Gauribidanur Radio Facilities

• Gauribidanur radio facilities are a network of radio astronomical instruments in Karnataka, India, specializing in low-frequency solar, pulsar, and transient observations.
• They employ integrated methods like 2D solar imaging, dynamic spectroscopy, and advanced beamforming to capture fine astrophysical details.
• The facility’s complementary design and precise calibration practices enable diverse studies from quiet-corona tracking to pulsar detection and transient event analysis.

Gauribidanur radio facilities are a cluster of radio astronomical instruments at the Gauribidanur observatory in Karnataka, India, near Bangalore and reported at approximately $13.6^\circ$ N, $77.4^\circ$ E. In the literature, the site is presented primarily as a low-frequency solar radio observatory operated by the Indian Institute of Astrophysics, but also as a long-used location for pulsar and transient work, instrumentation development, remote-operation test beds, and radio-frequency environment surveys (Ramesh et al., 2020, Gireesh et al., 2021, B et al., 31 Jan 2026, Likhit et al., 22 Jan 2025).

1. Site, institutional setting, and observational environment

The observatory is described as being in southern India, about $100$ km north of Bangalore, and at coordinates reported as $13.6^\circ$ N, $77.4^\circ$ E, $13.603^\circ$ N, $77.428^\circ$ E, and $13.604^\circ$ N, $77.427^\circ$ E in different papers. For the solar facilities, the site is explicitly identified as an Indian Institute of Astrophysics observatory; in earlier 34 MHz pulsar papers, the low-frequency telescope is described as jointly operated by the Raman Research Institute and the Indian Institute of Astrophysics (Ramesh et al., 2021, Likhit et al., 22 Jan 2025, Maan et al., 2014).

The geographic latitude is operationally important. Solar studies note that the Sun passes close to zenith, and one GRAPH observing strategy therefore uses data within $\pm 15$ min of meridian transit to minimize ionospheric refraction. A CME study on 2016 May 1 reports a solar elevation of about $77.4^\circ$0 during observation, and explicitly uses this geometry to reduce ionospheric refraction and scintillation (Ramesh et al., 2020, Ramesh et al., 2021).

The site is characterized very differently depending on application. Solar and pulsar papers describe minimal radio frequency interference in specific bands such as 50–80 MHz, and one GRASP study states that radio frequency interference in the observations is minimal. By contrast, a dedicated 30–300 MHz cosmology-oriented RFI survey describes Gauribidanur as the reference site “closer to cities,” with mean horizon obstruction $77.4^\circ$1, peak obstruction $77.4^\circ$2, and blockage solid angle $77.4^\circ$3 sr; the same survey concludes that the site’s proximity to urban regions and very low horizon obstruction make it very susceptible to terrestrial RFI, with about $77.4^\circ$4 of the 30–300 MHz band affected at some time (Kumari et al., 2019, Bane et al., 2022, Agrawal et al., 12 Apr 2025).

This suggests that the observatory’s radio environment is strongly regime-dependent: bands and beam patterns optimized for solar and pulsar measurements can remain productive even when the site is unsuited to horizon-sensitive global 21-cm cosmology experiments (Agrawal et al., 12 Apr 2025).

2. Core solar radio instrumentation

The solar facilities are repeatedly described as an integrated suite centered on imaging, dynamic spectroscopy, and polarimetry. In one long-term quiet-corona study, the observatory is said to host the Gauribidanur RAdioheliograPH (GRAPH), Gauribidanur LOw-frequency Solar Spectrograph (GLOSS), Gauribidanur RAdio Spectro-Polarimeter (GRASP), Gauribidanur Radio Interferometric Polarimeter (GRIP), and an e-CALLISTO station, with the earlier GRASS instrument also cited in the bibliography (Ramesh et al., 2020).

Facility	Primary role	Representative characteristics
GRAPH	2D solar imaging interferometer	T-shaped array; 53 and 80 MHz simultaneous imaging; $77.4^\circ$5 MHz bandwidth; $77.4^\circ$6 ms images
GLOSS	Dynamic spectroscopy	Used for burst detection; reported at 330–30 MHz in a later CME-shock study
GRASP	Spectro-polarimetry	Reported at 90–50 MHz in one type II study and 30–10 MHz in a later study
GRIP	Integrated interferometric polarimetry	80 MHz work with $77.4^\circ$7 half-power beamwidth in one CME study
e-CALLISTO	Metric-decimetric dynamic spectra	45–450 MHz, about 400 frequencies per sweep, 0.25 s cadence in one automated-burst study

GRAPH is described as a T-shaped low-frequency interferometer or radioheliograph. In one quiet-corona study it was used simultaneously at 53 MHz and 80 MHz with synthesized beams of $77.4^\circ$8 at 53 MHz and $77.4^\circ$9 at 80 MHz, $100$0 MHz bandwidth, and $100$1 ms temporal resolution; a CME thermal-emission study likewise describes GRAPH as operating in the 30–110 MHz band and using a 1024-channel digital correlator (Ramesh et al., 2020, Ramesh et al., 2021).

