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Author Topic: Do mobile phones work if you're travelling at the speed of sound?
benskia
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If someone on the ground tried to phone a passenger on a jet, which is going at the speed of sound, would they be able to talk together?

Weird questions huh? But I just want to check that I dont have a serious clanger waiting to be pointed out to me by someone who knows better.


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ChrisOwens
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Since cell phones pick up electrmagnetic signals from a network, I am not sure why the sound barrier would even be a problem.
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pantros
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yes phones work at the speed of sound which is tragically slow compared to the speed of phone signals which travel at the speed of light.

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Matt Lust
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um yes and no

Yes because Mobile phones don't use sound waves to communicate. They use a variation on radio waves that do in fact travel at the speed of light (in a vaccum that is) so yes they could in theory work

but

No because the key word isn't "mobile" is cellular. This is important because it refers to the footprint of the towers that are in cells like honeycomb. And since because a plane flying supersonic is likely to be shifting from cell to cell rather quickly it seems to me to be rather unlikely that you could use a standard mobile phone but if you wanted to give your character a sat phone like the ones marketed by the now defunct iridium* sat phone service it could work.

*Historical Aside: Iridium tried to get off the ground in the mid to late 90's when cell phones were just catching on. They had plans for a total constellation of 33 satellites to prvoide global coverage. I think they only got something in the neighborhood of 8 or so up before going under but i could be wrong.


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Survivor
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You aren't allowed to leave your personal cellphone on while traveling by commercial aircraft, for a number of good reasons.

Howevever, a "cellphone" tied into the aircraft's own communications equipment can be used, so you can call a passenger in-flight or a passenger could call you (this service typically costs extra).

As for whether a conventional "ground" cellphone can work in-flight, the answer is "sometimes". When flying over an area where coverage is provided by many closely packed cells, the hand-off's between various towers would increase the chances of a dropped signal dramatically (compared to using it in your car or something). On the other hand, if you were flying over an area with larger cells (like a rural area), it wouldn't be such a big problem. The fact that you're at a different altitude than the system is designed to accomodate and you're inside of a large metal container that has its own electronics will also have an impact.

In any case, if your phone rings in-flight and you answer it, you'll be in serious trouble with the carrier and perhaps with the feds. It's much better to use the plane's communication system in most cases.


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Corky
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For whatever it may be worth, people on Flight 93 (on September 11, 2001) were able to use their cell phones to call out of the plane.

So I would think it could be done in the other direction (calling from ground to plane).

Most passenger jets don't fly above the speed of sound, by the way. I think the Concorde was the only passenger jet that could even reach that speed, and they only flew it that fast over the ocean because supersonic flight causes sonic booms all along the route, and the people on the ground would complain like crazy. (Not to mention things like fuel costs.)


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Robert Nowall
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A couple of thoughts / comments...

If you're using your cell phone in a moving plane (and also cars), you'll only stay in touch as you're in range of the particular cell phone tower that's picking it up. On 9 / 11, when Barbara Olsen called her husband Ted, she called and they talked, then they were cut off when the plane moved out of range, then she called again and they talked until the plane again moved out of range, a couple of times until Ted Olsen didn't hear from her again.

The banning of passenger use of personal communications devices and most other electronic equipment derives from worries about interfering with the plane's own electronic equipment. Most of this worry dates from a plane crash in the late fifties or early sixties, where an early commercial jet was thought to have been brought down by a portable shortwave set someone brought on board. (Only they were never able to duplicate the effect in the lab.) Cell phones were used by the plane passengers on 9 / 11 because almost everybody has them now, and such was the stress of the incident that those particular rules were junked, if the passengers even knew about them.

I thought planes that flew faster than the speed of sound only made sonic booms going up past or coming down below that speed, not when they were actually above it.

(Oh, one other thing. Don't mistake me for an expert in any of this. I just have a lot of useless supposed facts rattling around inside my skull. I could easily be wrong about just about anything.)


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Corky
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Sonic booms come from any object that is travelling at any speed above the speed of sound.

Edited to add: And they are continual as the object moves along at supersonic speeds. They may seem to be short in duration because they have moved past the hearer.

http://www.sky-flash.com/boom.htm

[This message has been edited by Corky (edited January 23, 2006).]


