Posts on the Alpisto engineering blog filed under #time-sync.
Every ADS-L receiver takes its clock from GNSS. My vario has no GNSS by design — the position comes from the phone in your harness. So I took the UTC second from the phone too, over Bluetooth, and measured how bad that channel really is. Good enough with room to spare: the share of frames the device caught went from 15 % to between 50 and 100 %.