LimeSDR-XTRX how to use GPS chip on board

[tomasS]

Newer gateware revisions expose the GNSS UART interface to the host machine at /dev/ttyLimeUART0 (baud rate 9600). This allows directly monitoring all NMEA messages emitted by the chip, as well as configuration, if needed.
I’d recommend Gateware revisions 3.5+

This, as well as some other GNSS related questions were actually discussed in another recent discussion:

Specific set up for limeSDR XTRX for location data across X5

Your current understanding is correct. The 1980 timestamp you’re seeing without a lock is data taken directly from the ZDA message. Gateware assumes data provided by ZDA messages is always valid (unless checksum fails), so as soon as the GNSS chip achieves lock and starts emitting correct time information, it will be used for timestamps.