Localization and ,litigation of radio frequency interference for interferometric arrays

Steeb, Jan-Willem W (2018-12)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

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.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/104989
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