Radio astronomy, the survey and study of naturally occurring astronomical radio signals, is a challenging field in terms of engineering requirements. The typical astronomical signal of interest is incredibly faint, resulting in very low signal-to-noise ratios (SNRs) on the order of -30 dB or lower. To detect such signals, one must have an uncommonly low and stable receiver noise temperature, high gain through large aperture reflectors, and state-of-the-art signal processing algorithms. One must also be able to mitigate the effects of interference, the presence of which, even if extremely weak, can completely mask the faint astronomical signals of interest. To this end, this work presents the development of and results from a new broadband phased array feed (PAF) named the Focal L-Band Array for the Green Bank Telescope (FLAG). This instrument is able to form multiple simultaneous beams to survey a large patch of sky instantaneously, and has a minimum system noise temperature (Tsys) of 16.83 K. This PAF also has the potential to use spatial filtering techniques to place pattern nulls in the direction of interfering signals through the use of an orthogonal projection. This work will also present an improved method for computing an orthogonal projection operator, which is able to place a spatially broad null in the direction of a moving RFI source. A formal derivation of some detection and estimation theory properties for astronomical radio transients is also presented, which formalization is lacking within the astronomical community. This includes maximum-likelihood detectors and estimators and a Cramér Rao bound (CRB) analysis of astronomical transient parameters.

In recent years, the Brigham Young University (BYU) Radio Astronomy Systems group has developed phased-array feeds and the data acquisition processing systems necessary to perform radio astronomy observations. This thesis describes the development and testing of a real-time digital beamforming system that reduces both the time required to process phased-array feed data and the disk space used to record this data compared to post-processing beamforming systems. A real-data experiment is also discussed in this thesis, which focuses on some of the data post-processing required for one of BYU's data acquisition systems.Radio-frequency interference mitigation techniques for phased-array feed radio astronomy have been studied for several years, but the effect that these techniques have on weak-signal detection is not well understood. This thesis provides analysis of a simulated weak-source observation for the Green Bank 20-meter telescope and BYU 19 element phasedarray feed with radio-frequency interference present. Interference mitigation techniques are shown to reduce the detectability of weak sources compared with the no interference case, but it is also shown that a weak source can be detected that would otherwise be masked by interference.

Land mines are a major humanitarian problem with millions of active mines in place around the world. Since these mines can have little metal in them, novel detection techniques are needed. Nuclear Quadrupole Resonance (NQR) is one such technique. Unfortunately, NQR is highly succeptible to radio frequency interference (RFI). A significant contribution of this thesis is the development of a custom, experimental data acquisition system designed and built specifically for capturing RFI at frequencies significant to NQR land mine detection systems. Another major contribution is the development of data analysis techniques for determining the number of reference antennas required to effectively cancel out RFI at frequencies and in environments typical of an NQR land mine detection system.

Thesis (PhD)--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: The Square Kilometre Array (SKA) and its demonstrator MeerKAT are being designed to operate over a wide frequency range and are expected to achieve greater sensitivity and resolution than existing telescopes. The radio astronomy community is well aware of the negative impact that radio frequency interference (RFI) has on observations in the proposed frequency band. This is because weak radio signals such as those from pulsars and distant galaxies are difficult to detect on their own. The presence of RFI sources in the telescope’s operating area can severely corrupt observation data, leading to inaccurate or misleading results. Power-line interference and radiation from electric fences are examples of RFI sources. Mitigation techniques for these interference sources in the SKA system’s electromagnetic environment are essential to ensure the success of this project. These techniques can be achieved with appropriate understanding of the characteristics of the noise sources. Overhead power-line interference is known to be caused mainly by corona and gap-type (commonly known as sparking noise) discharges. Sparking noise is the dominant interference for the SKA. It is mainly encountered on wooden pole lines, which are usually distribution lines operated at up to 66 kV AC in the South African network. At this voltage level, the voltage gradients on the lines are insufficient to generate conductor corona. The power requirements for SKA precursors will be below this voltage level. The