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Smartphone GPS questions:

1. Is the practical limit on GPS accuracy/resolution set by the satellites or by the handset?
2. What is the accuracy of the iPhone 4 & 4S, and recent (2010+) high-volume Android handsets?
3. Where can I learn more about this?

Tim McCarty's profile photoAltay Akgun's profile photoDaniel Orellana's profile photoChristophe Versieux's profile photo
Aha! This is interesting. According to this link, the iPhone uses 3 methods to determine location:
1. GPS receiver
2. Cell tower triangulation
3. WiFi triangulation

So I modify my query thus: What is the practical accuracy of the CoreLocation framework in providing physical location of a handset? And how does that compare to the analogous services available on decent Android handsets?
I've heard that iPhones actually get a fix faster than Tom Toms from a cold boot because they can get a fix using cell towers and wifi at the same time as the satellites. Pretty cool.
Hmm. I'm just trying to think of what's a reasonable distance to set some sort of alert to go off notifying you that 'you are here!' Anecdotal experience with GMaps and Mapquest apps tells me that accuracy/resolution is ~10 m outdoors... looking for more info is all.
In practice, GPS accurracy and time to get the first fix are determined by the following factors:
- Satellite geometry: How many satellites are in the visible sky and how they are relatively located in the sky.
- Obstructions: Forest or other kind of obstacles affecting signal reception.
- Multipath effect: Tall buildings or canynons where signal bounces given wrong readings
- Receiver Chipset: Newer chipsets are able to use more satellites to compute location. They are also able to use Augmentation systems (EGNOS/WAAS) and assisted data (AGPS)
- Receiver antenna: Antenna size and geometry.

Other factors such as Cell tower triangulation and WiFi are different location systems, Smartphones use these systems together with GPS data to improve the location estimation in CERTAIN areas.

What does that means? A good GPS device with a modern chip under ideal conditions (open area, no tall buildings, good satellite signal) argues that you can expect aprox 3m accurracy 50% of the time. I'm not that optimistic and I definetely estimate 10m.

In dense urban areas with tall buildings, it could be quite worse and unpredictable. Thats because navigation systems such as TomTom use "matching" algoritms to match the computed position with a preloaded map.

Hope this help.
THanks for the thorough explanation Daniel.
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