Location, location, location. It's a crucial factor, whether it involves real estate or simply where you're standing. Using technology to determine a location isn't all that new a concept. For years, people have used multiple receivers with directional antennas and triangulation—or radiolocation—to track down anything that emits a radio wave. And since World War II, radar has been indispensable within military operations. Now, turn things around. What happens when your technology can determine your own location? Some of these electronic methods have been around for ages, such as Loran for ship location. And then there's the 24-satellite Global Positioning System (GPS), which has been in operation for a while, too. At first, GPS was built for the military. But now hundreds of civilian applications exist, and it's become the heart of many new commercial location technology systems.
GPS 101
GPS is, in fact, the 800-pound gorilla of location technology. Its 24 satellites operate in six planes positioned 55° with respect to the equator (Fig. 1). The satellites have a period of 12 hours at a height of 20,000 km or about 11,000 miles. Each satellite broadcasts signals in the microwave L band (1.57542 and 1.2276 GHz) that contain codes and time information, all of which can be picked up by receivers anywhere on earth.
At any given time, four to six overhead satellites allow the receivers to select three or four signals that are used to make calculations, which in turn yield latitude, longitude, altitude, and velocity information. Because the signals are synced to onboard atomic clocks, they provide ultra-accurate timing information.
The level of location accuracy varies with the exact location and the receiver, but it's typically in the 10- to 50-m range. The GPS system is maintained by the 50th Space Wing's 2nd Space Operations Squadron at Schriever AFB in Colorado. The military uses GPS for navigation, search and rescue, and precision targeting of bombs and missiles. Civilian uses include navigation, surveying, and cell-phone location.
The U.S. Coast Guard's NAVCEN operates the Coast Guard Maritime Differential GPS (DGPS) Service, which consists of two control centers and over 60 remote broadcast sites. It broadcasts correction signals on marine radiobeacon frequencies to improve the accuracy of GPSderived positions. It boasts 10-m accuracy in all established coverage areas. Usually, though, the positional error of a DGPS position is 1 to 3 m, greatly enhancing harbor entrance and approach navigation. Also, a special receiver is required to use DGPS.
A newer version of GPS is Assisted or Aided GPS (A-GPS). Pioneered by Qualcomm, it improves a GPS receiver's ability to produce accurate location information, even when it's out of sight of satellites. Typically, most GPS receivers can't pick up the signal indoors or in the "canyons" of large cities.
A-GPS is used in many newer cell phones with Enhanced 911 capability. Special fixed GPS receivers in local areas near the cell carrier acquire the satellite data and send it to the GPS-enabled cell phone via the network. This ultimately speeds up acquisition and computing time. Originally, these complex GPS receivers cost thousands of dollars each. Today, you can buy a basic satellite receiver for less than $200. Furthermore, semiconductor manufacturers can now supply a GPS receiver on a chip for less than $10, making it possible to build GPS capability into many other items. This has spurred GPS usage in a wide range of location services.
Dozens of different handheld GPS receivers, providing both latitude and longitude information, proliferate today (Fig. 2). But who carries around a map with such coordinates? Consequently, receiver manufacturers provide maps tied to the satellite data. The receiver's LCD screen displays a map of the immediate area with your position noted.
Such receivers from Garmin, TomTom, and other manufacturers cost about $1000, but they're indispensable if you absolutely have to know where you're going. Some receivers let you map a route to your destination, and the receiver provides step-by-step directions. This capability is now built into many newer highend luxury cars, too.
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