SDR ALERT decoder

What it is

ALERT and iFLOWS field stations across NSW report river and rain readings on 151.5 MHz as short AFSK bursts. This project decodes them: a desktop Python decoder, and an Android app that hosts the RTL-SDR over USB and runs the whole DSP chain on the phone -- no PC, no network, no server. It reports the sensor ID, the value, and the four raw bytes they came from, so a reading can be checked rather than trusted.

• Three protocols, not interchangeable: ALERT Binary (no checksum), Enhanced iFLOWS (6-bit CRC), and ALERT2 (a different physical layer).
• A 6-bit CRC is a 1-in-64 filter, not proof. Accepting on CRC alone invented 33 phantom stations in a capture containing four -- so CRC lowers the vote bar, it never replaces consensus.
• Measured off-air against an independent record: ALERT Binary produced a confidently wrong reading (1.946 m for a known 0.666 m) with full consensus, because it has nothing to fail. Enhanced iFLOWS did not, across the same session.
• No site register is bundled. Sensor registers are agency data; the decoder reports what came off the air and an operator supplies their own labels.

How it works

Parts

Software-defined receiver RTL-SDR Blog V3 RTL2832U + R820T2, sampled at 1.024 Msps on 151.5 MHz. Product page ↗

Front-end bandpass filter Mini-Circuits SBP-150+ 140-160 MHz lumped LC bandpass, 50 ohm, SMA. Optional; keeps strong out-of-band signals out of the dongle. Product page ↗

Code

github.com/agmurf/sdr-alert-decoder ↗

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.