Localization and litigation of radio frequency interference for interferometric arrays

Thesis (PhD)--Stellenbosch University, 2018.ENGLISH ABSTRACT: Radio telescopes have increased exponentially in sensitivity ever since the first single dish radio telescopes were built in the 1930's. This trend continues with the development of next generation telescopes such as the Square Kilometre Array (SKA). Parallel to the development of radio telescopes, has been the rapid expansion of telecommunication technologies. Consequently, radio telescopes are becoming more sensitive in an environment with ever increasing radio frequency interference (RFI). The ideal solution to RFI that is detected by a radio telescope is to locate its source and then have it removed. Removal of the source is usually only possible if it is occurring in a protected band or the radio telescope is in a radio quiet zone. Unfortunately, most of the radio spectrum has been allocated to active communication services and not all radio telescopes are in radio quiet zones. The alternative is to mitigate its effect using methods such as spatial RFI mitigation. The contributions of this PhD dissertation are twofold: firstly, a source localization algorithm that takes into account the constraints and advantages of the arrays used for radio astronomy has been developed; and secondly, existing spatial RFI mitigation techniques have been adapted to take into account the bandwidth of the RFI signals. The computationally efficient localization algorithm that was developed is best suited for interferometric arrays with low array beam sidelobes. Two variants of the algorithm were developed, one that works for sources in the near-field and the other for far-field sources. In the near-field, the computational complexity of the algorithm is linear with search grid size compared to cubic scaling of the state-of-the-art 3-D MUSIC method. The trade-off is that the proposed algorithm requires a once-off a priori calculation and storing of weighting matrices. In an experiment using a station of the Low Frequency Array (LOFAR) a hexacopter was flown around the array, at a mean radial distance of 190 m, broadcasting a signal. The mean error in distance between the estimated position of the hexacopter and the GPS position of the hexacopter was 2 m for a wavelength of 6.7 m. The non-narrowband RFI mitigation method developed consists of a second order filter that is used to mitigate powerful RFI with bandwidth sufficient to cause aberrations that are below the noise, but with power that competes with the astronomical sources. The second order filter consists of a first order subspace subtraction filter combined with a flat frequency response model for the RFI source. Taking into account mutual coupling as well as a calibration step to account for unknown complex gains, the algorithm was found to process approximately 1.6 times more bandwidth than using just a first order subspace subtraction filter.AFRIKAANSE OPSOMMING: Sedert die eerste enkelskottel radioteleskope in die 1930's gebou is, het die sensitiwiteit van radioteleskope eksponensieël toegeneem. Hierdie tendens gaan voort met die ontwikkeling van volgende generasie teleskope, soos byvoorbeeld die Square Kilometer Array (SKA). In parallel met die ontwikkeling van radioteleskope, het telekommunikasietegnologieë ook vinnig uitgebrei. Gevolglik word radioteleskope meer sensitief in 'n omgewing met toenemende radiofrekwensie-inmenging (RFI). Die optimale oplossing vir RFI is om die bron daarvan op te spoor en te verwyder. Verwydering van die bron is gewoonlik net moontlik as dit teenwoording is in 'n beskermde band of as die radio teleskoop in 'n radio-stil gebied is. Ongelukkig is meeste van die radio spectrum toegeken aan kommunikasiedienste en nie alle radio teleskope is in radio-stil gebiede nie. Die alternatief om die effek daarvan te mitigeer deur middel van metodes soos ruimtelike RFI-mitigasie. Die bydraes van hierdie doktorale proefskrif is tweeledig: eerstens, die ontwikkeling van 'n bronlokaliseringsalgoritme wat die beperkings en voordele van die skikkings wat gebruik word vir radio astronomie in ag neem en tweedens, die aanpassing van bestaande ruimtelike RFI mitigeringstegnieke om die bandwydte van die RFI seine in ag te neem. Die berekeningsdoeltreffende lokaliseringsalgoritme wat ontwikkel is, is die beste geskik vir interferometriese skikkings met lae samestelling-bundel sylobbe. Twee weergawes van die algoritme is ontwikkel, die eerste hanteer bronne in die nabyveld en die ander hanteer vêrveld bronne. In die nabyveld is die berekeningskompleksiteit van die algoritme lineêr met soektogroostergrootte in vergelyking met die kubieke skalering van die 3-D MUSIC-metode. Die nadeel is dat die voorgestelde algoritme 'n eenmalige a priori berekening en stoor van gewigsmatrikse vereis. In 'n eksperiment by 'n stasie van die Low Frequency Array (LOFAR), het 'n heksakopter oor die skikking gevlieg met 'n gemiddelde radiale afstand van 190 m en 'n sein uitgesaai. Die gemiddelde fout in die afstand tussen die beraamde posisie van die heksakopter en die GPS-posisie van die heksakopter was 2 m vir 'n golflengte van 6.7 m. Die nie-smalband RFI mitigasie metode wat ontwikkel is, fasiliteer die de-finieering van 'n tweede-orde filter wat gebruik word om kragtige RFI met bandwydte verwante krag onder die geruis, maar met krag wat met die astronomiese bronne kompeteer, te mitigeer. Die tweede order filter bestaan uit 'n eerste orde subruimte verminderingsfilter gekombineer met 'n plat frekwensie responsmodel vir die RFI bron. Met inagneming van wedersydse koppeling asook 'n kalibrasie stap om vir onbekende komplekse antenna aanwinste voorsiening te maak, is gevind dat die algoritme ongeveer 1.6 meer bandwydte kan verwerk as 'n eerste orde subruimte verminderingsfiter

Targeted searches for prompt radio transients using the Murchison Widefield Array

