rasoberry-noaa/ISS.md

This project now includes full reception and decoding of SSTV transmissions from the ISS.

ISS reception and scheduling

Reception is quite easy: The ISS TX works on 145.800 Mhz, Narrow FM. So we just need to tune our RTLSDR receiver and that’s all.

Scheduling is governed by the SCHEDULE_ISS inside .noaa.conf. Setting it to true will schedule future ISS passes.

ISS decoding

This is the difficult part: Decoding PD120 needs custom software. Right now the best Linux software for SSTV (in mu humble opinion) is QSSTV. QSSTV is great, works with a lot of modes, has auto slant and signal detection. On the other hand, QSSTV only works in GUI mode as it’s written in QT (hence Q-SSTV) and that’s not situable for this project as we don’t run any Window Server.

Surfing the Internet I’ve found Martin Bernardi’s (and team) final work about a PD120 modulator - demodulator so I put hands on slightly modifying it to adjust the project to my needs.

Decoding behavior

There are a few things to take care about

1. A single ISS pass during a SSTV event could have zero to three SSTV transmissions (based on an average pass time of 10 minutes) so we need to find a way to detect the transmission header to fire up the decoder and store each image.

2. A simple way to detect the header on an audio file (once digitized with scipy.io) is to sample the header as an absolute numeric representation, apply a threshold during certain amount of time. So if the header is present, mark the stream and do the same for each new header that ocurrs in about this_timestamp + 120 seconds so we don’t get any false positives in the middle of the PD120 carrier.

3. A Satellite transmission to the earth has doppler efect so the frequency changes over the time. This is a problem when you plot audio as data. So I had to include a high pass filter to remove the DC offset from the audio recorded by rtl_fm.

Upgrading procedure

There’s a migration script that handles the required modifications over the database as well as the web files. Take in mind that you need to have the web panel version of raspberry-noaa running.

1. cd /home/pi/raspberry-noaa
2. git fetch -pt && git pull
3. cd migrations/
4. ./20201292-iss.sh