In the paper, a theoretical model is established to calculate the energy spectrum response (ESR) of x-ray MCP image converters, and an experiment for the measurement of the ESR is designed and finished. The model predictions are in good agreement with experiment.
Image intensifying cameras have been found to be extremely useful in low-light-level (LLL) scenarios including military night vision and civilian rescue operations. These sensors utilize the available visible region photons and an amplification process to produce high contrast imagery. It has been demonstrated that processing techniques can further enhance the quality of this imagery. For example, fusion with matching thermal IR imagery can improve image content when very little visible region contrast is available. To aid in the improvement of current algorithms and the development of new ones, a high fidelity simulation environment capable of producing radiometrically correct multi-band imagery for low- light-level conditions is desired. This paper describes a modeling environment attempting to meet these criteria by addressing the task as two individual components: (1) prediction of a low-light-level radiance field from an arbitrary scene, and (2) simulation of the output from a low- light-level sensor for a given radiance field. The radiance prediction engine utilized in this environment is the Digital Imaging and Remote Sensing Image Generation (DIRSIG) model which is a first principles based multi-spectral synthetic image generation model capable of producing an arbitrary number of bands in the 0.28 to 20 micrometer region. The DIRSIG model is utilized to produce high spatial and spectral resolution radiance field images. These images are then processed by a user configurable multi-stage low-light-level sensor model that applies the appropriate noise and modulation transfer function (MTF) at each stage in the image processing chain. This includes the ability to reproduce common intensifying sensor artifacts such as saturation and 'blooming.' Additionally, co-registered imagery in other spectral bands may be simultaneously generated for testing fusion and exploitation algorithms. This paper discusses specific aspects of the DIRSIG radiance prediction for low- light-level conditions including the incorporation of natural and man-made sources which emphasizes the importance of accurate BRDF. A description of the implementation of each stage in the image processing and capture chain for the LLL model is also presented. Finally, simulated images are presented and qualitatively compared to lab acquired imagery from a commercial system.
Absorption type solar blind ultraviolet (SBUV) filters with transmittance levels of 10 - 20%, FWHM band widths of 16 - 20 nm and blocking levels exceeding 12 OD have been developed for use with image intensified CCD (ICCD) cameras with CsTe, RbTe and CsRbTe photocathodes. Solar blind UV ICCD cameras equipped with these filters produce no measurable signals when directly viewing the sun at noon time in a summer day. Images of fire, gunfire flashes and high voltage transmission line corona were obtained in full daylight at high signal to noise ratio, by using these imagers. A solar blind filter was constructed for BCCD camera with UVAR coating. Images of fire and low power light sources can be obtained in full daylight when an appropriate solar blind filter is coupled to a BCCD camera. Results with bispectral imagers, combining solar blind UV images and images of the visible scene, are reported. For visualization of very weak UV sources use of solar blind image intensifiers is of advantage. This advantage can in principle also be achieved by EBCCD technology.
A high performance Gen III Image Intensifier has been integrated into a high performance cooled integrating CCD Camera. Performance measurements have been performed on the system showing the ultimate performance at the lowest light levels possible. Performance extension of the camera has been achieved with very little change in image quality over the unintensified camera.
Electron Bombarded Charge Coupled Devices (EBCCD) which utilize a high performance Gallium Arsenide (GaAs) photocathode have been fabricated and characterized for performance and tube operating life. The EBCCD utilized an 11 mm diagonal, backside illuminated, frame transfer CCD compatible with RS170 video output. The CCD incorporated lateral anti-blooming structures optimized for backside operation. The EBCCD tube was proximity focused and operated with high gain (greater than 150) at low electron landing voltages (less than 2 kV). The EBCCD was integrated in a gated camera system with fast rise and fall times (less than 50 ns). Predicted operating life in a gated camera system as determined by accelerated tests is in excess of 12,000 hours, limited by photocathode degradation.
Many of the earlier technical problems related to the development of electron bombarded CCD (EBCCD) cameras are being solved by new methods. The early work on EBCCD cameras is reviewed, and results from recent developments in this field are discussed. It appears that sufficient progress has been made to state that the new era of EBCCD imager technology for low-light-level and photon-counting applications has begun.
