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The science programs carried out at the NRT over the past 4-5 years have shown the urgent need for one or several of the following specifications (Colom et al. 2011, Borsenberger et al. 2012 and 2013, Martin et al. 2013¹⁾):

• a very broad band acquisition, to explore the full range of frequencies available at the receiver ;
• a large number of spectral channels, to gain in spectral resolution ;
• the implementation of RFI mitigation techniques, including during flux density calibration periods ;
• off-line and (when possible) real-time Stokes parameters in a few spectral bands.

To meet these specifications, we have designed a new spectrometer, WIBAR, using a Berkeley CASPER Board Roach1 (see the CASPER website at https://casper.berkeley.edu/) for the analog-to-digital signal conversion. However, the huge number of channels over a very wide band makes it impossible to process with standard CPUs. Instead, we use Graphics Processing Units (GPUs) which are particularly well adapted for real-time Fast Fourier Transforms of long arrays (see Borsenberger et al. 2012). The operations require four acquisition PCs which host standard GTX590 dual graphics cards, as described in the figure below.

Regarding polarisation, accurate measurements of the Stokes parameters are routinely carried out at the NRT since the upgrade made in 2000 (see Wolak et al. 2013), using horn rotations and the linear and circular polarisation outputs of the receiver. In the near future, the numerical determination of the Stokes parameters will be implemented, as requied by some galactic programs. Simple radio frequency interference mitigation techniques can be implemented in real time, while more complex ones can be performed off-line using either waveforms or spectral data at high time resolution. Sigma-clipped filtering algorithms are already implemented for continuum observations in the post-processing software.

This spectrometer – or an updated version, fully integrated in the NRT acquisition and data processing system – could replace the standard correlator should there be a hardware failure. JMM 20130826

http://wibar.obspm.fr/documents/poster2013CASPER_borsenberger_etal.pdf (Poster CASPER - JMM 20130909)

(v.201907) Pierre Colom, Alain Lecacheux (Observatoire de Paris, LESIA), Christophe Belleval, Jean Borsenberger, Jean-Michel Martin, Eric Gérard, Wim van Driel, Jean-François Roig (Observatoire de Paris, GEPI) and Guy Kenfack (Observatoire de Paris, USN).
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