to the finite temporal response of adaptive-optical systems, steering commands must be based upon time-delayed wavefront measurements. The dynamics of the ABL target engagement scenario and turbulence-induced anisoplanatism cause the time-delayed wavefront measurements to be decorrelated from the required wavefront tilt correction. This decorrelation leads to suboptimal beam steering when time-delayed wavefront measurements are used directly in the control loop. To assess beam steering performance, we calculate the aperture-averaged mean square residual phase error after tilt correction, as a function of measurement time delay, and show that this quantity is minimized by the minimum mean square error (MMSE) estimate of tilt given a set of time-delayed wavefront measurements. Therefore, a temporal MMSE tilt estimator is constructed using time-delayed tilt plus higher-order aberration measurements. We define the tilt correction interval (tau) _c as the time delay at which the residual phase error is 1% larger than its value at zero time delay. Our results show that (tau) _c using the MMSE tilt estimator with modal measurements through Zernike 15 is over 3 times larger than (tau) _c using time-delayed tilt measurements only. Accordingly, the beam steering bandwidth for ABL target engagements is reduced by more than 70% when higher order modal measurements are used in optimal tilt correction.

Theoretical analysis of the temporal cross-correlation properties of turbulence-induced phase aberrations predict that while tilt is uncorrelated with most higher-order modes at zero time delay, the magnitude of the cross-correlation increases through some maximum value before going to zero at long time delays. To test this theoretical prediction, we employ laboratory measurements of phase induced on a laser beam propagating through a channeled flow of turbulent gas. From these measurements, the temporal cross-correlation of various aberration modes was estimated. Theoretical expressions were derived for these cross-correlations, and the results of these calculations agree well with experimental measurements. Both the experimental and the theoretical results demonstrate that the magnitude of cross- correlation between tilt and higher-order modes is non-zero over a range of temporal delay. Thus, higher-order model measurements may be incorporated into an optimal estimator for tilt, enhancing estimator performance.

We have pointed out that such two terms as `a laser guide star' (LGS) and `an effective scattering volume' are scientific synonyms. The first term had been introduced earlier by specialists in atmospheric optics and laser sounding. Corresponding mathematical apparatus allows one to estimate correlation characteristics of a LGS. We have considered the mutual correlation function of random angular displacements of a plane wave image and the centroid displacements of a focused Gaussian laser beam.

In Airborne Laser (ABL) like systems, the phase information is typically reconstructed from wavefront tilt sensor data using a least-squares approach. The solution to the phase reconstruction problem provides a smooth unwrapped phase surface that is not necessarily equal to the original wrapped phase surface in an optical propagation sense. Still, the computed phase surface furnishes information that can be used in determining the required phase conjugation for partial correction of an atmospherically distorted laser source. In this paper, we examine the effect of two phase reconstruction algorithms; unweighted least-squares and intensity weighted least-squares on the performance of an idealized ABL-like adaptive optics systems under ABL-like atmospheric conditions. The performance of each phase reconstruction algorithm will be investigated using MZA's Airborne Laser Simulation under atmospheres with Rytov numbers ranging from 0.233 to 3.725. Since the unwrapping process creates a phase surface without the 2(pi) discontinuities of the original phase, the r₀ and phase variance can be calculated from the unwrapped phase. These predictors of the phase correlation performance will be related to the performance measures; Strehl and power-in- the-bucket at the target.

The measurement sensitivity of CO₂ differential absorption LIDAR (DIAL) can be affected by a number of different processes. We will address the interaction of two of these processes: effects due to beam propagation through atmospheric turbulence and effects due to reflective speckle. Atmospheric turbulence affects the beam distribution of energy and phase on target. These effects include beam spreading, beam wander and scintillation which can result in increased shot-to-shot signal noise. In addition, reflective speckle alone has a major impact on the sensitivity of CO₂ DIAL. The interaction of atmospheric turbulence and reflective speckle is of great importance in the performance of a DIAL system. A Huygens-Fresnel wave optics propagation code has previously been developed at the Naval Postgraduate School that models the effects of atmospheric turbulence as propagation through a series of phase screens with appropriate atmospheric statistical characteristics. This code has been modified to include the effects of reflective speckle. The performance of this modified code with respect to the combined effects of atmospheric turbulence and reflective speckle is examined. Results are compared with a combination of experimental data and analytical models.

We have conducted active-tracking experiments in support of the Air Force's Airborne Laser program. These tests were conducted using the 5.4-km horizontal propagation range at the Lincoln Laboratory Firepond facility. Target illumination was accomplished using a multibeam illuminator laser provided by Lockheed Martin Missiles & Space; the tracking was performed using an imaging tracker provided by the Air Force Research Laboratory. Experiments were conducted using a number of different illuminator configurations, and with a variety of track algorithms. Results from these tests have quantified the performance benefits of multibeam illumination.

