Question about GPS vs. A-GPS in terms of an iPhone

Discussion in 'iPhone' started by essential, Jun 6, 2008.

  1. essential macrumors regular


    Apr 8, 2008
    With the Engadget article today that says the iPhone will more than likely have an A-GPS, I did a couple searches on Google to figure out what A-GPS meant. Nothing directly answered my question, so I’m hoping some of you can shed some light.

    1) Is A-GPS a true GPS?

    2) What is better for a phone, what I read made it seem like A-GPS was better, so why doesn’t everything use A-GPS, what’s the drawback?

    3) Is there anything about having a standalone GPS that’s better than an A-GPS?

    4) Will we need to be connected to the cellular network to use A-GPS, or can an A-GPS connect to the satellites directly if we are not within a cellular network? Basically, if we temporally loose our signal, does the A-GPS cut out as well?

    5) Does it being A-GPS and not a standalone GPS increase the likelyhood that we could be charged monthly to use/unlock the GPS since it could be dependent on the cellular network?

    Thanks for any help.
  2. JBaker122586 macrumors 65816

    Jun 21, 2007
    Here's the way I understand it, though I am by no means an expert.

    Basically, the "A" part of AGPS means that it will first do basically what the iPhone's "Locate Me" feature currently does. That way, the satellite knows to only look in the circle that the cell tower gives it, greatly speeding up the time it takes to pinpoint your location. So yes, is it true GPS.

    A-GPS is better for a phone. It already has cell tower positioning in it (required by federal law), so it makes sense to use it to speed up the process. Some phones don't have it because it is a relatively new technology.

    Some A-GPS systems can ONLY work when connected to a cell tower, others will work (albeit slower) while out of cell range. Obviously, no one know what the iPhone will use. It seems that if the satellite already knows where you are, it wouldn't suddenly forget. Therefore, if you're moving, and the GPS is following you, but your cell signal drops, the satellite should be able to continue following you. I believe the cell towers are only used to find you originally.

    And no, I don't think it increases the likelihood of being charged monthly, as the "A" part is basically already done on the current iPhone.
  3. rogersmj macrumors 68020


    Sep 10, 2006
    Indianapolis, IN
    ^ That makes a lot of sense. I have a Garmin Nuvi and if I turn it back on in about the same place it was turned off, it picks up its location in seconds. If I travel for awhile and turn it on some distance away from where it was turned off, though, it can take upwards of a minute to figure out where it is. I was wondering how the iPhone would handle that problem, considering that not many are going to want to leave GPS on all the time.
  4. kdarling macrumors demi-god


    Jun 9, 2007
    First university coding class = 47 years ago
    The phone receives GPS signals, yes. Some can go standalone from there.

    The A (assistance) part means the phone asks for carrier server help. This can consist of such things as: getting a quick general location, being sent satellite info for that location to speed up signal searching, corrections for local atmospheric conditions, and even total calculation based on raw data received by the phone.

    A-GPS was/is much more important when phone GPS chips were dumber and slower.

    Advances in technology allows more powerful standalone solutions. The drawback is that carriers must have A-GPS servers, and that costs them money. Plus you have to be within your carrier range.

    Obviously standalone is desirable if you're on the ocean or otherwise out of cell range from your carrier. Many A-GPS units will fall back to standalone.

    GPS is one-way information broadcast from the satellites. There isn't any transmission back to them. For one thing, such a system would give away the user's position, which is not militarily desirable.

    (The satellite signal is very tiny and can be jammed easily or blocked by buildings. Their output is roughly equivalent to viewing a 25-watt light bulb from a distance of 10,000 miles.)

    Usually, yes. Carrier assistance servers cost money to maintain.

    The latest and greatest solution seems to be E-GPS (enhanced). It still requires carrier servers, but it's similar to Google Maps Locator application: it uses cell tower triangulation to get a first fast fix (or the only fix if indoors sometimes), then goes for more accuracy using GPS.
  5. t0mat0 macrumors 603


    Aug 29, 2006
    If the 3G iPhone was to have GPS, what would it have?
    I'm hearing the lines of argument being expense, power usage, problems with and not being a standard issue on phones.

    Are these strong enough arguments to conclude that the 3G iPhone won't have GPS?
  6. kdarling macrumors demi-god


    Jun 9, 2007
    First university coding class = 47 years ago
    As you know, many location based services (aside from navigation) are going to work best with instant location information. Who wants to wait even thirty seconds for a search result, for example, much less minutes for a cold started GPS response?

    GPS needs updated orbit info about five times a day, and can be unreliable due to obstructions. Therefore A-GPS is a given. A-GPS lets the phone ask the tower for updated orbit, timing, atmospheric information, allowing a GPS lock within 5-6 seconds (still a long time) instead of much longer.

    So if it were to have GPS, I would bet on a combined form of what it has now, and A-GPS: WiFi first if it's on. Then true tower based triangulation (not what Google Maps Locator does). And at the same time, A-GPS going for an exact fix. The best of all worlds.

    The only thing that would keep GPS out would be cost, the same thing that kept 3G out the first time around.

Share This Page