How Do You Lose a Boeing 777?

Air traffic control sees an aircraft primarily through its transponder reply. Turn that off — it has a switch in the cockpit, because there are legitimate reasons to change or disable a code — and the aircraft's data block disappears from the controller's screen. It does not vanish physically, but the system that identifies it is gone.

Primary radar does not need the aircraft's cooperation, but it has limited range and is concentrated around populated airspace and military installations. MH370 was tracked on Malaysian military primary radar for about an hour after its transponder stopped — which is how the turn back across the peninsula is known — and then it flew beyond that coverage.

Beyond primary radar and out of ADS-B ground receiver range, there was nothing.

ACARS sends operational data to the airline. In MH370's case it had also stopped reporting. It is not a tracking system, it is a maintenance and operations link, and it can be disabled.

One thing remained: the satellite data unit continued exchanging automated handshakes with an Inmarsat satellite. These were not position reports and were never designed to be — they are the network equivalent of the terminal confirming it is still connected.

The engineers who worked out what could be extracted from them did something genuinely novel. Each handshake carries a burst timing offset, which gives the aircraft's distance from the satellite, and a burst frequency offset, which reflects Doppler shift and constrains its speed and direction relative to the satellite.

From seven handshakes that carried no location data, they derived an arc thousands of kilometres long and a direction of travel. It is the reason anyone knows the aircraft went south rather than north.

The southern Indian Ocean along that arc is among the deepest and least surveyed water on Earth. The recorders' beacons ran 30 days, and the arc was not derived in time. Surface debris took sixteen months to reach an inhabited shore, by which point drift modelling could only broadly confirm the region.

Much less easily. Satellite ADS-B now receives position broadcasts over oceans. Aircraft on oceanic routes must report position every 15 minutes. Autonomous distress tracking transmits position when an aircraft's state becomes abnormal, and is designed to be independent of crew action. Recorder beacons last 90 days.

The gap MH370 fell through has been substantially closed — because MH370 fell through it.