Engineering problems involving the development of cheap, easy-to-use, and effective packet communications terminals for connectivity to low-earth-orbit satellites (PACSATs) are addressed. Two prototype terminals were developed: a base station for maximum duration connectivity to the satellite, and a smaller station for portability and low cost. Commercially available systems for related amateur radio uses were integrated for these prototypes. The prototypes illustrated areas for further development before widespread use of PACSATs can be realized. To better understand some of the issues for PACSATs, two analyses were developed. The first analysis approach characterizes PACSAT orbits so that tradeoffs are readily identified and quantified. Results from an analysis of one PACSAT orbit indicate conflicting needs for the two terminal types. The second analysis approach addresses issues involving the use of an increasingly popular method for transmitting 9600 baud FSK which combines the spectral efficiency of premodulation pulse shaping with the simplicity of using FM discriminators for demodulation. The spectrum of the transmitted signal is studied and the performance of the demodulator is compared to that of an appropriate coherent receiver. Doppler shifts on the channel is particularly important for PACSAT communications. An analysis approach that addresses the mistuning problem is presented. Results of this analysis identify changes that would better suit the needs of PACSAT terminals. / Master of Science

The use of communication satellites operating above 10 GHz has prompted research on the attenuating effects of rain on these signals. Frequency scaling, a technique where attenuation at one frequency can be scaled up (or down) to another frequency, can extend the known data base of propagation impairments. This thesis studies the ratios of attenuations in frequency pairs 14/11, 30/20, 30/11, and 20/14 GHz. The attenuation data were derived from a year-long propagation experiment which used a dual-polarized radar to estimate drop size distributions on a slant path between Blacksburg, Virginia, and the Intelsat VA F10 satellite. The effect of varying drop size distributions on attenuation ratios is discussed. An uplink power control simulation is developed using the attenuation ratios. Attenuation on the uplink path is estimated by scaling the measured downlink attenuation by the appropriate attenuation ratio. The evidence suggests that an uplink power control scheme using drop size distribution-based attenuation ratios offers improvement in satellite link reliability during rain storms. / Master of Science

This dissertation presents unique and valuable insight into the analysis of packet-switched data communication systems. The research described in this dissertation examines performance characteristics of two types of packet-switched data communication systems. The first system to be analyzed operates in a parallel processing environment where cooperating processors independently perform assigned tasks. In this environment, the packet delay performance is dominated by queuing delays. The second type of system examined operates in a low earth orbit (LEO) satellite communications network environment. In this type of network, delay performance is affected by both queuing and propagation effects. The objectives of this research are to study the effects of queuing and propagation on the average packet delay, the number of buffers required to implement the networks that interconnect the parallel processors, and the satellite resource utilization rates. For both types of communication systems, mathematical metamodels [Agr85] are developed to capture the effects on packet delay caused by incremental changes in network dependent parameters. Part I of this research performs average packet delay and buffer cost comparisons of the augmented shuffle exchange network (ASEN) and the multistage cube (MSC) network. It is shown that the packet delay associated with the ASEN is between 20 and 25 percent lower than that of a similar sized MSC network. In addition to the delay benefits of the ASEN, network implementation cost savings for the ASEN are shown to be 9 to 16 percent lower than the MSC. Innovative mathematical design tools are developed and applied to the parallel processing interconnection network environment. These tools are used for predictive modeling of packet delay given network dependent parameters. The simple and concise models are shown to have predictive accuracy within 1 percent of the observed simulation delay results. Part II of this research focuses on LEO satellite system communications. Six different constellations, providing whole-earth coverage are modeled and analyzed. The number of satellites within the constellations examined range from 36 to 77. The analysis of these LEO satellite systems consists of examining the packet delay characteristics of these dynamic systems as well how resource requests to the satellites are distributed. It is shown that when packet delay is the only design criteria, the differences in delay between the 36-satellite system and the 77-satellite are minimal and do not warrant the use the 77-satellite system over the 36-satellite system. The satellite resource utilization analysis captures the resource request load balancing characteristics of the systems. From a load balancing perspective, the 54-satellite system yields the best performance while the 36-satellite system the worst. A third unique aspect of the research presented in Part II is the application of metamodeling to the LEO satellite system environment. The metamodels developed reduce a complex 8-factor packet delay representation to simple, yet accurate, 2 and 3- factor relationships. These metamodel delay relationships are shown to have a predicted versus observed packet delay “best case” accuracy of 8 and 4 percent for the 3 and 2-factor models, respectively. Predicted versus observed packet delays are typically within 20 percent of agreement. This research makes two contributions to the state-of-the-art knowledge in packet-switched communications system analysis. First, the metamodeling of interconnection networks and LEO systems are first of their kind. These models can allow for reduced simulation trials and more expeditious design decision making. The second contribution is the development of an integrated LEO satellite system model not seen in previously published research. This model can be used to further advance research in the LEO satellite system environment. / Ph. D.

No description available.

