GPS is quite simple in operation. It's based on a network of 24 Navstar satellites owned and operated by the U.S. Department of Defense. Each satellite has four atomic clocks on board. We need only one, but having three extras for reliability is a great idea considering the cost of launching satellites.
Anyway, an atomic clock is very, very accurate, and each satellite simply sends the value of this clock down to Earth on a continual basis. We know precisely where each satellite is at any given moment, and thus we know the position of any given satellite relative to the others (up to 12 out of the 24) that can be seen at any point on Earth at any given moment in time.
Thus, if we have a receiver on Earth that can see at least four of the satellites at any moment in time, we can determine the location of the Earth station with great accuracy -- within a few meters is typical. While I'm an automotive purist and in general don't like gadgets in my car, I just got a car with built-in navigation, and I love it. I do get lost a lot, which used to be fun. Now getting lost is hard, and getting unlost is easy.
The problem with GPS is that it only works well outdoors, because the receiver needs a clear view of the sky. Indoor positioning requires other techniques. Perhaps surprisingly, unmodified wireless LAN hardware can be used, with appropriate software, to determine the location of any Wi-Fi device. I say "surprisingly" because I assume that most people would be surprised to find that a product designed for broadband data networking can be used for location and tracking with no hardware changes. Some of the companies active in this space are AeroScout Inc., Ekahau Inc., PanGo Networks Inc. and Newbury Networks Inc.
How does WLAN-based location and tracking work? Here are several possible techniques:
-- Time difference of arrival, which looks at the amount of time it takes signals from two or more access points (AP) to reach an individual client and does a speed-of-light calculation.
-- Angle of arrival, which measures the angle of a given signal received at multiple APs; a little triangle-based geometry, and we have the location.
-- Signal strength, which is normally an unreliable mechanism for determining relative location, since radio waves fade quite dramatically over distance, and other forms of fading, such as that resulting from multipath, can severely distort a signal's amplitude. But if we take enough samples over a period of time (even a couple of seconds), we can use signal strength to quite accurately determine location.
All these techniques work best when we can calibrate a given environment, using a kind of site survey to take readings at different locations and then build a database of values that aid in navigation, if you will.
Skyhook Wireless Inc. uses a related technique called radio frequency fingerprinting to extend the idea of WLAN-based location and tracking into metropolitan areas. It literally drives through metropolitan areas and notes the pattern of signals present from APs that just happen to be installed in a given area. Note that Skyhook doesn't use the APs for anything other than the signals it sends, and no traffic is sent over these APs. They are used quite literally as beacons: When a Skyhook client is in a given location, the client can use the pattern of AP signals to determine, quite accurately, his location.
Note that a key advantage of WLAN-based techniques is that they are totally unrelated to GPS. GPS isn't required at all, nor is a clear view of the sky. We've toyed with the idea of using a combination of the two to provide world-coordinate-based location with accuracy of a few meters to indoor locations. This is done by establishing the location of a given AP with respect to GPS or other Earth coordinates, and then using a WLAN location technique to dereference off this point -- voila, real-world coordinates everywhere indoors, in real time.
WLAN location and tracking systems can be used for security, service discovery and location-based services. They also can be used to locate and track high-value people and things (like interns or defibrillators in a hospital) and to track shipping pallets in manufacturing and logistics operations.
A big question in front of us, however, has to do with the politics of such functionality. Might we be violating someone's privacy? Perhaps you saw Sprint Nextel Corp.'s announcement of its "Family Locator" service, which allows the location of a cell phone handset to be tracked and displayed, ideally, just to authorized users. While there are accuracy issues with the Sprint product, I expect them to be resolved faster than the political questions. But that's a topic for another day, along with the whole field of location-based services, which I'll cover in a future column.