Originally Posted by
FE Hoppy
They have a very accurate position at 07 due to the VHF ACARS transmission. They use that datum with the next ping to work out the return time at that known range. High school maths then allows you to work out future ranges based on ping return times.
It's not rocket science although they are capable of that too!
But if it is a one-way transmission from a/c to satellite then this is subject to error based on the clock drift between the satellite and the a/c. Even using the most accurate clocks drift with say one part per million stability would drift 3.6 milliseconds an hour. A millisecond clock error translates to roughly 300km position difference. Over seven hours, the location information based on a single, one-way ping would be useless. The satellite would have a high accuracy clock, but the a/c would have something normal, say 10ppm. Therefore, I guess they must to be talking about a two way communication, but I haven't seen the protocol spelled out anywhere. The details of this protocol matter a lot to determining the accuracy of the distance measurements.
Edited:
Ok I found this on the WSJ
After not receiving new data from the 777 after its automated reporting system was switched off, the automated satellite pings—the digital equivalent of a handshake—originated at a ground stations and was transmitted up to the orbiting satellite high above the Earth's equator. The satellite relays the ping down to the aircraft below, effectively asking the jet if it is still able to send and receive data. After receiving it, Flight 370 transmitted a return ping back up to Inmarsat, which in turn relayed it to the ground station.
To get a handle on the error we need to know where and how the ping transmit/receive time was measured and the total round trip time.