Today's mobile phone technology troubles 4156

I pray that my attributions are correct.

Robert Coe

There is a world of difference between "it tends to work with no problems" and "I can stake my reputation and my fortune to guarantee that it will never cause a problem." Public safety agencies want the latter kind of buttertion. Everything that has been said here is much closer to the first.

The shape of the fuselage does cause a focusing of stray RF in the chickenpit area, where the avionics are located.

There really are several issues here, and they interact in ways that tend to uphold the ban:

1) The worst case spot level of random cellphone emissions accidentally focused at any place in the airframe is hard to measure, model or predict, because there is a large variety of portable equipment types which - being handheld - can be anywhere at any given time, and at least in theory can cause all sorts of intermodulation products with stray emissions from the aircraft's own equipment. 2) The aircraft's equipment was not designed for an environment full of unpredictable random RF. Rather, it was designed for and tested in an environment where the type and location of every other piece of equipment is fixed and known. Specific combinations of equipment are certified for use on the individual aircraft. 3) There is at least a theoretical problem of performance impairment in the cellular system when a mobile is in many locations at once. This is the reason for the FCC part of the ban. 4) The airline has invested in the AIRFONE system, which is costly for pbuttengers to use, and which nobody would use if there was an alternative.

Problem number 2 goes away gradually, as newer designs of all kinds of electronics become better at RF compatibility. As avionics become miniaturized and modular and similar to other modern computing gear, it also becomes low-power, and better shielded, and thus more tolerant of stray RF and less likely to leak stray RF that may form the above mentioned intermodulation products.

Problem number 4 will eventually be the source of the solution. The AIRFONE "PBX" will be replaced by a GSM microcell, which will be tested as part of the avionics package. As the cell is nearby, the handsets' TX power will drop down to a few milliwatts. Pbuttengers will be able to use their own phones, but they will still be forced to use the airline's chosen carrier, probably at exorbitant prices.

Nick Spalding

It will happen first at the smaller carriers who have not already sunk money into AIRFONE and similar systems, and it will happen slower in the US where there are still non-GSM phones that may try to crank up their TX power to try to reach an AMPS base rather than than attach to the GSM microcell.

Aside from all that, there has recently been speculation that using commercial cell phones aloft just cannot work at all. I think this is a case of location-dependent variation. When AMPS and NMT950 were first introduced in the age of bag phones, cell sites were few and far between. They had large towers with large antenna apertures that probably could operate with airborne handsets. As usage went up, this infrastructure was gradually replaced by smaller cells with low power, each covering a much smaller area, so that many more subscribers could be accommodated in a given metro area by reusing the same channels .n many minicells simultaneously. In some areas, the old analog towers are still in place, used as fallback for rural coverage. In other areas they have been completely dismantled.

Lars Poulsen