The U.S. Dept. of Defense (DOD) is currently developing and testing a number of High Energy Laser (HEL) weapons systems. DOD range safety officers now face the challenge of designing safe methods of testing HEL's on DOD ranges. In particular, safety officers need to ensure that diffuse and specular reflections from HEL system targets, as well as direct beam paths, are contained within DOD boundaries. If both the laser source and the target are moving, as they are for the Airborne Laser (ABL), a complex series of calculations is required and manual calculations are impractical. Over the past 5 years, the Optical Radiation Branch of the Air Force Research Laboratory (AFRL/HEDO), the ABL System Program Office, Logicon-RDA, and Northrup-Grumman, have worked together to develop a computer model called teh Laser Range Safety Tool (LRST), specifically designed for HEL reflection hazard analyses. The code, which is still under development, is currently tailored to support the ABL program. AFRL/HEDO has led an LRST Validation and Verification (V&V) effort since 1998, in order to determine if code predictions are accurate. This paper summarizes LRST V&V efforts to date including: i) comparison of code results with laboratory measurements of reflected laser energy and with reflection measurements made during actual HEL field tests, and ii) validation of LRST's hazard zone computations.
Simulation development for Laser Weapon Systems design and system trade analyses has progressed to new levels with the advent of object-oriented software development tools and PC processor capabilities. These tools allow rapid visualization of upcoming laser weapon system architectures and the ability to rapidly respond to what-if scenario questions from potential user commands. These simulations can solve very intensive problems in short time periods to investigate the parameter space of a newly emerging weapon system concept, or can address user mission performance for many different scenario engagements. Equally important to the rapid solution of complex numerical problems is the ability to rapidly visualize the results of the simulation, and to effectively interact with visualized output to glean new insights into the complex interactions of a scenario. Boeing has applied these ideas to develop a tool called the Satellite Visualization and Signature Tool (SVST). This Windows application is based upon a series of C++ coded modules that have evolved from several programs at Boeing-SVS. The SVST structure, extensibility, and some recent results of applying the simulation to weapon system concepts and designs will be discussed in this paper.
The Air Force Research Laboratory is interested in developing techniques for characterizing and discriminating satellites in Low and Geo-synchronous Earth Orbit (LEO and GEO). Certain materials used in constructing satellites possess unique polarization and wavelength dependent properties that may be useful for satellite discrimination and classification. In this work, we use the TASAT simulation to produce polarization renderings of detailed satellite models, with active and passive illumination, to predict polarization signatures of satellites in various Earth orbit scenarios. TASAT is a detailed tracking and controls simulation developed for modeling electro-optic tracking and imaging scenarios. Polarization renderings from passive illumination provide Stokes parameters representative of material polarization effects for the observed wavelength bands. Active illumination allows the incident polarization state to be changed. Thus, with suitable illuminating states and corresponding Stokes measurements, Mueller matrices may be formed from the active satellite returns, providing additional polarization signature information. Degree-of-polarization (DOP), diattenuation and retardance values calculated from the Stokes parameters and Mueuller matrices provide the polarization signature needed to test for satellite discrimination. We examine the variation of these polarization signatures for different satellite models situated in LEO and GEO observation scenarios. Signature variations for a visible and IR wavelength are considered. The results provide an indication of the feasibility of using material polarization properties for satellite discrimination to within the accuracy of our current materials database and polarization rendering capabilities.
Simulation development for EO Systems has progressed to new levels with the advent of COTS software tools such as Matlab/Simulink. These tools allow rapid reuse of simulation library routines. We have applied these tools to newly emerging Acquisition Tracking and Pointing (ATP) systems using many routines developed through a legacy to High Energy Laser programs such as AirBorne Laser, Space Based Laser, Tactical High Energy Laser, and The Air Force Research Laboratory projects associated with the Starfire Optical Range. The simulation architecture allows ease in testing various track algorithms under simulated scenes with the ability to rapidly vary system hardware parameters such as track sensor and track loop control systems. The atmospheric turbulence environment and associated optical distortion is simulated to high fidelity levels through the application of an atmospheric phase screen model to produce scintillation of the laser illuminator uplink. The particular ATP system simulated is a small transportable system for tracking satellites in a daytime environment and projects a low power laser and receives laser return from retro-reflector equipped satellites. The primary application of the ATP system (and therefore the simulation) is the determination of the illuminator beam profile, jitter, and scintillation of the low power laser at the satellite. The ATP system will serve as a test bed for satellite tracking in a high background during daytime. Of particular interest in this simulation is the ability to emulate the hardware modelogic within the simulation to test and refine system states and mode change decisions. Additionally, the simulation allows data from the hardware system tests to be imported into Matlab and to thereby drive the simulation or to be easily compared to simulation results.
