After yesterdays failed field test I came home and spent last night debugging the code. Two issues caused the fail:

1. Random and persistent watchdog reboots (WDT reset)

2. SD card failure

The first was the bigger issue I have been meaning to fix for some weeks.  After testing various watchdog resets in various locations in the code (manual delays trigger reset) I finally figured out it was the serial communication.  I had the hardware serial at 9600 and this when dumping a big list of MACs would WDT reset.  Nice.  

Dont ask how long this took me to figure out. Much sad...

The second was actually a hardware problem.  I did not have a pull-up on MISO for the SPI. On one prototype this was just fine but on the breadboard one- this constantly had SD card access errors.  Easy fix with a 10K resistor and its now stable like a rock.  

I updated the KiCAD schematic a bit today while I was adding the pull-up resistor.  It looks decent to my eyes so hopefully others can follow it too. 

Hope to do another field test of the ground unit tomorrow now that the code is running smooth and stable :)

I weeeen!