GRASP and GLOSS provide the dynamic spectral context that GRAPH itself does not. For a 2016 March 16 split-band type II burst, GRASP is described as observing in the range $100$2 MHz with $100$3 MHz per channel and $100$4 ms integration, using a crossed log-periodic dipole and yielding Stokes $100$5 and $100$6 dynamic spectra. In a later joint VELC–radio study, GLOSS and GRASP are used together to provide continuous spectral coverage from 330 down to 10 MHz, with GLOSS covering 330–30 MHz and GRASP 30–10 MHz (Kumari et al., 2019, Kathiravan et al., 24 Feb 2026).

GRIP serves a complementary role as an integrated polarimetric monitor. In the 2016 May 1 CME study it is described as an 80 MHz transit interferometer with a half-power beamwidth of $100$7, larger than the solar disk, and is used to measure whole-Sun Stokes $100$8 and $100$9 and thus the degree of circular polarization (Ramesh et al., 2021).

The Gauribidanur e-CALLISTO station extends the spectral coverage upward. In a two-year automated burst-detection study it is described as operating over 45–450 MHz at Gauribidanur, with about 400 frequencies per sweep, nominal 0.25 s time resolution, 1 ms integration time, $13.6^\circ$0 kHz radiometric bandwidth, and dynamic range $13.6^\circ$1 dB. The same study notes that the FM broadcast band at $13.6^\circ$2 MHz is excluded because of severe RFI (Singh et al., 2019).

Although the historical emphasis is on low frequencies, the observatory has also been used to prototype a high-frequency solar interferometer. One development paper states that the Indian Institute of Astrophysics has been carrying out routine observations of solar coronal radio emission at $13.6^\circ$3 MHz at Gauribidanur, and then describes a new $13.6^\circ$4 GHz two-element interferometer built from commercial Dish TV antennas to explore radio observations of the chromosphere (Gireesh et al., 2021).

3. Pulsar and transient facilities at decametric and metric wavelengths

A distinct strand of Gauribidanur instrumentation is the decametric pulsar and fast-transient program based on the east–west arm of the Gauribidanur radio telescope. In this line of work, the telescope is operated near 34–34.5 MHz with a very large effective collecting area and wide field of view. One Geminga study states that the east–west arm has beamwidths of $13.6^\circ$5 in right ascension and $13.6^\circ$6 in declination at 34 MHz, an effective collecting area $13.6^\circ$7 at instrumental zenith, single linear polarization in the east–west direction, and raw 2-bit, 4-level Nyquist-sampled voltage recording; the same paper describes offline conversion to 512 channels across 1.53 MHz with $13.6^\circ$8 ms time resolution (Maan, 2015).

Earlier and related 34 MHz work used the Portable Pulsar Receiver for 2-bit, 4-level digitization, 256 channels across 1.05 MHz, and $13.6^\circ$9 ms time resolution in J1732–3131 searches; later deep-search work at 34 MHz used 1024 channels across 1.53 MHz with $77.4^\circ$0 ms resolution (Maan et al., 2011, Maan et al., 2014).

A later LPDA-based prototype extends pulsar observations to 50–80 MHz. That instrument is described as a dedicated 16-LPDA north–south array with 5 m spacing, divided into two groups of eight antennas, using broadband log-periodic dipoles with $77.4^\circ$1, gain $77.4^\circ$2 dBi, half-power beamwidths of $77.4^\circ$3 in the E-plane and $77.4^\circ$4 in the H-plane, and a fan beam of roughly $77.4^\circ$5 in right ascension by $77.4^\circ$6 in declination at a representative frequency of 65 MHz. The digital receiver samples at 90 MHz, uses a 4-tap polyphase filterbank with a 2048-point FFT, and yields raw channel spacing $77.4^\circ$7 kHz and effective FWHM per channel $77.4^\circ$8 kHz (Bane et al., 2022).

The next step in this transient program is the GAuribidanur Pulsar System (GAPS) with a 1-bit raw-voltage backend. In that work, eight LPDAs are used on a north–south baseline, the analog band is narrowed to 50–70 MHz for the reported tests, the sampling rate is 90 MSPS, and only the sign bit of each sample is retained. The paper emphasizes two advantages of this arrangement: simultaneous observations of any set of desired directions in the sky with multiple offline beams, and archival of observed data with minimal resources for future re-analysis (Bane et al., 2024).

