Originally Posted by
BOAC 
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Yes, everyone, I do understand the margins of error (as yet unknown) on this '40 degree arc', but I still reckon that the odds of the a/c tracking ACROSS the arcs and at least one ping being at some other angle would be high.
A very valid point, why would anyone navigate along a route exactly the same distance (elevation) from a satellite?
The 40 degree arc is based upon the
altitude of the plane at the LKP. That's why the satellite polling the device terminal has to contain altitude data. The physical radius of the radio waves (the band) is limited. So if the polling has the wrong altitude the signal will miss the plane entirely and the connection is going to be dropped. So a difference in the arc implies a difference in the altitude of the plane and nothing more. Moreover, since the polling take place only once an hour it only tells us something about the altitude of the plane at that precise moment in time, not what happened in between.