Detection Algorithm

FFT Harmonic Signature

Batear reads audio from an ICS-43434 I2S MEMS microphone at 16 kHz and runs a 1024-point real FFT (via ESP-DSP SIMD-accelerated routines) to produce a power spectral density (PSD) with 15.625 Hz/bin resolution.

Multi-rotor drones produce a characteristic acoustic signature: a strong fundamental frequency (f₀, tied to motor/propeller RPM) plus clearly visible harmonics at 2×f₀ and 3×f₀. Batear exploits this by scanning the PSD for peaks that exhibit this harmonic ladder pattern.

Detection Pipeline

Harmonic Analysis

For each candidate f₀ (180–2400 Hz), the detector checks whether energy peaks also exist near 2×f₀ and 3×f₀ relative to the noise floor.

Confidence Scoring

A 0–1 heuristic combining SNR, h2/h3 ratios, and exponential moving average smoothing reduces false alarms from transient sounds.

Hysteresis

Alarm requires 2 consecutive positive frames; clearing requires 8 consecutive negative frames — eliminating flicker.

ESP-DSP Accelerated

The ESP32-S3's SIMD instructions keep the full FFT + harmonic scan well under 10 ms per frame.

Alert Transmission

When a drone signature is confirmed, an AES-128-GCM encrypted alert is transmitted over LoRa to the gateway. See the LoRa Protocol page for packet format details.