The High Altitude Balloon Experiment (HABE) is being developed by the U.S. Air Force Research Laboratory, Space Vehicle Directorate at Kirtland Air Force Base, to investigate technologies needed to perform acquisition, tracking, and pointing (ATP) functions against boosting missiles in near-space environments. HABE is designed to demonstrate ATP sequence steps that start with acquisition of a missile plume, transition through passive IR tracking of the plume, and handover to precision tracking, which employs an active laser illuminator and imaging camera to image and track the missile nose. The Inertial Pseudo Star Reference Unit provides inertially stabilized line-of-sights (LOSs) for the illuminator laser, active fine track camera, and the marker scoring. The latter serves to measure and score the payload's pointing performance. The payload will be operated and carried aloft under a large, scientific balloon. The engagement parameters and timelines for the HABE ATP payload are consistent with scenarios encountered in space-based missile defense applications. In HABE experiments, target missiles will pass at ranges from 50 to 200 km. The performance goals of the ATP payload's LOS stabilization and marker laser pointing are required to exceed 1 microradian RMS or better in jitter, drift, and accuracy (two-axis, one sigma metrics), a requirement which stresses testing capabilities.

SVS has recently completed a phase II small business innovative research (SBIR) project called Low Cost Space Imager. As part of the SBIR project, a sparse aperture telescope design concept was developed. This design includes an optical control system capable of correcting the primary segments to within 38 nm piston and 17 nrad tilt as required by the optical tolerance analysis. The optical system utilizes a common secondary and primaries arranged in a Golay-6 configuration. The primaries are spherical, which eliminates the need for translation and rotation control. A laboratory experiment to validate the controls concept has ben completed. This experiment culminated in the demonstration of autonomous capture, alignment, and phasing of an optical system with a three segment primary to tolerances consistent with the space optical system. The implementation of the controls scheme in the laboratory experiment is done using Matlab/Simulink for controller design and code generation. The code is implemented real- time on a VME based computer system. Closed loop piston control, which utilizes a four-bin sensing scheme, of an actuated mirror to 25 nm RMS mirror motion has been demonstrated. Additionally, autonomous capture and phasing of three segmented primaries has been demonstrated. The technique for the phasing capture involves real-time implementation of image processing techniques to measure the white light fringe visibility in the far field.

A high speed tracker is being developed for the Airborne Laser (ABL) program. This paper outlines the parallel processing concept that is being considered for design on the program. A sensor array to be built specially for the ABL program will implement parallel channels to read out the sensor data. This 16-wide parallel data path will be followed through the processing, including pixel processing and application of the tracking algorithms. To implement the tracking algorithm in a fewer number of processors, subregions of the sensor data are tagged during pixel processing as being of interest (e.g. within the track gate) and only those regions are assigned to processors for further processing. The paper includes discussion on methods for grouping these subregions. Approaches for implementing two candidate tracking algorithms using these methods, and a discussion of the plan for implementing this concept.

The development of high-average-power solid-state lasers with good beam quality has been limited primarily by thermo- optic distortions in the laser gain medium. Cooling Yb:YAG gain media to cryogenic temperatures promises to significantly reduce thermo-optic distortions relative to 300 K Nd:YAG. Preliminary results have been obtained for a diode-pumped, cooled Yb:YAG laser operating at 1.03 micrometers , and the experiments to date verify a large reduction in thermo-optic effects.

A series of ground and flight tests were conducted to test the feasibility and performance of a gimballed active television. The laser light was provided to the gimbal via a fiber optic cable from a remote semiconductor laser. A high power, divergent beam was used to illuminate a scene providing enhanced performance in poor weather, the recording of (ship, aircraft or automobile) registry and augmentation to existing night vision devices. The flight tests were conducted over land and water and the results are discussed. The tests verified the search and rescue application, the ability to view into an automobile from a helicopter and the ability to read alphanumerics at ranges of 250 meters.

We report scintillometer measurements for near-infrared propagation over a 51.4-km path, at an average altitude of 760 m over mountainous desert terrain. We present time series and histograms of normalized irradiance variance and path-averaged C²_n values. We compare the results with an average vertical C²_n profile constructed from past balloon measurements. We also present preliminary results on the temporal power spectra of normalized irradiance.

A single-ended, range-resolved, refractive turbulence sensor concept was investigated for ground-based and airborne platforms. This technology is of interest to the Air Force's Airborne Laser (ABL) program, because it will enable the determination of optimal engagement paths for the weapons laser. In this paper we describe the performance of a range- resolved refractive turbulence profiler which is based upon a coherent laser radar array receiver technology. We present Monte-Carlo simulation performance predictions for several sensor configurations, including a one micron ABL sensor and an eye-safe two micron ground-based sensor. In addition to its refractive turbulence sensing function, this innovative sensor will be capable of measuring wind velocity and characterizing wind turbulence.

