Skip to main content
SpringerLink
Log in

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

  • Original Article
  • Published:
Experimental Astronomy Aims and scope Submit manuscript

Abstract

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.

This is a preview of subscription content, log in via an institution to check access.

Access this article

Log in via an institution

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Instant access to the full article PDF.

Institutional subscriptions

Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Fig. 11
Fig. 12
Fig. 13
Fig. 14
Fig. 15
Fig. 16
Fig. 17

Similar content being viewed by others

References

  1. Azeredo-Leme, C.: Clock jitter effects on sampling: a tutorial. IEEE Circuits Syst. Mag. 11(3), 26–37 (2011). https://doi.org/10.1109/MCAS.2011.942067

    Article  Google Scholar 

  2. Baars, J.W.M., Genzel, R., Pauliny-Toth, I.I.K., Witzel, A.: The absolute spectrum of CAS A - an accurate flux density scale and a set of secondary calibrators. Astron. Astrophys. 61, 99–106 (1977)

    ADS  Google Scholar 

  3. Bailey, D.k.: On a new method for exploring the upper ionosphere. Terr. Magn. Atmos. Electr. (Journal of Geophysical Research) 53, 41 (1948). https://doi.org/10.1029/TE053i001p00041

    Article  ADS  Google Scholar 

  4. Balanis, C.A.: Antenna theory: analysis and design Wiley-Interscience (2005)

  5. Barkana, R., Loeb, A.: A method for separating the physics from the astrophysics of high-redshift 21 centimeter fluctuations. The Astrophysical Journal Letters 624 (2), L65 (2005). http://stacks.iop.org/1538-4357/624/i=2/a=L65

    Article  ADS  Google Scholar 

  6. Becker, R.H., Fan, X., White, R.L., et al.: Evidence for reionization at z6: Detection of a gunn-peterson trough in a z = 6.28 quasar. Astron. J. 122(6), 2850 (2001). http://stacks.iop.org/1538-3881/122/i=6/a=2850

    Article  ADS  Google Scholar 

  7. Chipman, J.S.: Gauss-Markov theorem, pp 577–582. Springer, Berlin (2011). https://doi.org/10.1007/978-3-642-04898-2_270

  8. Cohen, A., Fialkov, A., Barkana, R., Lotem, M.: Charting the parameter space of the global 21-cm signal. Mon. Not. R. Astron. Soc. 472, 1915–1931 (2017). https://doi.org/10.1093/mnras/stx2065

    Article  ADS  Google Scholar 

  9. Condon, J.J., Ransom, S.M.: Essential radio astronomy (princeton series in modern observational astronomy) princeton university press (2016)

  10. Dicke, R.H.: The measurement of thermal radiation at microwave frequencies, pp 106–113. Springer, Netherlands (1982). https://doi.org/10.1007/978-94-009-7752-5_11

  11. Fan, X., Carilli, C.L., Keating, B.: Observational constraints on cosmic reionization. Annu. Rev. Astron. Astrophys. 44, 415–462 (2006). https://doi.org/10.1146/annurev.astro.44.051905.092514

    Article  ADS  Google Scholar 

  12. Florides, G., Kalogirou, S.: 1 annual ground temperature measurements at various depths

  13. Greenwood, P.E., Nikulin, M.S.: A guide to Chi-Squared testing (wiley series in probability and statistics) Wiley-Interscience (1996)

  14. Hampel, F.R.: The influence curve and its role in robust estimation. J. Am. Stat. Assoc. 69(346), 383–393 (1974). http://www.jstor.org/stable/2285666

    Article  MathSciNet  MATH  Google Scholar 

  15. Helmboldt, J.F., Kassim, N.E.: The evolution of cassiopeia a at low radio frequencies. Astron. J. 138, 838–844 (2009). https://doi.org/10.1088/0004-6256/138/3/838

