210 research outputs found
Low cost attitude control system scanwheel development
In order to satisfy a growing demand for low cost attitude control systems for small spacecraft, development of low cost scanning horizon sensor coupled to a low cost/low power consumption Reaction Wheel Assembly was initiated. This report addresses the details of the versatile design resulting from this effort. Tradeoff analyses for each of the major components are included, as well as test data from an engineering prototype of the hardware
Spectral anonymization of data
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 87-96).Data anonymization is the process of conditioning a dataset such that no sensitive information can be learned about any specific individual, but valid scientific analysis can nevertheless be performed on it. It is not sufficient to simply remove identifying information because the remaining data may be enough to infer the individual source of the record (a reidentification disclosure) or to otherwise learn sensitive information about a person (a predictive disclosure). The only known way to prevent these disclosures is to remove additional information from the dataset. Dozens of anonymization methods have been proposed over the past few decades; most work by perturbing or suppressing variable values. None have been successful at simultaneously providing perfect privacy protection and allowing perfectly accurate scientific analysis. This dissertation makes the new observation that the anonymizing operations do not need to be made in the original basis of the dataset. Operating in a different, judiciously chosen basis can improve privacy protection, analytic utility, and computational efficiency. I use the term 'spectral anonymization' to refer to anonymizing in a spectral basis, such as the basis provided by the data's eigenvectors. Additionally, I propose new measures of reidentification and prediction risk that are more generally applicable and more informative than existing measures. I also propose a measure of analytic utility that assesses the preservation of the multivariate probability distribution. Finally, I propose the demanding reference standard of nonparticipation in the study to define adequate privacy protection. I give three examples of spectral anonymization in practice. The first example improves basic cell swapping from a weak algorithm to one competitive with state of-the-art methods merely by a change of basis.(cont) The second example demonstrates avoiding the curse of dimensionality in microaggregation. The third describes a powerful algorithm that reduces computational disclosure risk to the same level as that of nonparticipants and preserves at least 4th order interactions in the multivariate distribution. No previously reported algorithm has achieved this combination of results.by Thomas Anton Lasko.Ph.D
CYCLOSTATIONARY FEATURES OF PAL TV AND WIRELESS MICROPHONE FOR COGNITIVE RADIO APPLICATIONS
Frequency spectrum being a scarce resource in communication system design,
spectrum sharing seems to be the solution to an optimal utilization of frequency
spectrum. The traditional fixed frequency allocation is not suitable for futuristic
networks that demand more and more spectrum for new wireless services. Cognitive
radio is a new emerging technology based on spectrum sharing concept.
Spectrum sensing is a vital task in this emerging technology by which it is able to
scan the frequency spectrum to identify the unused spectrum bands and utilize them.
In this thesis, we discuss spectrum sensing in the context of IEEE 802.22 Wireless
Regional Area Network (WRAN). In order to do so, we develop the co-existence
scenario with three cases according to geographical positions of primary services and
secondary service. In WRAN application, the SUs utilize the unused channel in TV
spectrum, which means that the primary users are TV service and other FCC part 74
low power licensed devices. We focus on special case of Analog TV-PAL service and
wireless microphone service as part 74 devices. Before discussing the spectrum
sensing technique, we propose architecture for sensing receiver. The concept of noise
uncertainty is also introduced in this context. The cyclostationarity theory is
introduced and we explain the motivation behind using the theory for spectrum
sensing and the reason that makes the cyclostationary features detector a powerful
detection technique in cognitive radio. We obtain the cyclostationary features of these
primary signals using spectral correlation function. Based on these features, we
develop two algorithms for spectrum sensing and their performances are evaluated in
comparison with energy detector which is considered as the standard simple detector.
Given that the cyclostationary features are unique for a particular signal; these
features can be used for signals classification. In our case, we use those features to
decide if the licensed channel is used by TV service or wireless microphone service.
