On the mystery of the numbers that point south.

Inmarsat is a company that provides telephone and data services over an older set of satellites.  I am very familiar with Inmarsat because I established the first Internet link from Antarctica to New York over the southern Inmarsat Satellite.  The one we are looking at for this data.

The technique used by the engineers very basic and is not a trick and does not really need much calculation.  A signal can be measured coming from a moving object.  If the frequency it is transmitting is shifted higher then the object is moving towards the receiver.  If the frequency is lower than that transmitted then the object is moving away from the receiver.  The amount of frequency shift tells us how fast the transmitter is moving away from or to the receiver.

The satellite receiver has a very accurate frequency lock.  It tracks how far off the transmitter is so it can lock to the transmitter and track the signal. The message is transferred to the earth station along with the frequency adjustment data.  This data can easily tell us how fast and how far the transmitter is moving in relation to the receiver.  By calculating these numbers over the repeated contacts with the plane we are able to determine the track of the plane with extremely good accuracy.

What the engineers and data cannot accurately tell us is the exact place the plane went down.  We only know where the plane was on its last transmission.  The plane went down somewhere in the next 30 minutes.  At 400K that would be quite a distance.