The solution to the problem of a satellite with a small constant thrust and under the influence of a central force field presents difficulties due to equations non-linearity. An investigation was made to determine what range of values of the thrust parameter could be utilized to obtain a valid approximate solution for the case of tangential thrusting. The investigation was accomplished by developing programs for the 1620 High Speed Digital Computer, since the solution to the exact and approximate equations would otherwise be exceedingly laborious. Under the assumption of no atmosphere, and neglecting the earth's oblateness, the study showed that the validity of the Kryloff and Bogoliuboff method was dependent on both the specific thrust aud the vehicle speed. For a Speed corresponding to an orbit of smaller eccentricity it was determined that the method of Kryloff and Bogoliuboff remained valid for larger values of W than when V was a velocity corresponding to an orbit with a greater eccentricity. The method of Kryloff and Bogoliuboff represents a practical approach to the solution of satellite motion both from the aspect of ease of application and reasonable calculation times. / Master of Science

The improved spatial resolution of the data from the SPOT satellite provides a substantially better basis for monitoring urban land use and growth with remote sensing than Landsat data. The purpose of this study is to delineate the road network in 20-m resolution SPOT-images of urban areas automatically. The roads appear as linear features. However, most edge and line detectors are not effective in detecting roads in these images because of the low signal to noise ratio, low contrast and blur in the imagery. For the automatic recognition of roads, a new line detector based on surface modelling is developed. A line can be approximated by a piecewise straight curve composed of short linear line-elements, called linels, each characterized by a direction, a length and a position. The approach to linel detection is to fit a directional surface that models the ideal local intensity profile of a linel in the least square sense. A Gaussian surface with a direction of invariance forms an adequate basis for modelling the ideal local intensity profile of the roads. The residual of the least squares fit as well as the parameters of the fit surface characterize the linel detected. The reliable performance of this line operator makes the problems of linking linels more manageable. / Master of Science

Land mobile satellite systems that are currently being designed for implementation in the next decade will need to operate in the presence of propagation effects such as vegetative shadowing and multipath that will cause signal fading. This paper discusses the statistical modeling and simulation of the land mobile satellite fading environment. Simple models are developed to approximate the complex analytical expressions for the fade distributions. The Average Path Model, which relates the physical parameters of the vegetation along the path to the propagation model parameters, is verified and shown as a useful model for estimating the propagation parameters. Discrepancies between the VT Propagation Simulator and the analytical models are resolved and results comparing secondary fading statistics from the simulator to measured data are given. Results of a study using the propagation simulator to simulate spatial diversity to combat vegetative fading are given. / Master of Science

The Land Mobile Satellite System (LMSS) to be available in the 1990 time frame will provide connection between mobile vehicles and the conventional terrestrial communication network. The design is dependent on the propagation characteristics of the land mobile satellite signal. Unlike fixed satellite links, there is blockage in the line of sight path, mainly in the form of vegetative shadowing. The focus of this study is to develop models for the fading of the received satellite signal. A brief review of the physics and statistics associated with mobile propagation is presented. This is followed by a review of current literature and experiments. The modeling of the cumulative distribution function for a totally vegetatively shadowed mobile (VS distribution) is presented. The VS distribution is then used in a model for the cumulative distribution function of a partially shadowed mobile. The complete model for partially shadowed routes permits calculations for arbitrary combinations of open and forested terrain. Comparisons are made to data reported for partially shadowed routes. The deterministic path model (DPM) developed in an earlier effort is a geometrically based tool for determining the signal path length through a stand of trees. It is expanded to give approximate expressions for the statistical parameters describing the fading of the line-of-sight component of the received signal. New expressions for the secondary statistics of a totally vegetatively shadowed mobile are derived. These new expressions are then used in models for the level crossing rate and average fade duration of a partially shadowed mobile. Comparisons are made to data reported for partially shadowed routes. / M.S.

