The real time clock (RTC) in sub.net is very accurate but for critical applications like synchrophasor measurements a better system is required. An embedded GNSS receiver is used for this which requires an external antenna located with a good view of the sky.
The receiver has 32 channels and a fast time to first fix (TTFF). The time available from GNSS is UTC which is equivalent to GMT. A local time offset may be set in the system configuration.
The most important aspect of GNSS for continuous, accurate operation is the location of the aerial. This must be above the roof line with a maximum view of the sky.
The receivers use both the GPS and Glonass range of satellites. Because there are twice as many sources as GPS alone the timing error is reduced and there is better coverage if the antenna has a restricted view.