One major reason why cellular phones should be switched off onboard aircrafts is related to security. 10 grams of plastic explosive hidden inside can blow off a head. Calling that particular phone triggers the "bomb", think about what it can do the plane. Israeli services used the system to "liquidate" a major Palestinian figure not so long ago...he received the brand new phone as a gift from a "friend".
Cellphones, be it NMT, GSM, AMPS, TACS, TDMA, CDMA, work generally well inside aircrafts, although at altitude ones needs to be pretty close to a window. Pilots do use their cellphones quite often.
An integral satellite-phone, be it Iridium, Globalstar, Inmarsat or Thuraya, will not work inside a plane, unless it is connected to an external antenna.
(which is the case with sat-equipped planes)
Cellphones (max 2W) and sat-phones (usually 2W but up to 25W) are subject to very strict and intricate tests (US FCC, European ETS, etc..) before being approved. The same principle goes of course for all electronic equipment onboard a plane.
Now, whatever "clean" an electronic device is, if it is designed to radiate energy, it will do so. This means in practice that i.e. a GSM phone will radiate up to 2W on a specific frequency, either during a call or during regular handshaking sessions with the network (which are transparent to the user).
These Watts are radiated by the antenna to produce a magnetic field (expressed in microvolt/meter).
2W of (effectively) radiated energy corresponds to a magnetic filed of 3.5V at a distance of 2 meters, 0.7V at 10 meters, pretty insignificant in any case (EXCEPT when directional antennas are used).
In addition, unwanted energy will also be produced at "harmonic" frequencies, i.e. at multiples of the "fundamental" frequency. These harmonics are created by non-linear devices in the electronic circuits. Eventhough filtering takes place, they are not totally cancelled but their level is several thousand times lower than the fundamental, we are talking microWatt or even nanoWatt here, close to nothing, and very substantially lower than all the fields produced by high power stations (AM, FM
broadcast, military stations, satellite ground stations, and the top of it: ultra high power submarine communication stations).
So, how can a tiny cellphone be found guilty of "interfering" with airborne equipment ? if everything was working as it should, it cannot.
Aircraft devices do not work in the same frequency bands of course, so no "interference" possible due to "signal pick up".
Actually there is only one possibility:
* the aircraft's device is not immune or insufficiently immune to a magnetic field when confronted to it. In the case of a cellphone, the distance between the phone and the device must be extremely small, talking cm here !
If the device's performance (and its associate wiring) is perfectly in accordance with the rules, a problem cannot occur.
THIS IS IT
, any unshielded or non-linear component can "pick up" that magnetic field and distribute it further down the line of circuits.
(an example of non-linear component is a "bad solder" point)
Conclusion: in theory nothing should happen, in practice noone will dare to say that everything is perfect in an aircraft.
Add to this the fact that allowing cellphone usage would obviously result in what everybody can imagine...
Not convinced ? think about all the aircraft systems that do radiate energy (VHF
, transponder, radar, TCAS, etc....), quite a bunch more than several cellphones.
Sorry if I was long...hoping it helped address some questions and correct misconceptions.