The tricky parts (that we don't really know as non-SpaceX employees) are:
- how accurate is the clock onboard the satellites? Given that it's likely an OFDM signal the timing is probably pretty good, but given that they're launching zillions of them they probably don't all have atomic clocks onboard
- how accurately is SpaceX tracking their orbits? Kind of a similar answer here... they're doing beamforming to the ground terminals, so it has to be pretty good but we don't really know how good.
- how many SVs are actually visible at a time? We need a minimum of four but the more the better. If there's lots visible we can somewhat work around the first two issues statistically but if there's a limited number than the orbit and clocks need to be super accurate.
Also—how accurately do those satellites track their own position? Unlike the high-orbit GNSS constellations, LEO satellites would bounce around a bit from orbit to orbit, as they're relatively close to the earth and sensitive to uneven distributions of mass. I don't know the exact magnitudes, but I understand they're large by GNSS standards.
Starlink satellites use on-board GPS receivers for extremely accurate (centimeter level) measurements of their position. The orbits which SpaceX reports to the world (for collision avoidance) are based on these measurements.
