iPhone users’ experience with GPS is so quick, so instant-on, that Apple’s Wednesday response about location tracking on iOS might almost seem baffling:
Calculating a phone’s location using just GPS satellite data can take up to several minutes. iPhone can reduce this time to just a few seconds by using Wi-Fi hotspot and cell tower data to quickly find GPS satellites.
Several minutes? Doesn’t my iPhone take just seconds to figure out where I am?
Well, yes… but only when it engages in a set of tricks to avoid a lengthy process that was de rigueur when GPS receivers first appeared. In simplifying matters, Apple’s not being entirely accurate about how this all works and what it’s doing. So let me explain where Wi-Fi and cell phone towers fit into the equation.
Early GPS receivers took 12.5 minutes from a cold start to obtain a lock; later locks in the same region could still take minutes. If you turned a GPS receiver off for a few weeks or moved it more than a few hundred miles, a cold start might be required again.
GPS relies on two factors to create a set of accurate coordinates for where you’re standing: time and space. GPS satellites broadcast precise time signals using a built-in atomic clock along with their current location. They also broadcast the location of all other satellites in the sky, called the almanac.
Every 30 seconds, a GPS satellite broadcasts a time stamp, its current location and some less precise location information for other GPS satellites. It takes 25 of these broadcasts (thus, 12.5 minutes) to obtain the full list of satellite locations. This information has to be decoded for a receiver to then properly interpret signals from the satellites that are within range.
If you know the position of four satellites and the time at which each sent their position information, you—or, rather, your GPS receiver—can calculate to within 10 meters the latitude, longitude, and elevation of your current location along with the exact current time. With three satellites, you lose elevation, but a device can still track movement fairly accurately. Standalone GPS receivers can lock in simultaneously on multiple satellites, and track more than four. Other techniques can improve accuracy, too.
But, heck, I don’t have 12.5 minutes. I’m a busy man! Give me that location faster!
So GPS chip and gear makers came up with a host of ways to shorten the wait, called Assisted GPS (AGPS). Instead of relying on live downloads of position data from satellites, future locations can be estimated accurately enough to figure out rough satellite positions, and get a fix at which point even more up-to-date information is retrieved. These estimates can be downloaded via a network connection in seconds or even calculated right on a device.
The current time can also be used as a clue. With a precise current time, fragmentary satellite data can be decoded to gain a faster lock or figure out the appropriate information to use. In CDMA networks, such as that used by Verizon, GPS-synchronized atomic time is required for the network’s basic operations, making it a simple matter to have such information available. (In fact, CDMA cell towers have GPS units built in to maintain better atomic time synchronization.)
These extras are what makes GPS into AGPS. Though a lot of people misunderstand AGPS and think it’s some faux GPS system, that’s not the case: AGPS requires a GPS receiver to work. Apple’s iPhone and 3G iPad models include AGPS, as do nearly all competing devices with GPS chips, notably Android phones. (AGPS allows the use of much cheaper and simpler GPS circuits in phones, reducing cost and battery drain.)
This is where Apple’s statement on Wednesday deviates from full accuracy. Apple uses AGPS for native GPS-lock improvements, and Wi-Fi network and cell tower locations are additional factors in providing a fast initial connection along with improving GPS accuracy.
Cellular carriers have extremely precise GPS measurements of the locations of all their towers. With a database of such towers, you can take measurements of the signal strength of those within range—which may be dozens—and trilaterate to find an area that overlaps among them. (Trilateration involves overlapping regions to find an intersecting area; triangulation uses the measurement of angles to find a center point.)
But cell towers are too far away from one another to provide GPS-like precision, and they don’t work well in less-populated areas, even suburbs, where less coverage is necessary than in an urban environment.
The flow of iPhone freebies continues with today with Phone Disk, a little Mac utility that enables you to access your iPhone or iPod touch’s file system from your desktop. Mounting your iPhone might sound a bit perverse, but it’s more geeky than it is kinky. With Phone Disk running on your Mac, you simply tether your iPhone or iPod touch it seamlessly mounts your filesystem so you can access your mobile device with your Finder or other desktop application. Why would you want to do this? Well, you could more easily open and edit your photos with your desktop image editing software, like Photoshop, which is much more powerful than any iPhone app for editing photos.