Originally Posted by
glenbrook
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
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.
ground to sat two fixed positions, sat to aircraft, aircraft to sat one fixed one moving, sat to ground two fixed items.
sat ground and vice versa absolutely irrelevant (the should be a constant). it matters not one jot whether the satellite has an hourly clock or the ground has the hourly clock, the only part that can measure the distance in time and relate it to distance is the sat-aircraft section.
The timing is almost certainly on the satellite why depend on extra transmissions from/to GS which may be down.
You fly into the coverage of a satellite, your presence is detected just like you going to an area of a new phone mast, it knows your there and both of them periodically check to see if you still are. So the satellite has detected a potential service user why tell the ground , it just logs it and checks in an hour