The Air Force Research Laboratory at Kirtland Air Force Base examined the characteristics of the refractive index structure parameter (C²_n) within a strong and persistent layer of turbulence using observations obtained from a 49.25 MHz radar and an instrumented C-135E aircraft at the Atmospheric Profiler Research Facility at White Sands Missile Range, NM on January 23, 1997. The aircraft measurements sensed the atmospheric temperature structure parameter (C²_T) with fine-wire aerothermal probes for deriving C²_n while the radar measurements provided C²_n from Bragg scatter at turbulent scales in the clear air. The aircraft results provide horizontal spatial information at the specific altitudes flown while the radar-obtained values show temporal profile information. Flight legs approximately 200 km long were flown along the wind direction at eight different altitudes from 11.01 km to 12.21 km MSL. The turbulent layer and direction of flights were selected from the VHF radar-obtained C²_n and wind measurements prior to take-off. Presentations include a range-height display of the patterns of refractivity turbulence obtained from the aircraft measurements and a range-height display derived from the radar observations corresponding to the aircraft results. Both range-height displays were produced by assuming Taylor's hypothesis and applying the actual wind profile to the time-height data. The evolution and persistency of features is discussed. A statistical evaluation comparing the two different methods of sensing C²_n is presented. Salient features of the aircraft sensors and radar are discussed.

The performance of Airborne Lasers is sensitive to the 3D, nonstationary, intermittent, anisotropic structure of atmospheric turbulence. The upper (stratospheric) layers are dominated by the background stable stratification, as a result of which the turbulence tend to be characterized by thin layers of high refractive index variations known as `pancakes'. In this paper we survey theoretical results and experimental measurements of turbulence spectra and correlations for pancakes in rotating stratified environments. Anisotropic turbulence models developed are benchmarked against both laboratory experimental data bases as well as field data to be gathered in the balloon and the EGRETT campaign of measurements.

We study tropospheric aerothermal probe data by using the orthogonal Haar wavelet averages of the dissipation to segment the data. Segmenting the data in this way allows us to isolate regions of distinct mean dissipation. We then use the Haar wavelet transform to derive spectra and structure functions for the segmented regions, thereby recovering Kolmogorov statistics. We also comment on wavelet derived structure functions and point out data anomalies only visible in the wavelet domain.

We present a new approach for analyzing local power law processes and apply it to temperature measurements from the upper atmosphere. We segment the data and use the wavelet scale spectrum to estimate the parameters of the power law, the scale factor and the exponent. These parameters vary from segment to segment. Part of this variation is due to the non-stationary of the data. Another part is due to estimation errors that depend on the segmentation. In this paper show how to remove effectively these segmentation dependent variations.

The optical measurement of atmospheric turbulence strength is often subject to the effects of scintillation saturation. In the saturation regime, the intensity variance no longer increases with turbulence strength. In some instances, the observed behavior with respect to turbulence strength shows a flattening of the variance followed by a shallow drop-off with further increases in turbulence. The Air Force Research Laboratory requires a robust instrument for determining path-integrated turbulence effects on a laser propagating over a 50 km path under a variety of clear-air atmospheric conditions and turbulence strengths. The approach described here is based on the measurement of the Mutual Coherence Function (MCF). The MCF doesn't saturate since it depends on the total wave structure function, a function consisting of a non-saturating phase term as well as the saturating log- amplitude term. The sensor itself is an interferometer which measures fringe visibility. In addition to a measure of turbulence strength, or C²_n, the wave structure function itself can be determined. The latter feature is an especially important one for determining whether or not Kolmogorov statistics prevail.

The refractivity structure parameter C_n² characterizes the strength of refractive index fluctuations in the propagation path of electromagnetic waves and specifies the magnitude of the spatial power spectral density of turbulence-induced phase aberrations. In addition, the integrated value of C_n² along the propagation path determines the performance of the adaptive optical system. In this study, we explore the statistical relationship between the refractivity structure parameter and synoptic scale meteorological measurements. To this end, we have employed simultaneous measurements of the structure constant and meteorological data associated with fronts and jet streams at the synoptic scale. Using linear regression techniques, the degree of association between the meteorological data and radar-observed structure parameter values was computed. The parameters most strongly related to the structure constant are indicated; however, further research is needed to define the exact relationship between the refractivity structure constant and synoptic-scale meteorological data.

A new atmospheric screen generator is developed for use in performance calculations of adaptive optics and imaging systems. The generator is valid over a wide range of atmospheric turbulence parameters and incorporates both phase and amplitude effects. The new screen generator accounts for diffraction effects caused by turbulence and incorporates the phase, amplitude, and cross statistics of a weak turbulence model. The second order statistics of the phase and amplitude perturbations are based on the auto- correlation functions developed by Lee and Harp and the cross-correlation of the phase and amplitude perturbations derived in this paper. The correlations are derived by modeling the turbulence as a number of layers of randomly varying refractivity perpendicular to the propagation path. As the field propagates through the medium, diffraction occurs at each of the layers. A Fourier series expansion of the wavefront phase and amplitude is used. The screen generator uses the power and cross spectral densities of the phase and amplitude perturbations. The mean square value and the structure functions of the wavefront phase and amplitude are calculated in a Monte Carlo experiment and shown to be within 1% of the theoretical value.