When using flat windows between an air medium and one with higher index of refraction, the surface becomes optically
active and a number of aberrations are induced. One affecting the optical control of a remotely-piloted underwater
vehicle is the apparent pincushion distortion resulting from Snell's law at the interface. Small wide-angle lenses typically
have the opposite problem, a barrel distortion caused by limitations in the number of lens surfaces and the constraints of
cost. An experimental calibration is described in which the barrel distortion of the lens compensated for most of the
inherent pincushion of the change in medium. ZEMAXTM models will be used to elucidate this phenomenon with a
published lens design.* With careful selection of the lens and additional corrector, the resultant image can be made
almost rectilinear, thus easing steering control and automatic target recognition.
KEYWORDS: Cameras, Image enhancement, Video, Image processing, Signal to noise ratio, Video surveillance, Video compression, Calibration, Image compression, Nonuniformity corrections
Underwater video has long been known for its murkiness and low contrast. Modern recording techniques only exacerbate
the problem because of the lossy compression methods applied to the video signals. Various image enhancement tools
have been implemented in software and hardware, but most of these assume that the user is processing uncorrupted video
and that the image has a long dynamic range. Simple preparatory steps can be applied before the enhancement to derive a
better image. These will be discussed, with some examples shown processed with a Matlab. GUI. A comparison will
be made with a commercial video-enhancement unit.
The Airborne Littoral Reconnaissance Technology (ALRT) program has successfully demonstrated the Wide-Field Airborne Laser Diode Array Illuminator (ALDAI-W). This illuminator is designed to illuminate a large area from the air with limited power, weight, and volume. A detection system, of which the ALDAI-W is a central portion, is capable of detecting surface-laid minefields in absolute darkness, extending the allowed mission times to night operations. This will be an overview report, giving processing results and suggested paths for additional development.
The Airborne Littoral Reconnaissance Technologies (ALRT) project has developed and successfully demonstrated a nighttime operational minefield detection capability using commercial off-the-shelf high-power Laser Diode Arrays (LDAs). The Coastal System Station's ALRT project, under funding from the Office of Naval Research (ONR), has been designing, developing, integrating, and testing commercial arrays using a Cessna airborne platform over the last several years. This has led to the development of three test bed variants, as reported on last year: the Airborne Laser Diode Array Illuminator prototype (ALDAI-P), the original commercial array version (ALDAI-C), and the most recent wide field-of-view commercial version (ALDAI-W). Using the ALDAI-W variant because of its increased operational capabilities with higher altitudes and wider field of views, ALRT recently demonstrated nighttime operation by detecting minefields over several background variations, expanding Naval reconnaissance capabilities that had been previously limited to daytime operation. This paper describes the demonstration and shows results of the ALDAI-W test.
Multispectral, hyperspectral, and polarization filters have been shown to provide additional discriminants when searching for mines and other obstacles, but they demand more illumination for the sensing system. Conventional CCD video cameras, when used through such filters, fail at sunset or soon after. It is tempting to employ an automatic-gain intensified camera to push this time deeper into the night (especially with artificial illumination) but relating the response between different images or different channels, possibly taken at different gain, is not as straightforward as it is with a bare-silicon CCD. Over the last several years, Coastal Systems Station has developed a set of simple system characterization and calibration procedures that enable using an intensified video camera as a serviceable imaging radiometer. Parameters from this calibration procedure are easily inserted into predictive models and images are directly comparable using them. These methods will be described, especially as they apply to the camera used in the recent Airborne Laser Diode Array Illuminator (ALDAI) tests. Minimum data that must be monitored in the camera will also be listed.
The Airborne Littoral Reconnaissance Technologies (ALRT) Project has demonstrated a nighttime operational minefield detection capability using commercial off-the-shelf high-power Laser Diode Arrays (LDAs). Historically, optical aerial detection of minefields has primarily been limited to daytime operations but LDAs promise compact and efficient lighting to allow for enhanced reconnaissance operations for future mine detection systems. When combined with high-resolution intensified imaging systems, LDAs can illuminate otherwise unseen areas. Future wavelength options will open the way for active multispectral imaging with LDAs. The Coastal Systems Station working for the Office of Naval Research on the ALRT project has designed, developed, integrated, and tested both prototype and commercial arrays from a Cessna airborne platform. Detailed test results show the ability to detect several targets of interest in a variety of background conditions. Initial testing of the prototype arrays, reported on last year, was completed and further investigations of the commercial versions were performed. Polarization-state detection studies were performed, and advantageous properties of the source-target-sensor geometry noted. Current project plans are to expand the field-of-view coverage for Naval exercises in the summer of 2003. This paper describes the test collection, data library products, array information, on-going test analysis results, and future planned testing of the LDAs.
