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This thesis describes the approach to designing a Low Earth Orbit CubeSat constellation capable of nearly constant coverage. The software package Satellite Tool Kit is used to create simulated multi-satellite systems that maintain a communication link between Tenby, Pembrokeshire, Wales and tactically chosen locations in the United States of America. The research will attempt to find the constellation capable of maintaining a set of design parameters (such as signal to noise ratio and altitude), with the minimum possible number of CubeSats. The downlink location, antenna design and the orbital planes are the negotiable parameters in the system, with little to no set constraints, and thus will be altered until the most favorable system is successfully designed.

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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.

In this thesis, application of linear control methods to control the attitude of a Low-Earth Orbit satellite is studied. Attitude control subsystem is first introduced by explaining attitude determination and control components in detail. Satellite dynamic equations are derived and linearized for controller design. Linear controller and linear quadratic regulator are chosen as controllers for attitude control. The actuators used for control are reaction wheels and magnetic torquers. MATLAB-SIMULINK program is used in order to simulate satellite dynamical model (actual nonlinear model) and controller model. In simulations, the satellite parameters are selected to be similar to the actual BILSAT-1 satellite parameters. In conclusion, simulations obtained from different linear control methods are compared within themselves and with nonlinear control methods, at the same time with that obtained from BILSAT-1 satellite log data.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: A low earth orbit satellite system can be useful in numerous communication applications where physical connections are not possible. Communication time available from any point on earth to the satellite is less than one hour per day. This one hour is fragmented into smaller time slots due to the satellite orbiting. This is not much time to transfer data and there is even less time available to transfer data when there are other external factors affecting the system. It is thus crucial to optimise the satellite communications link so that more data can be transferred per orbit. The goal of this thesis is to improve the performance of a low earth orbit satellite communication channel by varying certain parameters of the system, such as the protocol used, modulation scheme, packet size, transmission power etc. and then to observe how these parameters influence the system. The protocols that were chosen to be implemented are CSMA-CA, CSMA-CA with DSSS technology and Round-Robin Polling. A simulator for each protocol was designed with the Opnet platform, so that specific parameters could be changed and the results observed, in order to optimise the communications link between the satellite and ground stations. The results showed that there is no particular configuration of modulation scheme, packet size, transmission power etc. presenting the best overall solution for LEO satellite communications. It must be considered what the specific LEO satellite application would be used for and the characteristics required by that specific application. A suitable configuration must subsequently be chosen from the set of configurations available to satisfy most of the application requirements. / AFRIKAANSE OPSOMMING: ’n Satelliet met ’n lae wentelbaan kan gebruik word in verskeie kommunikasie toepassings waar fisiese verbindinge nie noodwendig moontlik is nie. Die kommunikasietyd van enige punt van aarde af na die satelliet, is minder as een uur per dag. Hierdie tyd word nog verder verklein omdat die satelliet besig is om, om die aarde te wentel. ’n Uur is glad nie baie tyd om data oor te dra nie en in realiteit is daar nog minder tyd beskikbaar as daar eksterne faktore op die sisteem inwerk. Dus is dit baie belangrik om die satelliet kommunikasiekanaal te optimiseer sodat soveel moontlik data as moontlik oorgedra kan word per omwenteling. Die doel van hierdie tesis is om die deurset van die kommunikasiekanaal van n lae wentelbaan satelliet te optimiseer, deur verskeie parameters te verander soos, protokol wat gebruik word, modulasie skema, pakkie grootte, transmissiekrag ens. en dan waar te neem hoe dit die sisteem beïnvloed. Die protokolle wat geïmplementeer is, is CSMA-CA, CSMA-CA met DSSS tegnologie en Round-Robin Polling. ’n Simulator vir elke protokol was ontwerp in die Opnet simulasie platform, sodat die spesifieke parameters verander kon word om die resultate te bestudeer met die doel om die kommunikasiekanaal tussen die satelliet en grond stasies optimaal te benut. Die resultate het bewys dat daar geen spesifieke konfigurasie van modulasie skema, pakkie grootte, transmissiekrag ens. is wat die algehele beste oplossing is nie. Die spesifieke applikasie waarvoor die lae wentelbaan satelliet gaan gebruik word moet geanaliseer word sowel as die spesifieke karakteristieke van daai applikasie. Daarvolgens moet n unieke konfigurasie opgestel word wat meeste van die applikasie se behoeftes bevredig.

