Geometric Phase

The geometric phase^* is the most important systematic error that we know of [1][2].
The motional magnetic field (resulting from traveling through the high electric field) combines with non-zero magnetic field gradients to produce a shift in the precession frequency proportional the electric field strength – a false EDM!
The frequency shift is proportional to the gradients, so the first defense against this effect is to reduce the gradients.
Using Helium-3 as the co-magnetometer provides additional handles on this effect thanks to its serendipitous temperature-dependent properties.

^{^*“A relativistic effect that bites you in the [rear] at 5 m/s!”}