Let’s be clear: The debris reports are ambiguous at best. The ocean is covered with “marine litter.” The fact that planes and satellites have spotted objects on the surface of the ocean is not surprising. Nothing has been seen that closely matches objects associated with an aircraft or with this flight in particular.
The conclusion that the plane went down in the southern Indian Ocean is based on an innovative interpretation of the “handshake” between the airplane and a ground station as relayed by a geostationary satellite. We have known for nearly two weeks that this satellite information led investigators to believe that the plane kept flying for at least six hours. Last week in our story on satellites, we reported that the northern corridor was eliminated not through any technical analysis, but simply through a survey of the radar coverage on the Asian mainland, which picked up no sign of a stray Boeing 777. But over the weekend, there was a new development, as we reported last night:
For six hours after all communication from the airplane ceased March 8, it continued to engage in a computer “handshake” with a satellite that orbits the Earth 22,000 miles above the surface. The satellite always remains above the same point on the equator in the Indian Ocean. Inmarsat, the satellite company, examined the “pings” transmitted from the airplane every hour and, according to the company’s senior vice president, Christopher McLaughlin, detected a Doppler shift in the radio waves. (A Doppler shift is what causes the perceived change in the sound of, say, a passing train or ambulance.)The analysts concluded that the airplane was moving away from the satellite. That, by itself, did not reveal whether the plane was flying north to Asia or south to the Indian Ocean. But, with help from an unnamed European aerospace expert and from Boeing, Inmarsat scrutinized the pings from other Malaysia Airlines Boeing 777s flying to the north and to the south.“In looking at the two plots, the correlation between the pings that we found and the southern route plot is absolutely the most compelling. The northern route has no correlation,” McLaughlin said.And there was more information squeezed from those enigmatic signals: The pings indicated that the plane was flying at a constant altitude and speed, which McLaughlin said “sort of points to it being on autopilot.”The plane typically flies on autopilot at 450 nautical mph, he said. That gave authorities a rough idea of how far the jet is likely to have flown. There were six hourly pings after the plane disappeared, but not a seventh because the aircraft presumably ran out of fuel.
Today, Malaysian authorities released more technical information about this investigation. This originates with the British Air Accidents Investigation Branch (AAIB). An excerpt:
In recent days Inmarsat developed a second innovative technique which considers the velocity of the aircraft relative to the satellite. Depending on this relative movement, the frequency received and transmitted will differ from its normal value, in much the same way that the sound of a passing car changes as it approaches and passes by. This is called the Doppler effect. The Inmarsat technique analyses the difference between the frequency that the ground station expects to receive and that actually measured. This difference is the result of the Doppler effect and is known as the Burst Frequency Offset.The Burst Frequency Offset changes depending on the location of the aircraft on an arc of possible positions, its direction of travel, and its speed. In order to establish confidence in its theory, Inmarsat checked its predictions using information obtained from six other B777 aircraft flying on the same day in various directions. There was good agreement.While on the ground at Kuala Lumpur airport, and during the early stage of the flight, MH370 transmitted several messages. At this stage the location of the aircraft and the satellite were known, so it was possible to calculate system characteristics for the aircraft, satellite, and ground station.During the flight the ground station logged the transmitted and received pulse frequencies at each handshake. Knowing the system characteristics and position of the satellite it was possible, considering aircraft performance, to determine where on each arc the calculated burst frequency offset fit best.The analysis showed poor correlation with the Northern corridor, but good correlation with the Southern corridor, and depending on the ground speed of the aircraft it was then possible to estimate positions at 0011 UTC, at which the last complete handshake took place. I must emphasise that this is not the final position of the aircraft.There is evidence of a partial handshake between the aircraft and ground station at 0019 UTC. At this time this transmission is not understood and is subject to further ongoing work.No response was received from the aircraft at 0115 UTC, when the ground earth station sent the next log on / log off message. This indicates that the aircraft was no longer logged on to the network.Therefore, some time between 0011 UTC and 0115 UTC the aircraft was no longer able to communicate with the ground station. This is consistent with the maximum endurance of the aircraft.
Obviously this does not answer the question of what happened on the plane as it was flying toward China, or where it ultimately wound up. It just indicates that it went south, far into the southern Indian Ocean. There is nothing out there, and thus all signs point to a crash at sea. It’s a terrible and tragic event, and still slathered in mystery.
Now comes more analysis to try to refine the likely location of the plane when it made its final handshake. Navy vessels will drag a ping detector to see if they can pick up a signal from the plane on the ocean floor. Time is precious: The pinger runs on batteries and will go silent after roughly 30 days.
I spoke to Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics, and an expert on satellites, and he said this looks like a robust analysis. But he makes a key point: “In terms of finding the debris, you’d really like to narrow it down to a hundred miles or so, and you can’t do that because small changes in assumptions about the airplane’s speed are going to change the search area by thousands of miles. All this analysis does is tell you that yes it is in the southern Indian Ocean, it’s on the southern track, it’s west of Australia, but how far west, how far south, we can’t do that.”
[FYI, we’re working up a complete story (not an A-blog) on this issue and should have it ready this afternoon.]