INMARSAT has several services for airliners. The Aero-H and Aero-H+ services
both accomodate ACARS and other voicec and data services for airline operation.
Aero-H service operates on the INMARSAT I-3 constellation that has global and
regional beams. Aero-H+ can operate on either I-3 or I-4 satellites. The I-4
satellites have a global beam as well as multiple small overlapping spot beams,
similar to a cellular network. INMARSAT can dynamically assign bandwidth
(spectrum) to spotbeams as required, allowing a very high level of frequency
re-use. The spotbeams are not steered on the satellite. The system also uses a
form of uplink power control. The mobile terminal sets initial transmit power
based on elevation angle to the satellite. The network control system keeps
track of all terminals logged into the network. The power setting, and
therefore approximate elevation angle, are reported back to INMARSAT NCS. If
there were two transmissions at large enough differences in report!
ed elevation an approximate bearing and speed could be calculated. That would
at least focus the search to a single wide line.
However, looking at the arc on the map and assuming it is accurate, this looks
to be too big to be in an I-4 spotbeam which means they may be operating on the
I-3 constellation which doesn't require as much information as the I-4 does.
I'm not as familiar with the H and H+ services as I am with the BGAN service so
I don't know the details of how an H or H+ terminal behaves on the two
different satellite versions. I truly hope they have more information than is
being released.
Howie, AB2S
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