Various military weapons including icbMs, cruise missiles and precision-

guided munitions. Artillery projectiles with embedded GPS receivers able

to withstand accelerations of 12,000g's or about 117,600 meters/second2

have been developed for use in 155 mm howitzers.

● Search and Rescue: Downed pilots can be located faster if they

have a GPS receiver.

● Reconnaissance and Map Creation: The military use GPS exten-

sively to aid mapping and reconnaissance.

● The GPS satellites also carry a set of nuclear detonation detectors

consisting of an optical sensor (Y-sensor), an X-ray sensor, a dosimeter,

and an electromagnetic pulse (EMP) sensor (W-sensor) which form a ma-

jor portion of the United States Nuclear Detonation Detection System.

Fortunately, an executive decree in the 1980s made GPS available

for civilian use also. Land-based applications are more diverse. The scien-

tific community uses GPS for its precision timing capability and a myriad

of other applications. Surveyors use GPS for an increasing portion of their

work. GPS offers an incredible cost savings by drastically reducing setup

time at the survey site. It also provides amazing accuracy. Basic survey

units can offer accuracies down to one meter. More expensive systems can

provide accuracies to within a centimeter! Recreational uses of GPS are

almost as varied as the number of recreational sports available. GPS is be-

coming increasingly popular among hikers, hunters, snowmobilers, moun-

tain bikers, and crosscountry skiers, just to name a few. GPS is rapidly be-

coming commonplace in automobiles as well. Some basic systems are al-

ready in place, providing emergency roadside assistance at the push of a

button (by transmitting your current position to a dispatch center).

The NAVSTAR system (the acronym for Navigation Satellite Tim-

Ing and Ranging, the official u.S. Department of Defense name for gps)

consists of a space segment (the satellites), a control segment (the ground

stations), and a user segment (you and your GPS receiver).The space seg-

ment, which consists of at least 24 satellites (21 active plus 3 operating

spares) is the heart of the system. The satellites are in what's called a «high

orbit» about 12,000 miles above the Earth's surface. Operating at such a

high altitude allows the signals to cover a greater area. The satellites are

arranged in their orbits so a GPS receiver on earth can always receive from

at least four of them at any given time. The «control» segment does what

Its name implies — it «controls» the gps satellites by tracking them and

then providing them with corrected orbital and clock (time) information.

There are five control stations located around the world — four unmanned

monitoring stations and one «master control station». The four unmanned

receiving stations constantly receive data from the satellites and then send

that information to the master control station. The master control station

«corrects» the satellite data and, together with two other antenna sites,

sends («uplinks») the information to the GPS satellites. The user segment

simply consists of you and your GPS receiver. As mentioned previously,

the user segment consists of boaters, pilots, hikers, hunters, the military

and anyone else who wants to know where they are, where they have been

or where they are going.