609 research outputs found

    The Gaia reference frame for bright sources examined using VLBI observations of radio stars

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    Positions and proper motions of Gaia sources are expressed in a reference frame that ideally should be non-rotating relative to distant extragalactic objects, coincident with the International Celestial Reference System (ICRS), and consistent across all magnitudes. For sources fainter than 16th magnitude this is achieved thanks to Gaia's direct observations of quasars. At brighter magnitudes it is difficult to validate the quality of the reference frame due to the scarcity of comparison data. This paper examines the use of VLBI observations of radio stars to determine the spin and orientation of the bright reference frame of Gaia. Simultaneous estimation of the six spin and orientation parameters makes optimal use of VLBI data and makes it possible to include even single-epoch VLBI observations in the solution. The method is applied to Gaia Data Release 2 (DR2) using published VLBI data for 41 radio stars. Results for the 26 best-fitting sources indicate that the bright reference frame of Gaia DR2 is rotating relative to the faint quasars at a rate of about 0.1 mas/yr, significant at 2-sigma level. This supports a similar conclusion based on a comparison with stellar positions in the Hipparcos frame. The accuracy is currently limited by the small number of radio sources used, by uncertainties in the Gaia DR2 proper motions, and by the astrophysical nature of the radio stars. While the origin of the indicated rotation is understood and can be avoided in future data releases, it remains important to validate the bright reference frame of Gaia by independent observations. This can be achieved using VLBI astrometry, which may require re-observing the old sample of radio stars as well as measuring new objects. The unique historical value of positional measurements is stressed and VLBI observers are urged to ensure that relevant positional information is preserved for the future.Comment: 17 pages, 5 figures. Revised version incorporating a Corrigendum published by A&A. Tables 2-3, Figures 3-5, and Sections 3-5 have been substantially revise

    AMPA experimental communications systems

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    The program was conducted to demonstrate the satellite communication advantages of Adaptive Phased Array Technology. A laboratory based experiment was designed and implemented to demonstrate a low earth orbit satellite communications system. Using a 32 element, L-band phased array augmented with 4 sets of weights (2 for reception and 2 for transmission) a high speed digital processing system and operating against multiple user terminals and interferers, the AMPA system demonstrated: communications with austere user terminals, frequency reuse, communications in the face of interference, and geolocation. The program and experiment objectives are described, the system hardware and software/firmware are defined, and the test performed and the resultant test data are presented

    Kinematics and Robot Design IV, KaRD2021

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    This volume collects the papers published on the special issue “Kinematics and Robot Design IV, KaRD2021” (https://www.mdpi.com/journal/robotics/special_issues/KaRD2021), which is the forth edition of the KaRD special-issue series, hosted by the open-access journal “MDPI Robotics”. KaRD series is an open environment where researchers can present their works and discuss all the topics focused on the many aspects that involve kinematics in the design of robotic/automatic systems. Kinematics is so intimately related to the design of robotic/automatic systems that the admitted topics of the KaRD series practically cover all the subjects normally present in well-established international conferences on “mechanisms and robotics”. KaRD2021, after the peer-review process, accepted 12 papers. The accepted papers cover some theoretical and many design/applicative aspects

    The use of computer-aided design techniques in dynamic graphical simulation

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    Willed action and its impairment in schizophrenia

