It appears Meteor MN2-2 uses OQPSK demodulation, where as the older Meteor MN2 uses QPSK demodulation. The current 3rd party LRPT demodulation scripts for gr-satnogs only support QPSK demodulation.
I’m trying to adapt the 3rd party LRPT demodulator that we use with gr-satnogs to enable OQPSK demodulation.
The difference between OQPSK and QPSK is that the former uses exclusively 90 degree phase changes, but at twice the symbol rate. So far I’ve managed to get a clean constellation diagram by increasing the symbol rate by a factor of 2 to deal with the half symbol phase changes. Unfortunately using only half of the decoded bits does not seem to work for OQPSK input.
Any suggestions for how to fix this?
I’ve extracted 10s of data from a Meteor MN2 and Meteor MN2-2 pass from station #39 and uploaded them to:
The gnuradio flowgraph is at https://www.dropbox.com/s/i5ebj0lkhegqv0w/offline_lrpt_mn22.grc?dl=0 (Note that this has a 144ksps symbol rate and uses the ‘Keep 1 in 2’ block before the constellation decoder).