StormyKnight
Would the 40 degrees be determined by received signal strength as suggested?
That's an interesting picture but I'm not sure the signal strength method is the only method available for "triangulation" in this case. I'll outline some general methods that are used in various systems, commenting also on TelcoAG's posts at 07:39 and 09:02 UTC today:
1. The signal strength method is a possibility but I believe its accuracy may be rather poor.
2. Another possible method is to measure the signal timing differences from two satellites, if data from both are available ( "reversed GPS principle" as someone said).
3. A third method may be to utilize the "Time Alignment" procedure that is widely used in e.g. GSM. Here, the system makes a signal timing adjustment to align the arrival time of radio packets from each active transmitter so that they align with the time slots in the receiver's (the satellite) TDMA (time division multiplex) channel frame. This adjustment value is an indication of the transmitter-receiver distance and can be used to plot a circle where the transmitter is located. Use 2 satellites and you get two cross bearings, one north and one south.
The caveat with this method, hovever, is that it is normally used only when you actually establish a connection to send a payload. For a periodic "ping" you normally don't need to allocate a timeslot.
4. It is quite conceivable that the Inmarsat satellite can measure the "elevation angle" of the received signal directly. Newer satellites have narrow individual "spot beams" to increase capacity, and obviously the system can keep track of which beam is allocated to any particular transmission.
5. Finally, the "ping time" itself (i.e. the roundtrip delay), if measured with enough precision, tells the distance between transmitter and receiver and can conceivably be used to plot a range circle as with the time alignment method.