Spectrum

The air around any laptop is full of broadcasts anyone may receive — aircraft position reports at 1090 MHz, ship transponders at 162 MHz, weather satellites at 137 MHz. A $40 RTL-SDR dongle can hear all of it.

Turning that into something a curious person can use takes a pile of single-purpose tools, each with its own driver setup and jargon. And a waterfall full of unexplained blips is the moment most people give up.

One streaming DSP core — decimator, mixer, FM and AM demodulators, squelch, waterfall — feeds small stateful receivers that take any chunk size, so a radio, a file and a simulation are interchangeable. The ADS-B receiver is my own Mode S codec and PPM demodulator with CRC-24 one-bit repair and global-then-local CPR positions, so aircraft need no station location. The AIS receiver demodulates both channels from one capture with per-burst carrier-offset removal, HDLC framing and NMEA output.

Satellite passes come from SGP4 on Celestrak elements. Spectrum decides when to record, records SigMF cleanly, and hands LRPT decoding to SatDump, detecting whether it is talking to the 1.2 or the 2.0 command line. The identifier measures bandwidth, burst timing, keying and symbol rate, checks the band plan, and scores 11 classes of weighted rules, showing the reasons that fired. Claude can give a second opinion on the spectrogram; if it fails, the rule result stands.

I had no radio, so the tests decode signals rather than bits. A generator models the physical layer — pulse timing with sub-sample offsets, Gaussian-filtered GMSK, carrier offsets, noise — and a simulated sky with ground truth drives both the demo and every end-to-end test.

• On the demo's synthetic sky, 203 of 204 transmitted ADS-B frames decode with a valid CRC (3 repaired by one-bit correction), all 8 aircraft are located with a median position error of 58 m, and all 6 ships decode from 60 AIS packets.
• AIS decodes with up to ±7 kHz of tuning error; ADS-B positions tested at four locations on four continents, with the codec checked against published Mode S reference frames.
• The identifier names a 433.92 MHz on-off-keyed sensor with high confidence and explains it in plain English; its 11 classes are validated on synthetic signals only.
• 101 tests and a clean fresh-clone install; CI green on Python 3.10–3.13 and macOS on the first run.