M.Ing. / The increase of multimedia and broadband communications in recent years has brought about the creation and implementation of synchronousdigital hierarchy (SDH) in the terrestrial network. The implementation of SDH in the satellite sector is a natural extension to improve the current available networks. It will drastically increase the flexibility and usability of the current under-utilized satellite sector. Unfortunately, SDH has still not been implemented in the satellite sector due to certain inherent synchronization problems of SDH and satellites. This thesis investigates some of these synchronization problems. A simulation tool was created that makes an accurate assessment of SDH synchronization in the satellite sector. The tool was used to study SDH links over goestationary (GEO), medium earth orbiting (MEO) and low earth orbiting (LEO) satellites. Various changeable SDH parameters were investigated to determine their influences. Parameters like the bit rate and elastic store length or hysteresis lengths were considered.

No description available.

No description available.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The telecommunications link between a LEO satellite and a rural ground station with a non-tracking antenna, has a strongly varying link quality and a short communications window. The satellite acts as a store-and-forward node between ground stations. The TC-SDLP and an FTP protocol form a shallow protocol stack, which excludes unneeded protocol functionality and the resulting overhead. Coding gain, introduced by BCH FEC in the TCSDLP, allows for link quality improvement. The core of this thesis is an improvement of the TC-SDLP to maximise effective payload data throughput, or goodput. This improvement was achieved by creating an optimal segment length selection metric based on the BER. Since the BER is not determinable from within the TC-SDLP, the metric was twice determined; once based on the FER and finally based on time delays. The work includes an extensive background study, which consists of space standardisation, orbital physics, error detection and correction, space datalink protocols, data throughput and culminating in the protocol stack design. The project specific link budget calculation is presented. The optimal segment length policy was mathematically determined. Asimulation model of the TC-SDLP was used as a proof of concept for the effective throughput and give a performance benchmark. Finally a TC-SDLP implementation offers a real world performance demonstration. / AFRIKAANSE OPSOMMING: Die telekommunikasie skakel tussen ’n lae aardomwenteling (LEO) sateliet en ’n plattelandse grondstasie met ’n nie-volg antenna, het ’n skakelkwaliteit wat in ’n groot mate varieer en ’n kort kommunikasievenster. Die sateliet tree op as ’n stoor- en- aanstuur node tussen grondstasies. Die TC-SDLP en ’n leêr oordrag protokol (FTP) vorm ’n vlak protokol stapel, wat onnodige protokol funksionaliteit en die gevolglike opkoste uitsluit. Kode aanwins, wat deur die BCH FEC in die TC-SDLP, aangebring word, verbeter die skakelkwaliteit. Die kern van hierdie tesis is ’n verbetering van die TC-SDLP om sodoende die ware deurvoer van nuttige vragdata te maksimimeer. Hierdie verbetering is bereik deur die skep van ’n optimale segmentlengte-seleksie metode gebaseer of die bit fout tempo (BER). Aangesien die BER nie bepaal kan word vanuit die TC-SDLP nie, is die maatstaf twee keer bepaal; die eerste keer is die bepaling gebaseer op die raamwerk fout tempo (FER) en die finale bepaling op tyd vertragings. Die tesis sluit ’n omvattende agtergrondstudie in, wat bestaan uit ruimte standardisering, wentelbaan fisika, die opspoor en regstel van foute, ruimte inligtingskakel protokol en deurstuur van data wat uitloop op die protokol ontwerp. Daar word aangedui hoe die berekening van die begroting vir die skakel van toepassing op die spesifieke projek, gedoen is. ’n Wiskundige analise van die optimale segmentlengte s ook gedoen. ’n Simulasie model van die TC-SDLP is gebruik as ’n bewys van die konsep vir die ware deurset en gee ’n prestasie maatstaf. Laastens bied die TCSDLP implementering ’n ware wereld prestasie demonstrasie.

published_or_final_version / Electrical Engineering / Master / Master of Philosophy

Approved for public release; distribution is unlimited. / NPSAT1 is a gravity-gradient friendly, prolate body designed to fly at 600 Å 40 km inclined to 34.5 degrees. The satellite uses a magnetic 3-axis active attitude control system (ACS) using magnetic torque rods that interact with the Earth's magnetic field. This thesis accomplishes three goals. The first objective was to verify and to validate the magnetic attitude control system program and model developed by Leonard. The verification and validation process was completed in two steps. The first step accomplished an independent modeling of the Earth's magnetic field using MATLAB. The second step completed a verification via inspection of Leonard's ACS SIMULINK model. The verification confirmed that Leonard's modular sub-components of the disturbance torques, the quaternion vectors, the Euler angles, the spacecraft kinematics and dynamics, and the ACS control laws conformed to current ACS empirical theory. The second goal was to establish a laboratory used to demonstrate the ACS robustness and ability to perform as designed. The laboratory was created to house an air-bearing platform that simulates NPSAT1 characteristics. The third goal was to perform hardware-in-the-loop experiments with the NPSAT1 ACS software and model. Hardwarein- the-loop tests were performed to the magnetic torque rods, torque rod driver circuit board, micro-controller computer, and control interfaces. Specifically, solenoid current tests, magnetic field determination tests, and digital-to-analog conversion tests were completed. / Lieutenant Commander, United States Navy

