61 research outputs found

    Joe Pawsey and the Founding of Australian Radio Astronomy

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    This open access book is a biography of Joseph L. Pawsey. It examines not only his life but the birth and growth of the field of radio astronomy and the state of science itself in twentieth century Australia. The book explains how an isolated continent with limited resources grew to be one of the leaders in the study of radio astronomy and the design of instruments to do so. Pawsey made a name for himself in the international astronomy community within a decade after WWII and coined the term radio astronomy. His most valuable talent was his ability to recruit and support bright young scientists who became the technical and methodological innovators of the era, building new telescopes from the Mills Cross and Chris (Christiansen) Cross to the Parkes radio telescope. The development of aperture synthesis and the controversy surrounding the cosmological interpretation of the first major survey which resulted in the Sydney research group's disagreements with Nobel laureate Martin Ryle play major roles in this story. This book also shows the connections among prominent astronomers like Oort, Minkowski, Baade, Struve, famous scientists in the UK such as J.A. Ratcliffe, Edward Appleton and Henry Tizard, and the engineers and physicists in Australia who helped develop the field of radio astronomy. Pawsey was appointed the second Director of the National Radio Astronomy Observatory (Green Bank, West Virginia) in October 1961; he died in Sydney at the age of 54 in late November 1962. Upper level students, scientists and historians will find the information, much of it from primary sources, relevant to any study of Joseph L. Pawsey or radio astronomy. This is an open access book

    The Proof of Concept of The Hurricane Imaging Radiometer: Hurricane Wind Speed and Rain Rate Retrievals

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    This dissertation presents the proof of concept for the Hurricane Imaging Radiometer (HIRAD), where remote sensing retrievals of the 2-dimensional wind and rain fields for several hurricanes are validated with independent measurements. A significant contribution of this dissertation is the development of a novel statistical calibration technique, whereby the HIRAD instrument is radiometrically calibrated, using modeled brightness temperatures (Tb) generated using a priori hurricane wind and rain fields that are statistically representative of the actual hurricane conditions at the time of the HIRAD brightness temperature measurements. For this calibration technique, the probability distribution function of the measured HIRAD Tb\u27s is matched to the modeled Tb distribution. After applying this Tb calibration, hurricane wind speeds and rain rates are retrieved for six hurricane surveillance flights between 2013-2015. These HIRAD results are compared with available, statistically independent, surface measurements from in-situ GPS dropwindsondes and remote sensing: Stepped Frequency Microwave Radiometer (SFMR), and the High-Altitude Imaging Wind and Rain Aerial Profiler (HIWRAP). Since there is good agreement in the intercomparisons, it is concluded that the HIRAD hurricane measurement technique performs as intended, after the corresponding Tb images are properly calibrated. Furthermore, based upon the above comparisons, it is concluded that the retrieved HIRAD 2-dimensional wind field improves upon the a priori calibration source, regardless of quality of this model used in the calibration. This shows that HIRAD is not simply replicating results of the calibration source, but rather, it adds useful information

