India's Giant Metrewave Radio Telescope Near Pune Has 30 Dishes of 45 Metres, Became an International Facility in October 2001 and Supplied Indian Data to the 29 June 2023 Gravitational-Wave Background Search

The Giant Metrewave Radio Telescope (GMRT) has thirty dishes, each 45 metres across, at a site about 80 km north of Pune. The National Centre for Radio Astrophysics (NCRA), part of the Tata Institute of Fundamental Research (TIFR), runs it and calls it the world’s largest radio telescope operating at metre wavelengths. Its data fed the European and Indian pulsar-timing result announced on 29 June 2023, which reported evidence for a faint background of gravitational waves.

The dishes sit in a Y. Fourteen are packed into a central square about 1 km across, and sixteen more run along three arms of about 14 km each. The observatory’s December 2025 status document gives baselines, the distances between pairs of dishes, from 100 m to 25 km. The telescope is at Khodad, about 10 km east of Narayangaon on the Pune-Nasik highway.

Why a telescope for metre waves, and why this field

At 150 MHz, one of the GMRT’s original frequencies, a radio wave is about 2 metres long. The observatory’s own account says the project chose this part of the spectrum because man-made radio interference was, in its words, considerably lower there in India. It also says no large facility anywhere in the world had been built to exploit these wavelengths.

The Khodad site was picked after a search across India. The criteria the GMRT page lists are low radio noise from human activity, good communications, nearby industry and education, and a latitude far enough north of the geomagnetic equator for a reasonably quiet ionosphere while still seeing much of the southern sky.

The 30 dishes work as one instrument. Signals from all 435 possible pairs are multiplied together over several hours, which lets the array build images with the sharpness of a single dish 25 km across. The original design used six frequency bands between 50 and 1,420 MHz.

Rope trusses and wire mesh

A solid 45-metre dish is heavy and catches the wind. The GMRT team used what it calls the SMART design, short for Stretch Mesh Attached to Rope Trusses. Thin stainless-steel wire ropes are stretched between 16 tubular steel parabolic frames and tensioned into a mosaic of flat facets that approximates a parabola. A mesh of 0.55 mm stainless-steel wire, with grid squares of 10 by 10 mm in the centre and 20 by 20 mm further out, forms the reflecting surface.

The observatory says the design cuts wind forces so much that the force and torque on a 45-metre GMRT dish are similar to those on a conventional 22-metre dish. It gives the dish weight as about 80 tonnes, with a 40-tonne counterweight, on a 15-metre concrete tower. The IEEE, which dedicated a Milestone plaque to the observatory on 30 March 2021, records that the whole programme was conceived, designed and made in India, and that the drive and servo systems came from technical collaboration with the Bhabha Atomic Research Centre in Mumbai. The IEEE proposal also says the GMRT was one of the first radio observatories to carry telescope signals over optical fibre instead of radio links.

Swarup’s group and the timeline

Govind Swarup came to TIFR in 1963 at the invitation of Homi Bhabha, whose own story is in our account of the atomic programme. His group built the Ooty Radio Telescope, which NCRA’s history page says was completed in 1969. NCRA’s obituary says he conceived and directed the design and construction of the GMRT from 1984 to 1996, became its project director in 1987 and was NCRA’s first Centre Director in 1993. He died on 7 September 2020.

The IEEE Milestone page gives the construction record. Land acquisition finished in 1990 and building began in 1991. The first antenna went up in 1993, and the first-light observation was made in 1994. All 30 antennas were standing by 1995. Test observations of astronomical objects began in early 1996, and the first high-quality radio galaxy image came that year. Regular observations started in 1997, and the GMRT was declared open to astronomers worldwide in October 2001.

The sources disagree on the start of operations. NCRA’s history says the telescope “went into routine operation in 2000”. The 2017 paper by NCRA’s Yashwant Gupta and colleagues says “since its commissioning in 2001”. The IEEE page uses 1997 for regular observations and October 2001 for the international opening. Those are different milestones, and each source’s date is given as it states it.

NCRA says astronomers from more than 25 countries use the telescope. The 2017 paper counted about 40 refereed papers a year using GMRT data, with about half the principal investigators from India and the rest from about 30 other countries.

The 2019 upgrade

The original receivers covered limited bands with a maximum bandwidth of 32 MHz. A major upgrade, first proposed during the 2007 to 2012 plan period, raised the instantaneous bandwidth to 400 MHz and replaced the old bands with wide receivers. The observatory’s December 2025 document lists four of them: 120 to 250 MHz, 250 to 500 MHz, 550 to 850 MHz and 1,000 to 1,460 MHz. It says the upgraded GMRT (uGMRT) has worked with all 30 antennas since March 2019.