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hoptoad
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notes on usage:
Benskia is a Brit. Clearly you Hatrackers know that in UK, Australia and numerous other places 'cellphones' are called 'mobile phones'.

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Matt Lust
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I understand that hop but I just wanted to focus on "mobile" vs "cell" because the distinction in diction is important as myself and several people have pointed out in the inability of continued contact to be used because of the plane's speed causing you to drop after you leave the cell your in.
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Kathleen Dalton Woodbury
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I first encountered wide usage of them when I spent three weeks in continental Europe in 1999. They called them "handies" in Austria, and, I think, in some of the other countries (France, Italy, Switzerland, Germany) that I visited.
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rcorporon
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Just as a side note, they are called "keitei's" (pronounced kaytie) in Japan.

You may return to your regular posting .


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Survivor
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Well, the diversion onto cellphones notwithstanding, that wasn't part of the original question.

If you call someone on their personal mobile phone, whether it's a sat-phone or whatnot, then they won't be allowed to talk because the steward/esse/s will confiscate the phone in question. They're pretty lax sometimes about allowing people to turn their phones on in-flight (which is technically just as prohibited), but they definitely will take your phone away if you start yakking.

Most commercial flights have phones for passenger use, though. These phones are tied into the plane's communication system, so they're not subject to the problems of cell usage (or rather, the plane's communication system is designed to work in-flight).


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Robert Nowall
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Well, it doesn't mean that they won't work on an airplane, just that passengers are forbidden to use them by regulation.

(Oh, and Corky? Thanks for the link to the info on supersonic speeds. It answered a lot of other questions I had besides correcting my mistaken impression. The only sonic booms I hear regularly come from the shuttle...)


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franc li
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You also are forbidden so that the in-flight calling service can charge you $5 a minute or whatever it is. I guess it is possible that something like a cell phone could mess up some highly sensitive avionics equipment.
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Survivor
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There was an airplane that crashed and they blamed it on a shortwave radio that one of the passangers had brought on board. Yeah, it doesn't seem terribly likely, but that's what they came up with at the time and since then all passenger devices which emit RF are prohibited by law.
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Corky
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You're welcome, Robert.

By the way, I had the privilege of being at the Cape when a shuttle landed. Didn't look fast enough in the right direction to see it, but I heard the sonic booms (yes, plural--one boom! quickly--as in a fraction of a second later--followed by another).

I have asked around about why there would have been two sonic booms, and the best we can come up with is that maybe the tip of the nose generated the first boom and the front edge of the tailfin generated the second one.

So, please tell me, have you ever heard a double boom when the shuttle's come in for a landing? And if so, do you always hear a double boom, or only sometimes?


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benskia
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Hi Corky.
Funnily enough, I've been reading around the web about sonic booms.
One thing I wanted to know in particular is whether the people on board the plane hear the boom. They dont apparantly.

But, I remember on a couple of the sites it did say that sometimes you could get a double boom. One from the point of the plane & then one from any other bit of the aircraft that sticks out more prominantly & causes a second pressure wave, outside the radius of that created by the nose.

So, yeah - you can have a double one.

Incedently - the guy who is receiving the call in the aircraft is okay to receive it. It's outside of normal passenger jet circumstances.

Thanks.


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Robert Nowall
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Yes, it has always been a distinct double boom when I've heard it. Quite loud, and always at a time when the shuttle was about to come to earth. (Not every time a shuttle passed by, though...I'd assumed there were variations in the flight path and it was closer when it did boom.)

In Tom Wolfe's account of Chuck Yeager's first supersonic flight in "The Right Stuff," I believe it mentions the double boom, and that it was in some way a predicted phenomenon, but didn't as I recall go into clinical detail. (I say "I believe" and "as I recall" because my copy of "The Right Stuff" is inaccessible behind a big pile of boxes in my so-called library right now, so I can't check it.)

I've never seen a shuttle launch or landing up close, somewhat to my regret. Though I've never spotted a landing from where I usually am in Florida, if launch weather is particularly clear, I can see the trail left behind as it rises, all the way up past booster separation. (I could see the recent Pluto launch, as well.)

(All, alas again, a little off the topic of whether you can make a call from a jet traveling at the speed of sound. I offer my apologies.)


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