Multisource Self-calibration for Sensor Arrays

Calibration of a sensor array is more involved if the antennas have direction dependent gains and multiple calibrator sources are simultaneously present. We study this case for a sensor array with arbitrary geometry but identical elements, i.e. elements with the same direction dependent gain pattern. A weighted alternating least squares (WALS) algorithm is derived that iteratively solves for the direction independent complex gains of the array elements, their noise powers and their gains in the direction of the calibrator sources. An extension of the problem is the case where the apparent calibrator source locations are unknown, e.g., due to refractive propagation paths. For this case, the WALS method is supplemented with weighted subspace fitting (WSF) direction finding techniques. Using Monte Carlo simulations we demonstrate that both methods are asymptotically statistically efficient and converge within two iterations even in cases of low SNR.Comment: 11 pages, 8 figure

Fundamental Imaging Limits of Radio Telescope Arrays

The fidelity of radio astronomical images is generally assessed by practical experience, i.e. using rules of thumb, although some aspects and cases have been treated rigorously. In this paper we present a mathematical framework capable of describing the fundamental limits of radio astronomical imaging problems. Although the data model assumes a single snapshot observation, i.e. variations in time and frequency are not considered, this framework is sufficiently general to allow extension to synthesis observations. Using tools from statistical signal processing and linear algebra, we discuss the tractability of the imaging and deconvolution problem, the redistribution of noise in the map by the imaging and deconvolution process, the covariance of the image values due to propagation of calibration errors and thermal noise and the upper limit on the number of sources tractable by self calibration. The combination of covariance of the image values and the number of tractable sources determines the effective noise floor achievable in the imaging process. The effective noise provides a better figure of merit than dynamic range since it includes the spatial variations of the noise. Our results provide handles for improving the imaging performance by design of the array.Comment: 12 pages, 8 figure

Calibration Challenges for Future Radio Telescopes

Instruments for radio astronomical observations have come a long way. While the first telescopes were based on very large dishes and 2-antenna interferometers, current instruments consist of dozens of steerable dishes, whereas future instruments will be even larger distributed sensor arrays with a hierarchy of phased array elements. For such arrays to provide meaningful output (images), accurate calibration is of critical importance. Calibration must solve for the unknown antenna gains and phases, as well as the unknown atmospheric and ionospheric disturbances. Future telescopes will have a large number of elements and a large field of view. In this case the parameters are strongly direction dependent, resulting in a large number of unknown parameters even if appropriately constrained physical or phenomenological descriptions are used. This makes calibration a daunting parameter estimation task, that is reviewed from a signal processing perspective in this article.Comment: 12 pages, 7 figures, 20 subfigures The title quoted in the meta-data is the title after release / final editing

Retractable Hammock

This Final Design Review will recap the objectives, background, design ideation, and concept development of the Retractable Hammock senior capstone project and continue to report on the final design and manufacturing that resulted in the final iteration of the Retractable Hammock

Self-report and parent-report of physical and psychosocial well-being in Dutch adolescents with type 1 diabetes in relation to glycemic control

BACKGROUND: To determine physical and psychosocial well-being of adolescents with type 1 diabetes by self-report and parent report and to explore associations with glycemic control and other clinical and socio-demographic characteristics. METHODS: Demographic, medical and psychosocial data were gathered from 4 participating outpatient pediatric diabetes clinics in the Netherlands. Ninety-one patients completed the Child Health Questionnaire-CF87 (CHQ-CF87), Centre for Epidemiological Studies scale for Depression (CES-D), and the DFCS (Diabetes-specific Family Conflict Scale). Parents completed the CHQ-PF50, CES-D and the DFCS. RESULTS: Mean age was 14.9 years (± 1.1), mean HbA(1c )8.8% (± 1.7; 6.2–15.0%). Compared to healthy controls, patients scored lower on CHQ subscales role functioning-physical and general health. Parents reported less favorable scores on the behavior subscale than adolescents. Fewer diabetes-specific family conflicts were associated with better psychosocial well-being and less depressive symptoms. Living in a one-parent family, being member of an ethnic minority and reporting lower well-being were all associated with higher HbA(1c )values. CONCLUSION: Overall, adolescents with type 1 diabetes report optimal well-being and parent report is in accordance with these findings. Poor glycemic control is common, with single-parent families and ethnic minorities particularly at risk. High HbA(1c )values are related to lower social and family functioning

Role of negative ion resonances in electron scattering from atoms and molecules

Transient negative ions (resonances) formed during the collision of an electron with an atom or molecule have been extensively studied for over thirty years. The continued interest in these states, both experimentally and theoretically, stems from the profound effects that they can have on electron scattering cross sections and the role that electron-electron correlations play in their formation and quasi-stability. A selective discussion of examples of such resonances, involving one, two and three excited electrons is given for a wide range of atomic and molecular systems. © CSIRO 1999