Text 1387, 64 rader
Skriven 2007-05-09 17:59:00 av Michiel van der Vlist (2:280/5555)
   Kommentar till text 1367 av Ward Dossche (2:292/854)
Ärende: Airbus
==============
Hello Ward!

On Wednesday May 09 2007 10:18, you wrote to Roy Witt:

 WD> I think that was what was meant in the announcement of Emirates which
 WD> you refered to: using microcells. Working for a company which offers
 WD> cell-phone services I've tried switching on my mobile in-flight and
 WD> hardly ever got a signal while overhead land in Europe, even at
 WD> minimal altitudes.

GSM does not go well with high altitudes and high speeds. It was designed for
terrestrial use at terestrial speeds. There are three problems:

1) GSM has a speed limit. For the time slots to work properly, the mobile has
to keep tracj of what is called the "timing advancement". If you have a handy
with e network monitor it will tell you the timing advancement which gives you
the distance to the bas cell in units of 550 meters. When on the move, the
timing advancement must be adjusted. If the speed is in excess of 300 kph, and
one is moving directly towards the base cell, the timing advancement mechanism
can not keep track and the connection is lost.

2) The timing advancment is kept in an 8 bit number, therefore the maximum
distance to the base cell is about 35 km. At altitude one can have a good
signal from base celsl further awar=, but one can not connect. I have
personally observed it standing on the falaise at Calias. Good signals from
Brittish providers, but no connect. Too far away...

3) There is a liniut to the numbe of base station a mobile can handle. If it
sees to many at the same time, it gets confused. When flying over populated
areas - most of Western Europe - this is often the case. Balloon pilots that
the GSM generally does not work at altitudes over 3000 meter.

 WD> It's not that I'm suggesting the airborne phonecalls of 9/11 couldn't
 WD> happen

In the US the use a different system. The older analog system NMT did not have
those limitations either.

 WD> because a friend of mine who flies 747's had his cell-phone go
 WD> off at 30,000 feet over Madrid Spain after he forgot it in his
 WD> pilot-jacket. But just picking-up the phone at 30,000 ft doesn't mean
 WD> it would work without a glitch just like that.

Indeed...

 WD> It is perfectly OK to use a GSM-phone in an aircraft while aloft.

But it would not work unless there is a microcell in the aircraft that is
connected to the rest of the world via a link not subject to the limitayions
inhrent in GSM. F.e. a satellite link

 WD> That's what Emirates are also talking about ... GSM-phones in the
 WD> hands of the (flying) public and operated via on-board micro-cells.
 WD> The only operational problem discovered was the heat-generation of
 WD> such cells and the associated cooling-issues as well as hightened
 WD> fire-hazard.

I do not believe that. Such a micro cell should not have any more heat issues
than a handheld cell phone.

Cheers, Michiel

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