    Article  ADS  Google Scholar 

  16. Huang, Y.: Radiation efficiency measurements of small antennas, pp 1–21. Springer, Singapore (2014). https://doi.org/10.1007/978-981-4560-75-7_71-1

  17. Kester, W.: Understand SINAD, ENOB, SNR, THD, THD + N, and SFDR so you don’t get lost in the noise floor. MT-003 Tutorial. http://www.Analog.com/static/importedfiles/tutorials/MT-003.pdf (2009)

  18. Malhotra, S., Rhoads, J.E.: Luminosity functions of ly emitters at redshifts z = 6.5 and z = 5.7: Evidence against reionization at z6.5. The Astrophysical Journal Letters 617(1), L5 (2004). http://stacks.iop.org/1538-4357/617/i=1/a=L5

    Article  ADS  Google Scholar 

  19. McGreer, I.D., Mesinger, A., D’Odorico, V.: Model-independent evidence in favour of an end to reionization by z6. Mon. Not. R. Astron. Soc. 447, 499–505 (2015). https://doi.org/10.1093/mnras/stu2449

    Article  ADS  Google Scholar 

  20. Meys, R.: A wave approach to the noise properties of linear microwave devices. IEEE Trans. Microwave Theory Tech. 26(1), 34–37 (1978)

    Article  ADS  Google Scholar 

  21. Mirocha, J., Harker, G.J.A., Burns, J.o.: Interpreting the Global 21 cm Signal from High Redshifts. I. Model-independent constraints. Astrophys. J. 777, 118 (2013). https://doi.org/10.1088/0004-637X/777/2/118

    Article  ADS  Google Scholar 

  22. Mirocha, J., Harker, G.J.A., Burns, J.o.: Interpreting the Global 21-cm signal from high Redshifts. II. Parameter Estimation for Models of Galaxy Formation. Astrophys. J. 813, 11 (2015). https://doi.org/10.1088/0004-637X/813/1/11

    Article  ADS  Google Scholar 

  23. Monsalve, R.A., Rogers, A.E.E., Bowman, J.D., Mozdzen, T.j.: Calibration of the EDGES High-band Receiver to Observe the Global 21 cm Signature from the Epoch of Reionization. Astrophys. J. 835, 49 (2017). https://doi.org/10.3847/1538-4357/835/1/49

    Article  ADS  Google Scholar 

  24. Morales, M.F., Wyithe, J.S.B.: Reionization and cosmology with 21-cm fluctuations. Annu. Rev. Astron. Astrophys. 48(1), 127–171 (2010). https://doi.org/10.1146/annurev-astro-081309-130936

    Article  ADS  Google Scholar 

  25. Narula, S. C., Korhonen, P.J.: Multivariate multiple linear regression based on the minimum sum of absolute errors criterion. Eur. J. Oper. Res. 73(1), 70–75 (1994)

    Article  MATH  Google Scholar 

  26. Nelder, J. A., Mead, R.: A simplex method for function minimization. Comput. J. 7(4), 308–313 (1965). https://doi.org/10.1093/comjnl/7.4.308

    Article  MathSciNet  MATH  Google Scholar 

  27. Neu, T.: Clock jitter analyzed in the time domain, part 1 Analog Applications (2010)

  28. Nuttall, A.H.: Some Windows with Very Good Sidelobe Behavior. IEEE Trans. Acoust. Speech Signal Process. 29, 84–91 (1981)

    Article  ADS  Google Scholar 

  29. Offringa, A.R.: Algorithms for radio interference detection and removal. University of Groningen, Ph.D. thesis (2012)

    Google Scholar 

  30. Papoulis, A.: Probability, random variables, and stochastic processes. McGraw-Hill Kogakush (1981)

  31. Patra, N., Bray, J.D., Roberts, P., Ekers, R.d.: Bandpass calibration of a wideband spectrometer using coherent pulse injection. Exp. Astron. 43, 119–129 (2017). https://doi.org/10.1007/s10686-017-9523-8

