المؤلفون: Cain, Stephen C.

المصدر: Faculty Publications

مصطلحات موضوعية: Fourier optics, Imaging systems, Optical systems, Ptychography, Wave propagation, Optics

الوصف: This paper describes a new Fourier propagator for computing the impulse response of an optical system with a curved focal plane array, while including terms ignored in Fresnel and Fraunhofer calculations. The propagator includes a Rayleigh-Sommerfeld diffraction formula calculation from a distant point through the optical system to its image point predicted by geometric optics on a spherical surface. The propagator then approximates the neighboring field points via the traditional binomial approximation of the Taylor series expansion around that field point. This technique results in a propagator that combines the speed of a Fourier transform operation with the accuracy of the Rayleigh-Sommerfeld diffraction formula calculation and extends Fourier optics to cases where the receiver plane is a curved surface. Bounds on the phase error introduced by the approximations are derived, which show it should be more widely applicable than traditional Fresnel propagators. Guidance on how to sample the pupil and detector planes of a simulated imaging system is provided. This report concludes by showing examples of the diffraction patterns computed by the new technique compared to those computed using the Rayleigh-Sommerfeld technique in order to demonstrate the utility of the propagator.

وصف الملف: application/pdf

العلاقة: https://scholar.afit.edu/facpub/796Test; https://scholar.afit.edu/context/facpub/article/1811/viewcontent/osac_3_6_1579.pdfTest

الإتاحة: https://doi.org/10.1364/OSAC.394977Test;
https://doi.org/10.1364/OSAC.394977Test
https://scholar.afit.edu/facpub/796Test
https://scholar.afit.edu/context/facpub/article/1811/viewcontent/osac_3_6_1579.pdfTest

المؤلفون: Lin, Kevin J.

المصدر: Theses and Dissertations

مصطلحات موضوعية: Structural Health Monitoring, Acoustic Wave Propagation, Finite Element Method, Non-Destructive Testing, Mathematics, Structures and Materials

الوصف: In 2008, the Department of Defense published a guidebook for a methodology named Condition-Based Maintenance Plus (CBM+) which capabilities include improving productivity, shortening maintenance cycles, lowering costs, and increasing availability and reliability. This push replaces existing inspection criteria, often conducted as non-destructive testing (NDT), with structural health monitoring (SHM) systems. The SHM system addressed utilizes guided Lamb waves generated by piezoelectric wafer active sensors (PWAS) to detect the existence, size, and location of damage from through-thickness cracks around a rivet hole. The SHM field lacks an experiment testing how small changes in receiver sensor distances affect damage detection. In addition, prior research has shown that transmitter and receiver PWAS angles significantly affected the received signal. Experiments here used existing damage detection metrics to establish thresholds for detection. Tests with two transmitter angles θ = 9°; 27° and three receiver distances, linearly incremented by 5mm, illustrated that damage detection capabilities significantly changed as the receiver distances were incremented at both 50mm and 1000mm transmitter distances. For 1000mm, the PWAS was able to detect the damage at certain geometries. This work validates of the PWAS detection capabilities for small changes and motivates further pursuits for varying PWAS geometries for long distances.

وصف الملف: application/pdf

العلاقة: https://scholar.afit.edu/etd/2192Test; https://scholar.afit.edu/context/etd/article/3193/viewcontent/AFIT_ENC_MS_19_M_001_Lin_K.pdfTest

الإتاحة: https://scholar.afit.edu/etd/2192Test
https://scholar.afit.edu/context/etd/article/3193/viewcontent/AFIT_ENC_MS_19_M_001_Lin_K.pdfTest

المؤلفون: Hyde, Milo W., IV, Bose-Pillai, Santasri

المصدر: Faculty Publications

مصطلحات موضوعية: Diffusers, Imaging systems, Light wave propagation, Optics

الوصف: High-spatial-frequency optical fields or sources are often encountered when simulating directed energy, active imaging, or remote sensing systems and scenarios. These spatially broadband fields are a challenge in wave optics simulations because the sampling required to represent and then propagate these fields without aliasing is often impractical. To address this, two spatial filtering techniques are presented. The first, called Fresnel spatial filtering, finds a spatially band-limited source that, after propagation, produces the exact observation plane field as the broadband source over a user-specified region of interest. The second, called statistical or quasihomogeneous spatial filtering, finds a spatially band-limited source that, after propagation and over a specified region of interest, yields an observation plane field that is statistically representative of that produced by the original broadband source. The pros and cons of both approaches are discussed in detail. A wave optics simulation of light transiting a ground glass diffuser and then propagating to an observation plane in the near-zone is performed to validate the two filtering approaches.

