I’m not really a technical type, but I had a look at various articles and came up with this:
Phones in planes
How far from a base station can a phone operate?
22 miles. The problem is not strength of signal, but time delays.
“GSM’s wide frequencies give it scalability advantages, but the short time slots cause problems in keeping phones synchronized with each other. Radio signals take just over 0.003 microsecond to travel a kilometer (km), which adds up to a round-trip delay of around 0.4 microsecond for a phone only 60 km (40 miles) from the base station. The time slot only lasts 0.577 microsecond, so this delay is enough to make the phone miss its slot entirely, even though it would be unnoticeable to a human listener. In practice, GSM phones cannot be used more than 35 km (22 miles) from a BTS, no matter how strong the signal.”
http://www.networkmagazine.com/article/NMG20000517S0169United 93 was never more than 7 miles up.
OK, that’s the theory, what’s the practice?
Generally, BTS do not have a range in excess of 10km.
“The current generation of GSM base stations cannot communicate over distances greater than 35 km because the delay in receiving radio signals becomes too great. However, the decline of signal strength with distance places a practical limit on coverage of around 10 km. For these reasons an extensive network of base stations is needed to ensure coverage throughout the UK.”
http://www.sitefinder.radio.gov.uk/mobilework.htmHowever, much of the problem here is due to attenuation by landscape features, which do not pose a problem to a mobile phone in the air trying to connect to a BTS on the ground, at least after the signal gets out of the plane.
How powerful are most mobile phones?
Around 2 watts.
“Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts). The base station is also transmitting at low power.”
http://electronics.howstuffworks.com/cell-phone4.htmHow powerful are base stations? What is their range?
It depends. One base station can only handle a certain number of calls, so in cities there are lots of base stations with small cell areas, which gives a network a greater carrying capacity. In rural areas the demand on capacity is not so high, so there are less base stations, but they are more powerful than those in cities and cover a wider area. Therefore, if you take off over a built-up area, you may go out of range initially, but may be able to make a call later when over a more powerful rural base station.
Speed
Does the speed at which a plane travels affect a mobile phone?
To some extent.
“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.”
http://en.wikipedia.org/wiki/Mobile_phones_on_aircraftAs far as I’m aware, the phones on 9/11 didn’t hop frequencies, so that’s not an issue here. Perhaps the success rate depends on how good a specific phone is, i.e. a good, expensive phone may have better results than a cheapo one. Nevertheless, it seems that speed does not make mobile phone use impossible in aircraft.
Attenuation
What is it?
“In telecommunication, attenuation is the decrease in intensity of a signal, beam, or wave as a result of absorption of energy and of scattering out of the path to the detector, but not including the reduction due to geometric spreading.”
http://en.wikipedia.org/wiki/AttenuateCan a signal get through a plane’s fuselage?
Yes, but it may be weakened in doing so.
The German high-speed ICE train has problems with attenuation:
“All ICEs have "repeater" carriages, which are equipped with technology to enable mobile phone use (as the windows have a metal coating). Only in these carts cell phone use is possible. These carts are marked with a sticker of a symbolized mobile phone.”
http://en.wikipedia.org/wiki/ICEAs far as I know, Boeing’s windows do not have a metal coating. Perhaps it’s easier to make a call from a plane if you’re sitting by the window. I have problems with attenuation in the middle of my building, but not when I’m standing by the window.
History
Mobile phone use is now mostly banned on aircraft, was it allowed in the past?
Yes. But most countries which permitted it finally banned it after the crash of a Saab 340 out of Zurich in 2000, as the event was blamed on the receipt of a text message and subsequent call. At one point some airlines permitted calls in flight, but not during takeoff and landing. I guess that if it was permitted, then it was possible. Why bother permitting/banning something that’s impossible?
What is the fix now under discussion?
The problem is not that mobile phones don’t work on planes, it’s that they know they are a long way from their base station, so they transmit on full power, which makes interference with a plane’s instruments more likely. Also, the signal goes to multiple cells, which blocks the frequency for other users on the ground. The fix is to put a repeater on a plane. The phones would transit to the nearby repeater at low power, thus not interfering with the instrumentation, and the repeater would send the signal to a specific base station down below, without blocking the frequency everywhere.
“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 in an overall decrease in the cellular system's capacity.”
http://en.wikipedia.org/wiki/Mobile_phones_on_aircraftConclusion
I take this to mean that the supposed cell phone calls could theoretically have been made from United 93. However, this depends on the plane’s actual position vis-à-vis base stations on the ground and also, possibly, the specifics of the various phones used to make the calls.