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Raspberry NOAA

Meteor M2 Decoding

The Meteor M2 satellite uses a 72k QPSK 120hz carrier to send very high quality images.

Receiving

Currently, the satellite is using 137.1Mhz as the center frequency, Wide-FM @ 120Khz.

Historically this framework uses rtl_fm @ 288k and sox to capture the raw stream. Most likely, the 288k bandwidth may be lower.

More recently we have moved the framework to gnuradio mode to capture the raw stream. While at first the model still only works with RTL-SDR cards, this change of default to gnuradio is done to enable developers to work to introduce hardware SDR other than RTL-SDR and hopefully introduce them back to the repoistory and share new hardware options.

gnuradio Decoding Steps

  1. Capture the signal using .py scripts that were initially designed from the .grc gnuradio workflows included in the audio_processors folder in this repository, and then exported to python, manually modified to work with the Raspberry NOAA V2 variables. The .py scripts capture the signal from the hardware and produce a .s file (an experitmental format very similar to .qpsk.)
  2. Use artlav's medet_arm to generate a rectified decoded .bmp for each sensor from the .s
  3. Use Imagemagik to conver and merge the files to produce output .bmps

There is more info here: Gnuradio workflow README

rtl_fm Decoding Steps

Below are the steps used to decode the signal:

  1. Normalize audio stream with sox gain -n
  2. Use dbdexter's meteor_demod to convert the audio stream to QPSK symbols at a 72k rate
  3. Use artlav's medet_arm to generate a decoded dump and then a false color image
  4. Use dbdexter's meteor_rectify to correct the visible deformation on Meteor images (wrong aspect ratio)
    1. Some changes were made to rectify to export to compressed JPG and remove some prints