    Article  ADS  Google Scholar 

  32. Patra, N., Subrahmanyan, R., Raghunathan, A., Udaya Shankar, R.d.: 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

    Article  ADS  Google Scholar 

  33. Patra, N., Subrahmanyan, R., Sethi, S., Shankar, N.U., Raghunathan, A.: Saras measurement of the radio background at long wavelengths. Astrophys. J. 801(2), 138 (2015). http://stacks.iop.org/0004-637X/801/i=2/a=138

    Article  ADS  Google Scholar 

  34. Perley, R., Schwab, F., Bridle, A.: Synthesis imaging in radio astronomy. Astronomical Society of the Pacific, San Francisco (1989)

    Google Scholar 

  35. Pober, J.C., Liu, A., Dillon, J.S., et al.: What next-generation 21 cm power spectrum measurements can teach us about the epoch of reionization. Astrophys. J. 782(2), 66 (2014). http://stacks.iop.org/0004-637X/782/i=2/a=66

    Article  ADS  Google Scholar 

  36. Pozar, D.M., Kaufman, B.: Comparison of three methods for the measurement of printed antenna efficiency. IEEE Trans. Antennas Propag. 36(1), 136–139 (1988). https://doi.org/10.1109/8.1084

    Article  ADS  Google Scholar 

  37. Price, D.C., Greenhill, L.J., Fialkov, A., et al.: Design and characterization of the Large-Aperture experiment to detect the dark age (LEDA) radiometer systems. ArXiv e-prints (2017)

  38. Pritchard, J.R., Furlanetto, S.R.: 21-cm fluctuations from inhomogeneous x-ray heating before reionization. Mon. Not. R. Astron. Soc. 376(4), 1680–1694 (2007). https://doi.org/10.1111/j.1365-2966.2007.11519.x

    Article  ADS  Google Scholar 

  39. Pritchard, J.R., Loeb, A.: Constraining the unexplored period between the dark ages and reionization with observations of the global 21 cm signal. Phys. Rev. D 82(023), 006 (2010). https://doi.org/10.1103/PhysRevD.82.023006

    Google Scholar 

  40. Rogers, A.E.E., Bowman, J.d.: Absolute calibration of a wideband antenna and spectrometer for accurate sky noise temperature measurements. Radio Sci. 47, RS0k06 (2012). https://doi.org/10.1029/2011RS004962

    Article  Google Scholar 

  41. Rumsey, V.: Frequency independent antennas. In: 1958 IRE international convention record, vol. 5, pp 114–118. https://doi.org/10.1109/IRECON.1957.1150565 (1957)

  42. Sathyanarayana Rao, M., Subrahmanyan, R., Udaya Shankar, N., Chluba, J.: On the Detection of Spectral Ripples from the Recombination Epoch. Astrophys. J. 810, 3 (2015). https://doi.org/10.1088/0004-637X/810/1/3

    Article  ADS  Google Scholar 

  43. Sathyanarayana Rao, M., Subrahmanyan, R., Udaya Shankar, N., Chluba, J.: GMOSS: All-sky model of spectral radio brightness based on physical components and associated radiative processes. Astron. J. 153, 26 (2017). https://doi.org/10.3847/1538-3881/153/1/26

    ADS  Google Scholar 

  44. Sathyanarayana Rao, M., Subrahmanyan, R., Udaya Shankar, N., Chluba, J.: Modeling the radio foreground for detection of CMB spectral distortions from the cosmic dawn and the epoch of reionization. Astrophys. J. 840, 33 (2017). https://doi.org/10.3847/1538-4357/aa69bd

    Article  ADS  Google Scholar 

  45. Sault, R.J., Teuben, P.J., Wright, M.C.H.: A retrospective View of MIRIAD. In: Shaw, R.A., Payne, H.E., Hayes, J.J.E. (eds.) Astronomical data analysis software and systems IV, astronomical society of the pacific conference series, vol. 77, p 433 (1995)

