How were cell phone calls completed from hijacked planes on 9-11?

November 30, 2007 8:20 AM

9-11 Technology Filter: I have never been able to even get signal on my Blackberry 8200 (GSM) within 10 seconds of liftoff, so how were cell phone calls completed from hijacked planes on 9-11? Specifically, given the prevailing handset, carrier, and tower technologies present in the Boston, NY, and DC metro and intervening areas at the time, and given the flight paths and altitudes of the hijacked planes, how were passengers able to complete cellphone calls to loved ones?

On the radio this morning, I heard some conspiracy theorists posit that proof of the 9-11 hoax is that all the evidence of the hijackers comes from cell and AirFone calls from the planes. They claimed that one of the planes was not equipped with AirFones, but that nonetheless descriptions of that hijacking came from passengers calling loved ones and their own voice mails on their cell phones (I don't not remember which plane they claimed didn't have airphones). All I could find online that was remotely coherent was here but it is very light on the technical details which would seem to put this issue to rest conclusively.

Some my questions are as follows:

A. How was it possible to for passengers to complete these calls at high altitudes? Were phones and towers more powerful then? Did they use a different encoding scheme that was more robust? (Analog vs. Digital, CDMA vs. GSM, etc.) The more technical the response, the better.

B. Were any of these calls from passengers recorded? What about the infamous "Let's Roll" (or "let's roll it"?) call? Are the voice mails available online?

C. Is a map available of the flight paths of the four planes that (a) includes their altitude, and (b) includes tower locations from 2001 from the major wireless carriers?

D. Has anyone been able to complete a cell phone call from an aircraft at any time between a minute after takeoff and a minute prior to landing? What carrier and phone?

E. If you have ever worked near the top of a very tall building (i.e. much taller than surrounding buildings and far above the ground, i.e. Transamerica in SF, Sears Tower in Chicago, Empire State or WTC in NY, Petronas Towers, Taipei 101, etc), have you experienced poor service that was explained because of your floors height above the surrounding towers?

F. Do cell towers not broadcast or receive signals from above? I assumed that towers broadcast signal radially and would therefore be subject to the inverse square law. Am I wrong? Are towers in rural areas significantly higher and stronger than towers in urban areas? Why?

Sorry if this is too long or inappropriately formated for AskMe. There is very little I could find about this on the web, so any help would be appreciated, and as I said, the more technical, the better.

posted by Pastabagel to technology (20 comments total)

I don't know the answer to your specific question(s) but my phone (Verizon, so ... CDMA?) generally gets a decent signal in the air. So that part, at least, I think is feasible.

Yes, it's usually off, but I've accidentally left it on more than once.
posted by spaceman_spiff at 8:23 AM on November 30, 2007

seconding i have left my phone on by mistake and had decent signal.
posted by phil at 8:28 AM on November 30, 2007

I saw this very question asked recently somewhere else. One of the answers was that, apparently, most, if not all, of the calls made were actually from the in-flight phones located in the seatbacks.
posted by Thorzdad at 8:30 AM on November 30, 2007

There won't be any geographical obstructions to cellular signals once you're airborne, so that could play a role in signal strength. (of course, I am not an engineer)
posted by malaprohibita at 8:47 AM on November 30, 2007

Here's a study that tests cell phone completions from various altitudes. I make no claim that it's a valid scientific study.
posted by desjardins at 8:48 AM on November 30, 2007

I can answer E. Yes, if the building does not have repeaters you will get poor cell reception once you get above a certain height. Seems to be around 60 or 70 stories in my area with my network.
posted by jcwagner at 8:48 AM on November 30, 2007

Also, according to wikipedia, only two of the calls from the Flight 93 were from cell phones - the rest from Air Phones.
posted by desjardins at 8:49 AM on November 30, 2007

generally gets a decent signal in the air. So that part, at least, I think is feasible.