TASAT is a detailed tracking and controls simulation developed for modeling electro-optic tracking and imaging scenarios. In our work, the polarization rendering capabilities of TASAT have been exploited to arrive at a methodology for modeling coherent polarized speckle backscatter from an illuminated object. For coherent illumination, we form a complex combination of the polarized rendered fields with random phase and propagate them to the far field to simulate polarized speckle. The speckle return is then analyzed using a four-channel polarimeter model to yield four Stokes parameter fields. We review the approach used in developing the TASAT polarization rendering model and its extension to obtain polarized speckle and Stokes parameter fields. We then show that the simulation provides results which agree with theory and which illustrate polarization measurement variations with object constituent material properties and different object models. Stokes parameter spatial statistics are used to analyze simulation results. Our results suggest that these statistics may be useful in characterizing the effective polarization properties of object materials and for providing a diagnostic signature for some object.s
The satellite imaging experiment (SIE) is a tracking and imaging experiment conducted during the last half of 1993 (June through December). We have obtained results from a high fidelity simulation called the time-domain analysis simulation for advanced tracking (TASAT) and also from the experiment itself that demonstrate closed loop passive tracking of stars and satellites. TASAT accurately predicts the residual track error for these objects by modeling the detailed physics of tracking through the atmosphere. In particular, an `orbit' appropriate to a star or satellite, an image rendering function, atmospheric point spread functions in the presence of adaptive optics, detailed sensors with noise, and high bandwidth active control loops all combine inside TASAT in a coupled, realistic fashion to predict active and passive cross sections, atmospheric tilt and higher order degradations, and residual track errors. We will discuss the present state of the simulation, results from TASAT that are germane to the SIE, and results from the experiment itself.
TASAT is a complete end-to-end system simulation of tracking and pointing systems. It can currently model ground-based (GB), space-based (SB), and kinetic energy weapon (KEW) systems at a very high level of fidelity to assess system performance and design tradeoffs. It is primarily a time-domain analysis tool, but it can also perform frequency-domain analysis for performance and stability analysis. TASAT was built as a modular set of interacting routines that permit much more than end-to-end analysis. Specifically, subsystem and even component level analyses are available. The code treats all aspects of tracking and pointing systems using realistic, anchored imagery in a multiwavelength simulation. Some of the functions modeled include orbit propagation or launch trajectories, image rendering with high fidelity scattering calculations, atmospheric or optical blur point-spread functions (PSFs), image formation via convolution, realistic focal plane sensors including dead bands, sensor noise, and analog-to- digital conversion, and control system response. For GB applications, the atmospheric model is a novel treatment of the time average PSF after application of an adaptive optics system. Also, atmospheric tilt is modeled exactly. The code has applications beyond GB, SB, and KEW systems. It will treat imaging systems, tactical and strategic surveillance systems, and radar range gating. The paper provides an overview of the simulation architecture and presents results from analyses of each of the principal systems modeled in TASAT.
A number of tracking and pointing applications require extremely precise referencing of the optical line of sight (LOS) relative to some small portion of the vehicle to be tracked. Since the referencing must be performed in a vehicle fixed coordinate system and the optical image is degraded due to disturbances such as atmospheric blurring, sensor noise, and diffraction, the referencing becomes quite difficult. These degradations, along with potentially coarse spatial quantization of the optical image [large pixel size driven by signal-to-noise ratio (SNR) considerations], also limit the ability of a human operator to interact with the optical system in real time to control the LOS. Several concepts to determine the real-time LOS control (vehicle intensity moments, neural networks, optical correlators, etc.) have been suggested in past studies, but generally have proven insufficiently sensitive or too complex to implement in a real-time system. The preliminary concept presented here centers on using a correlation tracker combined with a precomputed image sequence as a straightforward means to maintain a precision LOS. The concept employs the high SNR image within the correlation tracker reference map to make the relatively low bandwidth LOS corrections required. The corrections are determined by correlation of the tracker reference map imagery with a precomputed image sequence and thus provide the accuracy associated with the high SNR map image and high SNR precomputed image sequences. [The satellite tracking problem provides the tracker with viewing/aspect angle geometry, which is generally deterministic, within the uncertainty bounds of ephemeris and satellite attitude information, and thus stimulates the use of precomputed (simulated) imagery.] The precomputed image sequence provides the LOS control through registration of the desired vehicle fixed coordinate on the image with a fiducial point on the image array. We will discuss the theoretical basis, potential advantages, implementation, and performance [as determined by Time-Domain Analysis Simulation for Advanced Tracking (TASAT)] of the concept.