Pulse per second signal (1PPS)
For a basic introduction of this feature, you can see for example:
Basically, is a clock signal very accurate. You can setup it widely, and despite their name ("1 pulse per second"), the period is not limited to "1 second", so, can be any value.
For use this feature you need FW 1.26 at least and last version of "SmartOS Config"
With respect to the RTKITE board, both board versions RTKITE v2 and v3 (including Heading version) are valid but only the v3 have dedicated footprint connector for this, but not mounted. You can solder the coaxial connector (MMCX Female straight) or just solder wires if you prefer (center is 1PPS signal, the rest of pins are GND). You have also to solder a R0 0603 (a 0 Ohm jumper) in the resistor of picture.