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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, the detector sends an alert. The path depends on the deployment model:

  • LoRa Detector — An AES-128-GCM encrypted alert is transmitted over LoRa to the gateway. See the LoRa Protocol page for packet format details.
  • Wired Detector — The DroneEvent_t is published directly as JSON to the MQTT broker over Ethernet. No encryption overhead is needed since the network is wired.

Gateway → Home Assistant

The gateway decrypts the LoRa packet and pushes a MqttEvent_t into a FreeRTOS queue. On a separate core, MqttTask picks up the event and publishes a JSON payload over MQTT to the broker.

Wired Detector → Home Assistant

The wired detector's EthMqttTask receives DroneEvent_t items directly from AudioTask via a FreeRTOS queue and publishes them as JSON over MQTT. No gateway is needed — the detector connects directly to the MQTT broker over Ethernet (optionally via PoE for single-cable deployment). The network interface supports both DHCP (default) and static IP — see Configuration for setup.

The wired detector also exposes a REST API on port 8080 for remote management: device info, detection status, NVS configuration updates, and OTA firmware updates — upload a new binary via POST /api/ota and the device reboots with automatic rollback protection. See the REST API reference for endpoint reference.

Home Assistant discovers both device types automatically via MQTT Discovery — no manual YAML needed. See Configuration → MQTT / Home Assistant for topics, payloads, and setup.