· 1 min read
Why my first Meteor-M2 recording couldn't be decoded
I recorded a Meteor-M2-3 pass as FM-demodulated audio instead of raw IQ. Here's why LRPT needs baseband, and how I record now.
Meteor-M2-3 and M2-4 transmit LRPT, a digital phase-modulated signal near 137 MHz. My first pass looked fine in the waterfall, but SatDump found nothing to lock onto. I had recorded with SDR++ in WFM mode, so the file held demodulated audio, not the radio signal. FM demodulation throws away the phase information LRPT depends on, and no software can bring it back.
What’s actually in the file
| Recording mode | What’s stored | LRPT decodable? |
|---|---|---|
| WFM audio | demodulated audio | No |
| Baseband IQ | raw complex samples | Yes |
Recording baseband instead
Record the raw IQ stream at the satellite’s frequency, then let SatDump demodulate it:
# 137.9 MHz, 1.024 MS/s, 8-bit IQ — [TODO: confirm the frequency for the satellite you track]
rtl_sdr -f 137.9e6 -s 1.024e6 meteor-pass.cu8Checklist for the next pass
- Fresh TLEs for pass prediction.
- Baseband recording, not audio.
- A sample rate that covers the signal bandwidth.
Related project
Ongoing[TODO: start year] – presentWeather Satellites with SDR
Receiving and decoding LRPT images from the Meteor-M2-3 and M2-4 weather satellites with an RTL-SDR, SDR++ and SatDump.
Role: Solo project
- #sdr
- #rf
- #satellites