This document describes the theory, design and implementation of a digital receiver designed for the ACTS propagation experiments. The Virginia Tech Satellite Communications Group is designing, constructing and distributing eight ACTS propagation terminals (APTs) under a contract with NASA. The terminals will measure the received signal power from the ACTS satellite beacons (20 GHz, 27.5 GHz) in various climates at different elevation angles. The resulting signal power measurement, radiometer and weather measurement data will be used to characterize atmospheric effects on signal propagation at Ka band. Each APT system will contain two identical, independent digital receivers; one 20 GHz channel and one 27.5 GHz channel. The algorithm implemented on each receiver utilizes the results of multiple fast Fourier transforms to reliably identify the carrier tone amidst neighboring modulation tones. The acquisition procedure, can reliably identify the carrier signal at signal-to-noise ratios down to 10 dB in a 20 Hz bandwidth (23 dBHz in a 1 Hz bandwidth, -26 dB relative to clear air conditions on the APT system) in 2 seconds. The receiver then uses a comb filter, two FIR filters and additional FFTs to produce power measurements accurate to 0.1 dB at signal-to-noise ratios down to 5 dB in a 20 Hz bandwidth (18 dBHz in a 1 Hz bandwidth, -31 dB relative to clear air conditions on the APT system) at 1 Hz and 20 Hz sampling rates. The algorithm also provides several supplemental functions including a software selectable detection bandwidth from 2 Hz to 50 Hz (1 Hz increments), spectrum analyzer type output for a 303.333 kHz bandwidth centered on the current carrier frequency and carrier frequency estimates accurate to ±O. 5 Hz. / Master of Science

The present work addresses the development of a method for the calculation of Convective heat-transfer and surface temperatures on heat-shields of Aeroassisted Orbital Transfer Vehicles (AOTVs) in hypersonic flow regimes. Inviscid flowfield solutions are obtained about the aerobraking shield on the AOTVs using axisymmetric Euler's equations. The flowfield solutions are coupled with laminar and turbulent boundary-layer equations and the heat-shield material properties to obtain convective heating rates and heat-shield wall temperatures. A method for obtaining non-dimensionalized solution of convective heat-transfer rates is obtained. This non-dimensionalized solution can be used for calculating convective heat-transfer rates and wall temperatures for various freestream conditions encountered during the aerobraking maneuvers. Calculations are carried out for perfect gas and equilibrium air cases, and the effect of wall catalysis on convective heat-transfer is also incorporated. The results are in good agreement with available experimental and numerical results for AOTVs. / Master of Science

Range and,interferometric observations have been examined for their potential, application in a geometric approach to determination of satellite ephemeri. The approach differs from the normal (dynamic) approach in that each satellite position is treated as an independent state variable or benchmark. Programs have been developed that simulate and format the input, data for the least squares estimation routines, and perform statistical analyses of those results. Random error, tropospheric refraction errors, and atomic clock errors have been considered, and the range observation adjustment program directed to solve for clock errors. / Master of Science

Virginia Tech has recently developed and constructed seven ground terminals for use in a propagation experiment employing the Advanced Communications Technology Satellite. This satellite was deployed by NASA in September of 1993. The ground terminals are used in the study of propagation effects caused by the atmosphere for 20 and 30 GHz signals. Each of these terminals contains one total power radiometer for each frequency. These radiometers are used to set clear sky attenuation reference levels for satellite beacon calibration. This thesis describes the design, implementation and testing of the radiometers built at Virginia Tech. In addition, a mathematical algorithm used for the prediction of clear sky attenuation along satellite paths is tested for correlation with radiometer predicted clear sky attenuation. Test data is compared to determine if the algorithm might serve as a radiometer replacement in future propagation experiments or other applications. / Master of Science

This paper discusses the total power radiometers in the propagation experiment at Virginia Tech using the OLYMPUS communications satellite. The experiment is set up to measure the received signal strength of beacons at 12.502, 19.770 and 29.656 GHz on board the OL YMPUS satellite in order to gather space-earth propagation statistics for a period of one year. The radiometers are used to set the clear sky reference levels for the beacon receivers. The paper begins with an overview of radio signal attenuation mechanisms, and a general discussion of radiometry. A description of the OLYMPUS radiometers follows, with sections on calibration techniques and the methods required to convert radiometer data into derived path attenuation for use in setting the clear air reference levels for the beacon receivers. Several novel techniques were used in the Virginia Tech radiometers including the use of voltage to frequency converters for integration purposes, and reference load calibration using a waveguide switch to swap the radiometer input from the antenna to a waveguide load of known and stable temperature. The penultimate chapter contains data recorded in the course of the experiment which was selected at random from the month of January 1991. The data is compared to corresponding free space beacon attenuation data and shows conclusively that the total power radiometer can produce highly accurate and stable attenuation measurements for extended periods of time. / Master of Science

For the implementation of the NASA EOS testbed programmable operations, a control/data acquisition system has been designed, implemented, and tested. The work is focused mainly on the hardware design/build of the custom interface board and software development for real-time control and data acquisition of the testbed. Also, an application program was designed to control, collect, and display the data and parameters of the testbed elements, and troubleshoot the hardware. An algorithm was chosen and applied to the battery simulator control after computer simulations. The final experiment results were obtained successfully, which proved the design to be correct, as the testbed could be operated under computer control. / Master of Science