GPS says I'm in a different state

Discussion in 'iPhone' started by Clete2, Apr 10, 2009.

  1. Clete2 macrumors 65816

    Clete2

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    #1
    Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 2_2_1 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5H11 Safari/525.20)

    So I fly fairly often. I have noticed something that happens every time I'm IN the airport. When I use GPS services and I'm in the Detroit airport, the GPS shows me as being in Harrisburg, PA. When I enter the plane, it will properly show me on the runway in Detroit.

    Must be some tower pipeline connections between DWI and MDT? E.g. The GPS falls back to triangulation??
     
  2. CocoaPuffs macrumors 68010

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    #2
    iPhone GPS is AGPS, which uses towers, routers, and etc. to determine your location. So it actually makes a lot of sense that your phone might say you're in a different state when you're in an airport.
     
  3. 1Zach1 macrumors 65816

    1Zach1

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    #3
    I don't think it can use satellites if you are indoors.
     
  4. PoitNarf macrumors 65816

    PoitNarf

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    #4
    This usually happens when a wireless access point moves locations. When the iPhone tries to figure out it's location, it will scan for broadcasting wireless access points. For whatever reason, if it finds a wireless access point and matches it's MAC address to the WiFi location database it assumes that the location of the WAP is 100% correct and does not bother double checking with mobile phone tower triangulation. You're probably indoors and away from a large enough window, so you're not getting a GPS signal at all which would fix the problem (when you're on the plane you probably do get a GPS signal and therefore the correct location).

    So it seems that whatever WAP or WAPs your iPhone is detecting when you try to find your location in the Detroit airport used to be located in Harrisburg. If you happen to know the MAC address of a WAP that reports the wrong location you can fix it here: http://www.skyhookwireless.com/howitworks/submit_ap.php
     
  5. kdarling macrumors demi-god

    kdarling

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    #5
    A-GPS is about the satellites. The other methods (cell, wifi) are not part of A-GPS, but of a total hybrid approach.

    That seems most likely.
     
  6. CocoaPuffs macrumors 68010

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    #6
    Thanks for clarifying, old freakish looking man.
     
  7. Haephestos macrumors regular

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    #7
    The original iPhone uses the cell tower / WiFi network triangulation method. The iPhone 3G actually has a GPS chip and can use satellite triangulation.
     
  8. Steve686 macrumors 68030

    Steve686

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    #8
    The 3G uses cell tower triangulation, as does the V1 iPhone.

    WiFi is NOT triangulation, but address of your router source.

    GPS, err triangulation...won't work without satellite signal. I would say inside an airport without open areas with line of sight to the satellites, you won't get this signal at any strength to give you a lock.
     
  9. kdarling macrumors demi-god

    kdarling

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    #9
    Yes, I suppose that's a bad pic. Updating now to one that includes my wife, who's much better looking than me ;)
     
  10. Clete2 thread starter macrumors 65816

    Clete2

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    #10
    Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 2_2_1 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5H11 Safari/525.20)

    Thanks for the input. I'm back in Detroit now for my return flight. I tried standing near the large windows to see if it would pick up GPS but it doesn't. Seems most likely about the routers. I am not connected to a WAP but I guess it scans the available WAPs. Thanks all.
     
  11. PoitNarf macrumors 65816

    PoitNarf

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    #11
    Yes, it will scan for any WAP MAC addresses it can detect regardless if you're currently connected to WiFi.
     
  12. dave006 Contributor

    dave006

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    #12
    The A-GPS solution in the iPhone 3G uses a combination of resource to determe your location. This is the basis of the iPhone's Location Based Service (LBS).

    A-GPS normally uses a GPS receiver that is mounted on the Cell tower. In addtion the Broadcom Chip in the iPhone 3G uses the advanced WiFi positioning solution provided by SKYHOOK Wireless to receive the MAC address of nearby WiFi access points and comparing them against a known database of geo-located points. This is why an earlier poster suggest that you might want to report the issue you have in the Detroit Airport.

    GPS needs a clear line of sight to the GPS orbital platforms so that is why it works once you get on the airplane and taxi away from the terminal buildings.

    Dave
     
  13. PoitNarf macrumors 65816

    PoitNarf

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    #13
    That doesn't make much sense. Why place a GPS receiver on the mobile phone tower? Just about all towers are stationary (some are mobile for temporary strengthening of signals), so their coordinates are already known and most likely put into a database that the location services check against.
     
  14. dave006 Contributor

    dave006

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    #14
    Yes, that is the exact reason for using the fix towers GPS receiver, for speed and trianglation. A true GPS requires multiple connections to the orbital platforms and up to 15 minutes to aquire a specific location. By receiving the fixed GPS location from the Cell tower, your iPhone 3G takes less time to calculate your position and with the addition of the Hybrid solution from SKYHOOk, it can be even faster to provide the basic "blue circle" for LBS while it generates the "blue pointer" to indicate your location (a faster processor in the iPhone would be great but that also means more power consumption).

    If you have ever use a standalone GPS unit, you would know that it can take up to 15 minutes from a cold start to aquire the GPS signal and calculate your exact postion. :D

    Added Apple Sales link on A-GPS for the iPhone 3G: http://www.apple.com/iphone/features/gps.html

    Dave
     
  15. PoitNarf macrumors 65816

    PoitNarf

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    #15
    Still, a GPS device mounted on the mobile phone tower itself doesn't serve any purpose. All that is required is that the exact location of the tower be stored in some sort of lookup database. My point was that the towers don't really move, so there's no need for a device to constantly keep track of their "non-movement".
     
  16. Steve686 macrumors 68030

    Steve686

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    #16
    To shorten the answer, it is called WAAS enabled GPS when you use ground based stations. Stands for Wide Area Augmentation System. It is rated to give you 3 meter accuracy.

    Currently there are less than 30 or so of these sites in North America(the only place it's available). They actually communicate with geostationary equatorial satellites(one of two) for a GPS lock.

    I don't think the iPhone 3G would be capable of WAAS, but I haven't seen a definitive answer on whether or not it has the tech built in. But I don't think it is so I will say that when Apple advertises A-GPS, it has nothing to do with true WAAS, and is using 3G/cellular(just like the V1 phone or when the 3G is indoors) data as the "assisted" part of it's GPS system and not any GPS antennae that may be on the cell towers.
     
  17. kdarling macrumors demi-god

    kdarling

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    #17
    A lot of misinformation in this thread. Here are some facts:

    * The iPhone uses an Infineon GPS chip. It is simply a receiver; the iPhone cpu does all the calculations and assistance server communication.

    * "A-GPS", as I said above, refers ONLY to GPS based locating. The other methods (WiFi and cell id) are NOT called A-GPS.

    * Cell towers have a GPS receiver mostly for helping with CDMA timing.

    If the carrier has a lot of A-GPS equipped phones (such as Sprint and Verizon do), then they can also use the tower GPS receiver to note any reflection or ionospheric delay corrections... between what the receiver says and the actual tower location... to help correct the phone's GPS output.
     
  18. PoitNarf macrumors 65816

    PoitNarf

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    #18
    Glad to see at least one of us knows what they're talking about. Thanks for the info, it all makes sense now. :D

    And yes, it is annoying that everyone throws all these different location finding methods under the "A-GPS" umbrella, which you have clearly stated only applies when a GPS signal is in fact being used.
     

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