Original URL: http://www.theregister.co.uk/2014/03/26/inmarsat_doppler_effect_helped_locate_mh370/

Inmarsat: Doppler effect helped 'locate' MH370

Mind you, we still haven't found the wreckage

By Richard Chirgwin

Posted in Bootnotes, 26th March 2014 03:01 GMT

As air and sea searches continue to try and identify and retrieve wreckage from Malaysia Airlines MH370's presumed crash site in the southern Indian Ocean, Inmarsat has briefed Reuters on how it estimated the path the flight took.

Since relatively early in the search for the flight, reference has been made to the seven “pings” Inmarsat satellites received from the aircraft after it stopped communicating with air traffic control.

Initially, those pings gave two possible tracks for MH370: one heading north-west, which gave rise to a huge number of conspiracy theories suggesting it had been hijacked and landed somewhere on that path; and one heading towards the Indian Ocean.

Inmarsat has now told Reuters it took a second look at the satellite pings, focussing on how much they deviated from the notional frequency of the transmitter, to try and decide which of the two paths the aircraft took.

If we ignore the Reuter's “19th century physics” angle, that's still an impressively accurate frequency resolution: taking the cruising velocity of the aircraft as around 250 m/second, its impact on a 1 GHz signal would be a change of around 0.08 per cent, even if MH370 had been flying directly at the satellite.

With Inmarsat at geostationary orbit, the Doppler effect on signals from the aircraft must truly be tiny. It's no surprise, then, that before releasing its results, Inmarsat passed its analysis to another company to check before making its announcement.

A discussion of the analysis has been published by the Malaysian Ministry of Transport, here.

The Sydney Morning Herald reports the area offering the best prospects to locate wreckage is probably in the vicinity of the Southeastern Indian Ridge, a zone of underwater volcanoes that hasn't been surveyed for more than 20 years. ®