GRAND@Nançay Prototype Array

• GRAND@Nançay is a prototype array designed to autonomously detect extensive air showers from ultra-high-energy cosmic particles.
• It validates advanced hardware, firmware, and triggering strategies in a moderate RFI environment using bespoke HorizonAntenna units.
• Insights from GRAND@Nançay drive calibration, trigger optimization, and design scaling for the global GRAND project.

GRAND@Nançay refers to the prototype array established at the Nançay Radio Observatory in France as part of the Giant Radio Array for Neutrino Detection (GRAND) project. Its primary role is to serve as a technology and methods testbench for the autonomous radio-detection of extensive air showers (EAS) induced by ultra-high-energy (UHE) particles—cosmic rays, gamma rays, and especially neutrinos. GRAND@Nançay is a compact prototype, characterized by high accessibility for laboratory groups and intensive technical validation, designed to de-risk the hardware, firmware, and triggering strategies foundational to the full-scale GRAND observatory.

1. Conceptual Framework and Detection Principle

GRAND targets the detection of radio signals produced by EAS generated when UHE particles interact in the atmosphere or, in the case of tau neutrinos, emerge after traversing the Earth and decay in the atmosphere (Earth-skimming channels). The coherent radio emission is due mainly to the geomagnetic deflection of charged shower particles, with a subdominant contribution from the Askaryan effect, and is strongest in the 50–200 MHz band (Kotera, 29 Aug 2024). The emitted electric field amplitude $E$ scales approximately linearly with the primary particle's energy:

$E \propto E_{\text{primary}}$

The radio wavefront for highly inclined events can be reconstructed via spherical or hyperbolic models to determine the arrival direction and shower core position.

Detection is based on sparse arrays of broadband antennas capable of self-triggered acquisition. The system is designed to discriminate transient nanosecond-scale signals ($\sim 100$ ns duration) from a stochastic background dominated by Galactic and anthropogenic radio frequency interference (RFI) (Guelfand, 3 Jan 2025).

2. Technical Design and Instrumentation

The GRAND@Nançay prototype consists of four detection units (DUs) deployed at the Nançay Radio Observatory (Guelfand, 3 Jan 2025, Kotera, 29 Aug 2024, Correa, 2023, Collaboration et al., 25 Sep 2025). Each DU is built around a “HorizonAntenna”—a tripolar (three-arm) bow-tie antenna optimized for maximizing response to near-horizontal, highly-inclined showers (Neto, 2023). The arms are oriented along the North–South, East–West, and vertical axes to ensure complete polarization measurement.

Technical architecture of each DU:

• Antenna Mounting: 3.5 m elevation above ground to optimize horizon sensitivity and suppress ground pickup.
• Analog Signal Chain: Employs a low-noise amplifier (LNA, ~23.5 dB gain) directly at the antenna, followed by a front-end board (FEB) implementing a 30–230 MHz bandpass and an FM (87–108 MHz) band-stop filter.
• Digitization: The FEB features a 14-bit ADC sampling at 500 Msamples/s, with three channels for the polarization arms and one spare channel.
• Trigger & DAQ: An SoC (FPGA + 4 CPUs) performs first-level triggering (FLT), further filtering, and event building. Notch filters at the FPGA level suppress persistent narrowband interference. Optical fiber links connect DUs to a central DAQ, which also provides remote power and configures the FEB (Correa, 2023).
• Communication/Infrastructure: Optical fiber instead of wireless for site-specific noise immunity. Lower physical antenna height relative to remote deployments addresses test-lab constraints (Collaboration et al., 25 Sep 2025).

The analog-to-digital signal mapping at the antenna output adheres to the relation

$V_{\text{oc}} = L_{\text{eff}} \cdot E$

where $V_{\text{oc}}$ is the open-circuit voltage, $L_{\text{eff}}$ is the vector effective length (frequency and angle dependent), and $E$ is the incident electric field (Collaboration et al., 25 Sep 2025).

3. Triggering Algorithm and Background Characterization

A major focus at GRAND@Nançay is the tuning of autonomous online triggers to efficiently identify air-shower signals while maintaining high background rejection, a prerequisite for cost-effective detection in large, sparse arrays (Correa, 2023).

Background Characterization:

• Spectral Analysis: Measurements of the mean power spectral density (PSD) on site reveal dominant vertical polarization ($Z$) channel background, primarily below 70 MHz, often up to 50× stronger than horizontal channels (Correa, 2023). Spectral lines due to local communications and digital broadcasting are observed; filtering effectiveness is validated via the suppression of the FM band (i.e., $S(f) = S_0(f) \cdot H(f)$, with $H(f) \ll 1$ in the FM domain).
• Transient Background: The system defines a transient as any voltage sample exceeding $|V| > 5\sigma$ (with $\sigma$ the trace standard deviation), counting as separate events if separated by over 100 ns (50 samples). Measured RFI transient background rates are $\sim$hundreds of Hz per DU—informing trigger rate targets (Correa, 2023).