aim of the research in this dissertation is to evaluate the power line sparking characteristics through measurements and simulation of line radiation and propagation characteristics. An artificially made sparking noise generator, which is mounted on a power line, is used as noise source and the radiation characteristics are measured. Measurements were carried out in different environments, which included a high-voltage laboratory (HV-Lab), a 40m test-line, and another 22-kV test line of approximately 1.5 km. The key sparking noise parameters of interest were the temporal and spectral characteristics. The time domain features considered were the pulse shape and the repetition rate. The lateral, longitudinal and height attenuation profiles were also quantified. Since sparking noise current pulses are injected or induced onto power line conductors, the line will act as an unintentional antenna. The far-field radiation characteristics of the line were evaluated through measurements on physical scale-model structures and simulations. 1/120th and 1/200th scaled lines, using an absorbing material and metallic ground planes, respectively, were simulated in FEKO. The measurements of the constructed scale models were taken in the anechoic chamber. Both measurements and simulations showed that the line exhibits an end-fire antenna pattern mode. Line length, pulse injection point and line configuration were some of the parameters found to affect the radiation patterns. The findings from this study are used to determine techniques to identify the sparking noise, and locate and correct the sources when they occur on the line hardware. Appropriate equipment is recommended to be used for the location and correction of sparking noise. / AFRIKAANSE OPSOMMING: Die Vierkante Kilometer Reeks (SKA) en sy demonstrasie projek, die Karoo Reeks Teleskoop (KAT), word ontwerp om oor 'n wye frekwensie-bereik te funksioneer. Beide sal na verwagting beter sensitiwiteit en resolusie as bestaande radioteleskope he. Die radio-astronomie-gemeenskap is deeglik bewus van die negatiewe impak wat radio-frekwensie steurnisse (RFS) op waarnemings in die voorgestelde frekwensieband het. Die rede hiervoor is dat swak radio-seine soos die van pulsars en verafgelee sterrestelsels inherent moeilik is om te bepaal. Die teenwoordigheid van RFS bronne in die teleskoop se onmiddellike operasionele gebied kan waarnemings nadelig beinvloed. Dit lei uiteindelik tot onakkurate of misleidende resultate. Kraglyne en uitstralings van elektriese heinings is voorbeelde van RFS bronne. Metodes om die oorsake van die steurnisse van die SKA se elektromagnetiese omgewing te verminder is noodsaaklik om die sukses van hierdie projekt te verseker. Dit vereis egter deeglike begrip van die eienskappe van hierdie bronne. Steurnisse as gevolg van oorhoofse kraglyne word hoofsaaklik veroorsaak deur korona en gapingtipe ontladings (algemeen bekend as vonkontladings). Vonkontladings word hier beskou as die belangrikste oorsaak van steurnisse vir die SKA. Dit word in die Suid-Afrikaanse netwerk hoofsaaklik aangetref op houtpaal-installasies, wat gewoonlik bestaan uit distribusie lyne wat tot en met 66 kV wisselstroom (WS) bedryf word. By hierdie operasionele spanning is die spanningsgradient op die lyn onvoldoende om korona op te wek. Die kragvereistes vir die SKA se voorafgaande projekte sal sodanig wees dat hierdie spanningsvlak nie oorskry sal word nie. Die doel van die navorsing omskryf in hierdie proefskrif is om die eienskappe van vonkontladings rondom kraglyne te evalueer. Dit word gedoen met behulp van metings en simulasies van uitstralings- en voortplantingspatrone wat met 'n spesifieke lyn geassosieer kan word. 'n Kunsmatige vonkontladingsopwekker word op 'n kraglyn geplaas en dien as bron om die uitstralingspatrone te meet. Metings is uitgevoer in verskillende omgewings, insluitende 'n hoogspanningslaboratorium (HV-Lab), 'n 40 m toetslyn en 'n 22 kV WS toetslyn van ongeveer 1.5 km lank. Die hoof vonkontladings eienskappe van belang is die temporale en spektrale eienskappe. Die tydgebiedeienskappe wat ondersoek is, is die pulsvorm asook die pulsherhalingskoers. Die laterale, longitudinale en hoogte-attenuasie profiele word ook gekwantifiseer. Aangesien stroompulse deur vonkontladings op die kraglyn geplaas of geinduseer word, sal die lyn as 'n ongewenste antenna optree. Die ver-veld uitstralingskenmerke van die lyn is ook geëvalueer deur gebruik te maak van fisiese skaalmodelstrukture en -simulasies. 1/120ste en 1/200ste geskaleerde lynmodelle, wat onderskeidelik 'n absorberende- en metaalgrondvlak bevat, was gebruik om 'n 3 spanlengte kraglyn te simuleer met behulp van FEKO. Metings van die fisiese skaalmodel strukture is in 'n anegoise kamer geneem. Beide die metings en die simulasies toon dat die lyn 'n endpunt uitstralingspatroon het. Lynlengte, die opwekkingsposisie van die stroompuls en die lynkonfigurasie is 'n paar van die parameters wat die uitstralingpatroon beïnvloed, soos in die navorsing aangedui. Die bevindinge van hierdie studie word gebruik om steurnisse as gevolg van vonkontladings op die kraglyn te identifiseer, op te spoor en uiteindelik reg te stel. Toepaslike toerusting word voorgestel wat gebruik kan word vir die identifisering en opsporing van vonkontladings.

