Mobile Phones on Aircraft
The use of mobile phones on
aircraft is generally forbidden during flight.
One reason given for this is that the mobile
phone could interfere with the sensitive
equipment on the aircraft. This could be
restated as "during development these aircraft
were not designed to accept signals from mobile
phones and there has not been sufficient testing
to be sure that they could" as can be seen from
plans to improve certification [1] (http://www.caa.co.uk/docs/389/srg_sys_00002-01-300103.pdf).
Some level of electromagnetic interference is
theoretically possible from active radio
transmitters such as mobile phones on aircraft.
Exactly how much and in what way is dependent on
the particular phone system in use and the plane
component in question. Whether that level of
interference should have any influence on
electronic systems which should be designed to
fly through lightning storms without falling out
of the sky is an entirely different question.
One area in which interference would be most
likely is in the radio-based audio equipment
used for voice communications between the
aeroplane and the ground. The mobile phone
transmitter is much closer to the receiver on
the aircraft than the ground station, but
operates at a lower power than the ground
station.
Some mobile phone systems such as GSM may cause
an irritating buzz (explained in the TDMA
article) which would certainly disrupt
communications from the pilot to ground. The
high speed of air travel may make interference
more likely than it would otherwise be. The
maximum speed of travel in a mobile phone system
is limited by several factors, frequency
changes, rate of change of timing offset, etc.
The speed of an aeroplane often exceeds these
(typically phones are designed for use in a fast
car) which means the mobile will fail to
register to the network and retry registration
repeatedly.
Older analogue systems simply broadcast at a
high power of up to several watts. This has the
potention to cause more general interference,
and since the voice signal is not encoded there
may be direct crosstalk into the communication
systems of the plane.
But perhaps the greatest issue is with the
principle of frequency reuse. Mobile phones are
designed to transmit only as far as the closest
cell tower. This allows the same frequencies to
be used by different phones in any non-adjacent
cells, a key component in allowing tens or
hundreds of thousands of people to use their
phones at the same time in a given metropolitan
area. From an altitude, distant cells are
visible to the mobile with no line-of-sight
attenuation from intervening obstacles. Because
the cells are in some cases several tens of
thousands of feet below the aircraft, the phone
will transmit at its maximum power (also
increasing the risk of interference with
electronic equipment on the aircraft). Since the
phone is occupying its frequency and channel in
all of the cells its signal reaches to, that
frequency and channel cannot be used by any
other phones in any of those cells because of
interference, resulting an an overall decrease
in the cellular system's capacity. This is the
primary reason the U.S. Federal Communications
Commission prohibits the use of mobile phones in
the air. (The U.S. Federal Aviation
Administration is more concerned with
interference to aircraft systems.)
All of the above having been said, according to
the BBC "most of the evidence is circumstantial
and anecdotal. There is no absolute proof mobile
phones are hazardous." [2] (http://news.bbc.co.uk/2/hi/uk_news/399154.stm)
Some airlines do allow use of mobiles phones in
flight, only restricting their use (and use of
all other electronic devices) during take off
and landing when communications with the ground
are most critical. Meanwhile the passenger
aircraft manufacturers, such as Boeing and
Airbus, have begun to introduce wireless
services on their planes (e.g. WLAN) and
radio-based satellite phones are a standard
installation on aeroplanes. Clearly there is a
direct airline industry advantage in having
control over communication systems from within
an aeroplane with no clear way for potential
competitors to certify their systems as safe for
use on board. Some articles have even gone so
far as to accuse the airline industry of pushing
the ban on mobile phones in order to increase
revenue from on-board telephones [3] (http://news.zdnet.co.uk/business/0,39020645,2074198,00.htm).
A number of new phones have an "aeroplane mode"
feature that presumably stops all incoming and
outgoing communications while still allowing the
user to play games, type notes etc.
A few U.S. airlines have announced plans ([4]
(http://money.cnn.com/2004/12/09/technology/personaltech/cellphones_inflight/)
and [5] (http://money.cnn.com/2004/12/14/technology/air_broadband/))
to allow mobile phones to be used on aircraft,
pending approval by the FCC and the FAA. The
idea is similar to that used in some cars on the
German ICE train: the aircraft will contain a
mobile signal repeater that will then transmit
the signals to a terrestrial-based system on
separate frequencies that don't interfere with
the cellular system. Since the repeater is
literally right next to the passengers, the
phones' output power would be reduced to the
lowest level possible, reducing interference
with cells on the ground.
That leaves one final concern: mobile-phone
etiquette. Many have brought up concerns about
being forced to sit next to a loud mobile phone
user for a several-hour flight, adding airlines
to the list of establishments trying to figure
out how to enforce common courtesy.
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