Where in the world?

Two current everyday uses for the technology are in a hide-and-seek game called geocaching, and to allow students to track University buses.

GPS devices use a process called triangulation to locate a point within centimeters of its actual position. Satellites orbiting the earth continually transmit signals telling devices exactly where they are located.

“From the coded information that a (GPS device) receives, it can compute the difference in distance between it and each one of the satellites it sees,” said Chris Ruf, a professor of electrical engineering and atmospheric, oceanic and space sciences.

The GPS device must be able to receive and transmit information to at least three satellites in order to calculate its distance from them. These distances are then used to draw a sphere around each respective satellite, which represents the center of a sphere. Next, the device calculates the distance between each satellite and itself, in order to define the intersection point of all three spheres.

The intersection point of the three spheres will then give the device’s exact position.

Because the process requires at least three lines of sight, several satellites are required to operate the computers effectively.

“There is a constellation of 24 GPS transmitter satellites in orbit” Ruf said. “Because of the way their orbits are interleaved, a GPS receiver on the ground can usually see three or more of them at any one time.”

Before 2000, the Department of Defense attempted to limit public use of GPS devices by decreasing their accuracy, making the process of pinpointing a location more difficult.

Former President Bill Clinton chose to abolish this policy in favor of an attempt to “encourage the acceptance and integration of GPS into peaceful civil, commercial and scientific applications worldwide.”

Little did Clinton know that his policy change would lead to the development of a highly popular activity called geocaching.

Within three days of this change, a cache had been hidden in Portland, Ore. for GPS users to locate, and was visited by two different GPS users the same day. Thus the chase now pursued by several University students was born.

Participating individuals setup caches – which are small storage boxes – all over the world and list the location of these boxes on the Internet at www.geocaching.com. Currently, 227,249 caches have been placed in 220 countries.

Anyone with access to a GPS system can then use the Internet coordinates to search for these caches. Once found, a cache may provide the visitor with a wide variety of rewards. According to the online website, companies will sometimes sponsor hard-to-find caches and load them with large monetary rewards.

Business School senior Mark Loesel, who participates in the activity, said that the typical cache will contain items that cost less than a dollar like, “golf balls, key chains, baseball cards, and bouncy balls.”

Loesel said that caches usually contain, “a pad of paper to log the date, what you took, and what you left.”

GPS devices that can be used for geocaching cost about $90. Students like Loesel usually go in groups of two to three looking for caches around Ann Arbor.

GPS is also being integrated into the University bus system. Ruf is leading a project, called Magic Bus, which will integrate the navigation system with the campus bus system.

Ruf said this project has two objectives. “One is to help transportation services monitor and optimize their bus routes and schedules.”

“Another is to provide riders with helpful real-time information about where the buses are and when they will arrive at particular stops,” he added.

The system will be accessible to students in a variety of ways: a live web feed, text messaging updates, and an automated AOL Instant Messenger buddy.