The ABCs of A-GPS

I own two GPS-enabled digital cameras: a new Panasonic Lumix DMC-ZS7 (12 MP); and a used Apple iPhone 3G (2 MP). Which camera is better for geotagging photos? The iPhone 3G. Why? In a word (an acronym in this case), A-GPS.

Beginning with the iPhone 3G, all iPhones feature a built-in GPS receiver (GPSr). The GPSr listens for GPS satellite signals and triangulates the position/location of the phone. Apple iPhones use Assisted GPS, also known as A-GPS, meaning the device makes use of a data connection -- either Cell or Wi-Fi -- to download assistance information such as GPS satellite ephemeris data, and location assistance. These three positioning technologies -- GPS, Wi-Fi, and Cell, or what I referred to as the "ABCs" -- are complementary: Each technology has advantages and disadvantages; together they enable the iPhone to determine location accurately and quickly, while making more efficient use of battery power. Let's take a quick look at how Cell positioning and Wi-Fi positioning work to enhance GPS positioning.

Cell Positioning

Mobile phones have a built-in cellular radio that communicates with a nearby cell site, also known as a cell tower. Each cell site has a unique identifier; your mobile- or cell phone knows the identifier for the site with which it is connected. By querying a database of cell site locations (stored in "the cloud"), it is possible to infer the approximate location of your cell phone. Location accuracy depends upon cell site density: Cell positioning is usually more accurate in urban areas (higher density) than in rural areas (lower density). Cell positioning works anywhere there is cell coverage (including places where GPS positioning doesn't work as well, such as "urban canyons"), and it works well indoors -- a big disadvantage of GPS positioning.

Wi-Fi Positioning

All Wi-Fi access points and Wi-Fi hotspots have a unique identifier. The iPhone scans to see which advertised Wi-Fi networks are within range of the device, but rather than associating with (or joining) a network, the phone queries a database of Wi-Fi network locations in order to find the nearest networks. Wi-Fi positioning is usually more accurate than Cell positioning because Wi-Fi networks have a smaller range (typically less than 100 meters). Also, Wi-Fi positioning works well indoors where most Wi-Fi networks are located.

In places where there is a high density of W-Fi networks, Wi-Fi positioning works remarkably well. Several months ago, I experimented with GeoLogTag Free, an app for geologging tracks and geotagging photos. I field tested GeoLogTag's geologging function using my Apple iPod touch (2nd generation), a Wi-Fi only device. Afterwards, I used Google Earth to display the geolog captured by GeoLogTag (blue line) and annotated my actual track (red line). As shown in a screen capture of Google Earth, the two tracks coincide reasonably closely. [Editor's Note: I used GeoLogTag Free on my iPhone 3G to geolog a similar GPX track; as shown in a screen capture of Google Earth, track accuracy is significantly better using A-GPS rather than Wi-Fi only.]

Summary

A comparison of the three positioning technologies reveals that GPS positioning is the most accurate (within five- to 10 meters of the actual location), Wi-Fi positioning is less accurate, and Cell positioning is least accurate. In this case, high accuracy comes at a high cost: GPS positioning takes the most time to acquire a position fix (ranging from 10-20 seconds when the satellite ephemeris is current, to 10-20 minutes when it is not) and consumes the most power. Wi-Fi positioning takes less time and consumes less power than GPS positioning. Cell positioning takes the least time and consumes the least power.

So, what's the bottom line? An Assisted GPS-enabled device such as the iPhone is able to determine location more accurately, more quickly, and more efficiently (in terms of power consumption and battery life) than a GPS-enabled device such as the DMC-ZS7. Since accurate, quick geolocation of photographs is very important to me, the digital camera I use more often is the iPhone -- the one I usually carry with me wherever I go.

Tech Tips: Does your device use A-GPS? See a List of devices with Assisted GPS.