Observations of the redshifted 21-cm HI fluctuations promise to be an important probe of the post-reionization era (𝑧 ≤ 6). In this paper we calculate the expected signal and foregrounds for the upgraded Ooty Radio Telescope (ORT) which operates at frequency 𝑣_𝑜 = 326.5MHz which corresponds to redshift 𝑧 = 3.35. Assuming that the visibilities contain only the HI signal and system noise, we show that a 3𝜎 detection of the HI signal (∼ 1 mK) is possible at angular scales 11' to 3° with ≈ 1000 h of observation. Foreground removal is one of the major challenges for a statistical detection of the redshifted 21 cm HI signal. We assess the contribution of different foregrounds and find that the 326.5MHz sky is dominated by the extragalactic point sources at the angular scales of our interest. The expected total foregrounds are 10⁴−10⁵ times higher than the HI signal.

We have used the Fisher matrix formalism to quantify the prospects of detecting the 𝑧 = 3.35 redshifted 21-cm HI power spectrum with the upcoming radio-imterferometric array OWFA. OWFA’s frequency and baseline coverage spans comoving Fourier modes in the range 1.8 × 10⁻² ≤ k ≤ 2.7 Mpc⁻¹. The OWFA HI signal, however, is predominantly from the range 𝑘 ≤ 0.2 Mpc⁻¹. The larger modes, though abundant, do not contribute much to the HI signal. In this work, we have focused on combining the entire signal to achieve a detection. We find that a 5 - 𝜎 detection of 𝐴_HI is possible with ∼ 150 h of observations, here $A^{2}_{\text{HI}}$ is the amplitude of the HI power spectrum. We have also carried out a joint analysis for 𝐴_HI and 𝛽 the redshift space distortion parameter. Our study shows that OWFA is very sensitive to the amplitude of the HI power spectrum. However, the anisotropic distribution of the k modes does not make it very suitable for measuring 𝛽.

The line-of-sight direction in the redshifted 21-cm signal coming from the cosmic dawn and the epoch of reionization is quite unique in many ways compared to any other cosmological signal. Different unique effects, such as the evolution history of the signal, non-linear peculiar velocities of the matter etc. will imprint their signature along the line-of-sight axis of the observed signal. One of the major goals of the future SKA-LOW radio interferometer is to observe the cosmic dawn and the epoch of reionization through this 21-cm signal. It is thus important to understand how these various effects affect the signal for its actual detection and proper interpretation. For more than one and half decades, various groups in India have been actively trying to understand and quantify the different line-of-sight effects that are present in this signal through analytical models and simulations. In many ways the importance of this sub-field under 21-cm cosmology have been identified, highlighted and pushed forward by the Indian community. In this article, we briefly describe their contribution and implication of these effects in the context of the future surveys of the cosmic dawn and the epoch of reionization that will be conducted by the SKA-LOW.

The Diffuse Galactic Syncrotron Emission (DGSE) is the most important diffuse foreground component for future cosmological 21-cm observations. The DGSE is also an important probe of the cosmic ray electron and magnetic field distributions in the turbulent interstellar medium (ISM) of our galaxy. In this paper we briefly review the Tapered Gridded Estimator (TGE) which can be used to quantify the angular power spectrum $C_\ell$ of the sky signal directly from the visibilities measured in radio-interferometric observations. The salient features of the TGE are: (1) it deals with the gridded data which makes it computationally very fast, (2) it avoids a positive noise bias which normally arises from the system noise inherent to the visibility data, and (3) it allows us to taper the sky response and thereby suppresses the contribution from unsubtracted point sources in the outer parts and the side lobes of the antenna beam pattern. We also summarize earlier work where the TGE was used to measure the $C_\ell$ of the DGSE using 150 MHz GMRT data. Earlier measurements of $C_\ell$ are restricted to $\ell \le \ell _{\max } \sim 10^{3}$ for the DGSE, the signal at the larger $\ell$ values is dominated by the residual point sources after source subtraction. The higher sensitivity of the upcoming SKA1 Low will allow the point sources to be subtracted to a fainter level than possible with existing telescopes. We predict that it will be possible to measure the $C_\ell$ of the DGSE to larger values of $\ell _{\max }$ with SKA1 Low. Our results show that it should be possible to achieve $\ell _{\max }\sim 10^{4}$ and $∼10^5$ with 2 minutes and 10 hours of observations respectively.

We use the Fisher matrix formalism to predict the prospects of measuring the redshifted 21-cm power spectrum in different $k$-bins using observations with the upcoming Ooty Wide Field Array (OWFA) which will operate at $326.5 {\rm MHZ}$. This corresponds to neutral hydrogen (HI) at $z=3.35$, and a measurement of the 21-cm power spectrum provides a unique method to probe the large-scale structures at this redshift. Our analysis indicates that a $5 \sigma$ detection of the binned power spectrum is possible in the $k$ range $0.05 \leq k \leq 0.3 \, {\rm Mpc}^{-1}$ with $1000$ hours of observation. We find that the signal- to-noise ratio (${\rm SNR}$) peaks in the $k$ range $0.1- 0.2\, {\rm Mpc}^{-1}$ where a $10 \sigma$ detection is possible with $2000$ hours of observations. Our analysis also indicates that it is not very advantageous to observe beyond $1000$ h in a single field-of-view as the (${\rm SNR}$) increases rather slowly beyond this in many of the small $k$-bins. The entire analysis reported here assumes that the foregrounds have been completely removed.