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 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.
Some cell-phone companies also offer GPS mapping services. Verizon's VZ Navigation service costs $10/month. Use your cell phone like a GPS receiver, and onscreen maps show you the way. Sprint Nextel provides a similar service for about the same price. Both carriers offer high-end phones already replete with a GPS receiver, thanks to the Federal Communications Commission's Enhanced 911 requirement. You also can install navigation software like ALK CoPilot LIVE, Destinator SP, and TomTom Navigator 5 on your own smart phone.
Some carriers even offer similar services to help parents locate their kids. First, the kids need to carry cdma2000 cell phones equipped with GPS from Sprint Nextel or Verizon. Sprint's Family Locator service costs about $10/month for up to four phones. Parents use their own cell phone to call the service, which reports the location child's cell phone using a nearby address and landmarks. It also can send a text message to the phone indicating that the tracking call was Nextel offers a similar unlimited service for month. Disney's Mobile family Locator Service operates in the same way as well.
The Impetus For Location Technology
In 1996, the FCC required all cell phones to include technology that makes it possible to locate them when a 911 call is made. Known as Enhanced 911 (E911), it became necessary after the number of 911 calls from cell phones exceeded a third all 911 calls. The 911 Public Safety Answering Points (PSAP) can easily locate calls from fixed phones whose addresses are automatically attached. A cell call, of course, can come from anywhere.
FCC expected to fix this problem by October 2001. Yet the carriers and phone manufacturers come close, so the deadline was extended to 31, 2005. But full compliance still hasn't been resulting in many waivers and some fines.
The carriers could go with one of two solutions, either via a handset or a network. The handset solution uses an internal GPS receiver that sends its location information back to the network, which forwards it to the PSAP. Carriers offering cdma2000, such as Verizon and Sprint Nextel, take this approach using Qualcomm chips. These chips also include the A-GPS feature. The big problem has been getting customers to give up their old phones and buy new ones that incorporate the new technology.
Cingular and T-Mobile chose a network solution for their GSM networks. They use a system called uplink-time difference of arrival (UTDOA). Developed by TruePosition, it employs special receivers called location measurement units (LMUs) on every cell tower, in addition to the existing antennas.
The U-TDOA system uses a process similar to radio direction-finding triangulation (Fig. 3). It calculates the location of a cell phone by measuring the difference in time of arrival of signals at different receiver sites. The time that an LMU detects the signal is a function of transmission-path distance between the mobile phone and each LMU. Precise timing information must be used to get the desired accuracy.
Most LMUs incorporate a highly precise clock derived from a GPS receiver at each cell site. Using DSP, the system then matches up the signals at pairs of LMUs and determines the difference in reception time. The three LMU measurements allow the system to compute the latitude and longitude of the phone, with an accuracy of better than 50 m. Information then is forwarded to the PSAP.
The beauty of the U-TDOA system is that it doesn't need any cellphone handset additions. It also works with GSM phones, as well as older analog and TDMA phones and the newer WCDMA 3G handsets. This system currently resides in just over 50,000 cell sites nationwide, but over 44,000 more will be installed in the coming year.
The FCC's goal is to have carriers using the handset solution to locate 911 callers within 50 m for 67% of their calls and to 150 m for 95% of all calls. In the network solution, carriers are expected to locate 911 callers to 100 m for 67% of calls and within 300 m for 95% of all calls. See www.fcc.gov/911/enhanced for more information.
Other Wireless Technologies
If you can locate a phone or other device with cell-phone technology, you can pretty much do it with any other wireless technology. For example, radio-frequency identification (RFID) is rapidly replacing bar codes. Most RFID tags are passive, lacking their own power source. Yet when an RFID reader or interrogator unit transmits an inquiry signal, the tags use the signal to power and transmit the unique electronic product code (EPC) stored in their memory back to the reader. Reading distances are short for most tags, from only a few inches to many feet with the newer UHF tags. Active RFID tags use an internal battery to provide power to transmit their code and location to readers hundreds of feet away.
The high cost of tags has held RFID back. But today, passive tag prices are on the decline, while read range continues to improve. Active tags are far more expensive, but they're still worth the investment on larger and more expensive items. RFID is great for inventory tracking, asset management, and shipping and transportation applications. It can track animals and humans, automatically collect tolls, and charge for gas at enabled pumps.
Because of its short read range, RFID is a difficult location technology. With networks of readers in desired locations, it's possible to get a reasonably accurate fix on a person or object within the covered regions using some kind of triangulation scheme. Active RFID tags improve location accuracy, as their longer range and need for fewer readers let the system cover a much greater area.
This technique is more effective with Wi-Fi. While the 802.11 wireless local area network (WLAN) standard is designed for wireless access to LANs and the Internet via hot spots, it also can be used to track laptops or other equipped items. Organizations with lots of Wi-Fi access points can use it to find a particular laptop, PC, PDA, or cell phone. If a Wi-Fi device appears within the range of any hot spot, its location can be identified.
With tens of thousands of hot spots already online in airports, hotels, and other public places, location has become fairly precise. The location possibilities improve further because of the many municipal Wi-Fi mesh networks being installed in cities around the country. G2 Microsystems developed a chip that incorporates a low-power 802.11b transceiver plus 125 kHz and the new 900-MHz RFID circuits for use in new active tags for location applications.
Location technology can take many new forms. A system from eTelemetry combines hardware and software to identify the switch port, department, physical location, and phone number of the person assigned to each IP address. It can give network managers feedback to locate network troubles, determine who is using the most bandwidth, and tackle other monitoring applications.
The evolution of location technology has, in fact, generated a whole new industry called Real Time Location Systems (RTLS). It creates wireless systems using RFID and Wi-Fi or other technologies to locate small electronic devices on people or things at any time. Using arrays of RFID readers and Wi-Fi access points, it's possible to blanket certain areas with location coverage.
Possibilities include hospitals, theme parks, parking lots, factories and warehouses, and postal or other shipping facilities. Research firm IDTechEx predicts this business will increase to $2.71 billion by 2016. The heart of this technology is not just the wireless hardware, but also the software that can collect and make sense of all tag and other data collected by readers.
Location-Based Service (LBS)
With such a huge location system in place, carriers have contemplated just how they can get some commercial payback for their mandatory investment. Cell-phone carriers have the most interest as a way of helping to pay for the huge investment in the E911 requirement. But what about those LBSs? "Concierge" service would identify nearby restaurants, shopping, or whatever. It would put consumers in touch with nearby services and facilities at any time. It even could broadcast advertisements—as you pass a restaurant or a store, it could trigger a related ad. The LBS business has yet to be fully established, but it's poised for action once the E911 service is nearly fully implemented.