Monday, December 13, 2010

iPhone/iPad & GPS

During the last Paris Boat Show in Paris, we got many questions relative to the iPhone and iPad GPS : main is does iPad have a real GPS ?

Indeed, it seems that Apple's first vague announcement wording and technical spec on its site have left people guessing about whether the iPad has a true GPS chip, and if so, on what models.

To sum up :

  • Location data - The iPad only wifi gets location data ONLY from wifi routers (and external BT receivers if used). iPad with 3G will get location from GPS satellites, cell towers (assisted = A in A-GPS) and wifi.
  1. The wifi only iPad does not have any GPS hardware in it. This is the reason GPS does not work on the wifi only iPad. Hence the technical reason.
  2. The 3G iPad has GPS hardware built in. You do not need 3G service or contract to use the iPad GPS. GPS uses satellites to detect and track location, not 3G services. The A-GPS works to provide a faster initial location than using GPS alone. But the GPS works fine without A-GPS.
  • Map data - In order to use the location data in a GeoGarage mobile app map, the map data must be first loaded in the cache memory to be used later with no internet/data connection. This is good for wifi only iPads with an external BT receiver and iPads with 3G (which have GPS) but no data connection. So to use a cache of the map data already downloaded for the area you are trying out, this requires that you pan around marine navigation zone on all the different zoom levels before turning off Wi-Fi.
So only the iPad 3G model has a GPS chip : (see iFixIt)
it is an AGPS chip - a true GPS chip that is assisted by cellular and WiFi signals, a Broadcom BCM4750 Assisted-GPS (A-GPS) chipset which is Broadcom’s first single-chip global positioning system (GPS) solution.
But that doesn't mean it's a low-quality GPS chip that needs help.

A-GPS actually provides superior speed and location (think indoors) capability.
The BCM4750 is tailored for mobile devices which have cellular connectivity for A-GPS to provide faster Time-To-First-Fix (TTFF) than when operating in standalone mode.
So A-GPS just adds faster time to fix from cold and warm startup.
Instead of waiting for the ephemeris/almanac data to come down over the GPS channel (there's some scheduling that dictates this), you just pull it over the Internet and supplant it.
If available, the cellular network also keeps pretty precise time (another thing which gets synced and takes a while on standalone GPSes), which speeds things up.
Lastly, because you know just about where you are (using Cell-IDs and WiFi triliteration through Skyhook -see coverage-/Google/Apple's list of MACs and GPS coordinates), you've already got a starting point for the optimization routine that is GPS.
All those things added up and you get a fix in under 10 seconds instead of minutes.

The AGPS chipset has massive parallel hardware correlators provide faster signal searches, accurate real-time navigation, improved tracking sensitivity and very low average power consumption.

Wi-Fi, Bluetooth, and GPS are all provided by Broadcom
and located beneath EMI shields on the front side of the board :
iPad 3G and iPhone 4 use the Broadcom BCM4750 receiver GPS

About Broadcom BCM4750IUB8 single-chip GPS receiver (sources iFixIt : 1 / 2 )"High-sensitivity, -157dBm assisted acquisition sensitivity (with coarse time assistance) and -162dBm tracking sensitivity, enabling indoor and deep urban operation"
It’s the low power consumption and the excellent tracking sensitivity of -162 dBm which no doubt convinced Apple to adopt the BCM4750 over the Infineon Hammerhead II used in the iPhone 3G and 3GS.
The chip consumes less than 15 mW while navigating with one second map updates which is ahead of other published competitive power consumption figures.

There is a preponderance of anecdotal evidence of this chips improved performance over the Hammerhead II. The published sensivity of the Hammerhead II is -160 dBm which doesn’t seem a lot but remember it is a logarithmic scale.
However, it’s important not to put too much weight on the random, unscientific testing available as they are generally under uncontrolled conditions and do not identify what mobile network assistance is provided.
The published steady state position accuracy of the BCM4750 is 2m.
The autonomous (no cellular network assistance) TTFF is market leading being as fast as 0.5s and providing a quick navigation start.
The excellent tracking sensitivity allows the BCM4750 equipped iPad to detect very weak signals including those partially blocked or reflected by buildings or other structures. (GPSTekReviews)

So Apple iPad purchasers can be confident that they have on-board an excellent GPS receiver to empower all the ever popular location based services and applications loaded on to their device.
 
Apple has integrated the UMTS, GSM, GPS, Wi-Fi, and Bluetooth antennas
into the stainless steel inner frame.

How to determine the accuracy and quality of the GPS signal ?

The GPS signals come down from the visible satellites and the receiver makes its mathematical determination about where is the location (see 'How the GPS works')
To determine the accuracy, the best way to do it is to log NMEA data for several hours with a position each second and then look at the scatter plot with dedicated software (SA Watch or GPSInfo).
The problem with the iPhone/iPad is that there is no way to do that currently, not even to log just the position every second because the programing library doesn't give the access to this data.
The iOS location API only provides "high level location data" : position (Lat/Lon/Alt/Pitch), fix quality, heading and speed but the number of satellites used, signal strength, accuracy data (HDOP or GDOP) are not available.
By the way, accuracy is also influenced by the number of GPS fixes per second delivered by the sensor : the iPhone 3G‘s update rate is typically around 1.5Hz but can reach values up to 2Hz (comparing to basic 1 Hz marine GPS).

iPhone 3GS/4 needs no 3G service to obtain a GPS position fix.
But it also appears that the various bug fix updates to the firmware are having a significant impact, both good and bad, on GPS accuracy.
Users have experimented some problems with GPS reception on iOS 3 and iOS4 as well (see Apple support discussions / Apple discussions), with lots of drop outs, making navigation on iPhone 3GS impossible.
In July, Apple silently booted Skyhook and Google from providing GPS data on iOS3.2 and later, creating their own GPS service instead.

How to add an external kit to plug another GPS ?

It's not possible to connect standard Bluetooth-enabled GPS to an iPhone.
Although iPhone has Bluetooth, the firmware denies access to Bluetooth GPS devices that are not specifically designed to work with iPhone.
With jailbroken devices, some applications allow users to add a Bluetooth GPS receiver on iPod Touch, iPhone and iPad. See :
Note : we do not support jailbroken installations and cannot give any guaranties on proper operation regarding all these devices listed for informational purposes only.

Is it possible to connect an external GPS ?

For the moment Apple has not open up their Bluetooth profile to allow to receive NMEA data from a Bluetooth GPS. (Tekzilla video)
So the other option is to send out raw NMEA-0183 data over TCP/IP via a Wireless Serial Server (a specific article will be written on the subject next) :
Note : in case of NMEA 2000 output, it's possible to convert the N2k data to NMEA0183 before broascasting over WiFi via some converter :

Using your favorite Marine GeoGarage iPhone/iPad application, you can preload a geographical area’s worth of nautical maps while you have a WiFi or 3G connection by scrolling around the map to cover your region of interest.
Then, when WiFi is out of range, and 3G is turned off to avoid roaming fees, Marine GeoGarage app shows it's an useful tool for using nautical charts directly with the embedded ihone/iPad GPS.
But don't forget to consider that our mobile application is not to be used as a primary mean for navigation.Links :

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