4 ms·

I did notice that the map was disabled on my flight back from Australia yesterday. Pretty annoying. By the way - GPS watches work just fine in a plane.

mrduncan 17y ago
By the way - GPS watches work just fine in a plane. It sounds like they are not allowed to be used according to that document though. I would imagine that cell phones also work just fine (I've never had the urge to try), but that doesn't mean their use is allowed. Edit: Misread the document, I stand corrected. This is just another one of the TSA's typical security illusions.
- rufo 17y ago
  The document specifically mentions "aircraft-integrated" GPS devices.
- mdasen 17y ago
  Cell phones are a bit of a different story. Cell phones were designed to be operated somewhere around 0-100MPH from the surface (or very close to it). A plane violates those assumptions. That's important in a couple ways. First, if you're in the air, you have line of sight to a lot more cell sites than you normally would. While there might be a couple sites at any give point that are a similar signal strength to you, here there could be hundreds. Basically, cellular wireless is based off the idea that your transmission only goes so far and therefore they can re-use the same spectrum in different locations (or, in the case of CDMA systems, not have every code talking over each other, but rather have a limited number of voices screaming). Being on a plane violates that. With a CDMA system, you'd either be drowned out by closer signals or you'd be causing a ton of interference for all towers within range and it'd be a lot of cell sites. With a TDMA system, you'd be transmitting on a frequency/time-slot that would be re-used on another tower that could similarly hear you. To explain that a bit more: in real life if you have: Tower FreqA :: Tower FreqB :: Tower FreqC :: Tower FreqA in a linear fashion and you're transmitting to the first tower (using frequency A), your signal won't reach the last tower (which is using frequency A to talk to a different customer). However, if you're flying above all the towers, all of a sudden you have line of sight like you've never had before and if you're transmitting to the first tower on frequency A, your signal is also likely being heard by the last tower as well meaning that the entire theory of re-use goes down the drain. Likewise, you're travelling at (potentially) 500 miles per hour. Travelling at 60MPH, you'll likely need a new cell site every few minutes (depending on geography and the like). At 500MPH, you could be getting new sites more often than not. Combined with the fact that you'd be in "range" of so many sites as noted above, it would become difficult for the network to determine which site you should be transmitting to/from. And that is important. Cellular wireless only works because the same finite bandwidth can be reused since you aren't using that bandwidth in all locations. So, unless microcells are used on the plane itself, wireless service is a terrible idea in the sky. A microcell in the plane would get the phone to transmit at a very low power (therefore not interfering with customers on the ground) and the cell site would be a constant distance from you. It's why even in countries where the aviation folk are moving toward cell phones on planes, they're doing so through microcells. I'm not saying that it would't work. I haven't studied it, but it wouldn't be a wonderful experience and would cause havoc for the network. EDIT: It seems that it's an FCC (in addition to FAA?) prohibition in the US (http://en.wikipedia.org/wiki/Mobile_phones_on_aircraft#USA_Channel_reuse http://en.wikipedia.org/wiki/Mobile_phones_on_aircraft#USA_C...). The article on Wikipedia also mentions airlines installing microcells (although not by name) to allow people to call on planes.
  - johns 17y ago
    I think he was referring to cell phone GPS
    - mrduncan 17y ago
      For the record, I was referring to transmitting information with another party (either text or voice) - should have been more specific.
  - mrduncan 17y ago
    Very interesting explanation, thanks! I would imagine that text messaging probably wouldn't be subject to these issues since it's just a quick burst of data. It would also be a little easier to get away with using too.
    - mdasen 17y ago
      Well, the issue is even having the phone on since your phone needs to been in communication to cell sites via the control channel (which determines how to send an incoming call to you, what tower you should be using, and allows for sending/receipt of text messages). So, you might not be causing problems for the traffic channel and the speed issue probably wouldn't be a problem, but you'd still be causing problems on the control channel even by having the phone on. Granted, if a few people did it, it might not matter, but if everyone did you'd have the exact same problem where you'd be defeating frequency re-use.
lallysingh 17y ago
By the way - a departure time, and arrival time, and a wristwatch all work fine in a plane.

