Once you have a reference you can use a control algorithm. like a PID, to adjust the phase of the grid that you are generating locally.

If you sense that the grid is running out of phase from what your GPS clock says is the true reference you can increase/decrease your power output a bit to increase/decrease its frequency and catch up with the phase error.

It would be analogous to a type 2 PLL. The 1 Hz GPS clock would be the reference clock, the 60 Hz grid would be the "PLL output" and the VCO would be the power turbine, or AC inverter.

> Generally grid backfeed capability for these systems is predicated on the presence of a stable voltage and frequency already on the grid.

You are using the reference clock to know precisely how misaligned the grid is and you use the reference to make small corrections to the generator power or inverter phase to restore phase alignment.

If you have an external stable reference clock (GPS) you know what the grid *phase* should be and it should be easier for network of multiple small inverters to keep the grid stable and to bring it up by themselves if there's a large blackout.