The Brigham Young University (BYU) Radio Astronomy Systems group, in collaboration with the National Radio Astronomy Observatory (NRAO), the Center for Astrophysics at West Virginia University (WVU), and the Green Bank Observatory (GBO) have developed, and commissioned, a broadband real-time digital back end processing system for a 38-element phased array feed (PAF) with 150 MHz of instantaneous bandwidth. This system is capable of producing coarse and fine channel correlations, and implements a real-time beamformer that forms 7 simultaneous dual-polarized beams. This thesis outlines the hardware and software development for the digital back end and presents on-telescope commissioning results. This system has been measured to provide an unprecedented low Tsys/η noise level of 28 K and can perform maps of galactic hydrogen observations in a fraction of the time of a conventional single horn feed. The National Radio Astronomy Observatory (NRAO) has recently announced the concept and development of the next generation Very Large Array (ngVLA), a large interferometric array consisting of 300 radio telescopes and longest baseline (distance between a pair of antennas) of 300 km. Large interferometric arrays have been shown to attenuate radio frequency interference (RFI) because it is decorrelated as it propagates across long baselines. This is not always sufficient, especially with dense core array geometries and with the ever-increasing amount of strong RFI sources. Conventional RFI projection-based mitigation techniques have performed poorly on large interferometers because of covariance matrix estimation error due to decorrelation when identifying interference subspace parameters. This thesis presents an algorithm that overcomes the challenge of decorrelation by applying subspace projection via subarray processing (SP-SAP). Each subarray is designed to have a set of elements with high mutual correlation in the interferer for better estimation of subspace parameters. In simulation, compared to the former approach of applying subspace projection on the full array, SP-SAP improves mitigation of the RFI on the order of 9 dB. A signal of interest is shown then to be observable through the RFI in a full synthetic image.

Localisation, which is a mechanism for discovering the spatial relationship between objects, is an area that has received considerable research and development in recent times. A common name given to localisation operations based on the absolute reference frame of Earth is Geolocation. One important example of geolocation research is E-911, where wireless carriers in the United States must provide the location of 911 callers. The operation of E-911 can be based on either a network configuration, or the Global Positioning System (GPS). With the importance of localisation being acknowledged, a review concerning the vulnerability of the Global Navigation Satellite System (GNSS) is provided as background and motivation for this research. With the current vulnerability of GNSS, this dissertation presents the results of a research program undertaken with the objective of developing an electromagnetic localisation technique that can determine the relative position of GPS Radio Frequency Interference (RFI) sources. Intended for operation in a hostile environment, blind and passive localisation methodologies must be incorporated into the developed model. In performing localisation research, a background of current techniques is provided in addition to a review of current electromagnetic propagation models. From the review of propagation models, the Parabolic Equation Model (PEM) was chosen for investigation concerning localisation. The selection of PEM is due to model properties that are required for blind/passive localisation. The localisation system developed in this research program is based on the integration of inverse diffraction propagation (IDP) within the parabolic equation model. The title chosen for the localisation method is Inverse Diffraction Parabolic Equation Localisation System (IDPELS). This thesis presents the simulation and field trial results of IDPELS. Under simulation, the terrain or obstacle profiles were not based on any geodetic datum. Any estimate provided by IDPELS under simulation is therefore a "Localisation" solution. In the field trials however, IDPELS operation is referred to as "Geolocation" as geodetic datum's where used to determine the receiver's position. Under simulation analysis, IDPELS operation was considered to provide good promise as it could simultaneously perform localisation on multiple transmission sources. In each investigated simulation scenario, a display of signals amplitude (dB units) is displayed over the entire region. By determining the field convergence regions, a localisation estimate of IDPELS is provided. By defining the convergence regions as