Radiometric observations of Venus have revealed a monotonic decrease of brightness temperature ( T b ) in the decimeter wavelength regime. Such a decrease has also been independently reported by the observations using interferometric radio‐telescopes, like the Very Large Array (VLA) in the USA and the Giant Metrewave Radio Telescope (GMRT) in India. In this work, we have carried out microwave radiative transfer (RT) simulations of thermal emission from the Venusian surface at decimeter wavelengths to examine the role of subsurface properties of Venusian regolith in the continuous reduction of T b at the microwave‐radiowave spectral domain. These simulations are compared against spectral microwave measurements by GMRT over a wide decimeter wavelength regime ranging from ∼23–128 cm. Good agreements are obtained for simulations that consider a two‐layer Venusian surface (a low‐loss medium overlaid over a reflecting/lossy medium) a situation that would arise if the subsurface layer has high dielectric properties due to the presence of semiconducting mineral assemblages such as pyrites, ferroelectric minerals, magnetite–hematite, magnetite–pyrite or magnetite–hematite–pyrite equilibrium assemblages. The thickness of the top layer, the possibilities of the formation of such layers and their geological and chemical evolutions are also discussed.

In this autobiographical account, I first describe my family, then childhood and education in India. During 1953–55, I worked in the new field of radio astronomy at the Division of Radiophysics of the Commonwealth Scientific and Industrial Research Organisation in Australia. During 1956–57, I worked at the Radio Astronomy Station of Harvard University at Fort Davis, Texas, where I made observations of solar radio bursts at decimeter wavelengths. I then joined Stanford University as a graduate student in 1957. I contributed to the successful operation of the Stanford Cross Antenna and then used it for studying microwave radio emission from the Sun. I was awarded the Ph.D. degree by Stanford University in 1960 and was then appointed as an Assistant Professor for three years. With an urge to contribute to evolving scientific endeavors in India, I joined the Tata Institute of Fundamental Research (TIFR) at Mumbai, India, in April 1963. In my stay of more than three decades at TIFR, I conceived of, and guided, construction of two of the world's largest radio telescopes, namely the Ooty Radio Telescope and the Giant Metrewave Radio Telescope. These instruments have led to several outstanding contributions and discoveries in the areas of radio galaxies, quasars, pulsars, and cosmology.

The folder contains all the data of the radiometric observation campaign of Venus using the GMRT carried out during 2004. The data have been used in the publication of the paper, "Radio observation of Venus at meter wavelengths using the GMRT, by Mohan et al (2017)". The data can also be obtained from the GMRT data archive (https://naps.ncra.tifr.res.in/goa/data/search) with proposal code 05BBA01.

