[Ground-station] challenges with correlation estimation block - flowgraphs, assumptions, next steps

[Michelle Thompson]

Here are the two flowgraphs that we believe aren't working due to the
Correlation Estimation Issue here
https://github.com/gnuradio/gnuradio/issues/1207

We don't think it's operator error, but we're relatively new to GNU Radio,
and would welcome any comment or critique that helps to solve this.

FM_ARAP_to_downlink.grc is the flowgraph we are using for transmit side.

What does it do?

It collects up four analog FM channels, digitizes, and transmits a time
division multiplexed signal out. This is, in general, a simple model of our
uplink for a Groundsat or payload.

We see the expected transmitted signal on a spectrum analyzer and on a
HackRF portapack and on the receiving system across the lab.

The receiving flowgraph is called TDM_downlink_rx.grc

What does it do?

It takes the time division multiplex signal and is supposed to break the
channels back out.

This demo has worked in the past, but not anymore, and definitely not over
the air now.

The problem seems to be the correlator, and it seems to be the same trouble
reported in the issue.

Here's a video of the lab session:
https://youtu.be/T6TwN2FvfAo

We want a DVB-S2/X correlator. I think we can also help fix this
correlation estimation block. In all cases, we want to comply with the tag
scheme produced, so that downstream blocks already in GNU Radio get what
they're expecting, if they are expecting these types of tags.

Thank you to all taking a look at this and helping get blocks together for
the Block Party at GRCon2018.

-Michelle W5NYV
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