  46. Sethi, S.K.: HI signal from re-ionization epoch. Mon. Not. R. Astron. Soc. 363 (3), 818–830 (2005). https://doi.org/10.1111/j.1365-2966.2005.09485.x

    Article  ADS  Google Scholar 

  47. Shaver, P.A., Windhorst, R.A., Madau, P., de Bruyn, A.G.: Can the reionization epoch be detected as a global signature in the cosmic background?. Astron. Astrophys. 345, 380–390 (1999)

    ADS  Google Scholar 

  48. Singh, S., Subrahmanyan, R., Udaya Shankar, N., et al.: First results on the Epoch of Reionization from first light with SARAS 2. The Astrophysical Journal Letters 845, L12 (2017). https://doi.org/10.3847/2041-8213/aa831b

    Article  ADS  Google Scholar 

  49. Singh, S., Subrahmanyan, R., Udaya Shankar, N., et al.: SARAS 2 constraints on global 21-cm signals from the Epoch of Reionization. ArXiv e-prints (2017)

  50. Sokolowski, M., Tremblay, S.E., Wayth, R.b., et al.: BIGHORNS - Broadband instrument for global hydrogen reionisation signal. Publ. Astron. Soc. Aust. e004, 32 (2015). https://doi.org/10.1017/pasa.2015.3

    Google Scholar 

  51. Srivani, K.S., Girish, B.S., Shankar, N.U., Subrahmanyan, R.: A precision spectrometer for measuring signals from the epoch of cosmological recombination. In: 2014 XXXIth URSI general assembly and scientific symposium (URSI GASS). https://doi.org/10.1109/URSIGASS.2014.6930031, pp 1–4 (2014)

  52. Straw, R., Cebik, L., Hallidy, D., Jansson, D. (eds.): The ARRL Antenna Book. ARRL Antenna Book, ARRL (2007)

  53. Stutzman, W.L., Thiele, G.A.: Antenna theory and design. 3 edn Wiley (2012)

  54. Vedantham, H.K., Koopmans, L.V.E., de Bruyn, A.G., et al.: Chromatic effects in the 21 cm global signal from the cosmic dawn. Mon. Not. R. Astron. Soc. 437, 1056–1069 (2014). https://doi.org/10.1093/mnras/stt1878

    Article  ADS  Google Scholar 

  55. Vedantham, H.K., Koopmans, L.V.E., de Bruyn, A.g., et al.: Lunar occultation of the diffuse radio sky: LOFAR measurements between 35 and 80 MHz. Mon. Not. R. Astron. Soc. 450, 2291–2305 (2015). https://doi.org/10.1093/mnras/stv746

    Article  ADS  Google Scholar 

  56. Voytek, T.C., Natarajan, A., Jáuregui García, J.M., Peterson, J.B., López-Cruz, O.: Probing the dark Ages at z 20: the SCI-HI 21 cm all-sky spectrum experiment. The Astrophysical Journal Letters 782, L9 (2014). https://doi.org/10.1088/2041-8205/782/1/L9

    Article  ADS  Google Scholar 

  57. Wales, D.J., Doye, J.P.K.: Global optimization by basin-hopping and the lowest energy structures of lennard-jones clusters containing up to 110 atoms. J. Phys. Chem. A 101(28), 5111–5116 (1997). https://doi.org/10.1021/jp970984n

    Article  Google Scholar 

  58. Weiner, M.M.: Monopole antennas. dekker (2009)

Download references

Acknowledgements

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.

Author information

Authors and Affiliations

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

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

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

    Saurabh Singh

Authors
  1. Saurabh Singh
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Ravi Subrahmanyan
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. N. Udaya Shankar
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Mayuri Sathyanarayana Rao
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. B. S. Girish
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. A. Raghunathan
    View author publications

    You can also search for this author in PubMed Google Scholar

  7. R. Somashekar
    View author publications

    You can also search for this author in PubMed Google Scholar

  8. K. S. Srivani
    View author publications

    You can also search for this author in PubMed Google Scholar

Corresponding author

Correspondence to Saurabh Singh.