This provides additional information for spectrum management and power control. Implementation issue is very important in cognitive radio generally and spectrum
sensing specially, hence we discuss the implementation of cyclostationary features
detector and compare its complexity with that of energy detector
Space-qualified submillimeter radiometer
The purpose of this research was to develop a reliable submillimeter wave spectrometer for space-borne high frequency spectral line work. The emphasis was on improving the efficiency of frequency multipliers to limit the system components to rugged, low power consumption solid-state devices. This research has allowed Millitech to develop increased efficiency and performance in Millitech's existing line of submillimeter components and systems. Millitech has fabricated and tested a complete solid-state spectrometer front end for use at 560 GHz (the 1(sub 10) to 1(sub 01) transition of water vapor). The spectrometer was designed with the rigors of flight conditions in mind. The spectrometer uses a phase-locked, solid-state Gunn diode oscillator as the local oscillator, employing a tripler to produce about 3 mW of power at 285 GHz, and a low noise second harmonic waveguide mixer which requires less than 2 mW of LO power. The LO (and the signal) is injected into the mixer by means of a quasioptical diplexer. The measured system noise temperature is 2800 K (DSB) over 400 MHz. The whole spectrometer front end is compact (21 cm by 21 cm by 24 cm), light (7.4 kg), and has a power consumption of less than 8 W. Other topics explored in this work include compact frequency agile phase lock loops, optical filters, and InP Gunn oscillators for low noise applications. As a result of this research, the improvement in the design of multipliers and harmonic mixers will allow their use as the LO power for a variety of satellite-borne receivers operating in the 200 to 600 GHz frequency range
Characterization Methodology, Modeling, and Converter Design for 600 V Enhancement-Mode GaN FETs
Gallium Nitride (GaN) power devices are an emerging technology that have only become available commercially in the past few years. This new technology enables the design of converters at higher frequencies and efficiencies than those achievable with conventional Si devices. This dissertation reviews the unique characteristics, commercial status, and design challenges that surround GaN FETs, in order to provide sufficient background to potential GaN-based converter designers.Methodology for experimentally characterizing a GaN FET was also presented, including static characterization with a curve tracer and impedance analyzer, as well as dynamic characterization in a double pulse test setup. This methodology was supplemented by additional tests to determine losses caused by Miller-induced cross talk, and the tradeoff between these losses and overlap losses was studied for one example device.Based on analysis of characterization results, a simplified model was developed to describe the overall switching behavior and some unique features of the device. The impact of the Miller effect during the turn-on transient was studied, as well as the dynamic performance of GaN at elevated temperature.Furthermore, solutions were proposed for several key design challenges in GaN-based converters. First, a driver-integrated overcurrent and short-circuit protection scheme was developed, based on the relationship between gate voltage and drain current in GaN gate injection transistors. Second, the limitations on maximum utilization of current and voltage in a GaN FET were studied, particularly the voltage overshoots following turn-on and turn-off switching transients, and the effective cooling of GaN FETs in higher power operation. A thermal design was developed for heat extraction from bottom-cooled surface-mount devices. These solutions were verified in a GaN-based full-bridge single-phase inverter
One way Doppler extractor. Volume 1: Vernier technique
A feasibility analysis, trade-offs, and implementation for a One Way Doppler Extraction system are discussed. A Doppler error analysis shows that quantization error is a primary source of Doppler measurement error. Several competing extraction techniques are compared and a Vernier technique is developed which obtains high Doppler resolution with low speed logic. Parameter trade-offs and sensitivities for the Vernier technique are analyzed, leading to a hardware design configuration. A detailed design, operation, and performance evaluation of the resulting breadboard model is presented which verifies the theoretical performance predictions. Performance tests have verified that the breadboard is capable of extracting Doppler, on an S-band signal, to an accuracy of less than 0.02 Hertz for a one second averaging period. This corresponds to a range rate error of no more than 3 millimeters per second
A study of defibrillator waveforms, vectors, and therapies Kevin Anthony Michael.
Includes abstract.Includes bibliographical references.The defibrillation studies were performed in a porcine model given its resemblance to the human heart and its resilience to repeated ventricular fibrillation inductions (VF) and defibrillations. A novel sequential preshock waveform was assessed in a prototype serial capacitance device. Single and multi-electrode defibrillation vectors were evaluated in intra-cardiac, subcutaneous (SQ), and intra-pericardial configurations. The burden of ITS and the factors improving detection were assessed. Anti-tachycardia pacing was used to evaluate therapies in this patient cohort. The diagnostic utility of the post pacing interval (PPI) after failed ATP episodes was then investigated to help discriminate between atrial fibrillation/tachycardia (AF/AT) and VT
Advanced industrial X-ray computed tomography for defect detection and characterisation of composite structures
X-ray Computer Tomography (CT) is well suited to the inspection of Fibre-Reinforced-Plastic (FRP) composite materials. However, a range of limitations currently restrict its uptake. The aim of the present research was to develop advanced inspection procedures that overcome these limitations and increase the scope of composite structures that can be inspected by industrial cone beam CT. Region of Interest (ROI) CT inspection of FRP laminated panels was investigated and two data completion methods developed to overcome reconstruction errors caused by truncated projection data. These allow accurate, highly magnified regions to be reconstructed on objects that extend beyond the Field-of-View (FOV) of the detector. The first method extended the truncated projection data using a cosine signal tailing off to zero attenuation. This method removed the strong 'glowing' artefacts but an inherent error existed across the reconstructed ROI. This did not affect the defect detectability of the inspection but was viewed as problematic for applications requiring accurate density measurements. The second method used prior knowledge of the test object so that a model could be created to estimate the missing data. This technique removed errors associated with ROI reconstruction thus significantly improving the accuracy. Techniques for extending the FOV were developed and applied to the inspection of FRP wind turbine blades; over 1.5X larger than the conventional scanning FOV. Two data completion methods were developed requiring an asymmetrically positioned detector. The first was based on the cosine tailing technique and the second used fan beam ray redundancy properties to estimate the missing data. Both produced accurate reconstructions for the 'offset' projection data, demonstrating that it was possible to approximately double the FOV. The cosine tailing method was found to be the more reliable. A dual energy image CT technique was developed to extend the optimum dynamic range and improve defect detectability for multi-density objects. This was applied to FRP composite/Titanium lap joints showing improved detectability of both volumetric and planar defects within the low density FRP. The dual energy procedure was validated using statistical performance measures on a specially fabricated multi-density phantom. The results showed a significant improvement in the detail SNR when compared to conventional CT scans.EThOS - Electronic Theses Online ServiceTWI LtdThe Engineering and Physical Sciences Research Board (EPSRC)GBUnited Kingdo
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