Trigger Design:

• Thresholds: The FLT is optimized to maintain $\leq$100 Hz per DU (an order of magnitude reduction compared to naïve 1 kHz single threshold triggers), with the following condition per channel:

$|V_{\text{sample}}| > 5\sigma\$

• FPGA Notch Filtering: Used to suppress persistent narrowband emission that could saturate triggers.
• Future FLT/SLT Framework: Final large-scale deployments will integrate a two-tiered trigger, with local FLT outputs forwarding candidates to a second-level trigger (SLT) that implements array-wide selection through geometrical and pulse-shape correlation (Correa, 2023).

Laboratory-injected simulated air-shower waveforms, generated via frameworks such as CoREAS or ZHAireS, are shown to be successfully recovered after the analog and digital chain—validating the FLT response and full electronics model (Correa, 2023).

4. Role in the GRAND Project Architecture

GRAND@Nançay is one of three currently operational GRAND pathfinder arrays, alongside GRAND@Auger (Argentina) and GRANDProto300 (China) (Guelfand, 3 Jan 2025, Neto, 2023, Kotera, 29 Aug 2024). Its specific mandate is:

• Hardware Validation: Feedback from operation at Nançay informs successive design and firmware iterations—especially concerning antenna response, filtering efficacy, and DAQ stability.
• Trigger Algorithm Optimization: Nançay provides an environment to test autonomous trigger performance, supporting refinement before deploying to larger and more remote arrays (Neto, 2023).
• Calibration and Cross-Prototype Consistency: Data acquired allow detailed calibration studies and cross-checks necessary for globally coherent analysis in expanded arrays.

While not designed for science-grade statistics, the initial field operation has confirmed core performance metrics including noise level, time synchronization, and event selection capability (Collaboration et al., 25 Sep 2025). Early data confirm that the system identifies relevant transient signals, and that the instrumental noise environment aligns with expectations (Guelfand, 3 Jan 2025).

5. Scientific and Technical Contributions

Beyond its technical development function, GRAND@Nançay plays a strategic role in the project's broader scientific agenda:

• Demonstration of Self-Trigger Viability: Proves that nanosecond-scale, highly-inclined EAS radio signals can be autonomously detected in a moderate-RFI field environment without external triggers (Neto, 2023, Guelfand, 3 Jan 2025).
• Performance Feedback: Power spectrum measurements and transient response inform ongoing optimization of array-wide analysis algorithms (e.g., background-adaptive thresholds, polarization discrimination).
• Technology Transfer: Successes and failures at Nançay direct the next stages of deployment at GRANDProto300, reinforcing the generalizability and robustness of hardware/firmware solutions (Collaboration et al., 25 Sep 2025).
• Preparation for Scaling: Critical lessons regarding trigger robustness, DAQ reliability, and site-dependent performance feed directly into the design of the GRAND10k arrays (future 10,000-DU deployments in each hemisphere) (Kotera, 29 Aug 2024, Guelfand, 3 Jan 2025).

The integration of lessons from allied prototypes—e.g., cross-validation strategies leveraging coincident detection with established observatories as at GRAND@Auger—are expected to be generalized for GRAND@Nançay as well (Errico et al., 10 Jul 2025).

6. Challenges and Open Issues

Key technical challenges identified at GRAND@Nançay include:

• Radio Frequency Environment: The relatively higher local RFI, due to proximity to infrastructure compared to remote deployments (e.g., the Gobi desert for GP300), necessitates ongoing trigger algorithm adaptation (Collaboration et al., 25 Sep 2025).
• Design-Site Specificity: Variations such as reduced antenna height and the use of optical fiber links vs. wireless communications need to be thoroughly characterized to ensure cross-prototype calibration, especially when merging datasets or translating design features to full-scale deployments (Collaboration et al., 25 Sep 2025).
• Event Discrimination: The FLT/SLT framework must avoid high rates of accidental triggers while preserving sensitivity to true EAS signals, a balance that will only be fully validated in larger, operational arrays (Correa, 2023).
• Scalability: Testing at Nançay underpins the system's readiness for deployment and robustness in GRANDProto300 (targeting 300 DUs) and ultimately for the distributed GRAND10k arrays (Correa, 2023, Guelfand, 3 Jan 2025).

7. Outlook and Future Developments

GRAND@Nançay is a foundation for the iterative, staged realization of the GRAND vision. Planned directions include:

• Scaling Up: Implementation and validation of improved FLT/SLT algorithms first at Nançay, followed by migration to larger arrays such as GRANDProto300 (Correa, 2023).
• Refined Analysis: Continued development of algorithms for real-time RFI rejection, directional and polarization-based discrimination, and possibly machine-learning approaches for transient identification (Kotera, 29 Aug 2024).
• Comprehensive Calibration: Systematic studies of the ambient radio environment, antenna response, and event timing are essential to inform simulation codes (e.g., ZHAireS, radio morphing) and to build analysis frameworks applicable to the eventual multi-site, global GRAND array (Kotera, 29 Aug 2024).
• Multi-messenger Synergy: By validating the self-trigger concept and instrument robustness at Nançay, the array supports GRAND's multi-messenger goals, enabling cross-correlation with neutrino, gamma-ray, and gravitational-wave events on a global scale (Guelfand, 3 Jan 2025).

In sum, GRAND@Nançay is a primary technical proving ground for the autonomous detection principles at the core of the GRAND project, with its outputs and lessons learned feeding directly into the validation and scaling of next-generation UHE particle observatories.