Appendix: A Measurement of the total efficiency of an antenna using the Global Sky Model

Appendix: A Measurement of the total efficiency of an antenna using the Global Sky Model

We describe here a method developed to derive the total efficiency of any antenna using a night sky observation with a radiometer. The method uses a global model for the sky brightness distribution. We have used Global MOdel for the radio Sky Spectrum (GMOSS) [43] as the model and the SARAS 2 receiver to measure the total efficiency of the SARAS 2 monopole antenna.

At any frequency, the calibrated measurement data can be decomposed into a sum of contributions from the foreground, ground and receiver systematics. Further, the data is measured with reference to a standard load, whose physical temperature over the observing time is recorded using a logger. The contributions from foreground and the reference load temperature are the only significant time-varying components in the data. Since the instrument is being calibrated every second, all the temporal variations in receiver gain are calibrated out. Though the ground temperature may vary over the observing time, this is a variation on the surface; the effective temperature of the ground emission corresponds to the temperature at an effective penetration depth. For ground contribution in the frequencies of interest, the effective penetration depth is \(\sim 2.5~ m\) [52, Chapter 3]. At this depth, the diurnal temperature variations have been found to be negligible [12]. Thus the contributions from instrument systematics as well as contributions from the ground are essentially time invariant and may be treated as constant additive signals in the spectrum.

The measured temperature, thus, can be represented as :

$$ T_{A}(\nu, t) = \eta_{t}(\nu) T_{\mathrm{W}}(\nu, t) + T_{\text{add}}(\nu) - T_{\text{ref}}(t), $$
(29)

where T A is the measured temperature, TW is the beam-weighted foreground that couples to the system through the total efficiency η t of the antenna, and Tref is the reference temperature. It is to be noted that we actually measure the physical temperature of the reference load Tp, which is linearly related to the actual temperature Tref. This is because there is a thermal resistance between the actual source of noise and the outer metallic body where the temperature is measured. The time-invariant component of the data consisting of the systematics and ground contributions is represented by Tadd.

Using GMOSS, at every frequency, we decompose the time series data at each frequency into three components:

  • a component correlated with temporal variations in the foreground brightness,

  • a component correlated with temporal variations in the reference load temperature, and

  • a component that is constant over time.

Thus, at any frequency ν we have the following equation:

$$ T_{A}(\nu, t) = \eta_{t}(\nu)T_{\mathrm{W}}(\nu, t) + a_{1} T_{\mathrm{p}}(t) + a_{2}(\nu), $$
(30)

where T A is the measured equivalent temperature, TW is derived from GMOSS as a weighted average of the model T B over the sky with a weighting by the antenna beam. Tp is taken from the reference load temperature measurements. Using these, we optimize for a1 using measurement data across time and frequency that includes sufficient LST range so that the antenna temperature varies significantly. η t and a2 are optimized for each frequency independently.

The resulting total efficiency across the band is shown in Fig. 8. We also show the optimization fits for total efficiency at four representative frequencies in Fig. 18. This method also provides an estimate of the additive signals in the system, a2(ν), which may be used as a tool to model the data.

Fig. 18
figure 18

The component of data that is correlated with GMOSS foreground predictions shown at four sample frequencies. The slope of the line at each frequency provides an estimate of the total efficiency at that frequency

Full size image

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Singh, S., Subrahmanyan, R., Shankar, N.U. et al. SARAS 2: a spectral radiometer for probing cosmic dawn and the epoch of reionization through detection of the global 21-cm signal. Exp Astron 45, 269–314 (2018). https://doi.org/10.1007/s10686-018-9584-3

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1007/s10686-018-9584-3

Keywords

Search

Navigation