LoRaWAN to SDI-12 bridge

What it is

A Milesight radar gauge already reports over a commercial LoRaWAN network. This project adds a private gateway that hears the same sensor directly, decodes it locally, and serves the level over SDI-12 to an ELPRO ERT-A2 field station -- which puts it on the ALERT VHF flood-warning network. If the LoRaWAN backbone fails mid-flood, the reading still reaches the radio.

• The sensors are provisioned ABP (Activation By Personalisation): the private gateway holds each device's session keys, so it decodes uplinks without a join exchange and without depending on any network server upstream. Keys live in a file outside the source.
• The fix that made it work was the channel plan. The sensor is AS923 with six channels added downward; the stock gateway preset adds six upward. They overlap on two channels, so out of the box the gateway heard 2 uplinks in 8. Correcting it took reception from 25% to 94%.
• Reception is measured by the device's own frame counter, never by wall clock. That separates 'we missed it' from 'it never sent it' -- and every loss in this project was at the receiver.
• Physical constants are derived, not guessed. Both ends of the height validity band were once invented, and both were wrong; one rejected an entire falling tide while the radio retransmitted a frozen level as though it were live.

How it works

Parts

Radar water-level sensor Milesight EM411-RDL 60 GHz radar level sensor, 0.3-12 m range, IP68, LoRaWAN AS923, provisioned ABP. Product page ↗

LoRaWAN concentrator RAK2245 Pi HAT Semtech SX1301 + 2x SX125x, 8 channels. Re-planned to 922.0-923.4 MHz to match the sensor. Product page ↗

Gateway host Raspberry Pi Zero 2 W Quad-core Cortex-A53, 512 MB. Packet forwarder, minimal LoRaWAN network server, decoder, SDI-12 slave. Product page ↗

WisBlock carrier RAK6421 WisMesh Pi HAT HAT+ carrier on the Pi's 40-pin header; carries the SDI-12 module. Product page ↗

SDI-12 interface RAK13010 WisBlock SDI-12 SDI-12 transceiver with a 12 V sensor supply. The 1200 baud 7E1 bus between the gateway and the radio. Product page ↗

ALERT field station ELPRO ERT-A2 ALERT1 / ALERT2 flood telemetry transmitter, VHF 151.5 MHz at 5 W. Reads the level over SDI-12 and puts it on air. Product page ↗

Code

github.com/agmurf/lorawan-sdi12-bridge ↗

MIT licensed. No agency sensor register and no key material is included. Field engineering on a live warning path, published because the reasoning may help others — not a product, no warranty.