Towards the Giant Radio Array for Neutrino Detection (GRAND): the GRANDProto300 and GRAND@Auger prototypes

This paper describes the design, operation, and initial results of three small-scale GRAND prototypes (GRAND@Nançay, GRAND@Auger, and GRANDProto300), demonstrating that their successful autonomous radio detection of air showers validates the instrumentation and methodology for the future Giant Radio Array for Neutrino Detection.

The Gist

Imagine the universe is a giant, noisy concert hall. In this hall, the most energetic particles in existence—cosmic rays, neutrinos, and gamma rays—zoom through space at nearly the speed of light. When these cosmic "rock stars" crash into Earth's atmosphere, they don't just make a sound; they create a massive, invisible splash of radio waves, like a stone thrown into a pond, but the ripples are made of electromagnetic energy.

The GRAND project (Giant Radio Array for Neutrino Detection) is a team of scientists trying to build a massive "listening device" to catch these splashes. Their ultimate goal is to figure out where these cosmic particles come from, a mystery that has puzzled physicists for decades.

But building a listening device that covers an area the size of a small country (tens of thousands of square kilometers) is a huge engineering challenge. You can't just build the whole thing overnight. So, the GRAND team is using a "test drive" strategy. They have built three smaller, prototype versions of their detector to see if the technology works in the real world before they commit to the massive final project.

This paper is a report card on those three test drives, specifically focusing on two of the biggest ones: GRANDProto300 in the Gobi Desert of China and GRAND@Auger in the mountains of Argentina.

The Three Test Drives

Think of the three prototypes as different training camps for the same team:

1. GRAND@Nançay (France): This is the "sandbox." It's a tiny, 4-antenna setup right next to a radio observatory. It's easy for European scientists to visit, tweak, and break things to learn how the system works. It's the lab bench where they test new ideas.
2. GRANDProto300 (China): This is the "desert marathon." Located in the remote Gobi Desert, it's currently a 65-antenna array (growing toward 300). It's designed to be tough, autonomous, and able to run for months without human help, just like the final giant array will need to do. It's testing if the system can catch cosmic rays on its own in a harsh, sandy environment.
3. GRAND@Auger (Argentina): This is the "duet." It's a 10-antenna array built right inside the existing Pierre Auger Observatory. The cool thing here is that it can "sing along" with the Auger detectors. If Auger sees a cosmic ray, GRAND@Auger can check if it hears the radio splash at the exact same time. This helps them double-check their work and prove their method works.

How the "Listening" Units Work

Each antenna in these arrays is called a Detection Unit (DU). Imagine a DU as a very smart, solar-powered weather station, but instead of measuring rain, it listens for radio waves.

• The Antenna (The Ear): It looks like a tripod with five arms. It's designed to catch radio waves coming from the horizon, which is exactly where the "splash" from a cosmic ray hits the ground.
• The Amplifier (The Volume Knob): The radio signals are incredibly faint. The unit has a special chip (Low-Noise Amplifier) that turns the volume up without adding too much static.
• The Brain (The Front-End Board): This is the computer inside the box. It digitizes the sound, filters out the "noise" (like radio stations or cell phones), and decides: "Is this a cosmic ray, or just a truck driving by?"
• The Power (The Sun): Since these units are in the middle of nowhere, they run on solar panels and batteries. They are built to survive extreme heat, sandstorms, and freezing winters.

The Challenge: Distinguishing the Signal from the Noise

The biggest problem with listening to the universe is that Earth is loud. We have cell towers, FM radio, satellites, and airplanes. It's like trying to hear a whisper in a rock concert.

The GRAND team had to teach their computers to be very picky. They use a self-trigger algorithm. Think of it like a bouncer at a club. The bouncer (the computer) ignores the general crowd noise. But if someone shouts a specific pattern (a sharp, short burst of radio waves that looks like a cosmic ray), the bouncer opens the door and records the event.

The paper shows that their "bouncers" are working well. They can ignore the background noise and catch the real signals, even in the noisy environment of Argentina.

What Did They Find?