    Multi-Frequency VLBI Observations of the Active Galaxy NGC 1052

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    Active galactic nuclei (AGN) are among the most energetic sources in the universe, a large fraction of which are visible across the entire electromagnetic spectrum. Historically a zoo of different types of AGN were categorized based on a variety of observational properties, which can be explained by one unification scheme. A subset of these sources is characterized by relativistic outflows, called jets. The standard model assumes intrinsic symmetry between the jet and the counter-jet. Radio interferometric observations provide the highest achievable resolution which is key to understanding the physics driving AGN jets. The scope of this thesis is to investigate the physical processes responsible for the launching and collimation of relativistic jets. This is achieved with Very Long Baseline Interferometry (VLBI) at centimetre and millimetre wavelengths of the double-sided relativistic outflows within the active galaxy NGC 1052. At a distance of only 20 Mpc, linear scales down to a few hundred Schwarzschild radii can be imaged with mm-VLBI. The orientation of both jets close to the plane of the sky makes NGC 1052 an ideal target to study the symmetry-paradigm predicted by the unification scheme. The thesis is organized as follows. The first two chapters will give an introduction on our current understanding of launching, collimation, and emission processes of AGN and their jets as well as an overview on the technique of VLBI. In chapters 3 trough 5 I will present the analysis and results of a multi-frequency and multi-epoch study on NGC1052. Chapter 6 summarizes these findings and places them within the context of current AGN/jet scholarship. Additional information on the analysis is provided in tabular and graphical form in the appendices A and B. During my thesis work I developed a set of python scripts for calibration and analysis, which are presented in appendix C. In the following I give a short overview on the main results from this dissertation. Observations of NGC1052 at 22 GHz and 43 GHz over 4 years suggest an intrinsic asymmetry between both jets, which evolve east- and westwards in the plane of the sky. Based on a study of the outflow velocities, the eastern jet was found to be significantly faster than the western jet. Overall faster velocities were found compared to earlier estimates performed at lower frequencies. As the observing frequency increases regions are imaged at closer proximity to the jet spine. Therefore, these results point towards a transversal velocity gradient within both jets. The images from this study were used as input information for relativistic hydrodynamic simulations of the relativistic jets in NGC1052. The simulations favor a scenario in which a slightly over-pressured jet, resulting from a pressure-mismatch between the jet and the ambient medium at the nozzle, penetrates into a decreasing-pressure ambient medium. A molecular torus has been included in the simulations to account for thermal absorption. Based on the simulation results the torus particle number density is estimated within the range 0.7–1.0×10^22 cm^−2 . This numerical estimate is consistent with estimates from X-ray and radio observations. In addition, multi-frequency VLBI studies from 1.5 GHz to 86 GHz trace the absorbing effect of this torus, which covers large parts of the western, receding jet. It results in an emission gap between both jets whose size decreases with increasing frequency. Observations and simulations draw a consistent picture of the frequency-dependent thermal absorption of the non-thermal particles in the jet due to the optically thick structure. The torus only has a very small impact on the 43 GHz emission (and higher frequencies). Both jets are extremely straight and unresolved, however, there is a slight change in the western jet direction at about 2 milliarcseconds, which cannot be observed in the eastern jet. This kind of structure can only be explained by asymmetries, intrinsic to the jet or arising from interactions with the ambient medium

    Instrumentation development of innovative radio-devices to improve the coming cycles of radio astronomy observations

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    Radio astronomy represents one of the most useful tools for investigating celestial objects such as sychrontronic emissions from quasar , molecular clouds in the interstellar medium, and a black hole event horizon . All this is possible due to the great sensitivity that astronomical receivers can achieve, and the high angular resolution that can be reached using interferometric techniques. However, despite the great effort made, radio astronomy is not exempt of limitations that prevent it from deploying its maximum capability in terms of resolution. Atmospheric phase fluctuations, mainly induced by turbulent currents, are primarily responsible. Failure to correct these phase fluctuations will impede that the maximum potential of radio astronomy can be realized. In this thesis work, a novel solution to solve the drawbacks related to phase fluctuations in high frequency observations is presented. The ALMA telescope in Chile , has been selected as a target. The idea is to use an external optical system at room temperature, which can illuminate a low and a high frequency receiver, simultaneously. In this way, the solution for the phase fluctuation can be transferred from low to high frequency, thus, extending the maximum baseline for interferometric observations at high frequencies

