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And as has already been posted, while the equipment may not be bench calibrated , Inmarsat are very likely to have had similar "ping" data from when the plane's postion was known with considerable precision - as when on the ground - to calibrate their analysis.
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"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."
Tuesday, March 25, 06:50 PM MYT +0800 Malaysia Airlines MH370 Flight Incident
MH370 Flight Incident | Malaysia Airlines (as of today on dynamic page 3)