Why bother
The Lamprey AUV is mute the moment it dives. Wi-Fi and LoRa die within centimetres of the surface — seawater eats RF. Every untethered underwater vehicle in the world talks the same way whales do: sound.
Commercial acoustic modems start around €3,000 per end. Our target is far humbler: 10–50 bits per second over 200 m — enough for “I’m alive, here’s my depth and heading, aborting now.”
The physics we’re up against
- Sound in seawater: ~1,500 m/s, so 200 m = 133 ms one-way latency.
- Usable band for our size: 20–50 kHz (higher = smaller transducer, more absorption).
- The real enemy is multipath: surface and bottom reflections arrive as delayed copies of your signal, smearing symbols into each other. In our 8 m deep test bay, echoes trail the direct path by up to 15 ms.
Hardware, version zero
- Transducers: automotive parking-sensor piezos (40 kHz), €4 each, potted in epoxy.
- TX: ESP32 driving a MOSFET H-bridge into a step-up transformer, ~25 Vpp at the element.
- RX: the same piezo into a two-stage op-amp band-pass (35–45 kHz), sampled at 200 kSa/s by the ESP32’s I2S ADC.
Modulation: chirps, not tones
Our first attempt — simple on-off keying of a 40 kHz tone — decoded perfectly in a water barrel and not at all in the sea. Multipath echoes filled every “off” gap.
The fix is the one sonar and LoRa both use: chirp spread spectrum. Each symbol is a frequency sweep (up-chirp = 1, down-chirp = 0), detected by correlating against a template. Correlation compresses the chirp into a sharp peak, and the delayed multipath copies become smaller, separate peaks you can ignore.
Symbol: 20 ms chirp, 36 → 44 kHz
Guard interval: 30 ms (let the bay stop echoing)
Rate: 20 bps raw → 12 bps after Hamming(7,4)Results so far
| Range | Water | Packet success |
|---|---|---|
| 25 m | Harbour, calm | 98% |
| 90 m | Bay, SS1 | 87% |
| 180 m | Bay, SS2 | 41% |
At 180 m the received chirp is barely above the snapping-shrimp noise floor (yes, really — shrimp are the loudest thing down there). More TX power and a resonant matching network are the next steps.
Honest assessment
Twelve useful bits per second sounds pathetic next to any radio link. But those bits cross a boundary radio cannot. A depth report every 5 seconds and a reliable abort command is the difference between an autonomous vehicle and an expensive offering to the sea floor.
Next note: matched transducer pairs and a proper wake-up receiver so the modem doesn’t drain the AUV battery while listening.