Originally Posted by
pax2908
Sorry, simple question, maybe already answered (blacksheep ?)
Does the Doppler analysis / projected track, depend, OR does not depend, on the accuracy
of the local clock (the clock of the SATCOM terminal on the aircraft) ?
My understanding of the two measurements is
1 Doppler - which is a measure of the frequency shift of the Aircraft signal when received at this earth station and is dependent on the accuracy of the 'transmitting frequency' probably more accurately the precise bit rate. For the system to work, this needs to be very very accurate but is not a 'clock' in the sense of having a specific time, just a very accurate period between each bit. This measure tells if the plane was generally North or South of the satellite. I believe based on Inmarsat knowing the satellite's movement. But doesn't tell us much about the aircraft's speed or direction of flight.
2 The Ping - which is a time delay and tells us on which arc on the Earth the aircraft is located. The general conversation implies that it is a round trip delay of a message packet, however, it appears there is also is a calculation the satellite does to tell the Aircraft station what delay it needs to use from the satellite's reference clock in order to fit into the assigned TDM slot. This would be a pretty accurate number I order for the system to work effectively (because the more accurate each station is in hitting its time slot, the less buffer is needed between slots and the more data that can be transmitted). So if the arc is determined using this piece of data, it should be pretty accurate and pretty much independent of the condition of the Aircraft station.
I am sure a real expert will correct any errors. But from my perspective it seems very unlikely that Inmarsat will have gotten the Doppler direction wrong or the arc wrong. However, where the aircraft actually was on the arc depends on the assumptions of how fast it was going, and how steady that velocity was.