This thesis analyses the current regulatory regimes in the European Union---in particular Germany---and the United States governing the authorization of global mobile personal communications by satellite (GMPCS). / Current satellite licensing regimes fail to take into account the international character of satellite telecommunications. The reliance on a national public interest standard does not properly address the interests of the community of states, industry and consumers and is not suitable to meet the aspirations of international space law. The international harmonization of frequencies and international standardization are indispensable for the introduction of these new services. The establishment of an international communications regulator with the power to adopt binding decisions if required is the most suitable way to address these problems. However, the development of an international public interest standard with clear policy objectives will be hard to achieve. / With regard to the different aspects of ground segment licensing, a gradual approach is the best way to balance the different interests and concerns in this field. The International Telecommunications Union GMPCS MoU and the development of European regulations show a feasible way to achieve a regulatory regime that facilitates the introduction of these new services. Lighter regulations, possibly the introduction of one stop shopping procedures, mutual recognition of licenses and the introduction of general authorizations are measures that can and should be taken at the international level. The interdependence of earth and space segment licensing may lead to the understanding that some aspects of earth segment licensing should also be regulated at international level. / The question will be whether States, industry and users are able to reconcile their interests and the sometimes opposing trends of international cooperation and international competition for the benefit of "all mankind".

The role of the South African National Defence Force (SANDF) has changed considerably in the last decade. The emphasis has moved from protecting the country's borders to peacekeeping duties in Central Africa and even further North. Communications between the peacekeeping missions and the military bases back in South Africa is vital to ensure the success of these missions. Currently use is made of satellite as well as High Frequency (HF) communications. There are drawbacks associated with these technologies (high cost and low data rates/interference respectively). Successful long distance ionospheric propagation in the low Very High Frequency (VHF) range will complement the existing infrastructure and enhance the success rate of these missions. This thesis presents a feasibility study to determine under what ionospheric conditions such low VHF communications will be possible. The International Reference Ionosphere (IRI) was used to generate ionospheric data for the reflection point(s) of the signal. The peak height of the ionospheric F2 layer (hmF2) was used to calculate the required antenna elevation angle. Once the elevation angle is known it is possible to calculate the required F2 layer critical frequency (foF2). The required foF2 value was calculated by assuming a Maximum Useable Frequency (MUF) of 20% higher than the planned operational frequency. It was determined that single hop propagation is possible during the daytime if the smoothed sunspot number (SSN) exceeds 15. The most challenging requirement for successful single hop propagation is the need of an antenna height of 23 m. For rapid deployment and semi-mobile operations within a jungle environment it may prove to be a formidable obstacle.

No description available.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005. / In this thesis, the maneuvering and orientation of an inspection/service satellite is investigated. This thesis will demonstrate a simplified satellite maneuver around another satellite (for docking purposes). This is illustrated with a cart on an air bearing table (frictionless environment) which simplifies the problem to two dimensions. A mathematical model of the system was designed and simulated (Matlab) for this maneuver. With the simulation, different strategies were considered to maneuver the cart in a circle around another object. The conclusion was made that approximating the circle with segments would conserve propellant. This strategy was implemented on the air bearing table.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2008. / Horizon sensing is an effective way to determine the pitch and roll of a LEO satellite and Earth horizon sensors that operate in the visible range of the electromagnetic spectrum are commonly used. These sensors have the disadvantage that they cannot operate when the satellite is in eclipse. Earth horizon sensors that operate in the infrared spectral range are a solution to take attitude measurements when the satellite is in eclipse. Until recently infrared detectors could only operate at very low temperatures and needed to be cryogenically cooled. The result was that their power consumption and physical characteristics (like dimensions and mass) were such that they were not suitable for use in small and medium LEO satellites. As a result of technology expansion in the field of infrared imagers the past few years, infrared imagers were developed which do not require cooling. The scope of this project was to develop and implement an Earth horizon sensor by using a low-cost, uncooled infrared imager. The performance and physical characteristics of various imager were evaluated and it was decided to select a low resolution thermopile imager mainly as a result of the cost limitations of the project. Software algorithms were then evaluated and selected for horizon detection and attitude determination. The Earth horizon sensor that was developed did not comply with the accuracy requirement (3s < 0.1o) that was set for the project because of the low resolution of the sensor. Methods to improve the accuracy were investigated and finally a sub-pixel edge estimation algorithm was developed and implemented which resulted in an improvement of 69% in the pitch accuracy and 49% in roll accuracy. With the sub-pixel edge estimation algorithm implemented the horizon sensor almost met the accuracy requirements (s < 0.0811o for pitch and s < 0.2944o for roll). This project confirms that, with further improvement to the design and test facilities, developing a low-cost, uncooled infrared Earth horizon sensor that meets the accuracy requirements is feasible.

Thesis (MEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / The aim of this thesis is to find an a ordable and upgradeable manner of increasing the data rate of current satellite systems. The compression, modulation and amplification techniques and telecommunication regulations restrict the data rate, which are bypassed by designing a parallel channel configuration. In order to test this solution a system is developed that is based on the existing standards of DVB-S and MPEG-2. The combination of these standards protect the data from interference, package the data for ease of storage, modulate the data to more effectively radiate the signal and shape the spectrum to adhere to telecommunication regulations. The spacing between the channels is reduced and the transmitted signal is amplified to inject interference into the system. The effect of this interference on the receiver is shown by transmitting image data and comparing the received image with the original.