Experimental Astronomy

, Volume 45, Issue 2, pp 269–314 | Cite as

SARAS 2: a spectral radiometer for probing cosmic dawn and the epoch of reionization through detection of the global 21-cm signal

  • Saurabh Singh
  • Ravi Subrahmanyan
  • N. Udaya Shankar
  • Mayuri Sathyanarayana Rao
  • B. S. Girish
  • A. Raghunathan
  • R. Somashekar
  • K. S. Srivani
Original Article


The global 21-cm signal from Cosmic Dawn (CD) and the Epoch of Reionization (EoR), at redshifts \(z \sim 6-30\), probes the nature of first sources of radiation as well as physics of the Inter-Galactic Medium (IGM). Given that the signal is predicted to be extremely weak, of wide fractional bandwidth, and lies in a frequency range that is dominated by Galactic and Extragalactic foregrounds as well as Radio Frequency Interference, detection of the signal is a daunting task. Critical to the experiment is the manner in which the sky signal is represented through the instrument. It is of utmost importance to design a system whose spectral bandpass and additive spurious signals can be well calibrated and any calibration residual does not mimic the signal. Shaped Antenna measurement of the background RAdio Spectrum (SARAS) is an ongoing experiment that aims to detect the global 21-cm signal. Here we present the design philosophy of the SARAS 2 system and discuss its performance and limitations based on laboratory and field measurements. Laboratory tests with the antenna replaced with a variety of terminations, including a network model for the antenna impedance, show that the gain calibration and modeling of internal additive signals leave no residuals with Fourier amplitudes exceeding 2 mK, or residual Gaussians of 25 MHz width with amplitudes exceeding 2 mK. Thus, even accounting for reflection and radiation efficiency losses in the antenna, the SARAS 2 system is capable of detection of complex 21-cm profiles at the level predicted by currently favoured models for thermal baryon evolution.


Astronomical instrumentation Methods: observational Cosmic background radiation Cosmology: observations Dark ages Reionization First stars 



We thank the anonymous referee for their valuable comments and suggestions. We thank RRI Electronics Engineering Group, particularly Kasturi S., Madhavi S. and Kamini P. A., for their assistance in analog and digital receiver assembly. We also thank the Mechanical Engineering Group (RRI), led by Mohamed Ibrahim, for manufacturing the antenna along with construction of chassis and shielding cages for analog and digital receivers. Santosh Harish and Divya Jayasankar took an active role in developing real-time data acquisition software and system monitoring hardware respectively. We are grateful to the staff at the Gauribidanur Field Station led by Ashwathappa H.A. for providing excellent support in carrying out field tests and measurements.


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© Springer Science+Business Media B.V., part of Springer Nature 2018

Authors and Affiliations

  1. 1.Raman Research InstituteSadashivanagarIndia
  2. 2.Joint Astronomy ProgrammeIndian Institute of ScienceBangaloreIndia

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