Their satellites appear to regularly send out a ping as a low level part of their communications protocol to see what receivers are in range and to keep channel

Their satellites appear to regularly send out a ping as a low level part of their communications protocol to see what receivers are in range and to keep channels to them alive. The aircraft's SATCOM system continued to respond to those pings for a number of hours after it disappeared, and the new location information is based on analysis of the series of pings that occurred.