These pulsar and transient facilities broaden the scientific scope of the observatory beyond solar radio physics. The 34 MHz telescope was used for low-DM fast transients associated with Geminga and for candidate pulsed emission from J1732–3131, while the 50–80 MHz and 1-bit LPDA systems were used to detect standard pulsars such as B1919+21, B0950+08, B0834+06, and B1133+16, as well as a type III solar burst in a beamformed transient mode (Maan, 2015, Maan et al., 2011, Bane et al., 2022, Bane et al., 2024).

4. Recent LPDA arrays, SKA-related test beds, and remote-operation systems

Recent work at Gauribidanur adds a newer family of LPDA-based arrays in the 130–350 MHz regime. The Gauribidanur Diamond Array Radio Telescope (GBD-DART) is described as a custom-developed LPDA array for bright pulsars and solar transients. One hardware paper describes a checkerboard layout of 64 LPDAs in a diamond-shaped tile of $77.4^\circ$9 m by $13.603^\circ$0 m, with north–south and east–west diagonals of about $13.603^\circ$1 m; a later pulsar-pipeline paper characterizes the phased system for pulsar observations as 32 off-axis dual-polarised LPDAs with nominal gain of 22 dBi between 130 and 350 MHz and a 15-degree HPBW at 175 MHz (B et al., 31 Jan 2026, B et al., 31 Jan 2026).

The present digital backend for GBD-DART restricts the instantaneous bandwidth to 16 MHz, and the current pulsar pipeline is configured for 170–196 MHz observations with daily cadence. The same pipeline paper states that receiver output voltages are taken from a transient buffer, that high- and low-resolution full-polar spectral data are produced, that full-polar folded archives are generated with coherent and incoherent dedispersion, and that custom Python routines, FFT libraries, DSPSR, PSRCHIVE, and PRESTO are all used in the processing chain (B et al., 31 Jan 2026).

A separate SKA-Low-related LPDA test bed at Gauribidanur uses eight LPDAs grouped into two 4-LPDA elements on an east–west baseline of $13.603^\circ$2 m. Simulations for a single 4-LPDA element at 200 MHz give gain $13.603^\circ$3 dBi, 3 dB beamwidth $13.603^\circ$4, and first sidelobe at $13.603^\circ$5 dB relative to the main lobe. That system is coupled to a Streamlit-based web interface using PyVISA and SCPI commands, making the instrument remotely operable through a secure WLAN or VPN and providing manual RFI masking, transit plotting, fringe-plot analysis, and download access to observational data (Likhit et al., 22 Jan 2025).

The 11.2 GHz two-dish prototype belongs to the same developmental trajectory. It uses two commercial Ku-band Dish TV antennas with a 2.5 m east–west baseline, 25 MHz external reference locking of the LNBs, and a 1-bit digital correlator. The paper explicitly frames this instrument as a complement to the routine low-frequency coronal observations at Gauribidanur and to optical observations at the Kodaikanal Solar Observatory (Gireesh et al., 2021).

Taken together, these newer installations show a broadening of the observatory from a specialized low-frequency solar site into a mixed facility for solar physics, pulsar polarimetry, SKA-style beamforming experiments, and remote observatory control. A plausible implication is that Gauribidanur is increasingly functioning not only as a science site but also as a hardware-and-software development platform (Likhit et al., 22 Jan 2025, B et al., 31 Jan 2026).

5. Observing modes, calibration practice, and data products

The operational style of the solar facilities is highly regular. GRAPH is reported to observe every day for $13.603^\circ$6 hr on either side of meridian transit, and several studies restrict analysis to $13.603^\circ$7 min around transit to minimize ionospheric refraction. GLOSS and GRASP are described in a later study as observing the Sun for $13.603^\circ$8 h every day, while the e-CALLISTO station at Gauribidanur operates daily from 02:30–11:30 UT (Ramesh et al., 2020, Kathiravan et al., 24 Feb 2026, Singh et al., 2019).

The solar data products are correspondingly diverse. GRAPH produces 2D radioheliograms at discrete frequencies, from which some studies derive 1D equatorial brightness profiles, multi-Gaussian decompositions, and beam-deconvolved source sizes via

$13.603^\circ$9

GLOSS, GRASP, and e-CALLISTO produce dynamic spectra, and GRIP produces integrated Stokes $77.428^\circ$0 and $77.428^\circ$1 time series or visibilities. Difference imaging is used in GRAPH CME work by subtracting maps taken $77.428^\circ$2 minutes earlier, specifically to enhance faint thermal structures against the quiet background (Ramesh et al., 2020, Ramesh et al., 2021).

