The Department of Defense began to deploy the global positioning system known as Navstar GPS in the 1980s. Its purpose was to keep track of troops and equipment and to provide precise positioning for weapons targeting. In order to keep unfriendly nations from having access to its full accuracy, the Defense Department “blurred” the satellites’ signals so that readings would be off by as much as 100 meters. It soon became apparent that the built-in error could easily be circumvented, and the true readings were being denied mainly to American commercial and scientific interests.

In a move hailed by astronomers and others, the Clinton administration halted the signal’s degradation as of May 1, 2000. This effectively improved the accuracy for most civilian GPS systems from 100 meters or more, to less than 20 meters. With the new generation of satellites scheduled for launch beginning in 2003, civilian accuracy will improve to under 10 meters anywhere on the planet.

Navstar GPS is a fleet of 24 satellites that orbit Earth at an altitude of 20,200 kilometers, roughly 12,500 miles, every 12 hours. Their orbital paths take them across a broad band encompassing 60 degrees north and south latitudes, respectively. A minimum of three satellites are in radio range of Earth-based receivers at all times.

Elizabeth Bretz, in the April 2000 issue of IEEE Spectrum, explains how these operate to give a nearly precise location to any receiver picking them up. Each satellite continually broadcasts an identifying code, the date and time. The receiver has a built-in almanac telling it where that particular satellite should be at the stated date and time. The time difference between when the signal was transmitted and received by the GPS receiver gives the distance to the satellite. These two pieces of information, taken together, give the location of the receiver on the Earth’s surface. It takes a minimum of three satellites to give latitude and longitude by a process called triangulation and a fourth one to determine altitude. By continuously monitoring its position, a receiver can provide both speed and direction of travel.

Bretz says air transportation will soon be directed primarily by GPS. Today, most aircraft fly from one radio beacon to another over land, while relying on inertial navigation while over the ocean. The former is time-consuming and the latter can result in errors, putting the aircraft several kilometers off course. Also, the cost of current navigational devices has resulted in approximately 10,000 small airports not having any landing aids at all.

Today commercial airlines

are starting to equip their aircraft with GPS systems and, because a GPS system can be bought for under $2,000, more than 20,000 private planes are

now equipped with them as well. Eventually, says Bretz, every

airport will be equipped with GPS, and radio beacons will become a thing of the past.

Many land vehicles are equipped with GPS systems used for a variety of purposes. Some help drivers find their way in a strange city by giving a street map on a monitor and directions on what turn to take next. Others, used by freight terminals, continuously track the location of delivery trucks. Bretz says that in Charlotte, N.C., the ambulance service is equipped with GPS units so that dispatchers can send the nearest one to the location

of a 911 call and also give continual directions. These will be placed on police and fire vehicles as well.

GPS systems will even play a role in such unlikely activities as farming. An article appearing in a 1992 issue of Agricultural Research explains how one satellite system will be used to monitor large acreages for needs such as fertilizers or the degree of acidity. This can vary greatly from place to place even in a single, large field. Tractors carrying GPS systems will be directed as to how much fertilizer, lime, or other ingredients to spread in a given location.

As with any successful operation, others soon will be duplicating the Navstar GPS. The European Union is planning a 21-satellite system to be named Galileo. This is largely to prevent U.S. companies from dominating European ground-vehicle and airline markets expected to reach $1 billion by 2004.

The Russians already have a system called Glonass, but only nine of its 24 satellites are in operation. With so many systems in place or planned, competition for available frequencies is fierce. For example, Galileo is planning to use a frequency at 1176.45 MHz already in use by Navstar GPS. It may take a United Nations conference to iron out who gets which orbital regions and radio frequencies.

Clair Wood taught chemistry and physics for more than 10 years at Eastern Maine Technical College in Bangor.