The dates of the upgrade vary by source. The IEEE page gives 2012 to 2018, and NCRA’s director Yashwant Gupta, quoted in the 2023 press release, says 2013 to 2019. The 2017 paper expected sensitivity to rise by up to three times.

What the GMRT has surveyed and tracked

Between April 2010 and March 2012 the GMRT observed the TIFR GMRT Sky Survey at 150 MHz. Intema and colleagues, in a 2017 paper in Astronomy & Astrophysics, describe the first alternative data release as covering about 36,900 square degrees, roughly 90 per cent of the sky, with about 0.62 million sources extracted at a 7-sigma threshold.

In 2023 Arnab Chakraborty and Nirupam Roy reported a 5-sigma detection with the uGMRT of hydrogen emission at the 21 cm line from a galaxy at redshift 1.3, about 9 billion years back in time. Their abstract calls it the highest-redshift detection of hydrogen in emission from an individual galaxy to date. A foreground elliptical galaxy acts as a gravitational lens and boosts the signal.

On 19 October 2016 the GMRT listened for the European Space Agency’s Schiaparelli lander as it descended to Mars. ESA’s blog before the landing described the GMRT signal as a “nice-to-have” real-time view. ESA’s page of 20 October 2016 says early GMRT signals suggested that most of the descent steps had gone as planned, and that the signals received by the GMRT and by ESA’s Mars Express both dropped out shortly before touchdown.

Pulsars as clocks

Pulsar timing is among the GMRT’s stated goals. The observatory’s goals page says accurate timing of a group of pulsars, the fast-spinning neutron stars that flash like lighthouses, could lead to detecting a weak background of gravitational radiation from the very early Universe.

The method works because a gravitational wave passing between a pulsar and Earth changes the arrival times of its pulses. The effect is tiny. The 2023 press release puts it at less than a millionth of a second, and a single pulsar’s clock has its own irregularities. Astronomers therefore watch many pulsars and look for a pattern shared across the sky. The press release says the waves involved oscillate with periods of one to ten years, about 10 billion times slower than the ones LIGO first detected in 2015. Kilometre-scale detectors such as LIGO and the planned LIGO-India sense short bursts from colliding black holes. A pulsar array looks for a steady hum instead.

The GMRT’s contribution is its low frequencies. The press release says the pulsar signal is distorted by the medium between the pulsar and Earth, and that this is best measured near 400 MHz, where it describes the uGMRT as currently the world’s most sensitive radio telescope. The Indian Pulsar Timing Array (InPTA), which Bhal Chandra Joshi of NCRA founded, uses the uGMRT to observe millisecond pulsars simultaneously in two bands. Its first data release, published in 2022, gave arrival times for 14 millisecond pulsars observed between 2018 and 2021 at 300 to 500 MHz and 1,260 to 1,460 MHz.

The 29 June 2023 papers

The European Pulsar Timing Array (EPTA) and the InPTA announced their results on 29 June 2023, in two papers in Astronomy & Astrophysics, the InPTA press release says. The EPTA’s second data release covers 25 millisecond pulsars. The search paper combined it with InPTA data for ten pulsars timed by both groups. It reports marginal evidence for a background in the full 24.7-year European dataset and evidence at about 3 sigma in a 10.3-year subset. Adding the Indian data gave consistent results with better modelling of noise. A companion paper found the signal consistent with a population of supermassive black-hole pairs, and disfavoured an origin in ultra-light dark matter.

The InPTA press release says the analysis used pulsar data collected over 25 years with six of the world’s largest radio telescopes, including more than three years of uGMRT data. A. Gopakumar of TIFR, chair of the InPTA consortium, is quoted there: “this is the first time that an Indian telescope’s data is used for hunting gravitational waves”. Gupta says high-precision pulsar timing of this kind was one of the science targets of the upgrade and that its first results arrived within a few years.

Three other collaborations reported results at the same time. The NANOGrav team in North America found, among 67 pulsars, correlations matching the Hellings-Downs pattern expected for gravitational waves. It reports a significance of about 3 to 4 sigma, depending on the test. The press release names the Australian (PPTA) and Chinese (CPTA) arrays as the others.

What the Indian dataset shows alone

The InPTA’s own second data release covers 27 millisecond pulsars over seven years. Its first paper, posted in June 2025, reports some of the most precise dispersion measure estimates so far, a measure of the free electrons a pulsar’s signal has crossed on its way to Earth. A search paper posted on 3 August 2026 is the first independent look for the background in InPTA data alone. It finds a Bayes factor of 2.5 for a common signal and no significant detection. It sets a 95 per cent upper limit on the background’s amplitude about an order of magnitude above the amplitudes reported by longer-running arrays. Simulations in the paper indicate that a baseline of at least 10 years is needed to recover a common signal. The data release paper says part of the dataset is being incorporated into the International Pulsar Timing Array’s third data release.