PEACC -- Precision Emitter for 21 cm Array Coherent Calibration

This paper introduces PEACC, a novel dual-source, GPS-synchronized digital calibration system deployed on a drone that enables high-fidelity, wideband coherent beam calibration for 21 cm intensity mapping experiments, significantly outperforming traditional auto-correlation methods in low-signal-to-noise regimes.

The Gist

Imagine you are trying to listen to a very faint whisper from a distant star (the 21 cm signal from the early universe). The problem is, the universe is incredibly loud with "static" from our own galaxy, the sun, and even cell phone towers. To hear that faint whisper, astronomers need to know exactly how their radio telescope "hears" sound—specifically, how sensitive it is in every direction and how it distorts the signal. This is called beam calibration.

Usually, to calibrate a telescope, you point it at a known, bright star. But for the new generation of radio telescopes designed to map the whole sky, there are no bright stars in the right places, and the telescopes are too wide to look at just one spot.

Enter PEACC (Precision Emitter for 21 cm Array Coherent Calibration). Think of PEACC as a high-tech "ghost speaker" that helps astronomers tune their instruments.

Here is how it works, broken down into simple concepts:

1. The Twin Speakers (The Dual-Source Architecture)

Imagine you are trying to test a microphone in a noisy room. If you just play a sound, the microphone picks up the sound plus all the room noise. It's hard to tell how much of the signal is the sound and how much is the noise.

PEACC solves this with a clever trick using two identical speakers:

• Speaker A (The Drone): This speaker is mounted on a drone and flies over the telescope, broadcasting a specific, complex pattern of "white noise" (like static) into the dish.
• Speaker B (The Reference): This is an identical speaker sitting right next to the telescope's computer. It broadcasts the exact same noise pattern at the exact same time.

Because the computer knows exactly what the noise should look like (from Speaker B), it can compare it to what the telescope actually heard (from Speaker A). By subtracting the known pattern from the received signal, the computer can isolate the telescope's specific quirks and errors, ignoring the background noise of the universe.

2. The "Drone" Delivery System

Why fly the speaker?
Think of the telescope's "hearing" (its beam) like a flashlight beam. It's super bright in the center (where it looks straight ahead) but gets very dim and fuzzy at the edges (the "sidelobes"). To map the whole flashlight beam, you need to shine a light from every angle.

A drone is the perfect delivery vehicle. It can fly in a straight line over the telescope, shining the "noise signal" from the center of the beam all the way out to the very faint edges. This allows astronomers to map the telescope's sensitivity from the center to the far edges, which is impossible with stationary equipment.

3. The "Perfect Sync" (Coherent Calibration)

The magic of PEACC isn't just that it flies; it's how perfectly the two speakers stay in sync.

• The Problem: If Speaker A and Speaker B are even a tiny fraction of a second out of step (like two drummers slightly off-beat), the signal gets messy.
• The Solution: Both speakers are connected to a "GPS Master Clock." Every second, the clock sends a "1-2-3" signal to both speakers to reset their rhythm. They generate the exact same random noise pattern every single second.
• The Result: Even though the drone is moving and the wind might be shaking it, the two signals stay perfectly synchronized. This allows the computer to "lock on" to the signal and ignore the rest of the universe's noise.

4. The "Ghost" Signal

The noise PEACC generates is special. It's designed to look like random static to any other radio telescope or cell phone. It's a "ghost" signal that only the specific telescope being tested can recognize because it knows the secret "password" (the random seed) used to generate the noise. This ensures the calibration doesn't interfere with other astronomers trying to listen to the universe.

Why This Matters

In the past, measuring the faint edges of a telescope's beam was like trying to see a candle flame in a blizzard while wearing thick gloves. The noise drowned out the signal.

PEACC acts like a noise-canceling headphone for the telescope. By using the "Twin Speaker" method, the researchers found that they could measure the telescope's sensitivity with 1% precision even in the very faint, noisy edges of the beam.

The Bottom Line:
This paper proves that we can now use a drone carrying a "smart noise generator" to fly over radio telescopes and map their hearing abilities with incredible precision. This is a massive step forward for the next generation of telescopes that will map the history of the universe, ensuring that when they finally hear that faint whisper from the Big Bang, they know exactly what they are hearing.

In short: PEACC is a drone-delivered, perfectly synchronized "ghost noise" that helps radio telescopes tune their ears so they can hear the universe's faintest secrets.

Technical Summary

1. Problem Statement

21 cm intensity mapping experiments (e.g., HERA, CHIME, HIRAX, SKA) aim to measure cosmological neutral hydrogen signals. A primary challenge in these experiments is separating the faint cosmological signal from bright synchrotron foregrounds. This separation requires precise characterization of the telescope's frequency-dependent beam shape (chromatic response).

Traditional beam calibration using point sources is difficult for compact interferometers due to limited spatial resolution and scanning capabilities. While drone-based calibration has emerged as a solution, existing methods often rely on switched or pulsed incoherent sources. These methods suffer from:

• Noise penalties: Requiring background subtraction which degrades Signal-to-Noise Ratio (SNR).
• Timing limitations: Difficulty in maintaining precise phase coherence between a moving source and a stationary receiver.
• Low SNR in sidelobes: Inability to accurately measure faint beam sidelobes where foreground contamination is critical.

The authors aim to develop a Precision Emitter for 21 cm Array Coherent Calibration (PEACC): a digitally synthesized, coherent, broadband noise source that can be deployed on a drone to enable high-fidelity beam calibration without background subtraction.