Yes, it's usually off, but I've accidentally left it on more than once.
posted by spaceman_spiff at 11:23 AM on November 30

I've seen my phone show a signal too. But has anyone actually completed a call? Every time I've tried to place a call, the signal meter drops to zero.
posted by Pastabagel at 9:13 AM on November 30, 2007

Not an expert, but I have heard that you'll get signal in the air, however the cell towers can't handle the fact you are able to connect to so many towers at once, and that it can't handle the hand off's correctly when you are in the air above the towers... therefore expecting a call to work would be difficult.
posted by thilmony at 10:59 AM on November 30, 2007

The cell phones calls from United 93 were made at low altitude, so low that cell phone towers on nearby hills and mountains were above the aircraft
posted by A189Nut at 11:06 AM on November 30, 2007

Listening to the walk-over, huh? I don't have anything to add, except that the link above from desjardins is from a 9-11 conspiracy site; not to disregard it, but they definitely have some bias.
posted by shinynewnick at 11:27 AM on November 30, 2007

As I said, I make no claim that it's a valid scientific study.
posted by desjardins at 11:34 AM on November 30, 2007

Oh I know, desjardins, nothing against you at all. Just stating the obvious for people who don't look through the rest of that site.
posted by shinynewnick at 11:44 AM on November 30, 2007

The IEEE conducted a study in 2003 on the effects of cell phone use on airplanes in flight. They discovered that, "Passengers are using cellphones, on average, at least once per flight." I usually refer conspiracy theorists to that doc when they start talking about the cell phone thing.
posted by autojack at 2:41 PM on November 30, 2007

I have a friend who is a commercial pilot. He makes phone calls while in the air all the time.
posted by ASM at 5:50 PM on November 30, 2007

Regarding F), cell towers use antennas that radiate mostly horizontally. This is so that they don't waste power beaming cell transmissions into space or into the ground (or people) below, and so that they don't pick up as much noise from things outside of their intended coverage area. (They usually only transmit through a 90-120 degree angle ("sector antennas"), too, so that each tower can cover three or more distinct cells. In less-dense areas they might use a single less-directional antenna in the center of a cell.)

There will still be some signal transmitted in other directions, and in any given direction the signal will fall off as inverse-square, just with a different starting point. I expect you'd still be able to use a cell phone from directly above a cell tower, but you wouldn't get remotely as much signal as you would expect if the antenna were omnidirectional. (It's actually impossible to build an antenna that's a truly isotropic radiator.)
posted by hattifattener at 6:11 PM on November 30, 2007

Oh, and the second half of (F): I could be wrong, but as I understand it, yes, they are. In dense areas like cities, you want to have small cells, because there are only enough frequencies (or CDMA inter-user interference margin) for a finite number of users. In rural areas, that's not as much of a problem, so it's cheaper to build a smaller number of cell towers each of which covers a larger area.
posted by hattifattener at 6:21 PM on November 30, 2007

Flight 93 audio
posted by hortense at 7:25 PM on November 30, 2007

ASM, does your pilot friend makes calls on his cell phone from the air, or from the AirFone?

hattifattener: thanks for the detailed response. Follow-up is there some upper bound on how fast you can travel between cells before the towers are not fast enough to hand your call off to the next cell you enter, because by the time the handoff to the new cell is completed you've already exited the new cell? Specifically, I'm wondering if cell handoffs are handled properly when handset is moving at 400-500 mph (the approximate speed of the aircraft according to subsequent analyses).
posted by Pastabagel at 7:11 PM on December 1, 2007

pastabagel: Presumably there is some upper bound, but I have no idea what it is. It probably depends on details of the particular carrier's network. Also a possible problem using cell phones at high speeds is the doppler shift of the signal; again, whether this is a problem depends on technical details of the base stations (how much shift they can accomodate) as well as what frequency you happen to be using (higher frequencies will show more shift).

I'm exceeding my expertise a bit here.

Googling around, this page talks about the subject a little (in the specific context of flight 91, even) and doesn't seem to have any obvious errors. OTOH, I would expect CDMA cell phones to be more sensitive to doppler shift than TDMA, GSM, or AMPS.
posted by hattifattener at 7:46 PM on December 1, 2007