During the recent nighttime testing of the Airborne Laser Diode Array Illuminator (ALDAI) system in a humid and aerosol-rich environment, it was found that the laser backscatter competed with the ground-reflected signal at higher altitudes. Such an effect was stronger in the polarization channel parallel to the outgoing illumination. Theoretical calculations indicated that most of the scattering was taking place in the first few feet of overlap between the outgoing illumination and the field of regard. To test this, the laser was pointed into a geometry which had no target field, that scattered solely into the air. Backscattered returns were reduced and plotted versus source-receiver separation distance.
The inferred line-spread function is an easy technique for measuring orthogonal components of the two-dimensional modulation transfer function (MTF), even from the air. However, it has been most commonly used for cameras for which the resolution is nowhere near the Nyquist frequency. The purpose of such limitation is so that the pixel sampling does not have a serious consequence on the measurement of the MTF. The binning capability of the purely digital DIPOL camera is used to demonstrate that using this method even in moderately oversampled systems does not impact results as long as certain averaging techniques are used. A brief tutorial of the normalization and pitfalls of the method will also be given so that this powerful and simple measurement will become more widely used. Example images will also be shown of mine simulators, together with polarization-product images.
The Joint Mine Detection Technology (JMDT) project, following successful field-based testing of its new Tunable Filter Multispectral Camera (TFMC) has now completed initial Airborne Testing of the TFMC at both the Coastal Systems Station and Eglin Air Force Base sites. An overview of the testing is presented along with the investigations into the advantages of a system utilizing the TFMC in airborne operational scenarios. The TFMC-like tuning flexibility was flight-tested using optimized wavelength combinations, which were found using field test data, over a variety of backgrounds and altitudes. The data revealed the suitability of background tuning, polarization, and mechanically co-registered channels as benefits to multispectral target detection. The data were also compared to that collected with an IMC-201 camera, using the six filters of the Coastal Battlefield Reconnaissance and Analysis (COBRA) Advanced Technology Demonstration (ATD) system, in order to determine improvements over existing capabilities.
A tactical unmanned aerial vehicle-size illumination system for enhanced mine detection capabilities has been designed, developed, integrated, and tested at the Coastal Systems Station. Airborne test flights were performed from June 12, 2001 to February 1, 2002. The Airborne Laser Diode Array Illuminator uses a single-wavelength compact laser diode array stack to provide illumination and is coupled with a pair of intensified CCD video cameras. The cameras were outfitted with various lenses and polarization filters to determine the benefits of each of the configurations. The first airborne demonstration of a laser diode illumination system is described and its effectiveness to perform nighttime mine detection operations is shown.
An automated band selection algorithm suitable for real-time application with fixed filter multispectral cameras is presented for multispectral target detection. Fixed filter multispectral cameras collect all bands regardless of the background. Background adaptive band is the selection of a subset of the bands for target detection processing. Fixed filter systems typically include a small number of general-purpose bands. The bands are chosen to enhance target-background contrast but are not keyed to specific target features. In some situations it is unlikely that all bands contribute to target discrimination. Using only a subset of the available bands can decrease false alarms while maintaining target detection performance and reduced processing requirements. The advantages are demonstrated using six band multispectral data and two distinct background categories.
The Coastal Systems Station, in concert with Xybion Corp. has developed a tunable-filter multispectral imaging sensor for use in airborne reconnaissance. The sensor was completed in late 1999, and laboratory characterization and field- testing has been conducted since. The Tunable Filter Multispectral Camera (TFMC) is an intensified, gated, and tunable multispectral imaging camera that provides three simultaneous channels of 10-bit digital and 8-bit analog video from the near-UV to the near-IR. Exposure and gain can be automatically or manually controlled for each channel, and response has been linearized for approximate radiometric use. Additionally, each of the three channels as a separate programmable liquid-crystal tunable filter with a selectable center wavelength settings to which can be applied 100 different retardances for each of three channels. This paper will present setups, analysis methods, and preliminary results for both the laboratory characterization and field- testing of the TFMC. Laboratory objectives include measures of sensitivity, noise, and linearity. Field testing objectives include obtaining the camera response as the lighting conditions approached sunset of a clear day, signal-to-clutter ratios for a multiplicity of channel wavelength combinations and polarizations against several backgrounds, and resolution performance in field-conditions.
