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SARAS: a precision system for measurement of the cosmic radio background and signatures from the epoch of reionization

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Abstract

SARAS is a correlation spectrometer purpose designed for precision measurements of the cosmic radio background and faint features in the sky spectrum at long wavelengths that arise from redshifted 21-cm from gas in the reionization epoch. SARAS operates in the octave band 87.5–175 MHz. We present herein the system design arguing for a complex correlation spectrometer concept. The SARAS design concept provides a differential measurement between the antenna temperature and that of an internal reference termination, with measurements in switched system states allowing for cancellation of additive contaminants from a large part of the signal flow path including the digital spectrometer. A switched noise injection scheme provides absolute spectral calibration. Additionally, we argue for an electrically small frequency-independent antenna over an absorber ground. Various critical design features that aid in avoidance of systematics and in providing calibration products for the parametrization of other unavoidable systematics are described and the rationale discussed. The signal flow and processing is analyzed and the response to noise temperatures of the antenna, reference termination and amplifiers is computed. Multi-path propagation arising from internal reflections are considered in the analysis, which includes a harmonic series of internal reflections. We opine that the SARAS design concept is advantageous for precision measurement of the absolute cosmic radio background spectrum; therefore, the design features and analysis methods presented here are expected to serve as a basis for implementations tailored to measurements of a multiplicity of features in the background sky at long wavelengths, which may arise from events in the dark ages and subsequent reionization era.

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Acknowledgments

The development of the SARAS concept has benefited greatly from the contemporaneous experimentation with implementations and trials of a variety of system configurations, which received outstanding support from staff of the Gauribidanur Radio Observatory and the Electronics Laboratory and Mechanical Engineering services at the Raman Research Institute. We thank Ron Ekers for his encouragement and participation in the effort.

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Authors and Affiliations

  1. Raman Research Institute, C V Raman Avenue, Sadashivanagar, Bangalore, 560080, India

    Nipanjana Patra, Ravi Subrahmanyan, A. Raghunathan & N. Udaya Shankar

  2. Joint Astronomy Programme, Indian Institute of Science, Bangalore, 560012, India

    Nipanjana Patra

  3. National Radio Astronomy Observatory, PO Box 0, Socorro, NM, 87801, USA

    Ravi Subrahmanyan

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  1. Nipanjana Patra
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  2. Ravi Subrahmanyan
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  3. A. Raghunathan
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  4. N. Udaya Shankar
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Correspondence to Ravi Subrahmanyan.

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Patra, N., Subrahmanyan, R., Raghunathan, A. et al. SARAS: a precision system for measurement of the cosmic radio background and signatures from the epoch of reionization. Exp Astron 36, 319–370 (2013). https://doi.org/10.1007/s10686-013-9336-3

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  • DOI: https://doi.org/10.1007/s10686-013-9336-3

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