GBD-DART-I : Pulsars and transient source observation between 130 MHz and 350 MHz at Gauribidanur

Gauribidanur Diamond Array Radio Telescope (GBD-DART) is a new small LPDA antenna array consisting of 64 short dipoles and associated receivers that has been custom developed and deployed at the Gauribidanur observatory (13.604 N, 77.427 E) to study bright Pulsars and Solar transients in the frequency range of 130-350 MHz. The LPDAs are arranged in a checkerboard layout, with opposite pairs combined to enable dual-polarised operation. A diamond-shaped (tilted square) array configuration was chosen to achieve high sidelobe suppression in the East-West and North-South directions. The tile measures 5.9 meters by 5.9 meters, with diagonals along both the North-South and East-West directions, each measuring about 8.4 meters. The LPDA array with one diamond-shaped tile has been fully commissioned and is operating in transit-observing mode, successfully detecting strong pulsars and solar flares over the last seven months. The present digital backend restricts the instantaneous bandwidth for observations to 16 MHz. The array operations are streamlined to facilitate remote operations. Apart from investigating Pulsar and Solar phenomena at low radio frequencies in selected sources, this work aims to provide a training platform for radio astronomy through simple-to-construct, low-cost radio telescopes. In this paper, we present details of the array, including antenna and array response studies, brief descriptions of front-end and backend instrumentation, and illustrative results from observations of both pulsars and solar flares. It will also provide brief details of future upgrade plans, particularly for the tiles and digital backend, to facilitate the observation of additional sources.

💡 Research Summary

The paper presents the design, implementation, commissioning, and early scientific results of the Gauribidanur Diamond Array Radio Telescope (GBD‑DART‑I), a low‑cost, small‑scale aperture array built to observe bright pulsars and solar transients in the 130–350 MHz band. The instrument consists of a single “tile” containing 64 short Log‑Periodic Dipole Antennas (LPDAs) arranged in a 45°‑rotated square (diamond) configuration. The dipoles are grouped in a checkerboard pattern, with opposite pairs combined to provide dual‑polarised (X and Y) feeds. This geometry yields a relatively flat gain of ≈11 dBi across the band, a 60° field of view, and strong sidelobe suppression (≈‑20 dB) in the east‑west and north‑south directions, as confirmed by CST simulations and on‑site measurements.

Site RFI surveys identified strong interference below 130 MHz and moderate bands around 250–270 MHz; consequently, a high‑pass filter eliminates frequencies <129 MHz and the operational upper limit is set at 350 MHz. The analog signal chain begins with low‑noise amplifiers (20 dB gain, 1.2 dB NF) at each dipole, followed by hierarchical 8‑way and 4‑way combiners that preserve the dual‑polarisation outputs. The combined signals are further amplified (total ≈52 dB gain) and band‑limited (130–350 MHz) before being converted to optical form by an RF‑over‑Fiber transmitter. A 250‑m, 12‑core single‑mode fiber carries the optical signal to the observatory building, where an RF‑over‑Fiber receiver reconverts it to RF. The final stage is a portable dual‑receiver (PDR) that selects any 16 MHz sub‑band within the 220 MHz instantaneous bandwidth, digitises it with 8‑bit ADCs at 33 MS/s, timestamps and packetises the data on a Virtex‑5 FPGA, and streams UDP packets to a recording PC.

Data are captured in a RAM‑based circular buffer using the GULP utilities, written to PCAP files, and processed in real time: FFTs generate correlation products, temporal averaging produces spectra, and the results are archived in HDF5 format. High‑time‑resolution voltage streams are saved separately for pulsar and transient analysis.

Four categories of commissioning observations are reported. (1) ORBCOMM satellite signals at 137 MHz were detected, allowing a direct measurement of the beam shape during transit. (2) A drift scan of the Sun at 200 MHz demonstrated the expected –20 dB sidelobes for an eight‑dipole sub‑array. (3) Multi‑day drift observations with the same sub‑array tracked sky temperature variations; the measured power matched predictions based on the 408 MHz sky map, confirming system stability and calibration. (4) Solar flare events were captured as rapid flux increases within the 130–350 MHz band, and strong pulsars (e.g., B0329+54) produced clear pulse profiles, establishing the array’s sensitivity (~250 Jy for a 0.5 s, 2 MHz integration). Additional tests with bright radio galaxies (Cygnus‑A, Taurus‑A, Virgo‑A) yielded comparable sensitivity estimates.

The current system is limited by a 16 MHz instantaneous bandwidth and the single‑tile configuration. Planned upgrades include adding a second tile to increase collecting area, expanding the digital backend to support wider bandwidths and higher‑resolution sampling, automating the real‑time processing pipeline, and developing educational modules for university training. These enhancements aim to transform the GBD‑DART concept into a versatile platform for low‑frequency pulsar timing, solar radio physics, and, ultimately, path‑finder studies for 21‑cm cosmology, all while maintaining a low‑cost, remotely operable design.