My thesis presents a targeted search for prompt radio transients from gamma-ray bursts (GRBs) and repeating fast radio bursts (FRBs) using the Murchison Widefield Array (MWA). I used the MWA rapid-response mode to follow up a large sample of GRBs, and derived some of the best constraints on relevant coherent emission models. I also performed an extensive search for low frequency FRB emission, and derived significant constraints on FRB emission models

Gigahertz Bandwidth and Nanosecond Timescales: New Frontiers in Radio Astronomy Through Peak Performance Signal Processing

Abstract In the past decade, there has been a revolution in radio-astronomy signal processing. High bandwidth receivers coupled with fast ADCs have enabled the collection of tremendous instantaneous bandwidth, but streaming computational resources are struggling to catch up and serve these new capabilities. As a consequence, there is a need for novel signal processing algorithms capable of maximizing these resources. This thesis responds to the demand by presenting FPGA implementations of a Polyphase Filter Bank which are an order of magnitude more efficient than previous algorithms while exhibiting similar noise performance. These algorithms are showcased together alongside a broadband RF front-end in Starburst: a 5 GHz instantaneous bandwidth two-element interferometer, the first broadband digital sideband separating astronomical interferometer.  Starburst technology has been applied to three instruments to date. Abstract Wielding tremendous computational power and precisely calibrated hardware, low frequency radio telescope arrays have potential greatly exceeding their current applications.  This thesis presents new modes for low frequency radio-telescopes, dramatically extending their original capabilities.  A microsecond-scale time/frequency mode empowered the Owens Valley Long Wavelength Array to inspect not just the radio sky by enabling the testing of novel imaging techniques and detecting overhead beacon satellites, but also the terrestrial neighborhood, allowing for the characterization and mitigation of nearby sources of radio frequency interference (RFI).  This characterization led to insights prompting a nanosecond-scale observing mode to be developed, opening new avenues in high energy astrophysics, specifically related to the radio frequency detection of ultra-high energy cosmic rays and neutrinos. Abstract Measurement of the flux spectrum, composition, and origin of the highest energy cosmic ray events is a lofty goal in high energy astrophysics. One of the most powerful new windows has been the detection of associated extensive air showers at radio frequencies. However, all current ground-based systems must trigger off an expensive and insensitive external source such as particle detectors - making detection of the rare, high energy events uneconomical.  Attempts to make a direct detection in radio-only data have been unsuccessful despite numerous efforts. The problem is even more severe in the case of radio detection of ultra-high energy neutrino events, which cannot rely on in-situ particle detectors as a triggering mechanism. This thesis combines the aforementioned nanosecond-scale observing mode with real-time, on-FPGA RFI mitigation and sophisticated offline post-processing.  The resulting system has produced the first successful ground based detection of cosmic rays using only radio instruments. Design and measurements of cosmic ray detections are discussed, as well as recommendations for future cosmic ray experiments.  The presented future designs allow for another order of magnitude improvement in both sensitivity and output data-rate, paving the way for the economical ground-based detection of the highest energy neutrinos.</p

High-latitude over-the-horizon radar applications

Thesis (Ph.D.) University of Alaska Fairbanks, 2020Over-the-horizon radar (OTHR) systems that operate at high-latitudes often must contend with multipath and pronounced diffusive scattering effects produced by the anisotropic, birefringent, and heterogeneous nature of the ionosphere. In this thesis, radar performance at high-latitudes is quantified and several applications for either mitigating the deleterious effects of multipath and diffusive scattering or deriving information about the state of the ionosphere are proposed. The first application is inspired by adaptive optics techniques in other fields and involves the coherent summation of the received plane wave spectrum in order to improve angular resolution and array gain. The second application involves deriving ionospheric E x B drift from applying spatial correlation analysis to ground clutter echoes. The third application is the development of a new spatial adaptive processing technique designed specifically to preserve the Doppler spectrum of angle-Doppler coupled clutter like that observed at high-latitudes.1. Introduction -- 2. Scintillation correction in phased array high-frequency radar -- 3. Ground clutter spatial correlation analysis: transverse ionospheric drift velocity -- 4. MV-SAP: Preserving angle-doppler coupled clutter -- 5. Conclusions & future work -- Appendix: Alternative derivation of ground clutter MC

Abstracts on Radio Direction Finding (1899 - 1995)

The files on this record represent the various databases that originally composed the CD-ROM issue of "Abstracts on Radio Direction Finding" database, which is now part of the Dudley Knox Library's Abstracts and Selected Full Text Documents on Radio Direction Finding (1899 - 1995) Collection. (See Calhoun record https://calhoun.nps.edu/handle/10945/57364 for further information on this collection and the bibliography). Due to issues of technological obsolescence preventing current and future audiences from accessing the bibliography, DKL exported and converted into the three files on this record the various databases contained in the CD-ROM. The contents of these files are: 1) RDFA_CompleteBibliography_xls.zip [RDFA_CompleteBibliography.xls: Metadata for the complete bibliography, in Excel 97-2003 Workbook format; RDFA_Glossary.xls: Glossary of terms, in Excel 97-2003 Workbookformat; RDFA_Biographies.xls: Biographies of leading figures, in Excel 97-2003 Workbook format]; 2) RDFA_CompleteBibliography_csv.zip [RDFA_CompleteBibliography.TXT: Metadata for the complete bibliography, in CSV format; RDFA_Glossary.TXT: Glossary of terms, in CSV format; RDFA_Biographies.TXT: Biographies of leading figures, in CSV format]; 3) RDFA_CompleteBibliography.pdf: A human readable display of the bibliographic data, as a means of double-checking any possible deviations due to conversion