The warfighter constantly needs increased accuracy from GPS and a means to increasing this accuracy to the decimeter level is a broadcast ephemeris message containing GPS satellite orbit and clock corrections. The ephemeris message is produced at the GPS MCS (Master Control Station) which receives GPS signal data from NGA and Air Force worldwide and uses sophisticated software to produce the orbit and clock corrections. The problem is getting the ephemeris message to the tactical user in a forward operating area. This thesis proposed a notional architecture for pushing the ephemeris message to the tactical user. It then modeled the architecture and simulated the broadcast of the ephemeris message to a tactical user using NETWARS. The baseline architecture was simulated and analyzed and then additional constraints were placed upon the network to simulate a real-world model. The simulation results demonstrated that the architecture was feasible for ephemeris message broadcast with the constraints on time intervals between broadcasts, residual traffic and message size.

The concept of controlling optical laser beams on spacecraft for acquisition, tracking and pointing purposes is quickly becoming a reality. As a result, fine pointing of laser beams is turn out to be an increasingly important research topic. A unique testbed was constructed in order to study and develop new methods for laser beam control. This testbed, the Moving Target-Source Test Fixture (MTSTF), hosts a laser source, the Extremely Agile Relaying Laser Source (EARLS), which has the capability of automatically acquiring and directing a laser beam onto a satellite simulator while in motion. The purpose of this thesis is to make the EARLS platform operational by developing a tracking control system. The ultimate goal is to point the laser beam at the satellite simulatorâ s receiving telescope and maintain the laser within the telescopeâ s limits in the presence of structural disturbance induced by the EARLS motion.

Satellite broadband has the potential to provide Internet connectivity to people and places that are unreachable using other wired/wireless technologies. It is therefore a critical component of any service seeking to provide universal coverage. A universal service needs to support a range of Internet applications. While there is existing work on bulk and interactive applications with a characteristic transmission rate, popular Internet applications such as web browsing, web video download or variable-rate voice result in a network traffic that varies its transmission rate. This thesis first investigates the interaction of satellite resource request mechanisms with the quality of service offered to these variable-rate applications using a satellite platform based on DVB-RCS2. It shows that a flexible QoS architecture can be achieved by a combination of resource request methods and network-layer queuing. Variable-rate traffic also poses challenges to the widely deployed Transport Control Protocol (TCP). Previous work analysed the interaction with the congestion control algorithms and proposed Congestion Window Validation (CWV) to constrain the congestion window to the amount of traffic a transport has sent. TCP JAGO studied this problem for variable-rate bursty applications and proposed new algorithms; but these were shown to have drawbacks and there were no guidance on how to implement. This thesis therefore presents, newCWV, a practical mechanism to provide an appropriate estimate of the available path capacity and corresponding congestion control behaviour. This benefits variable-rate applications with shorter transfer durations, but has a consequence of allowing larger traffic bursts into the network that can increase packet loss. Burst mitigation techniques, such as TCP pacing, are proposed to deal with this. These techniques are implemented and tested in the Linux TCP/IP stack, where newCWV improves the burst transfer time. This benefit is particularly significant for the large delay of broadband satellite systems.

Approved for public release; distribution is unlimited / This research is part of a continued effort to simulate the effects of total dose radiation on the performance of single and composite operational amplifiers using PSPICE. This research provides further verification that the composite operational amplifier has a superior performance to the single operational amplifier while operating in a radiation flux. In this experiment, a single and composite op amp were constructed in PSPICE and implemented in a finite gain amplifier circuit. The effects of ionizing radiation were simulated by varying the parameters of the components that made up the op amps. These component parameters were varied in ways that would mimic the response of the actual components that were irradiated in previous research. The simulations were incrementally run to simulate an increasing radiation dose. The results of these simulations were then compared with the results of an actual study conducted at Naval Postgraduate School where similar circuits were irradiated using the school's LINAC. This procedure proved to be an improved method for predicting the effects of total dose radiation for radiation hardened devices and provided additional confirmation of the superior performance of the composite op amp over the single op amp. / Lieutenant, United States Navy