JMDT is a Navy/Marine Corps 6.2 Exploratory Development program that is closely coordinated with the 6.4 COBRA acquisition program. The objective of the program is to develop innovative science and technology to enhance future mine detection capabilities. The objective of the program is to develop innovative science and technology to enhance future mine detection capabilities. Prior to transition to acquisition, the COBRA ATD was extremely successful in demonstrating a passive airborne multispectral video sensor system operating in the tactical Pioneer unmanned aerial vehicle (UAV), combined with an integrated ground station subsystem to detect and locate minefields from surf zone to inland areas. JMDT is investigating advanced technology solutions for future enhancements in mine field detection capability beyond the current COBRA ATD demonstrated capabilities. JMDT has recently been delivered next- generation, innovative hardware which was specified by the Coastal System Station and developed under contract. This hardware includes an agile-tuning multispectral, polarimetric, digital video camera and advanced multi wavelength laser illumination technologies to extend the same sorts of multispectral detections from a UAV into the night and over shallow water and other difficult littoral regions. One of these illumination devices is an ultra- compact, highly-efficient near-IR laser diode array. The other is a multi-wavelength range-gateable laser. Additionally, in conjunction with this new technology, algorithm enhancements are being developed in JMDT for future naval capabilities which will outperform the already impressive record of automatic detection of minefields demonstrated by the COBAR ATD.
KEYWORDS: Unmanned aerial vehicles, Video, Video surveillance, Land mines, Global Positioning System, Surveillance, Multispectral imaging, Target detection, Reconnaissance, Reconnaissance systems
The Coastal Battlefield Reconnaissance and Analysis)COBRA) system described here was a Marine Corps Advanced Technology Demonstration (ATD) development consisting of an unmanned aerial vehicle (UAV) airborne multispectral video sensor system and ground station which processes the multispectral video data to automatically detect minefields along the flight path. After successful completion of the ATD, the residual COBRA ATD system participated in the Joint Countermine (JCM) Advanced Concept Technology Demonstration (ACTD) Demo I held at Camp Lejeune, North Carolina in conjunction with JTFX97 and Demo II held in Stephenville, Newfoundland in conjunction with MARCOT98. These exercises demonstrated the COBRA ATD system in an operational environment, detecting minefields that included several different mine types in widely varying backgrounds. The COBRA system performed superbly during these demonstrations, detecting mines under water, in the surf zone, on the beach, and inland, and has transitioned to an acquisition program. This paper describes the COBRA operation and performance results for these demonstrations, which represent the first demonstrated capability for remote tactical minefield detection from a UAV. The successful COBRA technologies and techniques demonstrated for tactical UAV minefield detection in the Joint Countermine Advanced Concept Technology Demonstrations have formed the technical foundation for future developments in Marine Corps, Navy, and Army tactical remote airborne mine detection systems.
Noise and sensitivity measurements are compared for five intensified cameras used for airborne sensing of mines. Individual camera variations are discussed, as well as global similarities useful in camera modeling. The cameras generally have behaviors that closely match each other, but differ in sensitivity by factors of up to 12 for equal noise. The implication is that these intensified cameras-being short production-run items-display a great deal of individuality and that each camera must be carefully characterized before use. Reciprocity (product of radiance and exposure) and the variation of noise with radiance were also tested and found to be flat over three orders of magnitude, thus simplifying modeling.
Land mines usually appear in a wide-field airborne camera as small, low contrast objects. Factors such as the transfer function at relatively high spatial frequencies (as compared with half the pixel frequency) become critically important for detection as well as predicting performance for a notional detection system. An experimental method is described (using a general-purpose filter wheel) for obtaining better resolution and contrast values. MathcadTM documents that automate this process somewhat will also be presented.
This paper reports results of a recent field experiment using a prototype system to evaluate the acousto-optic tunable filter polarimetric hyperspectral imaging technology for target detection applications.
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