[BenjiWiebe]

I'm not an interferometry expert, so I don't know what level of precision is required, but usually a GPSDO takes a lot more than a PPS output.

A PPS output makes for very accurate timekeeping over long time scales. A GPSDO is a very stable oscillator that gives you precise frequency.

Disciplining your oscillator with a PPS signal would be much harder than with the typically used 10 MHz signal. And I don't think an RPi would do very good at putting out a highly stable MHz signal disciplined to a PPS input.