(Ultra) Precise Astrometry today and tomorrow, with Next-generation Observatories

High precision astrometry provides the foundation to resolve many fundamental problems in astrophysics. The application of astrometric studies spans a wide range of fields, and has undergone enormous growth in recent years. This is as a consequence of the increasing measurement precision and wide applicability, which is due in turn to the development of new techniques. Forthcoming next generation observatories have the potential to further increase the astrometric precision, providing there is a matching improvement in the methods to correct for systematic errors. The EVN and other observatories are providing demonstrations of these and are acting as pathfinders for next-generation telescopes such as the SKA and ngVLA. We will review the perspectives for the coming facilities and examples of the current state-of-the-art for astrometry.Comment: ADS reference 2018evn..confE..54R from conference https://pos.sissa.it/34

Astrometric "Core-shifts" at the Highest Frequencies

We discuss the application of a new VLBI astrometric method named "Source/Frequency Phase Referencing" to measurements of "core-shifts" in radio sources used for geodetic observations. We detail the reasons that astrometrical observations of 'core-shifts' have become critical in the era of VLBI2010. We detail how this new method allows the problem to be addressed at the highest frequencies and outline its superior compensation of tropospheric errors

Astronomical verification of a stabilized frequency reference transfer system for the Square Kilometre Array

In order to meet its cutting-edge scientific objectives, the Square Kilometre Array (SKA) telescope requires high-precision frequency references to be distributed to each of its antennas. The frequency references are distributed via fiber-optic links and must be actively stabilized to compensate for phase-noise imposed on the signals by environmental perturbations on the links. SKA engineering requirements demand that any proposed frequency reference distribution system be proved in "astronomical verification" tests. We present results of the astronomical verification of a stabilized frequency reference transfer system proposed for SKA-mid. The dual-receiver architecture of the Australia Telescope Compact Array was exploited to subtract the phase-noise of the sky signal from the data, allowing the phase-noise of observations performed using a standard frequency reference, as well as the stabilized frequency reference transfer system transmitting over 77 km of fiber-optic cable, to be directly compared. Results are presented for the fractional frequency stability and phase-drift of the stabilized frequency reference transfer system for celestial calibrator observations at 5 GHz and 25 GHz. These observations plus additional laboratory results for the transferred signal stability over a 166 km metropolitan fiber-optic link are used to show that the stabilized transfer system under test exceeds all SKA phase-stability requirements under a broad range of observing conditions. Furthermore, we have shown that alternative reference dissemination systems that use multiple synthesizers to supply reference signals to sub-sections of an array may limit the imaging capability of the telescope.Comment: 12 pages, accepted to The Astronomical Journa

Análisis de los sistemas de control de potencia en redes LTE

Este trabajo analiza el funcionamiento de los sistemas de control de potencia desarrollados para su implantación en redes móviles celulares. Desde el punto de vista medioambiental, estos sistemas pueden tener un impacto clave en temas como el ahorro en el consumo energético, el control de las potencias máximas de emisión, etc. El análisis se centrará en la tecnología LTE (Long Term Evolution), máximo exponente de la denominada cuarta generación de las comunicaciones móviles o 4G, por lo que previamente se realizará una breve descripción de sus principales características

Measurement of core-shifts with astrometric multi-frequency calibration

VLBI is unique, among the space geodetic techniques, in its contribution to defining and maintaining the International Celestial Reference Frame, providing precise measurements of coordinates of extragalactic radiosources. The quest for increasing accuracy of VLBI geodetic products has lead to a deeper revision of all aspects that might introduce errors in the analysis. The departure of the observed sources from perfect, stable, compact and achromatic celestial targets falls within this category. This paper is concerned with the impact of unaccounted frequency-dependent position shifts of source cores in the analysis of dual-band S/X VLBI geodesy observations, and proposes a new method to measure them. The multi-frequency phase transfer technique developed and demonstrated by Middelberg et al. (2005) increases the high frequency coherence times of VLBI observations, using the observations at a lower frequency. Our proposed SOURCE/FREQUENCY PHASE REFERENCING method endows it with astrometric applications by adding a strategy to estimate the ionospheric contributions. Here we report on the first successful application to measure the core shift of the quasar 1038+528 A at S and X-bands, and validate the results by comparison with those from standard phase referencing techniques. In this particular case, and in general in the cm-wavelength regime, both methods are equivalent. Moreover the proposed method opens a new horizon with targets and fields suitable for high precision astrometric studies with VLBI, especially at high frequencies where severe limitations imposed by the rapid fluctuations in the troposphere prevent the use of standard phase referencing techniques.Comment: 6 pages, 2 figures. Proceedings of 17th Working Meeting on European VLBI for Geodesy and Astrometry (April 2005