Space based bifocal relay mirrors are potentially an enabling/enhancing piece of any architecture making use of long-range laser propagation. Inherent in the bifocal concept is dual line of sight control. This is especially challenging in this space-based application due to spacecraft attitude control issues. This paper presents a summary of the research into acquisition, tracking, pointing (ATP) and control technologies relevant to a bifocal relay mirror system as well as the development of a laboratory experimental test bed to integrate the advanced optics systems onto a Three-axis spacecraft simulator. The relay geometry includes a cooperative source and either a cooperative or non-cooperative target depending on the application. The described test bed is a joint effort with the Air Force Research Laboratory (optics) and the Naval Postgraduate School (spacecraft simulator).
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.
In last years proceedings, the above authors reported a basic limitation on the maximum effective bandwidth when tracking through atmospheric turbulence. This limitation, called the optical frequency, was shown to be an upper limit on tilt detection. This paper will further expand on this fundamental limitation. Further testing at the MIT/Lincoln Laboratory has provided more insight into the optical frequency as well as other tracking limitations. It will be shown in this paper that scintillation appears to be dominant above the optical frequency and that by wisely selecting the bandwidth of the tracking system, one can exclude some of the noise of scintillation, while still performing the best possible tracking.
Tracking through a turbulent atmosphere poses several challenging problems. The authors have recently conducted a series of tracking tests at a MIT/Lincoln Laboratories facility where a complete tracking and adaptive optics system is available in a laboratory. The atmosphere is simulated using seven precision rotating phase screens. A great deal has been learned about tracking algorithms and their response under a scintillated atmosphere. Data will be shown to describe a key limitation to high bandwidth tracking. This effect, called the `Optical Frequency', appears to be an upper bound on track bandwidth when using an image based tracking system.
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.
The Absolute Radiometric Code (ARC) is a collection of Matlab functions tied together under a Matlab Graphical User Interface (GUI). ARC was developed as part of the Satellite Imaging Experiment conducted by the Air Force Research Laboratory at Kirtland, AFB, in order to get fast estimates of the Optical Cross Sections of various satellites. ARC uses multiple star measures to calculate the atmospheric and optical transmission of the system. The transmissions are then used to compute the optical cross section of an object. Generally, the optical transmission of a sensor system can be characterized quite well, so it serves as a sanity check on all ARC results. The atmospheric transmission changes considerably from night to night and even from hour to hour on the same night. ARC uses a collection of calibration stars at various elevation angles to determine the atmospheric transmission through the viewing times. The star calibration is generally taken several times during the experiment period.
Tracking through a turbulent atmosphere has several challenges. If the target is extended, such as a large target being illuminated with laser energy from the tracking aperture, the problems of scintillation and anisoplanatism cause significant concerns. The 'blotches' caused by scintillation can be interpreted by the tracking algorithms as tilt and incorrectly applied to the steering mirror. Similarly, anisoplanatism imparts tilt components from separated points, that may be independent, and not act as coherent tilt that is possible to correct with one mirror. The authors have been involved in a study that compares the performance of 14 different tracking algorithms under scintillated conditions. Image data, available from Lincoln Laboratory, has been used to calculate tilt from the different algorithms and do comparisons using Power Spectral Density analysis. The results show that different algorithms have significantly different performance characteristics.
Optical systems which transmit power density optical radiation present real design and implementation challenges. Sophisticated multi-disciplinary methods have been developed to support the design and analysis of the interaction of the optical radiation field and the transmissive and reflective optics which the field interacts with. The primary effect which results are distributed thermal deformation of the optical surfaces resulting in undesirable aberrations, loss of Strehl and beam 'quality'. Modern optical systems involve the use of adaptive optical systems which mitigate the effect of these aberrations within their spatial and temporal correction bandwidths. The authors have been involved in a study, which addresses the closed loop correction of thermally induced beam train aberrations using computational numerical models to describe the wavefront effects. This paper reports on the analysis methods used and describes some preliminary results.
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.