2. Methodology and System Architecture

Core Concept: Dual-Source Coherent Calibration

The PEACC system employs a dual-source architecture synchronized by GPS-disciplined oscillators (GPSDO):

1. Transmission Unit: Mounted on a drone, it transmits a Gaussian noise signal into the telescope aperture.
2. Reference Unit: Connected directly to the radio data acquisition system (correlator), it generates an identical copy of the transmitted signal.
3. Synchronization: Both units are triggered by a 1 Pulse-Per-Second (PPS) signal from independent GPSDOs. Upon each PPS trigger, both units load the same pre-saved deterministic seed from memory, ensuring they generate the exact same pseudo-random sequence simultaneously.

Hardware Implementation

• Platform: Xilinx Ultrascale+ RFSoC (specifically the RFSoC4x2 board). This was chosen over previous LimeSDR prototypes to support GHz-scale bandwidth, high sampling rates, and integrated DACs.
• Signal Generation: A custom FPGA design generates a 1.2 GHz bandwidth Gaussian noise signal. It uses a sum of 256 Linear Feedback Shift Registers (LFSRs) to approximate a Gaussian distribution (Irwin-Hall distribution) while maintaining a spectrally flat Power Spectral Density (PSD).
• Timing: The system uses Furuno GF-8804 GPSDOs. The architecture relies on a 10 MHz reference clock and a 1 PPS trigger to reset the signal generation every second, mitigating long-term clock drift.
• Deployment: The transmission unit is integrated into a DJI Matrice 300 RTK drone payload, including power regulation, a BicoLOG omnidirectional antenna, and the RFSoC board.

Experimental Validation

The system was validated in three stages:

1. Benchtop Tests: Direct coaxial connection between transmission and reference units to verify signal coherence and SNR.
2. Anechoic Chamber: Measuring the beam pattern of a CHIME-like cloverleaf feed antenna in a controlled environment to characterize angular response and sidelobes.
3. Drone Flight: Flying the PEACC payload over a 3-meter radio dish at Yale University to perform real-world beam mapping.

3. Key Contributions

• First Free-Space Coherent Calibration: This is the first published demonstration of a free-space coherent calibration signal synchronized only by clocks (GPSDOs), without physical cabling between the source and receiver.
• Drone Deployment: The first published deployment of such a coherent digital source on a moving drone platform.
• Dual-Source Architecture: Demonstrates that cross-correlating a known reference signal against the telescope input isolates the calibration signal directly, eliminating the need for background subtraction and avoiding the noise penalty associated with switched incoherent sources.
• Wideband Capability: The system operates over a 1.2 GHz bandwidth, covering key 21 cm observing bands (e.g., CHIME: 400–800 MHz, CHORD: 300–1500 MHz).

4. Results

Timing Jitter Performance

• Requirement: Previous simulations established a requirement of 1.7 ns timing jitter to maintain cross-correlation superiority over auto-correlation at low beam amplitudes.
• Measured Jitter:
  • Anechoic Chamber: Effective timing jitter was estimated at 0.75–1.08 ns.
  • Drone Flights: Effective timing jitter was estimated at 0.43–1.29 ns.
• Conclusion: The system comfortably meets the 1.7 ns requirement, even while the transmission unit is on a moving platform.

Beam Measurement Precision

• Cross-Correlation vs. Auto-Correlation: The cross-correlation channel consistently outperformed the auto-correlation channel across all Signal-to-Noise Ratio (SNR) regimes.
  • Anechoic Chamber: Achieved 1% precision in beam amplitude recovery down to -8.44 dB (cross-correlation) compared to the auto-correlation which was limited by system noise floor much earlier.
  • Drone Flights: Achieved 1% precision down to -8.8 dB and 10% precision down to -30 dB.
• Dynamic Range Limitation: The current precision is limited by the dynamic range of the ICEboard correlator (saturating at ~1/8th of full scale). Simulations suggest that with improved dynamic range, 1% precision could be achieved down to -20 to -30 dB.

Phase Measurements

• While amplitude recovery was successful, phase measurements were limited by long-term clock drift (timescales > 1 second). The phase data showed drift inconsistent with the drone's geometric trajectory during slow flights, indicating that while the 1-second reset maintains coherence, the clocks are not stable enough for absolute geometric phase recovery over longer durations without further development.

5. Significance and Future Outlook

• Feasibility of High-Fidelity Calibration: The work proves that digital, coherent calibration sources are feasible for next-generation 21 cm instruments, offering a practical path to improved foreground control.
• Sidelobe Characterization: The ability to measure beam sidelobes with high precision (1% down to -9 dB) is critical for understanding and mitigating foreground contamination in cosmological surveys.
• Scalability: The system is adaptable to various telescope configurations via configurable band selection.
• Future Improvements:
  • Continuous Transmission: Moving from 1-second pulsed operation to continuous transmission (with 1-second seed resets) to avoid undersampling at high drone speeds.
  • Dynamic Range: Upgrading correlators to handle higher signal-to-noise ratios to reach deeper sidelobes (-20 to -30 dB).
  • Phase Stability: Implementing more stable clocking systems or additional reference antennas to enable absolute geometric phase measurements.

In summary, PEACC represents a significant advancement in radio astronomical instrumentation, providing a robust, high-precision tool for calibrating the complex beams of 21 cm intensity mapping arrays, thereby enabling more accurate cosmological measurements.