The Reference Unit takes a reading every second of the degrees to the next voltage sine wave zero crossing. When a Field Unit calls in, the Field Unit compares the local angle measurement to the cataloged value for that matching second. Since the GPS time is precisely the same at each location and since the phase “A” voltage is in phase at each location, the angle comparison determines how many degrees the Field Unit voltage is away from the Reference Unit, which then determines the phase identification as well.