areas having the greatest signal amplitude values (i.e. ≥ 99%), elliptical areas as low as 3.2m² were considered to indicate an excellent localisation capability. With the theoretical validity of IDPELS operation in electromagnetics having been established under simulation, further investigation into the practical feasibility of the IDPELS was performed. The field trials positioned a continuous-wave (CW) transmission source at a known location. By measuring signal phasors along a straight section of road, the geodetic spatial-phase profile was used as the input signal for IDPELS. Road sections used were cross-wise to the transmitter's boresight. Many data sets were recorded, each being made over a sixty second time period. Different regions and ranges where used to continuously measure the spatial-phase profile of the signal with fixed antennas in a moving vehicle. Such a measurement process introduced an analogy with Synthetic Aperture Radar (SAR) processes. In quantitating the accuracy of the IDPELS geolocation estimate in field trials, the linear error of range and cross-range components was analysed. A free-space PEM model was chosen for development of IDPELS and hence, data sets demonstrating properties of a free-space environment were able to be considered suitable for testing of the geolocation method. Data sets demonstrating free-space propagation characteristics were measured at the base of the Mt Lofty ranges in South Australia, where the range and cross-range error are respectively 3.14m, and 0.15m. Such low error values clearly demonstrate the practical feasibility of IDPELS geolocation. With the practical feasibility of IDPELS having been established in this research program, a novel contribution to electromagnetic geolocation methodologies is provided. An important characteristic of any geolocation technique concerns its robustness to operate in a wide variety of possible environments. With continued development of IDPELS, the robustness of this passive/blind geolocation technique can be enhanced. Further assistance with geolocation of multiple transmission sources is also indicated to be available by IDPELS, as shown in the simulation analysis.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: This thesis describes the design and construction of a two-element digital correlation interferometer operating at 1.42 GHz. This instrument consists of two 92cm off set-parabolic reflectors arranged in an east-west baseline, with a maximum length of 24m. The reflectors are out-fi tted with helical beam antennas as primary feeds, and are further equipped with low-noise front-ends. A wideband dual-conversion superheterodyne receiver processes the astronomical signals before digitisation. The receiver is implemented as far as possible using a ffordable off -the-shelf technologies. The cross-correlation between the two antenna signals is measured using a 256MHz bandwidth digital FX correlator, and is implemented on a Reconfi gurable Open Architecture for Computing Hardware (ROACH) board. The preliminary observations made with this interferometer, suggests that it is possible to detect the sun, and some other much weaker sources such as Taurus A. The design of an interferometer in general is presented, from the formulation of the underlying instrumental requirements, to making meridian drift scan observations. The interferometer developed may serve as a demonstrator for other engineering students to gain a working knowledge of radio interferometry, which should prove invaluable when addressing the challenges the Square Kilometre Array (SKA) is faced with. / AFRIKAANSE OPSOMMING: In hierdie tesis word die ontwerp en konstruksie van 'n twee-element digitale korrelasie interferometer wat by 1.42 GHz ontvang beskryf. Die instrument bestaan uit twee 92 cm afsetparaboliese re ektorantennas met 'n oos-wes basislyn, met maksimum lengte 24 m. Heliese antennas word gebruik as primêre voer vir die reflektors, en verder maak die voorent gebruik van laeruis versterkers . Die wyeband superheterodineontvanger verwerk die astronomiese seine in twee stappe voordat dit digitaliseer word. Bekostigbare kommersiëele komponente word gebruik in die vervaardiging van die ontvanger. Die kruiskorrelasie tussen die twee antenna seine word digitaal gemeet deur 'n FX korreleerder met 'n 256MHz bandwydte op 'n ROACH bord. Die resultate toon aan dat dit moontlik is om die son waar te neem, sowel as sommige ander swakker bronne, soos Taurus A. Al die kwessies insluitende die instrumentale vereistes betrokke by die ontwerp van 'n interferometer word bespreek. Verder word die gebruik van interferometers om meridiaan meetings te doen bespreek. Die interferometer dien as 'n praktiese demonstrasie van radio interferometrie vir ingenieurstudente. Hierdie demonstrasie is voordelig vir studente om die uitdagings van die SKA te bestudeer.

Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The extension of the Karoo Array Telescope (KAT), MeerKAT, will be the most sensitive radio telescope in the southern hemisphere until it is superseded by the completion of the Square Kilometre Array (SKA). These instruments are to be constructed in the Karoo which is an area in South-Africa that is protected against Radio Frequency Interference (RFI) by the Astronomy Geographical Advantage (AGA) act. However, the telescope is also vulnerable to self-generated interference and specialised measurement systems are required to monitor RFI levels. The development of a ReAl Time Transient AnalYser (RATTY) is described and two Experimental Development Models (XDM) are compared. The first uses a mixing philosophy, and the second direct-sampling. The selection of these models was based on the evaluation of several analogue Front-End (FE) designs. A stripline-filter design process is presented along with the results obtained for custom filters developed for the FEs. Several analyses were compared to measurements performed with one of the devices and good agreement was shown between the system characteristics. Issues regarding the Spurious Free Dynamic Range (SFDR) of the FE designs were identified in the process and measurement-corrected simulations used to predict the achievable ranges. The outcome of the XDM comparison promotes the continued development of a direct-sampling strategy to fulfil the short-term goals of the project. A static calibration procedure is demonstrated for the mixing system and implemented to account for different FE configurations. An overview of the digital and software components of the RATTY system is given and Electromagnetic Compatibility (EMC) principles are applied during the construction of both systems. / AFRIKAANSE OPSOMMING: Die finale fase van die Karoo Reeks Teleskoop (KAT), MeerKAT, sal die sensitiefste radio teleskoop in die suidelike halfrond wees. Dit sal egter oortref word deur die vierkante kilometer reeks, wat die sensitiefste radio teleskoop ter wêreld sal word. Beide instrumente sal vatbaar wees vir radiofrekwensie steurings en sal opgerig word in ‘n wet-beskermde omgewing. Die teleskope is ook vatbaar vir radiofrekwensie steurings wat deur interne stelsels opgewek kan word. Dus word gespesialiseerde meetapparate benodig om die betrokke area en substelsels van die teleskope te monitor. Die ontwikkeling van ‘n meetinstrument vir die ontleding van kort-durasie tydseine (RATTY) word beskryf en twee eksperimentele ontwikkelings modelle word vergelyk. Hierdie modelle is gebaseer op die verfyning van voorafgaande ontwerpe vir die analoog substelsel van die instrument en hierdie proses word verduidelik. Die eerste model volg ‘n menger strategie waar die tweede model direkte-monstering implementeer. ‘n Dubbel-laag, mikrostrookfilter ontwerpsproses word beskryf en die gemete resultate vir die ontwikkelde filters word bespreek. Verskeie ontledings is aangewend en vergelyk met die gemete resultate van die stelsels. Hieruit word bevredigende ooreenkomste getref. Die beperkings van die modelle, weens interne distorsie, word geïdentifiseer in dié proses en verdere skattings word gemaak d.m.v. simulasies. Die eksperimentele modelle word vergelyk en die voorkeur van ‘n direkte-monsterings stelsel word gemotiveer. Die digitale en sagteware komponente word oorsigtelik behandel. Tydens die konstruksie van die modelle word die toepassing van elektromagnetiese verenigbaarheids beginsels verduidelik. Laastens word ‘n eenvoudige kalibrasie toegepas op die menger stelsel en ‘n toepassing daarvan word behandel.

Radio frequency interference has become a large problem to radio astronomers. This thesis proposes the idea that radio frequency interference can be mitigated using a phased array feed in conjunction with a large reflector. A phased array feed would allow radio astronomers to observe fainter signals than is currently possible, while at the same time enabling rapid sky surveys. A phased array feed was designed and simulated, and sensitivity optimization was performed on the array feed. It was shown that higher sensitivity can be achieved using a 7-element phased array feed than with a conventional waveguide feed. Simulations were ran using RFI mitigation algorithms on the array to show that interference cancellation can, in principle, be performed using a phased array feed. In addition to these simulations, improvements were made to a previously designed RF receiver so that radio astronomy observations could be made and interference mitigation algorithms tested on a receiver platform.

The Brigham Young University (BYU) Radio Astronomy group, in collaboration with Cornell University, the University of Massachusetts, and the National Radio Astronomy Observatory (NRAO), have in recent years developed and deployed PAF systems that demonstrated the advantages of PAFs for astronomy. However, these systems lacked the necessary bandwidth and acquisition times to be scientifically viable. This thesis outlines the development of a 20-MHz bandwidth system that can acquire for much longer periods of time and across much larger bandwidths than previous BYU systems. A report of the deployment of this system on the 305-meter reflector at the Arecibo Observatory in Puerto Rico is also summarized.The Commonwealth Scientific and Industrial Research Organisation (CSIRO) is currently constructing a PAF-equipped synthesis imaging array named the Australian Square Kilometre Array Pathfinder (ASKAP) that offers great promise for widening FOVs and enhancing RFI mitigation techniques. Previous work in RFI mitigation has demonstrated effective cancellation for synthesis imaging arrays under the assumption that the processing bandwidth is narrowband and correlator dump times are short. However, these assumptions do not necessarily reflect real-world instrument limitations. This thesis explores simulated adaptive array cancellation algorithm effectiveness as applied on the ASKAP instrument given realistic bandwidths and correlator dump times. The results demonstrate that active RFI mitigation performed across long baselines is largely ineffectual.