Geolocation and Assisted GPS

C urrently in development, numerous geolocation technologies can pinpoint a person's or ob-ject's position on the Earth. Knowledge of the spatial distribution of wireless callers will facilitate the planning, design, and operation of next-generation broadband wireless networks. Mobile users will gain the ability to get local traffic information and detailed directions to gas stations, restaurants, hotels, and other services. Police and rescue teams will be able to quickly and precisely locate people who are lost or injured but cannot give their precise location. Companies will use geolocation-based applications to track personnel, vehicles, and other assets. The driving force behind the development of this technology is a US Federal Communications Commission (FCC) mandate stating that by 1 October 2001 all wireless carriers must provide the geolocation of an emergency 911 caller to the appropriate public safety answering point (see http://www.fcc.gov/e911/). Location technologies requiring new, modified, or upgraded mobile stations must determine the caller's longitude and latitude within 50 meters for 67 percent of emergency calls, and within 150 meters for 95 percent of the calls. Otherwise, they must do so within 100 meters and 300 meters, respectively, for the same percentage of calls. Currently deployed wireless technology can locate 911 calls within an area no smaller than 10 to 15 square kilometers. An obvious way to satisfy the FCC requirement is to incorporate Global Positioning System (GPS) receivers into mobile phones. GPS consists of a constellation of 24 satellites, equally spaced in six orbital planes 20,200 kilometers above the Earth, that transmit two specially coded carrier signals: L1 frequency for civilian use, and L2 for military and government use. GPS receivers process the signals to compute position in 3D—latitude, longitude , and altitude—within a radius of 10 meters or better. Accuracy has increased substantially since the US government turned off Selective Availability, the intentional degradation of GPS signals , in May 2000. Because no return channel links GPS receivers to satellites, any number of users can get their positions simultaneously. GPS signals also resist interference and jamming. To operate properly, however, conventional GPS receivers need a clear view of the skies and signals from at least four satellites, requirements that exclude operation in buildings or other RF-shadowed environments. Further, it takes a GPS receiver starting " cold " —without any knowledge about the GPS constellation's state—as long as several minutes to achieve the mobile station location fix, a considerable delay for …