version history

20201224 initial release
working:
- receives fix information from GPS receiver u-blox NEO-M8N-0-10
- extracts time information and calculates maidenhead locator from GPS position
- decides whether to transmit or not depending on the timestamp
- uses Si5351A to send data stored in the tx_buffer

not working, buggy:
- calculate tx_buffer from given callsign, maidenhead locator and tx_pwr in dBm (fixed buffer so far, crashes when not commenting JTEncode.wspr_encode() out)

todo:
- display data on TFT display (all debugging and troubleshooting is being done using the serial monitor)
- add 1 button to toggle tx_enabled
- add 2 buttons to select frequency band
- extract JTEncode functionality to shrink code size (uses ~ 20 kB of 30 kB)