Posts on the Alpisto engineering blog filed under #gfsk.
An ADS-L receiver must know when each UTC second starts, and receivers normally get that from GNSS. My vario has no GNSS on purpose: its position comes from the pilot's phone. So I took the time from the phone too, over Bluetooth, and measured how accurate that is. It is accurate enough, with room to spare: the share of frames the device caught went from 15 % to between 50 and 100 %.