You may notice that tracking is more sporadic. This is because ADS-B is ground-based, over the water we rely on ADS-C and ACARS, which is satellite-based. An example below is from one of the hobbyists that feed satellite signals to ADSBExchange. The one in the back has been picking up signals for the current flight, this is a California-based ground station. In about 2 hours we will be picking up a signal from the UK one.

These ground stations pickup signals from Inmarsat satellites that then communicate with aircraft.