LRPT Decoding Flowgraph + Scripts

I figured it might be worth documenting what I’ve been playing around with for LRPT demodulation.

First off, the flowgraph:
This is based off this flowgraph, adapted to work with SatNOGS doppler correction. This flowgraph gets added into satnogsclient/scheduler/ where you would expect it to.

The flow-graph performs the appropriate resampling, produces a waterfall output, and performs QPSK demodulation, with 8-bit signed soft-bit data saved to /tmp/< decoded_data_file_path >.s

I haven’t yet found software suitable for decoding the LRPT bitstream in ‘realtime’. Instead, I’m using this decoder, being run from a post-observation script by this python code. The python code looks in /tmp/ for soft-bit files and runs medet_arm to produce a set of .bmp files. Depending on the settings, ‘convert’ (from imagemagick) is used to concatenate and convert the multiple outputs into a single png file. This png file is then placed in /tmp/.satnogs/data/ for upload.

At the moment I’m not using the patches to satnogs-client to allow auto upload, since it’s a bit broken with the NOAA demod (uploads the file mid-decode). Instead my post-observation script just uses rsync to copy any *.png files in /tmp/.satnogs/data/ to a webserver.

There are a few issues at the moment:

  • This post-processing approach doesn’t really fit in with the current SatNOGS client architecture. Ideally the processing of the soft-bit file could be performed on the network server, however the raw files are around 130MB/observation, which is a bit much to upload. I’m not really sure how feasible it would be to implement a complete LRPT demod (right to image output) that could run in real-time. I’m also not sure if it’s worth the effort, and I think this is a good use-case for a 2-part decoder (live capture, non-realtime post-processing prior to upload to network).

  • I’ve noticed that there are no timestamps passed to flow-graph in the decoded_data_file_path parameter, just ‘data’ and the observation ID. The NOAA flow-graph appears to get this data somehow, but I haven’t worked out where yet (probably just missed something). Once I figure this out the resultant images should integrate much nicer with the network observation pages.

Hopefully this proves useful to someone!


@vk5qi Can you check-in the generated python file for the grc in your git repo.

J Ferns

Just run:
grcc -d . lrpt_demod.grc

within the flowgraphs directory to produce a python file.


1 Like

A toasted USB Drive has forced a rebuild and I am finding the -d option in the grcc statement is no longer supported.

pi@raspberrypi:/datadrive/satnogs-extras/flowgraphs $ grcc -d . lrpt_demod.grc
usage: grcc [-h] [-o DIR] [-u] [-r] GRC_FILE [GRC_FILE …]
grcc: error: unrecognized arguments: -d

Any idea how I can install the flowgraph?

The flowgraph mentioned in this guide will not work on the latest satnogs version.

There is work by others progressing on an updated flowgraph that may work, but it’s not fully tested yet.