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So, I shut down modems 1-3 and then the rsync connection to 4 started working well, 40 kB/sec with no timeouts. So it seems that the cellular comm problems are all related to the fact that the antennas are too close togethertransmissions are interfering.
Would be nice to be able to test the systems at spacings similar to what they will be in the field. We'll see if that can be done.