    An Analysis of the Radiometric Quality of Small Unmanned Aircraft System Imagery

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    In recent years, significant advancements have been made in both sensor technology and small Unmanned Aircraft Systems (sUAS). Improved sensor technology has provided users with cheaper, lighter, and higher resolution imaging tools, while new sUAS platforms have become cheaper, more stable and easier to navigate both manually and programmatically. These enhancements have provided remote sensing solutions for both commercial and research applications that were previously unachievable. However, this has provided non-scientific practitioners with access to technology and techniques previously only available to remote sensing professionals, sometimes leading to improper diagnoses and results. The work accomplished in this dissertation demonstrates the impact of proper calibration and reflectance correction on the radiometric quality of sUAS imagery. The first part of this research conducts an in-depth investigation into a proposed technique for radiance-to-reflectance conversion. Previous techniques utilized reflectance conversion panels in-scene, which, while providing accurate results, required extensive time in the field to position the panels as well as measure them. We have positioned sensors on board the sUAS to record the downwelling irradiance which then can be used to produce reflectance imagery without the use of these reflectance conversion panels. The second part of this research characterizes and calibrates a MicaSense RedEdge-3, a multispectral imaging sensor. This particular sensor comes pre-loaded with metadata values, which are never recalibrated, for dark level bias, vignette and row-gradient correction and radiometric calibration. This characterization and calibration studies were accomplished to demonstrate the importance of recalibration of any sensors over a period of time. In addition, an error propagation was performed to detect the highest contributors of error in the production of radiance and reflectance imagery. Finally, a study of the inherent reflectance variability of vegetation was performed. In other words, this study attempts to determine how accurate the digital count to radiance calibration and the radiance to reflectance conversion has to be. Can we lower our accuracy standards for radiance and reflectance imagery, because the target itself is too variable to measure? For this study, six Coneflower plants were analyzed, as a surrogate for other cash crops, under different illumination conditions, at different times of the day, and at different ground sample distances (GSDs)

    Hurricane Imaging Radiometer (HIRAD) Tropical Rainfall Retrievals

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    The Hurricane Imaging Radiometer (HIRAD) is an airborne passive microwave remote sensor, developed to measure wind speed and rain rate in hurricanes. This dissertation concerns the development of a signal processing algorithm to infer tropical rainfall from HIRAD radiance (brightness temperature, Tb) measurements. The basis of the rain rate retrieval algorithm is an improved forward microwave radiative transfer model (RTM) that incorporates the HIRAD multi-antenna-beam geometry, and uses semi-empirical coefficients derived from an airborne experiment that occurred in the Gulf of Mexico off Tampa Bay in 2013. During this flight, HIRAD observed a squall line of thunderstorms simultaneously with an airborne meteorological radar (High Altitude Wind and Rain Profiler, HIWRAP), located on the same airplane. Also, ground based NEXRAD radars from the National Weather Service (located at Tampa and Tallahassee) provided high resolution simultaneous rain rate measurements. Using NEXRAD rainfall as the surface truth input to the HIRAD RTM, empirical rain microwave absorption coefficients were tuned to match the measured brightness temperatures. Also, the collocated HIWRAP radar reflectivity (dBZ) measurements were cross correlated with NEXRAD to derive the empirical HIWRAP radar reflectivity to rain rate relationship. Finally, the HIRAD measured Tbs were input to the HIRAD rain retrieval algorithm to derive estimates of rain rate, which were validated using the independent HIWRAP measurements of rain rate

    A review of RFI mitigation techniques in microwave radiometry

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    Radio frequency interference (RFI) is a well-known problem in microwave radiometry (MWR). Any undesired signal overlapping the MWR protected frequency bands introduces a bias in the measurements, which can corrupt the retrieved geophysical parameters. This paper presents a literature review of RFI detection and mitigation techniques for microwave radiometry from space. The reviewed techniques are divided between real aperture and aperture synthesis. A discussion and assessment of the application of RFI mitigation techniques is presented for each type of radiometer.Peer ReviewedPostprint (published version

    The Cosmic 21-cm Revolution Charting the first billion years of our universe

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    The redshifted 21-cm signal is set to transform astrophysical cosmology, bringing a historically data-starved field into the era of Big Data. Corresponding to the spin-flip transition of neutral hydrogen, the 21-cm line is sensitive to the temperature and ionization state of the cosmic gas, as well as to cosmological parameters. Crucially, with the development of new interferometers it will allow us to map out the first billion years of our universe, enabling us to learn about the properties of the unseen first generations of galaxies. Rapid progress is being made on both the observational and theoretical fronts, and important decisions on techniques and future direction are being made. The Cosmic 21-cm Revolution gathers contributions from current leaders in this fast-moving field, providing both an overview for graduate students and a reference point for current researchers

    National Astronomy Meeting 2019 Abstract Book

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    The National Astronomy Meeting 2019 Abstract Book. Abstracts accepted and presented, including both oral and poster presentations, at the Royal Astronomical Society's NAM2019 conference, held at Lancaster University between 30 June and 4 July 2019
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