Introduction: Venus is like Earth in its size, mass and bulk density. Yet, the distinctions are much more pronounced, as the surface is extremely dry with a temperature of ∼750 K and pressure of ∼90 bar. The CO2 dominated atmosphere and the globally covered H2SO4 clouds make the study of its lower atmosphere and surface thermo-physical properties extremely difficult. While the satellite-based remote sensing of its lower atmosphere has been limited to single polarization, the lander-based in-situ measurements were limited to localized regions and were severely affected by the planetary conditions. The Earth-based radio telescopes using interferometry have the potential for multi-frequency and dual-polarization (H and V) observations which can be used to study the thermo-physical properties of the surface and the lower atmosphere with reasonable spatial resolutions. Scientific Problem: More than 100 observations were carried out on Venus over a wide frequency range between mm-cm wavelength spectra from the Earth-based platforms (eg. black filled circles in fig. 1). But, observations at decimetre (dm) wavelengths, especially, beyond 70 cm were limited due to increased background noise, system contamination and reduced planetary emission. Thermal emission from the Venus at microwave spectral regime, observed by several investigators, showed a monotonous decrease of radiometric brightness temperature (Tb) beyond ∼6 cm wavelength (black filled circles in fig. 1), which are expected to emanate from the subsurface layers of Venus [1, 2]. Hence, there have been several attempts to explain this emission even though none were satisfactory (eg. [2, 3, 4]). The reason for the continuous Tb decrease was hypothesised to a cooler subsurface [2], due to the change in emissivity with wavelength, and linked to the dielectric constant and subsurface properties. Methodology: The observations of Venus using Giant Metrewave Radio Telescope (GMRT) interferometric techniques at 233.67 MHz 1280.67 MHz (or ∼123 cm 21 cm wavelength) channels were used to address this problem. GMRT is an interferometric array consisting of 30 antennas of 45 m aperture-diametre. The telescope operates in 5 frequency channels in the range of 150 MHz to 1280 MHz (or ∼200 cm ∼21 cm). Later, the results from these observations were used in a zero-order radiative transfer simulation to explain the planetary thermal emission. Results: Firstly, the archived data of GMRT observation of Venus carried out at 606.67 MHz, 332.9 MHz and 239.9 MHz (or ∼50 cm, ∼90 cm and ∼123 cm), conducted during 2004 were analysed. The results confirmed a further reduction of Tb beyond 70 cm wavelength [5] and the Tb values derived at the respective frequencies are 526± 22 K, 409± 33 K and <426 K. Further, based on these results, a dedicated, better SNR, GMRT observation campaign was conducted at multifrequency and multi-stokes (I,Q,U) for a longer duration (10 hr for each channel), when Venus was near to its inferior conjunction in July-September of 2015. The Tb derived at the respective frequencies 1297.67 and 607.67 MHz were 622± 43 K and 554± 38 K, respectively. The derived value of the Tb at 233.67 MHz placed an upper limit of 321 K. The results of the two observations are shown as red filled circles in the fig. 1. Using the multi-stokes data of the 606.67 and 1297.67 MHz, the dielectric constant of Venus surface (globally averaged) was derived as ∼4.5 [6] which is in agreement with those derived from the orbiter-based radar (Pioneer Venus [7] and Magellan [8]) observations. Radiative Transfer Simulation: The total emission simulation is accounted based on a zero-order radiative transfer (RT) model which accounts for the atmospheric and the surface thermal emission. The atmospheric part accounts for the induced absorption of CO2 and N2 based on [9], while the surface part accounts for the intensity of thermal emission from the surface/subsurface. The total brightness temperature (Tb) is the product of emissivity (e) and effective radiating temperature (Teff ) given by:

Research Laboratory (PRL) in Ahmedabad to interact with its faculty and students.I went to PRL several times during the next three years.There I also gave talks on radio astronomy, the Kalyan Radio Interferometer and the Ooty Radio Telescope (ORT).I had many discussions with the faculty and students at PRL regarding research being carried out in several fields, such as cosmic rays, atmospheric sciences.I also recall discussions with U. R. Rao about his work on cosmic rays and solar wind.During one of my visits to PRL, Sarabhai described to me his plans for building capabilities for space research and communication in India.He had a clear vision about it.He also took me to visit the Indian Institute of Management in Ahmedabad that was founded by him.It was a great experience.I also visited two sites that were selected by him for establishment of the proposed Space Applications Centre (SAC) at Ahmedabad.I was also told then that it was planned to import a 45 ft parabolic dish from Japan for microwave communication at the SAC.I told Sarabhai that it could be designed and built in India in a couple of years instead of being imported.Sarabhai became the Chairman of the Atomic Energy Commission after the untimely death of Homi Bhabha in January 1966 in a plane crash at the Mont Blanc in the Swiss Alps.In 1965, Bhabha had approved the construction of the ORT that was proposed by me.During a visit to TIFR, Sarabhai asked me the details of the construction being done for the ORT that consisted of a 530 m long and 30 m wide parabolic cylindrical reflector being placed on a hill at Ooty.Sarabhai made several valuable comments that we considered.In 1967, Sarabhai invited M. G. K. Menon, Yash Pal, I and Sitaram from TIFR to visit the Thumba Equatorial Rocket Launching Station (TERLS) where a rocket was being designed for launching in space.After we reached TERLS, Sarabhai introduced to us the teams that were designing various sub-systems of the rocket.Menon and Yash Pal visited the group that was designing the solid

Some countries in the greater Asian area, namely Australia, China, India, Japan and New Zealand, played important roles in the early development of radio astronomy from the 1940s through into the 1960s. In this paper—which is based on the Public Lecture that we presented in Pune during the ICOA-9 conference—we trace these early developments. We then finish this review paper by briefly surveying the exciting new radio astronomical developments that are currently occurring throughout the greater Asian region.

