I think we both agree on how Skyhook Wireless's Wi-Fi MAC address location lookup works in principle. The difference between your point of view and my own simply lies in whether we're willing to put all of this under the category of triangulation/trilateration. Uncertainty math and fuzzy algorithms are rarely considered essential elements of trigonometry and triangulation. But that's just my take, if you want to disagree that's fine by me as well.
Originally Posted by Viewfly
Ok, let me try and explain the Google thing again, because I don't think I made myself fully clear. Google has no idea where the GSM base stations are located, nor do they care. Google has no concept of 120 degree GSM base station sectors either. All they know is that your mobile is served by a particular sector with a particular Cell ID (CGI) and that their database shows that other mobiles before you have reported being served by that same sector while at such and such location. So they conclude that if other mobiles have seen that sector from points X, Y, and Z, then you're probably somewhere between (or near) those points, and that's what the blue circle indicates. It's a range of locations previously reported by other mobiles while being served from that same sector with the same Cell ID.
I don't know what Google is doing. Perhaps they only use one tower and give you the location of it. But as you said in your memo, the towers are sectored, so at the very least they have found you one angle, since for a 3 sectored tower, that maps you into a one of the 120 degree sectors. Where I live, I normally can be seen by 3 or 4 towers, each with different RSSI dbm's If google were smart (and they are) that information can give you a fuzzy idea of where you might be, better than using the id of one tower. If you were on the fringe of your tower (20 miles out) and not picked up by another cell, you must be pretty far in the boonies.
In my 20.5 mile example cited above, my iPhone was actually able to see several other sectors, all of which were closer to me than the serving sector. However, due to topography of the terrain, it just so happened that the farthest sector had the greatest signal strength and hence was my serving sector. The iPhone definitely saw neighboring sectors in the vicinity, which would have allowed for a much better location fix, but again, the way Google's GSM Cell ID location lookup works is to only correlate the Cell ID of the serving sector with previous reports for that same sector. Since I was in a somewhat contrived situation where my serving sector was actually the farthest away, it seems that they hadn't gotten any previous reports of that Cell ID being seen in that locale, hence the position fix I received from Google was limited to an area much closer to the serving base station, where users would routinely camp on that sector. I only brought it up as proof positive that Google's algorithms doesn't take any distance measurements into account, and hence can't possibly qualify as trilateration.
No question about it. AT&T's e911 system gets a much better location fix than Google Maps, but that's a network based system, and the information is only available to AT&T. The mobile can not determine it's own location using that same system. TruePosition is absolutely based on trilateration and is a proper application of that term, but there's nothing in the iPhone that's in any way special or unique to the TruePosition system. In fact, AT&T specifically chose a network based e911 location system so they wouldn't have to police and retrofit their entire fleet of mobiles deployed in the field, the way VZW, Sprint, ALLTEL, and every other CDMA carrier had to do.
And for certain, the ATT E911 TruePosition system does use timing signals, multiple towers when it can, and trilateration to find you.
Ok, thanks for clarifying that. However, if your comments were geared towards something that I didn't even cover in my original memo, it may have been slightly presumptive and preemptive of you to state that I "made some incorrect statements". If the scope of your statements is unrelated to the scope of my statements, then even if your statements are correct and different from my statements, you can't conclude that my statements were made in error. That's a logical fallacy.
My original comment was geared toward GSM cell tower E911 systems, that do use trilateration. Not really google's method.
Fair enough. I think we thoroughly established what each of us was trying to get across, all true, just referring to different things.
I'm not trying to have a tit for tat argument with you on language. I'm just pointing out that it is a lot more complicated that just returning which single wifi access point you are using, at least in dense urban environments. I concede that I don't know what Google is doing, but real GSM Cell tower location services are a form of tri or multi lateration.