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Locational Services and your Mobile Device: Part 1

Locational Services and your Mobile Device: Part 1
Can I tell you a secret? Okay not such a secret but lean in .. I used to be a geographer. No stop! I don’t know the capitols of every city. I studied all about cartography (making maps), GIS (Geographical Information Systems), GPS (don’t get me started on GPS) and in addition Computer Science with some Psychology thrown in just to keep me sane I guess.

Now then, let me ask you another question. Did you know that the cellular phone in your pocket is yelling “yoo-hoo! I’m right here” to your network provider right this instant?

Your cellular phone is continually seeking the best reception from the nearest and strongest signals it can receive from at least two if not more cell antennas or towers.

Slightly oversimplified but basically now you know. Cellular phones are radios; they can transmit and receive in full duplex and most radio signals rely on line-of-sight to enable this process.

So what does this mean to you? It means that your cellular phone has a measure of knowledge of where it and thus you are. The network antennas are fixed positions i.e. the cellular provider and anyone with a map, GPS etc. has its coordinates/ location.

You however with your cellular phone have the unmitigated cheek to keep moving. This tends to make things complicated as you and every other cellular phone moves in 3 dimensions.

Really though the theory and practice of locating a phone is based on triangulation: –
  • To establish the distance from your phone to one cell antenna imagine using the simple formula time x speed = distance i.e. if it takes 2 seconds to ping an antenna with a signal travelling at 1km/ second then the distance from the antenna is 2km.
  • Do the same thing to another antenna and just to be sure a third tower.
  • Let’s assume that we can establish the rough direction each antenna is from us and now if we were to draw a line that shows the distance calculated from each antenna then where the three lines intersect is the triangulation point of the cellular phone.
This is not exact or at least as exact as GPS but it works. More to follow in part 2.

I don't think that they use the time it takes for the signal to travel as a measurement. There are 2 problems with that…

Firstly, the phone and base station don't have a single clock that can be used for reference, so you'd have to work out the round-trip time, which means sending a signal and waiting for it to come back. This isn't a major technical problem, but it makes things a little more complicated.

Secondly, the time it takes for the processing at each end means that it is difficult to calculate how much of the time was actually the signal travelling, especially as both ends are capable of multitasking and might have queued the request for a short time.

I think the way that they actually calculate it is based on relative signal strength as opposed to timing. I'm not sure though.