After running these prototypes for about two years, the team learned a lot:

1. It Works: The hardware and software survived the harsh environments. The solar panels kept the batteries charged, and the computers didn't overheat (even in the Gobi heat).
2. The "Background Noise" Map: They mapped out exactly what kind of radio noise exists at each site. In China, it's quiet, mostly interrupted by planes. In Argentina, it's noisier, with lots of TV and radio signals. Knowing this helps them filter out the junk later.
3. Listening to the Galaxy: One of the coolest things they did was listen to the "hum" of our own galaxy. The Milky Way emits a constant radio glow (synchrotron emission). The GRAND antennas detected this hum perfectly. It's like tuning a radio to a station that is always on, proving their instruments are sensitive enough to hear the faintest cosmic whispers.
4. First Catch: They have already spotted some likely candidates for cosmic rays! In China, they found signals that look like high-energy particles. In Argentina, they found one that was seen by both the GRAND antennas and the Auger detectors at the same time. This is a huge victory because it proves their method is accurate.

The Future

This paper is essentially saying, "We built the prototypes, we tested them in the real world, and they passed the test."

The next step is to build the "Grand Finale": two massive arrays, one in the Northern Hemisphere and one in the Southern Hemisphere, each with 10,000 antennas. These will cover the entire sky, allowing scientists to finally solve the mystery of where the most energetic particles in the universe come from.

Thanks to these prototypes, the GRAND team knows their design is solid, their software is smart, and they are ready to build the biggest radio telescope for neutrinos the world has ever seen.

Technical Summary

1. Problem Statement

The origin of Ultra-High-Energy Cosmic Rays (UHECRs) remains one of the most significant unsolved questions in astroparticle physics. To address this, the Giant Radio Array for Neutrino Detection (GRAND) proposes a multi-messenger observatory designed to detect UHECRs, neutrinos, and gamma rays. The primary scientific challenge is the extreme rarity of these particles, requiring an observatory with an unprecedented effective aperture ($10^5$ km$^2$sr) to detect thousands of events annually.

GRAND plans to achieve this using vast arrays of radio antennas to detect the radio-frequency (RF) signals emitted by extensive air showers. However, scaling from current detectors (hundreds of antennas) to GRAND's target (tens of thousands) introduces critical technical hurdles:

• Autonomous Detection: The system must detect air showers without auxiliary particle detectors, relying solely on radio signals amidst stochastic noise and anthropogenic Radio Frequency Interference (RFI).
• Environmental Resilience: The hardware must operate autonomously for prolonged periods in harsh environments (extreme heat, sandstorms, rain) with limited power.
• Signal Reconstruction: The system must distinguish genuine air shower pulses from background noise and reconstruct shower properties accurately despite sparse antenna spacing (km-scale).

2. Methodology

To validate the GRAND concept before full-scale deployment, the collaboration established a staged construction strategy involving three prototype arrays deployed in 2023. This paper focuses on the two largest prototypes: GRANDProto300 (GP300) in Dunhuang, China, and GRAND@Auger (G@A) in Malargüe, Argentina.

Experimental Setup:

• GRANDProto300 (GP300): Located in the Gobi Desert (radio-quiet). It evolved from a 13-antenna stage (2023) to a 65-antenna stage (2025) arranged in a hexagonal layout covering ~30 km$^2$. It aims to test autonomous detection of inclined air showers.
• GRAND@Auger (G@A): Located within the Pierre Auger Observatory. It consists of 10 GRAND Detection Units (DUs) repurposed from existing Auger Engineering Radio Array (AERA) stations. This setup allows for coincident detection, validating GRAND's radio reconstruction against the established particle detectors of the Auger Observatory.

