Previous 1 2 3 Next 40 Replies Latest reply: Nov 3, 2008 3:38 PM by Danny Authement
ParagJ_11 Level 1 Level 1 (60 points)
What is the difference between GPS and assisted GPS? iPhone specs says, it has assisted GPS. Does it mean that it will not be able to pinpoint the street location positioning ?

iPhone 3G, Mac OS X (10.5.1)
  • Tamara Level 6 Level 6 (13,730 points)
    GPS uses satellites to get a fix on a position. A-GPS uses cell towers & wifi to get a quicker position while the GPS antenna gets a lock on satellites.
  • neuroanatomist Level 7 Level 7 (31,690 points)
    Yes, A-GPS will pinpoint your location (to within a few meters), and it will do so faster than un-assisted GPS. Accurate location requires a fix on 3 satellites, and the signals do not penetrate buildings (even the roof of a car can attenuate the GPS signal). So, if you are not in view of 3 satellites, A-GPS can estimate your location based on 2 satellites plus cell tower data (less accurate). If you aren't in view of at least 2 satellites, the iPhone uses WiFi (if available in the Skyhook database, I think coverage is minimal in AU) or cell-tower triangulation, which is not very accurate. Lower accuracy is represented by a larger circle on your map.

    Here's some background reading:

    http://en.wikipedia.org/wiki/A-GPS
    http://wmexperts.com/articles/gpsvs_agps_a_quicktutorial.html

    Hope this helps...
  • KBeat Level 3 Level 3 (790 points)
    FWIW: In side by side tests with a Hertz Neverlost system in my rental car, my iPhone 3G was able to pinpoint my location much, much faster at startup, and with the same accuracy. So the "assisted" part really does make a difference in getting your initial location.
  • kevlar x Level 1 Level 1 (0 points)
    I find the gps consistantly inacurate. I live in the uk and everytime I use it in open air it is always 10 - 15 meters out. Looks to me like a software problem, maybe incorrect co-ordinates in the google map app.
  • alba123 Level 1 Level 1 (0 points)
    i find that gps works in the edge mode

    i thought it had to be in 3g

    which is correct?
  • MobileDev Level 3 Level 3 (565 points)
    A-GPS uses cell towers & wifi to get a quicker position while the GPS antenna gets a lock on satellites.


    That describes a hybrid locating system. It is NOT the meaning of A-GPS.

    A-GPS is about GPS. It takes four satellites to get a full position. GPS needs to know which ones are in view, as well as their exact orbit (which constantly changes due to moon/earth/solar influences, so orbit info is updated from ground stations every five hours or so).

    The satellites rebroadcast their orbit data, but only at about 50 bytes per second. It takes about thirty seconds to download a particular sat's info, assuming you know which ones to listen to. (This is why multi-channel receivers are important, so it can try for many sats at one time.)

    Obviously it can take many minutes to "lock" into the satellites needed and get their orbit info. That's where A-GPS comes in. The assistance consists of the orbit information and often a first estimate of location. This allows the GPS to act as if it were already going for a while, and get a position solution within seconds instead of minutes... and using fewer satellites in some cases.
  • Tamara Level 6 Level 6 (13,730 points)
    It can work in either Edge or 3G.
  • steve copeland Level 1 Level 1 (55 points)
    Conventional GPS has difficulty providing reliable positions in poor signal conditions. For example when surrounded by tall buildings (as a result of multipath), or when the satellite signals are weakened by being indoors or under trees. Some newer receivers are better at handling these situations.
    In addition, when first turned on in these conditions, some non-assisted GPS units may not be able to download the almanac and ephemeris information from the GPS satellites, rendering them unable to function until a clear signal can be received continuously for up to one minute.
    An A-GPS receiver can address these problems in several ways, using an Assistance Server:
    The Assistance Server can locate the phone roughly by what cell site it is connected to on the cellular network.
    The Assistance Server has a good satellite signal, and lots of computation power, so it can compare fragmentary signals relayed to it by cell phones, with the satellite signal it receives directly, and then inform the cell phone or emergency services of the cell phone's position.
    It can supply orbital data for the GPS satellites to the cell phone, enabling the cell phone to lock to the satellites when it otherwise could not, and autonomously calculate its position.
    By having accurate, surveyed coordinates for the cell site towers, it can have better knowledge of ionospheric conditions and other errors affecting the GPS signal than the cell phone alone, enabling more precise calculation of position. (See also Wide Area Augmentation System)
    Some A-GPS solutions require an active connection to a cell phone (or other data) network to function, in others[2][3] it simply makes positioning faster and more accurate, but is not required.
    As an additional benefit, it can reduce both the amount of CPU and programming required for a GPS Phone by offloading most of the work onto the assistance server. (This is not a large amount for a basic GPS – many early GPSs utilized Intel 80386-class 16MHz CPUs or similar hardware.)
    High Sensitivity GPS is an allied technology that addresses some of the same issues in a way that does not require additional infrastructure. However, unlike some forms of A-GPS, high sensitivity GPS cannot provide instant fixes when the phone has been off for some time.
  • evdh Level 1 Level 1 (15 points)
    You even don't need edge or 3G
  • dehlers Level 1 Level 1 (0 points)
    Sure you do. Where do you think the map changes come from, they are not stored in your phone. No Edge or 3G or WiFi- NO MAPS
  • clarkkent26 Level 2 Level 2 (355 points)
    dehlers wrote:
    Sure you do. Where do you think the map changes come from, they are not stored in your phone. No Edge or 3G or WiFi- NO MAPS


    EVDH means you can use normal Gprs too when you are not in edge or 3g coverage. The little blue square with no symbols is Gprs.
  • Rick Thalhammer1 Level 1 Level 1 (25 points)
    This discussion is interesting, but I'm curious as to what the iPhone actually has/does. In my experience, it does not have true GPS functionality. Without some sort of internet/phone service, the app, "Maps" cannot determine location even when satellite coverage is unlimited. In other words, when you're out in the woods with an iPhone, the iPhone cannot find your location. Anyone have a different experience, suggesting my device might be malfunctioning? Or is the claim that the iPhone (3G) offers GPS a bit exaggerated/misleading/untrue?
  • tonytone Level 1 Level 1 (0 points)
    Rick Thalhammer1 wrote:
    This discussion is interesting, but I'm curious as to what the iPhone actually has/does. In my experience, it does not have true GPS functionality. Without some sort of internet/phone service, the app, "Maps" cannot determine location even when satellite coverage is unlimited. In other words, when you're out in the woods with an iPhone, the iPhone cannot find your location.

    Exactly, since the only iPhone app currently available that can display your location on a map is Google Maps...and of course as you are probably aware, the map data is downloaded to your phone only if your phone is able to establish a cell or Wifi signal--i.e., the map data (other than possibly any previously-downloaded cached data) is not stored anywhere on the phone.

    I'll let the experts answer the question as to whether the iPhone's GPS radio receiver is allowed to be active even though the phone itself is not getting a cell/wifi signal...
  • MobileDev Level 3 Level 3 (565 points)
    More than one person has reported being able to start Google Maps in an area without cell coverage (so of course they saw nothing), dropped a pin, and then looked at the map when they got back into coverage... and sure enough, the pin was at the right location.

    Their experience indicates that the GPS works outside of communication coverage.

    (The only remaining question is: did it have satellite info cached, that was still good, and thus didn't need to get assistance? Or did it really operate on its own.)
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