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Enhancing the European VLBI Network of Radio Telescopes

Deliverables

The sub-project was aimed at developing and making available to the radio astronomical community at large new state-of-the-art techniques of observational radio astronomy. The sub-project included tests of these techniques as one of its most important components. The new techniques under development in the sub-project are: - Multiple field centre correlation. This technique allows to significantly increase the amount of astronomical information which can be extracted from a given VLBI observation. The gain is achieved by processing the image within the primary beam of the largest radio telescope involved in the VLBI observation. This latter value for a typical EVN experiment at 18cm is about 0.5 arcmin, while traditional VLBI observation (i.e. without multiple field centre correlation) allows to reconstruct an image of a field of only about 0.5 arcsecond in diameter. - Cluster-Cluster VLBI technique is based on the use of several antennas located at the same site involved in VLBI experiment and used for simultaneous observations of different sources. This makes possible to translate calibration parameters obtained from an observation of a stronger reference source to a weaker target source thus increasing the sensitivity of the VLBI system. In this sense, the Cluster-Cluster technique is an enhancement of the interferometric phase-referencing technique. Within the framework of the current project, the Cluster-Cluster VLBI technique was successfully implemented using several European VLBI Network telescopes and the Very Large Array in the US. Pulsar gating in VLBI observations. The ability to determine sub-millisecond positions, forms the principal motivation of observing pulsars with VLBI. A series of VLBI observations can yield a pulsar's position, proper motion, and parallax with unmatched precision, the latter providing a model-independent distance. Combined with various other independent data, these results can be used to provide checks on models of the electron density in the solar neighborhood, study the characteristics of plasma turbulence in the ISM, provide calibrations to scattering-based distance scales, investigate whether useful links may be established between individual pulsars and putative progenitors, estimate decay time scales for a pulsar's magnetic field, or determine frame ties between the extragalactic (Earth-rotation based) and dynamic (Earth-orbit based) reference frames. The goal of pulsar gating on VLBI correlations is to increase the SNR of pulsar observations by integrating only when the pulsar is "on". Since the typical duty-cycle of a pulsar is 3-10%, gating can lead to a SNR increase of factors of 3-5. The technique of pulsar gating was successfully tested with the EVN MkIV correlator at JIVE and the auspices of the present project. Development of the Field System (FS) software, which provides the means of implementing various techniques of VLBI observations. The FS subproject concentrated on making new versions of the Linux kernel and Debian system distribution available and suitable for FS use. New versions are required to support new PC hardware and to enhance system robustness and network security. The subproject has released several new versions of the Linux kernel and Debian system distributions for FS use.
Three different filter types for the UHF-band have been studied as a possible concept design for an RFI-robust receiver. The successful results of the use of prototype filters in one of the WSRT receivers have lead to the application of this designs in all receiver systems of the Westerbork Synthesis Radio Telescope (WSRT), making it considerably more RFI-robust. The knowledge, obtained from a study on RF/IF-architecture, has been turned to practical use in the design of the WSRT new IF to Video Converters (IVC) system and of the signal chain of the third SKA technology demonstrator system THousand Element Array (THEA). The second part of the program on design aspects was essentially a research effort, mainly during the last 18 months of the project, devoted to a study on one specific active RFI-suppression technique, the Feed Forward System (FFS). This resulted in the demonstration of a prototype system in the laboratory in Dwingeloo for verification of the defined models for inter-modulation and noise performance. The measurement report on the prototype describes the good agreement between theoretical models and practical results.
The goal of this sub-project was to provide the European astronomical community with a set of measures that will lead to radio astronomical telescopes able to operate in the presence of strong radio frequency interference (RFI) signals. Monitoring facilities for RFI-signals were set up and operated at the European radio astronomy observing locations. The participants in the project were project coordinator ASTRON (WSRT) in Dwingeloo, The Netherlands, MPIfR (Effelsberg) in Bonn, Germany, JBO (Jodrell Bank) in Manchester, United Kingdom and IRA-CNR (Noto) in Italy. Monitoring activities have been performed by all participants. These monitoring activities had two goals. The first was to provide information on RFI-sources, which could be used as input information for the second part of the project. The second goal was to build an RFI database, from which the information could be used for scheduling of VLBI and local observations.
The study involved an investigation into the properties of optical fibre components suitable for high data rate transmission. Both analogue and digital transmission systems were investigated. It was found that analogue systems are unsuitable for the large (1000 km +) link distances involved due to dynamic range restrictions and therefore digital systems need to be used. It is likely that the existing networks using SDH transmission protocols will be used in any practical system since providing dark fibre for radio astronomy use alone is liable to be prohibitively expensive. Further studies have shown (conducted in consultation with the DANTE group) that Giga-bit links were becoming feasible over the next few years as the GEANT system for GRID was being installed and upgraded.

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