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The forest industry is a fundamental cornerstone of the Swedish economy employing over 70 000 workers on a national scale. In recent years, the industry has seen increased efforts to digitalize operations and management of forestry to reap economical rewards, improve efficiency and gain competitive advantages. Due to living nature of trees, forest-management is a critical activity within the industry; trees and forests maintained properly will reward the owner with not only greater quality and greater quantity of wood, but also a shorter realization of cash flow. Fully realized, data from UPM Skog suggests that proper forest management has the potential to increase forest returns with up to 50 percent. This thesis studies forest management in the context of digitalization with the aim of making forest monitoring more digital and intelligent. In particular, it studies unsupervised monitoring in detail, a subset of remote sensing. Applications and effects of unsupervised monitoring include but are not limited to; enable real-time management of forest inventory, improved planning, time logging to market-fluctuations and enhanced biodiversity. There are a multitude of technologies that could be utilized and applied to achieve unsupervised monitoring and the startingpoint for discussion are the technologies Internet of Things and Enterprise Blockchain. The research is carried out as a case-study utilizing qualitative semi-structured interviews as primary data collection tool combined with a rigorous literature study in the relevant technological areas. The findings of this thesis portray a trade-off between infrastructure, maintainability, precision and latency on the choice of communication technology. Further, it discusses various ways to work around the different drawbacks of specific technology choices and in what contexts they be more or less suitable. Blockchain technology can act as an enabler for multiple novel industrial applications due to is properties of immutability, transparency and trust; however, not critical for unsupervised monitoring applications in the short run. This research serves as a first step in exploring the area of Internet of Things and Blockchain applied to unsupervised monitoring of forestry. In doing so, the thesis addresses infrastructural considerations, economic feasibility as well as key implications in terms of change management. / Skogsindustrin är en hörnsten för svensk ekonomi och sysselsätter fler än 70 000 arbetare på en nationell skala. De senaste åren har industrin sett ökade satsningar mot att digitalisera förvaltning och operation av skog med syftet att nå ekonomisk avkastning, ökad effektivitet och förbättrad konkurrenskraft. Till följd av trädens levandenatur är skogsförvaltning en kritisk aktivitet inom industrin; trä och skog som underhålls på rätt sätt belönar ägaren med både höjd kvalitet på virke och högre kvantitet av virke samt snabbare realisering av kassaflöden. Data från UPM Skog uppskattar att god skogsförvaltning har potential att medföra upp till 50 procent högre avkastning från skogen. Denna uppsats studerar skogsförvaltning inom kontextet av digitalisering med syftet att göra förvaltningen mer digital och mer intelligent. Mer specifikt studeras tillsynslös övervakning (TÖ) i detalj, ett område som kan tolkas som ett subområde till fjärranalys. Tillämpningar och effekter av TÖ inkluderar men är inte begränsat till: möjliggöra real-tids förvaltning av skogsinnehav, förbättrad planering, optimerad avverkning efter marknadens fluktuationer samt också ökad biodiversitet. Det finns flertalet teknologier som kan användas och appliceras för att åstadkomma TÖ och startpunkten i denna uppsats är teknologierna Internet of Things och Enterprise Blockchain. Forskningen är utförd som en fallstudie och nyttjar kvalitativa semistrukturerade intervjuer som primärdata samt en rigorös litteraturanalys inom relevanta teknologiområden. Undersökningsresultaten av denna fallstudie påvisar en avvägning mellan infrastruktur, underhållbarhet, precision och latens på den valda kommunikationsteknologin. Vidare undersöks olika sätt att arbeta runt olika nackdelar med de specifika teknologivalen samt utreder i vilka kontext som teknologierna är mer eller mindre lämpliga. Blockchain kan möjliggöra flertal nya industriella applikationer med hjälp av sina egenskaper som oföränderlighet, transparens och tillförlitlighet; men kan inte bedömas vara kritisk för TÖ utifrån ett kort tidsperspektiv. Denna forskning är ett första steg i att utforska området Internet of Things och Blockchain applicerat mot TÖ i skog. I detta adresseras allt från infrastrukturella frågeställningar, ekonomisk genomförbarhet samt implikationer inom förändringsledning.