وصف الملف: application/pdf

العلاقة: https://scholar.afit.edu/facpub/118Test; https://scholar.afit.edu/context/facpub/article/1123/viewcontent/083107_1.pdfTest

الإتاحة: https://doi.org/10.1117/1.OE.56.8.083107Test
https://scholar.afit.edu/facpub/118Test
https://scholar.afit.edu/context/facpub/article/1123/viewcontent/083107_1.pdfTest

المؤلفون: Meier, Carlos A.

المصدر: Theses and Dissertations

مصطلحات موضوعية: Radar receiving apparatus, Radar--Interference, Radio wave propagation, Signal Processing

الوصف: It has always been an issue for an armed force, or government, to obtain complete radar coverage over an area of interest. Generally, this objective remains unaccomplished due to geographical, technical, and/or operational reasons (meaning topographic obstacles, transmitted power, extreme isolation, hierarchy of objectives, etc.) The fact of having vast areas of territory beyond radar coverage can be decisive in an armed conflict. With the recent resurgence of bistatic radar theory and applications, now in the form of Passive Coherent Location (PCL) systems, using existing signal sources (TC and Radio Stations) it is possible to decrease the blind zones of a country's air defense system, in a somehow economic and effective way. The general purpose of this thesis is to develop a methodology to determine possible receiver locations needed to implement a PCL system, while emphasizing low altitude coverage, specific terrain and atmospheric information, available signal sources and the needed for coverage.

وصف الملف: application/pdf

العلاقة: https://scholar.afit.edu/etd/4455Test; https://scholar.afit.edu/context/etd/article/5457/viewcontent/AFIT_GE_ENG_02M_17_Meier_C_ADA401442.pdfTest

الإتاحة: https://scholar.afit.edu/etd/4455Test
https://scholar.afit.edu/context/etd/article/5457/viewcontent/AFIT_GE_ENG_02M_17_Meier_C_ADA401442.pdfTest

المؤلفون: Allnutt, Richard A.

المصدر: Theses and Dissertations

مصطلحات موضوعية: Emergency communication system, Radio--Antennas, Radio wave propagation, Electrical and Electronics

الوصف: The NVIS system characterized in this work was designed to eliminate skip propagation by optimizing the design for contiguous coverage. The NVIS technique involves use of transmission and receiving antennas that create nearly vertical propagation and continuous coverage from the transmitter to a distance of 200 miles. Man portable, very low power transceivers (5 watts maximum) and horizontal dipole antennas five feet above the ground are used in an NVIS communication system for this work. The system is designed for the purpose of supporting communication with emergency workers in areas where other communication is difficult. Digital and analog effectiveness are compared at this low power range, and the human factors of communication error are described.

وصف الملف: application/pdf

العلاقة: https://scholar.afit.edu/etd/4466Test; https://scholar.afit.edu/context/etd/article/5468/viewcontent/AFIT_GE_ENG_02M_32_Allnutt_R_ADA401383.pdfTest

الإتاحة: https://scholar.afit.edu/etd/4466Test
https://scholar.afit.edu/context/etd/article/5468/viewcontent/AFIT_GE_ENG_02M_32_Allnutt_R_ADA401383.pdfTest

المؤلفون: Pollock, Peter R.

المصدر: Theses and Dissertations

مصطلحات موضوعية: Artificial satellites in telecommunications signal processing, Radio waves--Diffraction, Tropospheric radio wave propagation, Signal Processing

الوصف: This study focused on multipath communication propagation impairments to the LEOSAT communications channel. Two terrain diffraction models, based on the geometric theory of diffraction (GTD), were developed and applied to the space-to-ground communications channel. These models were used to predict the impact of terrain on the performance of three LEOSAT communication systems with designs based on the Iridium, Globalstar and Orbcomm implementations. The study verified the feasibility of applying models based on the GTD rather than empirical or statistical models, to approximate the effect of propagating signals over terrain. Both models confirm that signal blockage and multipath propagation, due to terrain diffraction, can be significant considerations for designers and users of such systems.