Venus was observed at frequencies of 1297.67MHz (23cm), 607.67MHz (49cm), and 233.67MHz (1.28m) with the Giant Metrewave Radio Telescope (GMRT) during the period of 2015 July 25 and September 6 when it was close to its inferior conjunction. Values of the brightness temperature (T-b) of Venus from these observations were derived as 622 +/- 43K, 554 +/- 38K for 1297.67 and 607.67MHz frequencies, respectively, which are in agreement with the previous observations. The attempt to derive the T-b at 233.67MHz affirms an upper limit of 321K which is significantly lower than the previously reported upper limit of 426K at the same frequency. We also present the dielectric constant (epsilon) values of the Venus surface estimated using the degree of polarization maps of Venus, derived from the GMRT polarization observations and theoretical calculations. The epsilon of the Venus surface was estimated to be similar to 4.5 at both the 607.67 and 1297.67MHz, close to the reported values of epsilon of 4 to 4.5 from the radar-based observations including the Magellan observations at 2.38GHz (12.6cm).

The radio-thermal emission from Venus as observed by ground-based interferometric radio-telescopes shows a significant spectral variation, with a gradual increase in brightness temperature from 1 mm to 6 cm and a decrease thereafter at higher wavelengths. The first time GMRT observations beyond 70 cm wavelength also reconfirm this decreasing trend in T-b with the increase in wavelength [7]. Efforts have been made to model this spectral variation in T-b [1], [10], but these models fail to explain the low-frequency radio-thermal emission from Venus (decrease in T-b with the increase in wavelength) and the problem still remains unresolved. The authors attempt to explain this problem using radiative transfer based model and radiometric observations of Venus focussing particularly on the higher wavelength T-b observations from the Giant meter radio telescope (GMRT). The GMRT brightness temperature (T-b) (at 0.21, 0.5, 0.9, 1.23 and 2 m) is observed to decrease with frequency. A radiative transfer model was developed and it is seen that a two layer Venusian surface model matches with the observations. Based on the simulation studies, the authors put forth a hypothesis that Venus may have an absorbing layer within the first few meter depth.

Firstly I plan to describe briefly growth of the Radio Astronomy group that was established at TIFR, Mumbai, in early 1963. The Ooty Radio Telescope (ORT) was designed and built indigenously and became operational in early 1970. It consists of a 530m long and 30 m wide parabolic cylindrical antenna that is located on a suitable hill with its axis of rotation parallel to that of the Earth, which allows it to be steered mechanically in hour angle for about ten hours. A phased array allows coverage from -45 to +45 degrees in declination. Using the lunar occultation observations, angular size of over 1000 radio galaxies with arc-second resolution were made by 1976 for the first time in the world. The observations supported the Big Bang model. An Ooty Synthesis Radio Telescope (OSRT) was operational during 1982-1986. Currently, ORT is being used for Inter-planetary Scintillation observations of compact radio galaxies and Quasars for daily measurements of the velocity of the Solar wind. An active programme for observations of Pulsars is also carried out.

In his guest editorial, Arunan 1 has pondered upon the criterion that was used by the selection committee of Raman Research Institute (RRI), (the Raman Trust) for appointing V. Radhakrishnan, son of C. V. Raman, as the Director of the Institute in 1971.

The Venusian surface has been studied by measuring radar reflections and thermal radio emission over a wide spectral region of several centimeters to meter wavelengths from the Earth-based as well as orbiter platforms. The radiometric observations, in the decimeter (dcm) wavelength regime showed a decreasing trend in the observed brightness temperature (T-b) with increasing wavelength. The thermal emission models available at present have not been able to explain the radiometric observations at longer wavelength (dcm) to a satisfactory level. This paper reports the first interferometric imaging observations of Venus below 620 MHz. They were carried out at 606, 332.9 and 239.9 MHz using the Giant Meterwave Radio Telescope (GMRT). The T-b values derived at the respective frequencies are 526 K, 409 K and <426 K, with errors of similar to 7% which are generally consistent with the reported T-b values at 608 MHz and 430 MHz by previous investigators, but are much lower than those derived from high-frequency observations at 1.38-22.46 GHz using the VLA. (C) 2017 Elsevier Inc. All rights reserved.