Lithium-ion batteries are a possible choice for the energy storage system onboard hybrid electric vehicles and low-earth-orbit satellites, but lifetime performance remains an issue. The challenge is to diagnose the effects of ageing and then investigate the dependence of the magnitude of the deterioration on different accelerating factors (e.g. state-of-charge (SOC), depth-of-discharge (DOD) and temperature). Lifetime studies were undertaken incorporating different accelerating factors for two different applications: (1) coin cells with a LixNi0.8Co0.15Al0.05O2-based positive electrode were studied with a EUCAR power-assist HEV cycle, and (2) laminated commercial cells with a LixMn2O4-based positive electrode were studied with a low-earth-orbit (LEO) satellite cycle. Cells were disassembled and the electrochemical performance of harvested electrodes measured with two- and three-electrode cells. The LixNi0.8Co0.15Al0.05O2-based electrode impedance results were interpreted with a physically-based three-electrode model incorporating justifiable effects of ageing. The performance degradation of the cells with nickelate chemistry was independent of the cycling condition or target SOC, but strongly dependent on the temperature. The positive electrode was identified as the main source of impedance increase, with surface films having a composition that was independent of the target SOC, but with more of the same species present at higher temperatures. Furthermore, impedance results were shown to be highly dependent on both the electrode SOC during the measurement and the pressure applied to the electrode surface. An ageing hypothesis incorporating a resistive layer on the current collector and a local contact resistance (dependent on SOC) between the carbon and active material, both possibly leading to particle isolation, was found to be adequate in fitting the harvested aged electrode impedance data. The performance degradation of the cells with manganese chemistry was accelerated by both higher temperatures and larger DODs. The impedance increase was small, manifested in a SOC-dependent increase of the high-frequency semicircle and a noticeable increase of the high-frequency real axis intercept. The positive electrode had a larger decrease in capacity and increase in the magnitude of the high-frequency semi-circle (particularly at high intercalated lithium-ion concentrations) in comparison with the negative electrode. This SOC-dependent change was associated with cells cycled for either extended periods of time or at higher temperatures with a large DOD. An observed change of the cycling behaviour in the second potential plateau for the LixMn2O4-based electrode provided a possible kinetic-based explanation for the change of the high-frequency semi-circle. / Litiumjonbatteriet är en möjlig kandidat för energilagring i hybridfordon och i satelliter i låg omloppsbana, men än så länge är livslängden på batterierna ett problem. Utmaningen ligger i att kunna förstå hur batteriet åldras genom att utforska hur åldringsprocessen accelereras av faktorer som laddningstillstånd, urladdningsdjup och temperatur. Livslängdsstudier för två olika typer av batterier tänkta för olika applikationer utfördes: (1) knappceller med positiva LixNi0,8Co0,15Al0,05O2-baserade elektroder studerades med en effektstödd (power-assist) hybridcykel från EUCAR, och (2) laminerade kommersiella celler med positiva LixMn2O4-baserade elektroder studerades med en satellitcykel, avsedd för en satellit med låg omloppsbana. Cellerna öppnades och de uttagna elektrodernas elektrokemiska egenskaper utvärderades i två- och tre-elektroduppställningar. Resultaten från elektrokemiska impedansmätningar för den positiva LixNi0,8Co0,15Al0,05O2-baserade elektroden tolkades med hjälp av en fysikalisk tre-elektrod modell som tog hänsyn till de i litteraturen främst föreslagna effekterna av åldring. Prestandadegraderingen av celler med nickelkemi var oberoende av cykel och laddningstillståndet där åldringen skedde, men starkt beroende av temperaturen. Den positiva elektroden visade sig vara den största orsaken till impedansökningen i batteriet. Ytfilmerna på den positiva elektroden hade en sammansättning som var oberoende av laddningstillståndet men beroende av temperaturen. Impedansresultaten från de uttagna elektroderna var starkt beroende av både laddningstillstånd och yttre tryck på elektrodytan. Det visade sig att det var tillräckligt att ta hänsyn till ett resistivt skikt på strömtilledaren och en lokal kontaktresistans mellan kolet och det aktiva materialet (som är beroende av laddningstillståndet) för att anpassa modellen till impedansdata mätt på de uttagna elektroderna. Prestandadegraderingen av celler med mangankemi påskyndades av både högre temperaturer och högre urladdningsdjup. Impedansen ökade något, då både högfrekvenshalvcirkeln och högfrekvensintercepten ändrades. Positiva elektroden hade en större degradering i kapaciteten och en större ökning i magnituden av högfrekvenshalvcirkeln (speciellt vid högre litiumjon koncentrationer i elektroden) jämfört med den negativa elektroden. Denna laddningstillståndsberoende impedans-ökning var kopplad till celler som hade cyklats under en längre tid eller vid en högre temperatur och med ett högt urladdningsdjup. Ökningen i magnituden av högfrekvenshalvcirkeln skulle kunna vara relaterad till kinetiska begränsningar eftersom cyklingsbeteendet vid andra spänningsplatån ändrades samtidigt för de LixMn2O4-baserade elektroderna. / QC 20100621