Photon counting systems were originally developed for astronomy, initially by the astronomical community. However, a major application area is in the study of luminescent probes in living plants, fishes and cell cultures. For these applications, it has been necessary to develop camera system capability at very low light levels -- a few photons occasionally -- and also at reasonably high light levels to enable the systems to be focused and to collect quality images of the object under study. The paper presents new data on MTF at extremely low photon flux and conventional ICCD illumination, counting efficiency and dark noise as a function of temperature.
During the noon hour on October 9, 1997 an extremely bright fireball (approximately -21.5 in stellar magnitude putting it into the class of a super-bolide) was observed over western Texas with visual sightings from as far away as Arizona to northern Mexico and even in northern New Mexico over 300 miles away. This event produced tremendously loud sonic boom reports in the El Paso area. It was also detected locally by 4 seismometers which are part of a network of 5 seismic stations operated by the University of Texas at El Paso (UTEP). Subsequent investigations of the data from the six infrasound arrays used by LANL (Los Alamos National Laboratory) and operated for the DOE (Department of Energy) as a part of the CTB (Comprehensive Test Ban) Research and Development program for the IMS (International Monitoring System) showed the presence of an infrasonic signal from the proper direction at the correct time for this super-bolide from two of our six arrays. Both the seismic and infrasound recordings indicated that an explosion occurred in the atmosphere at source heights from 28 - 30 km, having its epicenter slightly to the northeast of Horizon City, Texas. The signal characteristics, analyzed from approximately 0.1 to 5.0 Hz, include a total duration of approximately 4 min (at Los Alamos, LA) to greater than approximately 5 min at Lajitas, Texas, TXAR, another CTB IMS array operated by E. Herrin at Southern Methodist University (SMU) for a source directed from LA toward approximately 171 - 180 deg and from TXAR of approximately 321 - 4 deg respectively from true north. The observed signal trace velocities (for the part of the recording with the highest cross-correlation) at LA ranged from 300 - 360 m/sec with a signal velocity of 0.30 plus or minus 0.03 km/sec, implying a Stratospheric (S Type) ducted path. The dominant signal frequency at LA was from 0.20 to 0.80 Hz, with a peak near 0.3 Hz. These highly correlated signals at LA had a very large, peak to peak, maximum amplitude of 21.0 microbars (2.1 Pa). Our analysis, using several methods that incorporate various observed signal characteristics, total distance traveled, etc., indicates that the super-bolide probably had a source energy in the range between 10 - 100 tons (TNT equivlaent). This is somewhat smaller than the source energy estimate made using U.S. DoD satellite data (USAF news release, June 8, 1998).
A portable low earth orbit satellite (LEO) tracking mount is described which has dimensions of 21' X 15' X 10' and weighs 58 pounds. Using 22 bit encoders on 9.5' worm gears, an integral microcontroller is capable of adjustable slew rates to six degrees per second. With a CCD and tracking software LEO pointing is demonstrated on f/10 eight inch telescope to less than 8 urad for periods of 10 seconds, and 50 urad for entire orbit passes. A closed loop one Hz video tracker is also described with automatic tracking of mag 7/8 satellites using a 12' telescope. Requiring only one operator, this system can be transported by a small car and be operational in a few hours. Possible uses and other recent work is also explained.
The U.S. Space Command maintains a positional catalog of over 8000 man-made space objects. The basis of this catalog is the observational data collected by a worldwide network of radars and optical sites known collectively as the U.S. Space Surveillance Network which is operated by the U.S. Air Force, Army and Navy. This network was developed following the launch of the first SPUTNIK in 1957 at which time the Air Force was given the task by the U.S. Congress of maintaining a catalog of all the detectable objects in space-active and inactive satellites, spent boosters and other miscellaneous jetsam that constitute the dangerous portion of the space debris population. The fundamental mission of the space surveillance network is to keep reliable, up-to-date information on all detectable resident space objects (RSOs) in space. The sensors in the network are primarily ground-based except for a recent sensor that was deployed in space. The capabilities of the network are described in this paper. Specific examples will be used to demonstrate that the space surveillance network constitutes a capable and extensive remote sensing system for resident space objects and debris.