وصف الملف: application/pdf

العلاقة: https://scholar.afit.edu/etd/4677Test; https://scholar.afit.edu/context/etd/article/5681/viewcontent/AFIT_GSO_ENG_01M_01_Pollock_P_ADA392030.pdfTest

الإتاحة: https://scholar.afit.edu/etd/4677Test
https://scholar.afit.edu/context/etd/article/5681/viewcontent/AFIT_GSO_ENG_01M_01_Pollock_P_ADA392030.pdfTest

المؤلفون: Rounsavall, Paul C.

المصدر: Theses and Dissertations

مصطلحات موضوعية: Photonics, Spiral antennas, Antenna radiation patterns, Finite element analysis, Energy gaps, Electromagnetic wave propagation, Electrical and Electronics, Signal Processing

الوصف: Microelectromechanical Systems (MEMS) is a rapidly growing technology that lends itself particularly well to optical applications. An example of an optical MEMS device is the piston-action mirror that modulates the phase of reflected light. The phase of reflected light can be varied using thermal or electrostatic actuation to control the position of the mirror. In previous research, a modulation method to control thermally actuated mirrors was developed. This thesis presents the development, implementation, fabrication, and testing of a complimentary metal oxide semiconductor (CMOS) controller capable of directly interfacing between a digital system, such as a computer, and an electrostatically actuated MEMS mirror device. The controller pulse width modulates a supply voltage to vary the power applied to the MEMS mirror device. The MEMS mirror device responds with negligible position ripple to the applied average power of the pulse width modulation signal. By varying the duty cycle of the pulse width modulation signal, the position of the mirror is varied. This controller can be adapted to control other electrostatically actuated devices using the design and methodology described in this thesis. The implementation of this controller is a step toward the monolithic integration of a MEMS deformable mirror array with CMOS control electronics.

وصف الملف: application/pdf

العلاقة: https://scholar.afit.edu/etd/5262Test; https://scholar.afit.edu/context/etd/article/6264/viewcontent/AFIT_GE_ENG_99M_25_Rounsavall_P_ADA361750_no_vita.pdfTest

الإتاحة: https://scholar.afit.edu/etd/5262Test
https://scholar.afit.edu/context/etd/article/6264/viewcontent/AFIT_GE_ENG_99M_25_Rounsavall_P_ADA361750_no_vita.pdfTest

المؤلفون: Doggett, Matthew K.

المصدر: Theses and Dissertations

مصطلحات موضوعية: Tropospheric radio wave propagation--Mathematical models, Atmospheric Sciences

الوصف: The Variable Terrain Radio Parabolic Equation (VTRPE) computer model is a powerful and flexible program that provides calculations of the radar propagation conditions of the atmosphere. It is limited however, by the accuracy and resolution of the input data. This study quantifies the sensitivity of the VTRPE model to the accuracy and resolution of the atmospheric parameters that go into it. Also, two case studies are examined to test the utility of VTRPE in operational use. The sensitivity to measurements of pressure, temperature, and humidity was found to be dependent on the meteorological environment. In standard and subrefractive environments, average values of Root Mean Squared Error in calculating propagation pathloss were greatest for measurement errors in humidity. While the overall RMSE averaged only 0.5% to 5%, in certain regions the errors in calculating pathloss were as high as 20%. VTRPE was used to calculate possible height errors when birds were detected at long ranges from a WSR-88D radar in a ducting environment. While the radar assumes a standard atmosphere when calculating height, results from VTRPE suggest that this resulted in possible height errors of over 3 km. Another case study of detected anomalous propagation was examined to determine the effects of multiple soundings in the VTRPE calculation of propagation pathloss. In this example, the effect of assuming a homogeneous atmosphere resulted in propagation pathloss errors of up to 30%.

