单选题 2. Finding Yourself without GPS
As more mobile phones tap into the Internet, people increasingly turn to them for location-centric services like getting directions and finding nearby restaurants. While Global Positioning System (GPS) technology provides excellent accuracy, only a fraction of phones have this capability. What's more, GPS coverage is spotty in dense urban environments, and in—phone receivers can be slow and drain a phone's battery.
To sidestep this problem, last week Google added a new feature, called My Location, to its Web-based mapping service. My Location collects information from the nearest cell-phone tower to estimate a person's location within a distance of about 1,000 meters. This resolution is obviously not sufficient for driving directions, but it can be fine for searching for a restaurant or a store. "A common use of Google Maps is to search nearby," says Steve Lee, product manager for Google Maps, who likened the approach to searching for something within an urban zip code, but without knowing that code. "In a new city, you might not know the zip code, or even if you know it, it takes time to enter it and then to zoom in and pan around the map."
Many phones support software that is able to read the unique identification of a cell-phone tower and the coverage area that surrounds it is usually split into three regions. Lee explains that My Location uses such software to learn which tower is serving the phone—and which coverage area the cell phone is operating in. Google also uses data from cell phones in the area that do have GPS to help estimate the locations of the devices without it. In this way, Google adds geographic information to the cell—phone tower's identifiers that the company stores in a database. As the database grows, says Lee, the service will become more accurate. It will never be as accurate as GPS, but he expects that it could eventually find a person within a couple hundred meters.