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    Schizophrenia is a disabling psychiatric disorder characterised by positive symptoms (those which the patients experience and are abnormal by their presence such as hallucinations) and negative signs (when the patients lack some element of normal behaviour such as poverty of speech). Frith (1992) suggested that some of the signs and symptoms of schizophrenia reflect a dysfunction of "willed" actions while the processes involved in "stimulus driven" actions remain largely intact. The patients can perform routine acts elicited by environmental stimuli, but have difficulty in producing spontaneous behaviour in the absence of external cues. The aim of this thesis is to examine this hypothesis using a variety of experimental paradigms and procedures to asses willed initiation and preparation as well as willed suppression of action in schizophrenia. The aim of Study 1 was to assess the initiation and preparation of willed actions in 10 patients with schizophrenia and 13 controls using reaction time (RT) tasks that differ in the degree to which they involve volitionally controlled versus stimulus-driven responses. Study 2 examined performance of 11 patients with schizophrenia and 13 normal controls on two motor tasks (placing pegs in a pegboard and repetitive index finger tapping) under unimanual, bimanual and dual task conditions. The aim of Study 3 was to examine the above hypothesis by measuring movement related potentials (MRPs) prior to self-initiated and externally-triggered movements in three groups: 6 patients with schizophrenia with high ratings of negative signs, 5 patients with of schizophrenia with high ratings of positive symptoms and 6 normal controls. Studies 4, 5 and 6 examined willed suppression. Go no-go RT tests have both a relevant stimulus requiring a response and to-be-ignored stimuli requiring the response to be withheld, i.e. response inhibition. The aim of Study 4 was to examine the ability to withhold a response in conditions with increased complexity of decision-making for identifying 'go' stimuli in 14 patients with schizophrenia and 12 normal controls. The aim of Study 5 was to examine the ability to withhold a response in conditions with greater dimensional overlap between the non-target no-go and the target go stimuli in 14 patients with schizophrenia and 12 normal controls. The patients were divided into two groups, the 'high symptom' group consisted of the 7 patients with ratings of positive symptoms higher than the group median of 9, and the 'low symptom' group consisted of the 7 patients with positive symptom ratings below 9. The patients with 'high symptom' ratings had slower RTs than the controls which were significant for the SRT conditions of both tasks, and approached significance for the CRT conditions of both tasks. The differences in SRT or CRT between the 'low symptom' group and the controls were not statistically significant for either task. In Study 5 the controls showed greater slowing between CRT1 and CRT2 than between CRT2 and CRT3, whereas the RTs of 'low symptom' patients did not differ at all between CRT1 and CRT2 and slowed greatly for CRT3. Both patient groups had slower response times than controls in the Hayling test, and produced significantly fewer words in the alternating word fluency task compared to the controls. Performance on the cognitive tasks correlated with performance on the no-go tasks. Negative priming refers to the slowing of reaction times that occurs when an ignored distractor stimulus in a first trial (prime) becomes the target stimulus in the subsequent trial (probe) (Tipper, 1985). Unlike normal controls, patients with schizophrenia fail to show significant negative priming, that is, the significant delay of reaction times on probe trials are not present (Beech et al., 1989; David, 1995). The aim of study 6 was to examine the spatial negative priming effect in schizophrenia using a new paradigm that allows the effects of perceptual mismatch on RT to be considered independently of any spatial negative priming effects. The patients with schizophrenia did not show any spatial negative priming in conditions with or without perceptual mismatch. These results constitute the first unequivocal demonstration of impaired inhibitory processes in schizophrenia based on reduced negative priming effects. These results provide some overall but not uniformly consistent support for the hypothesis that patients with schizophrenia have an impairment in willed action while stimulus driven action remains intact. (Abstract shortened by UMI.

    Development and usage of a false color display technique for presenting Seasat-A scatterometer data

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    A computer generated false color program which creates digital multicolor graphics to display geophysical surface parameters measured by the Seasat-A satellite scatterometer (SASS) is described. The data is incrementally scaled over the range of acceptable values and each increment and its data points are assigned a color. The advantage of the false color display is that it visually infers cool or weak data versus hot or intense data by using the rainbow of colors. For example, with wind speeds, levels of yellow and red could be used to imply high winds while green and blue could imply calmer air. The SASS data is sorted into geographic regions and the final false color images are projected onto various world maps with superimposed land/water boundaries

    Name agreement in picture naming: An ERP study

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    Name agreement is the extent to which different people agree on a name for a particular picture. Previous studies have found that it takes longer to name low name agreement pictures than high name agreement pictures. To examine the effect of name agreement in the online process of picture naming, we compared event-related potentials (ERPs) recorded whilst 19 healthy, native English speakers silently named pictures which had either high or low name agreement. A series of ERP components was examined: P1 approximately 120ms from picture onset, N1 around 170ms, P2 around 220ms, N2 around 290ms, and P3 around 400ms. Additionally, a late time window from 800 to 900ms was considered. Name agreement had an early effect, starting at P1 and possibly resulting from uncertainty of picture identity, and continuing into N2, possibly resulting from alternative names for pictures. These results support the idea that name agreement affects two consecutive processes: first, object recognition, and second, lexical selection and/or phonological encoding

    Hydrogel and Soluble Polymers to Support Metal Ion Chemosensors

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    Most water systems contain metal ions. Some of these ions, such as lead, arsenic, and mercury, are extremely toxic. It is of great concern when those ions make their way into drinking water. There is a need for a device that can detect small amounts of dissolved metal ions in real-time. The Schwabacher group has designed such a device, based on azo dyes as the chemo sensors that change color in the presence of metal ions. These sensors can detect very small concentrations of metal ions into the parts per billion range. The sensor dyes are connected with covalent bonds to a hydrogel polymer solid support. The work herein describes the continuing development of hydrogel polymers for this application. These hydrogels are transparent and attached to glass for stability. The previous prototype worked but had some undesirable variability that can be improved upon. The length of the synthesis of the hydrogel components has been reduced achieving higher stability and better replicability
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