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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1980. / Microfiche copy available in Archives and Science. / Includes bibliographies. / by Roger Nelson Clark. / Ph.D.

This document discusses the measurements conducted at Virginia Tech, in Blacksburg, VA, and at COMSAT Labs, in Clarksburg MD, to determine the impact of multipath fading upon a circularly polarized L-Band satellite-to-handheld communications system using an omni-directional antenna. A significant finding was that motion of the satellite could induce fades of up to lOdB, well in excess of the small link margins available to satellite-to-handheld systems (typically ≤ 3dB). A method for ameliorating multi path fading was then sought. It was also found that the multipath fading effect was significantly different at as small a distance as half a wavelength from the original antenna position. Multipath signals could actually combine at the new position to form a small increase in signal to noise ratio (S/N), rather than a fade of as much as 16dB. Therefore it was determined that a small-scale antenna diversity system might offer a solution to the problem of multipath fading. / Ph. D.

A data acquisition and control system (DACS) has been designed and constructed for use during propagation experiments with the OLYMPUS satellite. OLYMPUS is a European Space Agency experimental satellite that broadcasts coherent at 12, 20 and 30 GHz and is viewed from Blacksburg, Virginia at an elevation angle of 13.9°. This low elevation angle yields a relatively long atmospheric path which serves to accentuate propagation effects. The DACS is a custom design which collects propagation, environmental and status information and periodically calibrates external equipment. Beacon signal strength is measured via a hybrid analog/digital receiver. The analog portion the receiver utilizes the coherency of the satellite beacons to track 20 and 30 GHz signals to the noise floor. The digital portion of the receiver is contained within the DACS and consists of a stand-alone microprocessor which filters the beacon signal to determine the power in a 3 Hz bandwidth. Additional DACS circuitry collects analog and digital input channels and controls external through digital output channels. Digital outputs are used to reference the collected to known by performing periodic calibrations on external equipment. Analog input channels are used to measure quantities including external temperature, wind speed wind direction, while digital inputs monitor alarm conditions. The VIEW program utility permits an operator to graphically view data in real time. In addition, collected data is stored to tape without an interruption in data collection. DACS operation has been virtually continuous since data collection was started on August 3, 1990. / Master of Science

System engineers are currently planning satellite communications links in the 20/30 GHz bands and frequency reuse through orthogonal polarizations. This thesis examines the system level constraints imposed by medium effects at these high frequencies of which the most serious is rain induced attenuation and depolarization. The effect of crosstalk between orthogonal channels is calculated in terms of a degradation of the carrier to noise ratio. This concept is combined with the results of rain propagation modeling to define a rain induced effective attenuation which incorporates the effects of depolarization and attenuation. Methods for computing the effective attenuation and link outage times caused by rain are outlined which are applicable to any earth to space millimeter wave communication link. The Millimeter Wave Communications Experiment is used as an illustrative example of these methods. / Master of Science

Fades on satellite to land mobile radio links are caused by several factors, the most important of which are multipath propagation and vegetative shadowing. Designers of vehicular satellite communications systems require information about the statistics of fade durations in order to overcome or compensate for the fades. Except for a few limiting cases, only the mean fade duration can be determined analytically, and all other statistics must be obtained experimentally or via simulation. This report describes and presents results from a computer program developed at Virginia Tech to simulate satellite path propagation of a mobile station in a rural area. The simulator was developed using 869 MHz balloon data provided by Wolfhard Vogel of the University of Texas at Austin and was tested using helicopter data provided by Wolfhard Vogel and Julius Goldhirsh of the Johns Hopkins University Applied Physics Laboratory. It generates rapidly-fading and slowly-fading signals by separate processes that yield correct cumulative signal distributions and then combines these to simulate the overall signal. This is then analyzed to yield the statistics of fade durations. / M.S.

The intent of this research is to provide a performance analysis of multiple access protocols in packet-switched Very Small Aperture Terminal (VSAT) satellite communication networks. This research consisted of three major thrusts. First, we analyzed the average time delay of the broadcast Time Division Multiplexing (TDM) outbound channel (hub to VSAT). Second, a throughput performance analysis of an asynchronous Direct-Sequence Code Division Multiple Access (DS-CDMA) communication system is carried out for the inbound line (VSAT to hub). Each channel was characterized by its bit error probability, and transmits fixed-length packets generated according to a Poisson process in an unslotted environment. Third, we presented a delay analysis of the ALOHA DS-CDMA/TDM channel to determine the total service time of a packet originating from either the VSAT or the hub station. In addition to its multiple access capability, this thesis is concerned with the use of direct-sequence spread-spectrum signaling primarily because of its ability to combat interference. Emphasis is placed on average throughput performance, and on the average packet delay after solving for the steady state probability generating function of the station queue size. Then, a discussion of the effect of finite buffer size, and an analysis relating the probability of buffer overflow to packet statistics and buffer size is presented. Because of the bursty nature of a traffic originating from the VSAT’s, the Automatic Repeat Request (ARQ) technique used for error control is the Stop-and-Wait (SW) protocol. It is used as a retransmission strategy in both the Asynchronous Time Division Multiplexing (ASTDM) and the ALOHA DS-CDMA channels. / M.S.