Doppler information could confirm MH370's southern arc

Recently released Inmarsat satellite information about Malaysia Airlines Flight 370 would seem to confirm that the airliner traveled in a southern arc before disappearing. Data previously made public suggested the airliner had traveled along either a northern or southern arc, although search efforts have focused on the southern path.

Jeff Wise writing in Slate comments, “Combined with previously released data, publicly available information, and a little vector mathematics,” a 47-page report released two weeks ago “has proved sufficient to lift the veil on Inmarsat’s calculations and reveal the ultimate fate of the plane.”

A month ago, writing in the Atlantic, Ari N. Schulman, executive editor of The New Atlantis: A Journal of Technology and Society, summarized the problems with the publically released data available at the time.

For example, Doppler readings suggested the plane was traveling at about 50 mph (relative to the satellite) before leaving the gate. (The satellite itself is not truly stationary but exhibits a north-south wobble that should have contributed little in the way of frequency shift to a stationary aircraft nearly due east of the satellite.)

And the last ping between the satellite and plane resulted in Doppler data that indicated that the plane was receding from the satellite at just 103 mph.

Writing today in Slate, Wise cites two sentences from the recently released information: “Inmarsat Classic Aero mobile terminals are designed to correct for aircraft Doppler effect on their transmit signals. The terminal type used on MH370 assumes a stationary satellite at a fixed orbital position.”

Because the plane's electronics pre-corrected the transmission frequency, writes Wise, beat-frequency offset (BFO) data cannot be directly used to figure out the plane's speed relative to the satellite.

However, knowing the movement of the satellite, the data can be used to determine the plane's path. Wise notes that during the plane's flight, the satellite was at first moving north and then south at increasing speed.

Writes Wise, “Because the satellite’s velocity error becomes so dominant toward the end of the flight, and because that error varies strongly with the latitude at which the plane happened to be, the BFO value basically tells you where along the final 'ping arc' the plane was when it neared the end of its flight. And this, we can assume, is why the authorities have been searching the particular stretch of ocean they’re looking at now.”

Of course, the plane has still not been found.

Question—why the pre-correction? Granted, you would want to keep the receiver bandwidth-per-plane on the satellite as low as possible to accommodate more planes. Anyone have any more information on this?