Orbital debris in low-Earth orbit in the size range from 1 to 10 cm in diameter can be detected but not tracked reliably enough to be avoided by spacecraft. It can cause catastrophic damage even to a shielded spacecraft. With adaptive optics, a ground-based pulsed laser ablating the debris surface can produce enough propulsion in several hundred pulses to cause such debris to reenter the atmosphere. A single laser station could remove all of the 1 - 10 cm debris in three years or less. A technology demonstration of laser space propulsion is proposed which would pave the way for the implementation of such a debris removal system. The cost of the proposed demonstration is comparable with the estimated annual cost of spacecraft operations in the present orbital debris environment. Orbital debris is not the only space junk that is deleterious to the Earth's environment. Collisions with asteroids have caused major havoc to the Earth's biosphere many times in the ancient past. Since the possibility still exists for major impacts of asteroids with the Earth, it shown that it is possible to scale up the systems to prevent these catastrophic collisions providing sufficient early warning is available from new generation space telescopes plus deep space radar tracking.
All long-duration spacecraft in low-earth-orbit are subject to high speed impacts by meteoroids and orbital debris. As a result, the threat of damage from such high-speed impacts is a major design consideration in the development and construction of long duration earth-orbiting spacecraft. Recent studies have shown that the nature of a spacecraft wall perforation can range from a flat hole with a jagged edge (i.e. a so-called cookie-cutter hole) to a hole accompanied by bulging, cracking, and petaling. If a cracking event were to occur on-orbit, unstable crack growth could develop which could lead to an unzipping of the impacted spacecraft module. It is, therefore, imperative to be able to determine whether or not a spacecraft wall perforation will be accompanied by petaling and cracking. This paper presents the results of a study whose objective was to develop an empirical model that could be used to determine whether a spacecraft wall perforation would be in the form of a petaled hole or a cookie-cutter hole. A petaling limit function was developed to predict the onset of petaling in terms of impact conditions and spacecraft wall system geometry.
This paper presents a study of the short-term collision risk posed to the resident Earth-orbiting population by fragments generated in hypothetical explosions in the geosynchronous ring. Debris clouds resulting from such explosions contain constrictions, which are regions of high fragment density caused by the Earth's central gravitational attraction. The highest density constriction, the pinch point, is almost fixed inertially and occurs at the breakup point. Hence most of the resident geosynchronous population flies near it. In this study, the computer programs IMPACT and DEBRIS were used to assess collision risk. Program IMPACT was used to model the explosions, and program DEBRIS was used to perform short-term debris cloud propagation and compute collision risk. The four cases considered are all based on the explosion of a generic liquid apogee kick stage which has never happened in the geosynchronous ring. The satellites for which collision risk was assessed were taken from an August 1995 update of the USSPACECOM Satellite Catalog. It was found that short-term collision risk was low, even though the entire geosynchronous population flies near the pinch point. This occurs because the fragments are spread over very large volumes in geosynchronous orbit. However, approximate extrapolation of these results indicates an increased collision risk in the intermediate time frame of several months to a year. Due to the lack of a cleansing perturbation in the geostationary region, collisions are statistically likely to occur.
This paper is the culmination of the research effort which was reported on last year while still in-progress. As previously reported, statistical methods for expressing the impact risk posed to space systems in general [and the International Space Station (ISS) in particular] by other resident space objects have been examined. One of the findings of this investigation is that there are legitimate physical modeling reasons for the common statistical expression of the collision risk. A combination of statistical methods and physical modeling is also used to express the impact risk posed by reentering space systems to objects of interest (e.g., people and property) on Earth. One of the largest uncertainties in the expressing of this risk is the estimation of survivable material which survives reentry to impact Earth's surface. This point was demonstrated in dramatic fashion in January 1997 by the impact of an intact expendable launch vehicle (ELV) upper stage near a private residence in the continental United States. Since approximately half of the missions supporting ISS will utilize ELVs, it is appropriate to examine the methods used to estimate the amount and physical characteristics of ELV debris surviving reentry to impact Earth's surface. This report details reentry survivability estimation methodology, including the specific methodology used by ITT Systems' (formerly Kaman Sciences) 'SURVIVE' model. The major change to the model in the last twelve months has been the increase in the fidelity with which upper- atmospheric aerodynamics has been modeled. This has resulted in an adjustment in the factor relating the amount of kinetic energy loss to the amount of heating entering and reentering body, and also validated and removed the necessity for certain empirically-based adjustments made to the theoretical heating expressions. Comparisons between empirical results (observations of objects which have been recovered on Earth after surviving reentry) and SURVIVE estimates are presented for selected generic upper stage or spacecraft components, a Soyuz launch vehicle second stage, and for a Delta II launch vehicle second stage and its significant components. Significant similarity is demonstrated between the type and dispersion pattern of the recovered debris from the January 1997 Delta II 2nd stage event and the simulation of that reentry and breakup.