Following my article from 8 November 2009 the accuracy of couple iNavX / iPhone, I tried to explore the subject, and I deliver to you the fruit of my investigations. Be careful it might be long !
The built-in GPS chipset in the iPhone is an A-GPS specially designed for mobile phones. This requires some explanations and a bit of technique.
The A-GPS system :
An example of a built-in A-GPS system is that of the iPhone 3G, that uses a chipset Infineon "Hammerhead II" for navigation.
A-GPS chipsets for smartphones :
March, the PMB2525 "Hammerhead II", developed by Infineon and Global Locate, was launched in January 2007 and fit out the iPhone 3G. It occupies a place < 50mm² and has a sensitivity of 160 Dbm
In June 2007 Broadcom bought Global Locate, and develops the chipset BCM4750, with a surface area of 35mm² and a sensitivity of 162 Dbm, Presented in July 2008, fit out recently TomTom.
On his side, Infineon and Epson associated deliver a new chipset PMB2540 "XPOSYS" in February 2009, six months before the launch of the iPhone 3GS. He surpassed his predecessor with a place occupied < 26mm² and a sensitivity of 165dBm.
It remains unclear if this last one has replaced the Hammerhead II on iPhone 3G, Apple keeps silence about it, but it is very likely when you compare the respective sizes of the GPS modules on the motherboards of the two devices and the clear improvement in the GPS position compared to the previous model. What is certain, however,, it's the presence of Broadcom's WiFi/Bluetooth combo module BCM4325 on the 3GS.
You should know that in an email exchange with a blogger in July 2008, Bob Borchers, Senior Director of Global Marketing for iPhone Products, indicated that the chipset of the 3G iPhone was low and that the antenna was small. He had also specified that the circle of visible metal around the camera lens was part of the GPS antenna. That time, Apple does not consider the iPhone as GPS to provide a step by step navigation solution (turn to turn).
iPhone software processing :
The iPhone uses a mix of two mechanisms of location : one based on the geolocation provided by cellular antennas, and the other based on the built-in A-GPS chipset.
So when location services are turned on the first time, The iPhone gives preference to the location provided by the nearest cellular antenna, but with an accuracy of up to a kilometer or more because one can be anywhere within the radius of that antenna. This location is represented by a blue button (blue-dot) circled by a bluish zone of varying extent depending on the calculated precision.
Once the iPhone fixes the position given by GPS, it takes precedence over the cellular network, The position icon then turns into a blue button surrounded by a blue ring animated by a rhythmic expansion movement.
Sometimes, even in an unobstructed space, the iPhone refuses to switch to GPS mode and you can wait a long time before getting the desired accuracy. The thing to do in this case seems to be to restart location services in the iPhone's settings and then relaunch the app. However, Apple seems to use some magic code not documented in Maps application (Google Maps) which allows you to always quickly obtain a precise position (GPS). One way is to first look up the position in Maps (icon at the bottom left) and then launch the application you want to use (iNavX, in example). It works in surprising ways !
Provisional conclusion :
It is therefore a safe bet that with chipsets with such theoretical performance on manufacturers' documentation, the problems of inaccuracies and/or temporary stalls that I have noticed are related to, not to the material integration, but to the software processing designed by Apple. Mainly processing cellular/GPS positioning mode selection.
With the iPhone 3GS, the accuracy of the position from the GPS is excellent, less than 5 meters high, found out with Maps on a long motorway journey. But as soon as you are in the city, where cellular signals are predominant, Accuracy sometimes degrades when switching to cellular geolocation mode. In coastal marine navigation, the same phenomenon must probably occur when the iPhone comes within range of a strong cellular network.
Unfortunately, There is no irrefutable evidence to date that allows me to support this inference. Perhaps we will see an improvement in a future iPhone firmware update, Who knows ?
[Update September 24 August 2010]
I have noticed at sea that the reception of the GPS has been much improved since the iOS update 4. No more unexpected fix loss, even near the coast where the cellular locaion can be activated. My hypothesis was therefore well founded, Apple has made a definite improvement in the software processing of location services.
[Update September 20 August 2011]
See the recent post : iPhone built-in GPS : a formidable accuracy !
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