Calibration methods vary by subsystem. GRASP flux calibration is reported to use the Galactic center. The 30–300 MHz RFI-radiometer survey developed its 50-$77.428^\circ$3 load calibration at Gauribidanur, with a 290 K termination replacing the antenna and explicit conversion from measured dBm to Kelvin. The 11.2 GHz interferometer uses the Moon as a brightness-temperature calibrator. In the new DART pulsar system, an artificial pulsed noise signal is injected at the first analog stage for instrumental polarization calibration, and PSRCHIVE pac is then used to derive and apply the calibration solution (Kumari et al., 2019, Agrawal et al., 12 Apr 2025, Gireesh et al., 2021, B et al., 31 Jan 2026).

The pulsar and transient systems rely heavily on stored voltage data and offline processing. At 34 MHz the telescope records 2-bit Nyquist-sampled raw voltages that are converted offline into filterbank format with user-selectable time and frequency resolution. In the 1-bit GAPS system, offline beamforming applies geometric delays

$77.428^\circ$4

and then phase corrections in the frequency domain to synthesize multiple beams from archived sign-bit voltages. In GBD-DART-II, UDP packet capture, DADA conversion, coherent and incoherent dedispersion, PRESTO searches, and PSRFITS archive generation are all part of the routine chain, with daily data reduction proceeding at approximately a 1:1 ratio with observation time (Maan, 2015, Bane et al., 2024, B et al., 31 Jan 2026).

6. Scientific uses, strengths, and limitations

The facilities have been used across a wide span of solar physics. GRAPH quiet-corona studies tracked a steady shrinkage of the equatorial solar-corona diameter at 53 and 80 MHz during 2015 January–2019 May and interpreted it as a decline in coronal electron density. GRAPH plus GLOSS, GRASP, and GRIP were also used to isolate faint thermal radio emission from a CME on 2016 May 1, yielding direct estimates of electron density, mass, kinetic energy, and magnetic field strength of the CME plasma. A GRASP–GRAPH–EUV–white-light campaign on 2016 March 16 produced an empirical coronal density law and a magnetic-field profile $77.428^\circ$5 G over $77.428^\circ$6 (Ramesh et al., 2020, Ramesh et al., 2021, Kumari et al., 2019).

The facilities are also sensitive to weak and unusual solar events. One quiet-Sun campaign coordinated Gauribidanur low-frequency radio data with Chandrayaan-2/XSM and SDO/AIA and found non-thermal type I radio bursts in close temporal association with sub-A-class X-ray flares and EUV brightenings, with estimated burst brightness temperature $77.428^\circ$7 K at 80 MHz. Another study used GLOSS and GRASP together with ADITYA-L1/VELC to constrain the onset height of a CME-driven shock to $77.428^\circ$8 from the start frequency of a type II burst (Ramesh et al., 2021, Kathiravan et al., 24 Feb 2026).

At metric–decimetric wavelengths, the Gauribidanur e-CALLISTO station underpins statistical burst studies and automation. An automated classifier developed on 2013–2014 Gauribidanur e-CALLISTO data achieved recall values of $77.428^\circ$9 for 2013 and $13.604^\circ$0 for 2014, and the same dataset showed that $13.604^\circ$1 of the type III bursts observed in 2014 were below 200 MHz (Singh et al., 2019).

For pulsar and transient astronomy, the 34 MHz east–west arm has enabled candidate periodic emission from J1732–3131, low-DM dispersed bursts from Geminga, and deep searches for radio-quiet gamma-ray pulsars. The later LPDA-based systems extend this to 50–80 MHz and 170–196 MHz, where DART now delivers daily-cadence polarimetric observations, rotation-measure estimates, single-pulse studies, and a $13.604^\circ$2-day monitoring sequence of the Crab pulsar’s spin-down (Maan et al., 2011, Maan, 2015, Bane et al., 2022, B et al., 31 Jan 2026).

A common misconception is that the observatory can be characterized by a single RFI label. The literature does not support that simplification. For solar and pulsar observations, several papers describe specific operating bands with minimal interference and show successful long-term science use. For global 21-cm cosmology, however, a dedicated 30–300 MHz survey found fully occupied FM and LEO-satellite bands, broad-band contamination across 30–75 MHz, and concluded that Gauribidanur is highly unsuitable for observations in the 40–200 MHz operation range of SARAS (Bane et al., 2022, Kumari et al., 2019, Agrawal et al., 12 Apr 2025).

In that sense, the defining feature of Gauribidanur radio facilities is not uniformity but complementarity: fixed-frequency arcminute solar imaging, broad-band dynamic spectroscopy, integrated polarimetry, raw-voltage transient capture, LPDA beamforming prototypes, and new full-Stokes pulsar pipelines coexist at one site, each exploiting a different part of the low-frequency parameter space (Ramesh et al., 2020, Bane et al., 2024, B et al., 31 Jan 2026).