Technical Implementation:

• Detection Unit (DU) Hardware:
  • Antenna: A "Horizon Antenna" with five arms (North-South dipole, East-West dipole, vertical monopole) optimized for the 50–200 MHz band.
  • Low-Noise Amplifier (LNA): Custom boards providing ~20–30 dB gain. GP300 uses a uniform matching network; G@A uses a high-frequency optimized matching to mitigate low-frequency RFI.
  • Front-End Board (FEB): Contains a 14-bit, 500 MSPS ADC, a System-on-Chip (SoC) with FPGA and CPU, and GPS timing. It handles signal filtering, digitization, and local triggering.
  • Power: Solar panels and batteries designed for one week of autonomy without sunlight.
• Data Acquisition (DAQ) & Triggering:
  • Local Trigger: Firmware on the FPGA identifies peaks exceeding a signal threshold (5$\sigma$) followed by a specific pattern of noise threshold crossings within a microsecond window to reject RFI.
  • Central Trigger: A central computer correlates local triggers across the array based on time coincidence and geometry to confirm an air shower event.
  • Unbiased Triggering: Periodic data collection (every 10s) independent of signal amplitude is used for background monitoring and calibration.
• Software: Custom firmware and PetaLinux-based DAQ software manage data flow, event building, and wireless transmission (via Ubiquiti Bullets/Rockets) to central storage.

3. Key Contributions

• Hardware Validation: Demonstrated the successful deployment and operation of GRAND DUs in two distinct, harsh environments (Gobi Desert and Argentine Pampas), validating the mechanical and electronic designs against thermal stress, sand, and moisture.
• Autonomous Triggering: Proved that a self-triggering algorithm can efficiently select air shower candidates while rejecting anthropogenic RFI, achieving trigger rates up to 1 kHz with high efficiency.
• RFI Characterization: Provided the first detailed characterization of the electromagnetic environment at the GRAND sites, identifying specific RFI sources (e.g., aviation, satellite comms, TV carriers) and quantifying their impact on different frequency bands.
• Galactic Synchrotron Measurement: Successfully detected and measured the Galactic synchrotron emission using the prototypes, establishing a method for absolute calibration of the antenna response.
• Coincident Detection: Achieved the first coincident detection of an air shower by both the GRAND@Auger radio array and the Pierre Auger particle detectors, validating the reconstruction capabilities of the GRAND system.

4. Results

• Operational Status: Both prototypes have operated for approximately two years. GP300 expanded from 13 to 65 antennas, and G@A operates with 10 units.
• Trigger Performance: The system maintains a stable trigger rate (~1 kHz) without losing functionality. The self-trigger algorithm successfully filters out long-duration signals and pulse trains typical of RFI.
• Background Analysis:
  • GP300: Generally radio-quiet, with significant RFI only in the 118–140 MHz band (aviation/satellite).
  • G@A: Higher background noise with numerous narrowband and broadband RFI sources (AERA beacons, TV, FM). Despite this, RFI-free bands were identified for analysis.
• Calibration: The power spectral density (PSD) measurements confirmed the detection of Galactic synchrotron emission, showing the expected amplitude peak when the field of view aligns with the Galactic center (16–20 hours Local Sidereal Time).
• Coincidence: One coincident event between G@A and the Pierre Auger Observatory has been detected, confirming the system's ability to reconstruct air showers in sync with established detectors.
• Candidates: Preliminary analysis of GP300 data has yielded the first likely candidates for UHECRs in the 10$^{16.5}$–10$^{18.5}$ eV range.

5. Significance

This paper marks a critical milestone in the transition from theoretical design to operational reality for the GRAND experiment.

• Feasibility Confirmed: The successful operation of the prototypes confirms that the GRAND instrumentation is capable of addressing its scientific goals, specifically the autonomous detection of UHECRs and neutrinos.
• Scalability: The adaptability of the hardware (e.g., different LNA designs for different RFI environments) and software (unified DAQ pipeline) proves that the technology can be scaled to the planned 10,000-antenna arrays in both hemispheres.
• Synergy: The G@A prototype demonstrates the value of hybrid observatories, where radio detection complements traditional particle detection to improve reconstruction accuracy and background rejection.
• Future Roadmap: The lessons learned regarding power management, thermal design, and RFI mitigation are being integrated into the design of the next batch of antennas for the full GRANDProto300 array, paving the way for the final 300-antenna stage and the ultimate 10,000-antenna observatory.

In conclusion, the GRAND prototypes have successfully validated the core detection principle, demonstrating that large-scale, autonomous radio arrays can effectively monitor the atmosphere for ultra-high-energy particles despite environmental and electromagnetic challenges.