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posted by LaminatorX on Wednesday May 21 2014, @03:54PM   Printer-friendly
from the Radio-Gaga dept.

The reason why FM receivers are present on smartphones is that they can be used to locate your position by noting a simple thing as signal strength of transmitters. More advanced methods makes use of SNR, frequency deviation and multipath interference characteristics. And the same method can be used for WiFi which of course makes collection of such data very useful for localization purposes where GPS etc isn't useful. Arrival time of a radio signal that is reported to the operator from many devices may also be used for the same purpose.

 
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  • (Score: 4, Informative) by kaszz on Wednesday May 21 2014, @04:44PM

    by kaszz (4211) on Wednesday May 21 2014, @04:44PM (#46015) Journal

    The difference being that with FM it's possible to locate indoors when cell transceivers are out of reach. And to do it with a room to room granularity.

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  • (Score: 4, Informative) by frojack on Wednesday May 21 2014, @08:55PM

    by frojack (1554) on Wednesday May 21 2014, @08:55PM (#46130) Journal

    Hold on there...

    Once you are indoors all you have is a relative strength indicator that can no longer be trusted, because some or all of your FM will have to come through the walls, which adds a variable and uncertain degree of attenuation.

    And this is true of being in a car, a forest, behind a hill or having any dissimilar amount and type of material (metal, wood, glass) between you and the station. These all attenuate signals to a differing degree. You might be in front of a window facing one tower, and behind a wall facing a different tower.

    Similarly, Arrival time of FM signals have no built in time reference [wikipedia.org]. (They don't contain a time code).

    Furthermore, when you actually look around a big city, you will find the vast majority of transmitter towers located on the same hill, which pretty much eliminates most attempts to triangulate.

    And finally, the FM receiver needs an antenna, (your ear-buds wire) in order to deliver anything approaching a usable signal.

    No, An FM tuner is in your phone because it is incredibly cheap to bundle it into either the cellular receiver or the wifi chipset. (These things are almost never a stand-alone chipset).

    This guy did a research paper. That's all. It doesn't exist in the real world.
    (There isn't a shred of evidence to support you last sentence in TFS.)

    Read the PDF you posted. He did this work all in a 12x12 static test room, with known transmitters, broadcasting known test-tones, known receivers, (his own) at set distances, through consistent materials.

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    • (Score: 2) by mcgrew on Thursday May 22 2014, @01:07AM

      by mcgrew (701) <publish@mcgrewbooks.com> on Thursday May 22 2014, @01:07AM (#46204) Homepage Journal

      An FM tuner is in your phone because it is incredibly cheap to bundle it into either the cellular receiver or the wifi chipset.

      Exactly. You're talking about adding a single capacitor to the receiving circuit. You can't get much cheaper than that.

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    • (Score: 2) by Foobar Bazbot on Thursday May 22 2014, @05:10PM

      by Foobar Bazbot (37) on Thursday May 22 2014, @05:10PM (#46459) Journal

      I mostly agree with you, but a few nitpicks...

      Similarly, Arrival time of FM signals have no built in time reference. (They don't contain a time code).

      True, at least for analog FM (I'm unsure regarding the various associated digital streams, such as RDS , DirectBand, etc.). However, they do carry a mostly non-repeating stream of data, and you can compare that to the same data recorded at a known distance from the tower. (Processing can be done at either end.) The communication with the known-location receiver may be a problem for real-time location, although when the same chipset contains an always-on link to the nearest cell tower, it's not wholly infeasible as part of a "lawful intercept" system.

      Furthermore, when you actually look around a big city, you will find the vast majority of transmitter towers located on the same hill, which pretty much eliminates most attempts to triangulate.

      Depends greatly on the size of the city, what local terrain is like, and how close nearby cities/towns are, but in the moderately populated, gentle hills of the part of the midwest I grew up in, one can usually receive tolerable signals from a couple of nearby towns.

      No, An FM tuner is in your phone because it is incredibly cheap to bundle it into either the cellular receiver or the wifi chipset. (These things are almost never a stand-alone chipset).

      I absolutely agree. However, it's not 100% clear that, being included because it's so cheap/easy, it isn't then opportunistically used to augment location data.

      This article does basically nothing to prove it, but it's distinctly possible that FM signals are used to help locate some mobile phones in some circumstances.

      • (Score: 2) by frojack on Thursday May 22 2014, @06:07PM

        by frojack (1554) on Thursday May 22 2014, @06:07PM (#46477) Journal

        This article does basically nothing to prove it, but it's distinctly possible that FM signals are used to help locate some mobile phones in some circumstances.

        True. But "Possible" covers a wide range of supposition. It might be possible but nobody is doing this in cell phones.

        It is surely "possible", and the process of using such has been recently patented [uspto.gov].

        Although that patent really only covers using this as a basis to get gross location (probably 10s of miles) as an aid to guesstimating location for jump-starting Assisted GPS. Further, the patent in question contains the seeds of its own uselessness, because it specifies a network connection to the internet, and if you have that, you already have either wifi or cell towers, which are already used in Assisted GPS, and either provide more accurate information.

        But the current crop of phones on the market don't have any antenna connected to the FM receiver unless you plug in your headphones, so they couldn't get a signal good enough to track anyone not having their earbuds plugged in.

        FM Transmitter locations are quite well known, (and public [fmscan.org]), Nobody I've ever heard of is monitoring and timing RDS signals of FM transmitters to arrive at a time queue. So the idea of a usable time signal to calculate distance seems extremely unlikely.

        That leaves relative signal strength alone, and lots of math, and a HUGE database, or an internet connection, which immediately obviates the need of FM.

        Note that FM is often delivered with local repeaters expanding a market area, each with their own built in delay, and signal strength. Some of these are shared by more than one station.

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