mrduncan 17y ago

By the way - GPS watches work just fine in a plane. It sounds like they are not allowed to be used according to that document though. I would imagine that cell phones also work just fine (I've never had the urge to try), but that doesn't mean their use is allowed. Edit: Misread the document, I stand corrected. This is just another one of the TSA's typical security illusions.

rufo 17y ago
The document specifically mentions "aircraft-integrated" GPS devices.
mdasen 17y ago
Cell phones are a bit of a different story. Cell phones were designed to be operated somewhere around 0-100MPH from the surface (or very close to it). A plane violates those assumptions. That's important in a couple ways. First, if you're in the air, you have line of sight to a lot more cell sites than you normally would. While there might be a couple sites at any give point that are a similar signal strength to you, here there could be hundreds. Basically, cellular wireless is based off the idea that your transmission only goes so far and therefore they can re-use the same spectrum in different locations (or, in the case of CDMA systems, not have every code talking over each other, but rather have a limited number of voices screaming). Being on a plane violates that. With a CDMA system, you'd either be drowned out by closer signals or you'd be causing a ton of interference for all towers within range and it'd be a lot of cell sites. With a TDMA system, you'd be transmitting on a frequency/time-slot that would be re-used on another tower that could similarly hear you. To explain that a bit more: in real life if you have: Tower FreqA :: Tower FreqB :: Tower FreqC :: Tower FreqA in a linear fashion and you're transmitting to the first tower (using frequency A), your signal won't reach the last tower (which is using frequency A to talk to a different customer). However, if you're flying above all the towers, all of a sudden you have line of sight like you've never had before and if you're transmitting to the first tower on frequency A, your signal is also likely being heard by the last tower as well meaning that the entire theory of re-use goes down the drain. Likewise, you're travelling at (potentially) 500 miles per hour. Travelling at 60MPH, you'll likely need a new cell site every few minutes (depending on geography and the like). At 500MPH, you could be getting new sites more often than not. Combined with the fact that you'd be in "range" of so many sites as noted above, it would become difficult for the network to determine which site you should be transmitting to/from. And that is important. Cellular wireless only works because the same finite bandwidth can be reused since you aren't using that bandwidth in all locations. So, unless microcells are used on the plane itself, wireless service is a terrible idea in the sky. A microcell in the plane would get the phone to transmit at a very low power (therefore not interfering with customers on the ground) and the cell site would be a constant distance from you. It's why even in countries where the aviation folk are moving toward cell phones on planes, they're doing so through microcells. I'm not saying that it would't work. I haven't studied it, but it wouldn't be a wonderful experience and would cause havoc for the network. EDIT: It seems that it's an FCC (in addition to FAA?) prohibition in the US (http://en.wikipedia.org/wiki/Mobile_phones_on_aircraft#USA_Channel_reuse http://en.wikipedia.org/wiki/Mobile_phones_on_aircraft#USA_C...). The article on Wikipedia also mentions airlines installing microcells (although not by name) to allow people to call on planes.
- johns 17y ago
  I think he was referring to cell phone GPS
  - mrduncan 17y ago
    For the record, I was referring to transmitting information with another party (either text or voice) - should have been more specific.
- mrduncan 17y ago
  Very interesting explanation, thanks! I would imagine that text messaging probably wouldn't be subject to these issues since it's just a quick burst of data. It would also be a little easier to get away with using too.
  - mdasen 17y ago
    Well, the issue is even having the phone on since your phone needs to been in communication to cell sites via the control channel (which determines how to send an incoming call to you, what tower you should be using, and allows for sending/receipt of text messages). So, you might not be causing problems for the traffic channel and the speed issue probably wouldn't be a problem, but you'd still be causing problems on the control channel even by having the phone on. Granted, if a few people did it, it might not matter, but if everyone did you'd have the exact same problem where you'd be defeating frequency re-use.

lallysingh 17y ago

By the way - a departure time, and arrival time, and a wristwatch all work fine in a plane.