Ph. D.

Doppler tracking of the Navy Navigation Satellite System (NNSS) gives solutions for polar motion as a by-product of the orbital computation process. The polar motion estimation process at the Defense Mapping Agency is reviewed and comparisons of the Doppler results with the polar motion series of the newer techniques, Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI), and the Bureau International de l'Heure (BIR) Circular-Dare made. TwoChandler periods of data are analyzed beginning with the start of the Main MERIT Campaign in September 1983. The two major components, annual and Chandler wobble, are removed. The resulting residuals, as well as those obtained by differencing all combinations of Doppler series and the three reference series, are smoothed by different techniques. The 'seven-point moving average' and the Vondrak smoothing algorithm are the two smoothers applied. A computational spectral analysis is presented that determines which periods dominate the residuals. / M.S.

This thesis presents the computer model FORWP that is capable of predicting attenuation and cross polarization at 10-30 GHz from dual-polarization radar data with a high degree of accuracy. FORWP uses a rigorous backscattering computer model BSCAT to infer rain drop size distribution along the slant path from radar measured reflectivity and differential reflectivity data collected along the path. Then, two semi-empirical models are developed which predict attenuation from radar measured reflectivity and differential reflectivity. These two semi-empirical models are used to evaluate FORWP. Finally, attenuation predictions of FORWP are compared to the two semi-empirical prediction models and measured link attenuation at 11.4 GHz for a rain event in southwest Virginia. / M.S.

The proposed use of communication satellites operating above 10 GHz has stimulated research into the effects of atmospheric rain and ice on the reception of these signals. This thesis examines the statistics of fade rate, fade duration, and interface intervals on 19 and 28 GHz communication links, at an elevation angle of 45 degrees. The study uses 2 years of data collected from the COMSTAR-D2 experimental propagation beacons at Blacksburg, Virginia. The results are shown to depend on frequency, elevation angle, time of year, rainrate, rainfall amount, and the signal polarization. The results are also shown to depend on the receiver time constants, the data acquisition system sampling rate, and the signal-to-noise ratio. The number of fade events and interface intervals was found to vary slightly when hysteresis was added to the data reduction program. / M.S.

Strategies for generalizing raster-based land cover data were investigated. Generalization strategies were studied as part of the U.S. Geological Survey's (USGS) Federal Land Information System (FLIS). Area filtering, category aggregation, resampling, and modal search and replace comprised the strategies tested on USGS Alaska Interim Land Cover data. Generalization of the land cover data was deemed necessary for two reasons: 1) reduction in the volume of homogeneous land cover regions required for computer memory storage, and 2) simplification of the highly complex land cover map. The generalization strategies were evaluated based upon how well they maintained the integrity of land cover information while minimizing the number of homogeneous land cover regions. Maintenance of land cover information after application of a generalization strategy was measured by omission and commission errors, percent unchanged, Cohen's statistic, and the number of land cover regions. / Master of Science

The work presented in this dissertation examines the performance of two satellite radio communication systems by computer simulation. Two simulations were separately performed for a spread spectrum chirp system as an analog communications system, and for the Land Mobile Satellite System (LMSS) channel as a digital communications system. For the simulation of analog communications, a spread spectrum system using chirp techniques called ‘Coded Multiple Chirp Spread Spectrum’ was proposed as a simple, cost-effective alternative for conventional spread spectrum systems. Its application as a spread spectrum overlay service on analog FM-TV was examined through the mutual interference analysis and spectral analysis using software programming. For the simulation of digital communications, various digital modulation schemes as well as channel encoding, block interleaving/deinterleaving, and differential encoding techniques were used for a thorough performance evaluation of a Land Mobile Satellite System under fading conditions. For this purpose, an LMSS fading channel simulator capable of simulating diverse fading characteristics for a satellite channel was designed and tested to yield various performance measures such as symbol error rate and average bit error rate. / Ph. D.

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No description available.

This thesis introduces the analysis of a novel device which, capitalizing on recent advances in gossamer solar sail technology, offers the possibility of propellantless satellite deorbiting and attitude control. By taking advantage of aerodynamic drag effects, a lightweight sail can rapidly deorbit a satellite. At the same time, the sail provides an ideal substrate for a large area magnetic torque coil for attitude control. Through the use of orbit propagation software, the performance of an implementation of this "MagSail" on a Low Earth Orbit (LEO) small satellite is simulated. The analysis is set forth in three parts. First the orbit decay profile of the satellite under the effects of atmospheric drag is presented. The results are interpreted for various initial orbits. Next, the actual torque generation of the MagSail is analyzed. Emphasis is placed on how various design parameters change the magnetic moment of the sail. Finally, a six degree of freedom simulation, combining both orbit propagation and PD attitude control demonstrates a possible implementation of the sail's attitude control capabilities. The work presented in this thesis provides an in-depth look at the deorbiting performance of large-area, low-mass LEO satellites. This research provides a theoretical framework for the development of compact, cost-effective propellantless propulsion and space debris mitigation systems.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2007. / The aim of this project was to design and test a partial attitude determination and control system for a nanosatellite. The reaction wheel system was designed and tested as an actuator for the nanosatellite. This reaction wheel system consists of four reaction wheels mounted in a tetrahedral formation. A rate sensor system was also designed and its viability for this space application was examined. The rate sensor system consists of 3 orthogonally mounted planes, each with three rate sensors mounted on it. Hardware-inthe- loop tests were used along with an air bearing experimentational setup, which created near frictionless circumstances, to prove the effectiveness of the designed reaction wheel setup. The results following from this project were the following: The reaction wheel system proved to be an adequate actuator for this nanosatellite application and the rate sensor systemwhich was analysed proved to be inadequate for a nanosatellite application.