وصف الملف: application/pdf

العلاقة: https://scholar.afit.edu/etd/5942Test; https://scholar.afit.edu/context/etd/article/6945/viewcontent/AFIT_GM_ENP_97M_04_Doggett_M_ADA323189.pdfTest

الإتاحة: https://scholar.afit.edu/etd/5942Test
https://scholar.afit.edu/context/etd/article/6945/viewcontent/AFIT_GM_ENP_97M_04_Doggett_M_ADA323189.pdfTest

المؤلفون: Yowell, Robert J.

المصدر: Theses and Dissertations

مصطلحات موضوعية: Ionospheric radio wave propagation, Mars (Planet)--Atmosphere, Atmospheric Sciences, Signal Processing

الوصف: This thesis presents a preliminary design of an ionospheric sounder to be carried aboard one or more of NASA's Mars Surveyor landers. Past Russian and American probes have indicated the existence of an ionosphere, but none of these missions remotely sensed this atmospheric layer from the surface. The rationale for utilizing a surface-based Martian ionospheric sounder is discussed. Based on NASA's choice of launch vehicle and power source, a low-weight, low-powered Chirp sounder using a horizontally-polarized dipole antenna is recommended for the sounder experiment. The sounder experiment should be conducted for at least one Martian year, in order to investigate significant changes in radio propagation during seasonal transitions. Specific data compression techniques are suggested in order to reduce the quantity of data transferred from each sounder. The Appendix presents an overview of Earth's ionospheric structure and solar cycle effects. Finally, a Matlab software model of a hypothetical ionogram as measured from the Martian surface is presented.

وصف الملف: application/pdf

العلاقة: https://scholar.afit.edu/etd/6207Test; https://scholar.afit.edu/context/etd/article/7210/viewcontent/AFIT_GE_ENG_96J_01_Yowell_R_ADA310746_no_vita.pdfTest

الإتاحة: https://scholar.afit.edu/etd/6207Test
https://scholar.afit.edu/context/etd/article/7210/viewcontent/AFIT_GE_ENG_96J_01_Yowell_R_ADA310746_no_vita.pdfTest

المؤلفون: Ghordlo, Ala

المصدر: Theses and Dissertations

مصطلحات موضوعية: Military communications, Radio wave propagation, Spread spectrum communications, Atmospheric Sciences, Signal Processing

الوصف: This research expanded Low Probability of Intercept (LPI) communications analysis in two areas. First, multimode communication was included to account for ground to ground and air to ground links in addition to the standard air to air links traditionally used in LPI analysis. The propagation equations for the three modes of interest were derived and included in LPI analytic models in the form of a mode quality factor to account for multimode LPI scenarios. This new quality factor was used in studying several communication and interception link combinations. Variations due to differences between the communication and interception modes were presented graphically. Second, atmospheric conditions were included to account for atmospheric attenuation. Previously, both links were assumed to be under the same atmospheric conditions. This assumption limits LPI analysis to scenarios where the receiver and interceptor are located close to each other. Therefore, the atmospheric quality factor had to be expanded to include scenarios where the communication link and the interception link are experiencing different and possibly fluctuating atmospheric conditions. The atmospheric propagation losses were accounted for by using the Liebe atmospheric propagation model to estimate atmospheric attenuation in dB/km for any practical atmospheric conditions. The atmospheric quality factor was then applied to the analysis of various scenarios for communication and interception links under similar and different atmospheric conditions. The results were represented graphically emphasizing the changes in LPI quality due to atmospheric conditions. It was apparent from the simulations, that LPI analysis was greatly enhanced by including the atmospheric quality factor. Finally, using both the mode and the atmospheric quality factors along with all the standard quality factors, a comprehensive LPI analysis was performed for many possible LPI scenarios.

وصف الملف: application/pdf

العلاقة: https://scholar.afit.edu/etd/5920Test; https://scholar.afit.edu/context/etd/article/6923/viewcontent/AFIT_GE_ENG_96D_03_Ghordlo_A_ADA319680.pdfTest

الإتاحة: https://scholar.afit.edu/etd/5920Test
https://scholar.afit.edu/context/etd/article/6923/viewcontent/AFIT_GE_ENG_96D_03_Ghordlo_A_ADA319680.pdfTest