In this chapter I recall my initiation into the field of radio astronomy during 1953–1955 at CSIRO, Australia; the transfer of thirty-two 6-feet (1.8-m) diameter parabolic dishes from Potts Hill, Sydney, to India in 1958; and their erection at Kalyan, near Bombay (Mumbai), in 1963–1965. The Kalyan Radio Telescope was the first modern radio telescope built in India. This led to the establishment of a very active radio astronomy group at the Tata Institute of Fundamental Research, which subsequently built two world-class radio telescopes during the last 50 years and also contributed to the development of an indigenous microwave antenna industry in India. The Ooty Radio Telescope, built during 1965–1970, has an ingenious design which takes advantage of India’s location near the Earth’s Equator. The long axis of this 530-m × 30-m parabolic cylinder was made parallel to the Equator, by placing it on a hill with the same slope as the geographic latitude (~11°), thus allowing it to track celestial sources continuously for 9.5 h every day. By utilizing lunar occultations, the telescope was able to measure the angular sizes of a large number of faint radio galaxies and quasars with arc-second resolution for the first time. Subsequently, during the 1990s, the group set up the Giant Metrewave Radio Telescope (GMRT) near Pune in western India, in order to investigate certain astrophysical phenomena which are best studied at decimetre and metre wavelengths. The GMRT is an array of 30 fully steerable 45-m diameter parabolic dishes, which operates at several frequencies below 1.43 GHz. These efforts have also contributed to the international proposal to construct the Square Kilometre Array (SKA). This chapter is a revised version of Swarup (Journal of Astronomical History and Heritage, 9: 21–33, 2006).

During the last six decades, several outstanding discoveries have been made in the field of radio astronomy. These have revolutionized our understanding of the mysteries of the Universe. Observations of radio galaxies and quasars that emit extremely powerful radio waves indicate presence of supermassive black holes at their centre. Discovery and detailed observations of the 2.7 K cosmic microwave background temperature have provided strong support to the Big Bang Model of the Universe, giving tight constraints on the relative contributions of baryons, dark matter and dark energy in the Universe. Observations of the emission line of neutral hydrogen from spiral galaxies provide information about the formation and evolution of galaxies. Over 150 molecules have been discovered in the interstellar medium, giving details about the physics and chemistry of the interstellar space; these are also ingredients of life in the Universe. Discovery of Pulsating Radio Sources (Pulsars) have provided strong support to the models of neutron stars that are end products of a star when its 'nuclear fuel' runs out. Observations of Pulsars also provide tests of the General Theory of Relativity. I also summarize some of the investigations that are being carried out currently with the Ooty Radio Telescope in South India and with the Giant Metrewave Radio Telescope near Pune; these are amongst the largest radio telescopes in the world. Finally, I describe some of the Key Questions today.

We describe observations of 20 square degree region towards the WMAP cold spot (l=207.80, b=-56.30; RA = 03h15m, Dec. = -19d35) made with the Giant Metrewave Radio Telescope at 625 MHz, and 325 MHz. We find deficiency in the number counts of radio sources towards the cold spot in a 3 sq. deg region at position (l= 206.600, b= -54.740), similar to that reported by Rudnick et al. (2007). We find that the average value of the spectral index of radio sources in the 3 sq. deg region (’Radio Cold Spot’) is significantly flatter than that elsewhere in the field observed by us of about 20 square degrees. Further, most of the radio sources in the ≈ 3 sq. deg region are relatively compact compared to radio sources elsewhere. It may be noted that the cold spot indicates significant deviation of the CMB from Gaussianity and has been confirmed by the recently released Planck data. We discuss significance of our results.

The IAU Working Group on Historical Radio Astronomy (WGHRA) was formed at the 2003 General Assembly of the IAU as a Joint Working Group of Commissions 40 (Radio Astronomy) and 41 (History of Astronomy), in order to: a) assemble a master list of surviving historically-significant radio telescopes and associated instrumentation found worldwide; b) document the technical specifications and scientific achievements of these instruments; c) maintain an on-going bibliography of publications on the history of radio astronomy; and d) monitor other developments relating to the history of radio astronomy (including the deaths of pioneering radio astronomers).

During the Rio General Assembly we held the following meetings of the Working Group: a Business Meeting, a Science Meeting on “The Development of Aperture Synthesis Imaging in Radio Astronomy”, and a Science Meeting on “Recent Research”.

We report 325 MHz observations of a approximate to 20 square degree region towards the WMAP cold spot (l= 207.8 degrees, b= -56.3 degrees), using the Giant Metrewave Radio Telescope (GMRT). Deficiency is observed in the number counts of radio sources at 325 MHz near the cold spot, similar to that has been reported using the NRAO VLA Sky Survey (NVSS) catalogue at 1400 MHz. The observed deficiency of the 325 MHz source counts at position (l= 206.60 degrees, b= 54.74 degrees) in the cold spot region is found to be marginal, yet noteworthy.