In the frantic days after Flight MH370 disappeared, authorities had little to go on.
The Aircraft Communication Addressing and Reporting System had been switched off but one signal remained.
Every hour, Britain's Inmarsat satellite was sending a message to a terminal on the plane to check that the connection was still good.
In response, the terminal was sending back a ping to say, yes, it was still there. The system works like a mobile phone communicates with towers so the network can reach it with an incoming call.
The pings identified the plane as Flight 370 but contained no direct location data.
However, it was possible to use the raw ping data to deduce location information. The time the ping takes to travel from the plane to the satellite 37,000km above the Indian Ocean varies according to how far it must go.
The satellite recorded the time each ping arrived and analysts could work out how far away the jet was. The positions were plotted in two arcs on the Earth's surface.
One swept north over land into Central Asia, the other south into the southern Indian Ocean.
The final ping was sent at 8.11am local time, seven hours after the jet took off from Kuala Lumpur.
That narrowed the likely crash sites to the end of each arc. But which end?
This is where an area of science well known to any WA driver snared by a radar trap or anyone who has listened to an emergency vehicle siren.
Both are examples of the Doppler effect. When the source of a radar wave, sound wave or radio wave is coming closer, the frequency rises.
In the case of Flight MH370, analysts were able to take advantage of the fact the satellite was not absolutely stationary in orbit. When the satellite moved closer to the flight path, the signal changed slightly. Only a southern path fitted the data.