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Malaysia Airlines Flight 370: Experts Unable To Replicate Inmarsat Analysis
McGruber (1417641) writes "The lynchpin of the investigation of the disappearance of Malaysia Airlines Flight 370 has been the pings from the plane to one of Inmarsat's satellites. The pings are the sole evidence of what happened to the plane after it slipped out of radar contact. Without them, investigators knew only that the plane had enough fuel to travel anywhere within 3,300 miles of the last radar contact—a seventh of the entire globe. Inmarsat concluded that the flight ended in the southern Indian Ocean, and its analysis has become the canonical text of the Flight 370 search. It's the bit of data from which all other judgments flow—from the conclusive announcement by Malaysia's prime minister that the plane has been lost with no survivors, to the black-box search area, to the high confidence in the acoustic signals, to the dismissal by Australian authorities of a survey company's new claim to have detected plane wreckage. But scientists and engineers outside of the investigation have been working to verify Inmarsat's analysis and many say that it just doesn't hold up."
The problem with this analysis is that the Doppler "spike" would not have come from a due South trajectory. It is most likely to have come from a change of trajectory almost directly towards teh satellite. The implication is that this was a "ping" that just happened to have occurred during a turning manouevre; given that the SATCOM terminal on the aircraft uses a high-gain steerable antenna, it is not surprising that an "unscheduled" ping took place during a turn, as the beam-steering unit reacquired the satellite.
The other massive confounder with this analysis is that the SATCOM terminal necessarily pre-compensates its transmissions for Doppler shift. The channel bandwidth in the Inmarsat Classic Aero system is sufficiently narrow that when received at the Satellite, the center frequency must be +/-250 Hz of nominal. As Doppler shift due to the expect motion of an aircraft is in the region of +/- 800 Hz, this can only be done by active pre-compensation.
You'll notice from the Inmarsat Data that the uncorrected Doppler shift is within 250 Hz of expected, indicating that some pre-compensation is present.
Without details of how the compensation works, analysis is very difficult, if not impossible. A scan of the patent literature suggests that both measured-Doppler compensation (i.e. the aircraft terminal measures Doppler shift on a satellite broadcast channel, and applies an equivalent compensation on its transmissions) and estimated Doppler compensation (i.e. the satellite terminal communicates with the aircraft's navigation reference unit to obtain heading, and velocity information, and then computes an expected Doppler shift which is applied to transmissions) may be in use.