EYES in the SKY.

It can navigate your hike through a thick forest or a drive to a friend's party, it can track the whereabouts of a wild elephant or your lost dog. Soon it could even tell your parents exactly where you are, 24 hours a day!

Think you can hide from the global positioning system (GPS)--the world's most powerful navigation and homing device? Read on ... and think again.

GPS, a network of 24 satellites and on-Earth receivers, has had its eagle eyes on the globe for nearly 20 years. But its inventors, the U.S. military, kept a super-precise version of the technology--used to pinpoint troop and missile movements--under wraps until last May. That's when former President Clinton signed an executive order that anyone--even you!--can use "military-strength" GPS to locate the position of anything on Earth--within just 6 meters (20 feet).

How does GPS work? Well, imagine you're motorcycling through Egypt's Sinai desert, as German adventurer Sebastian Birnbach did in 1995. Since an empty desert isn't known for road signs, Birnbach used a civilian version of GPS by first attaching a walkie-talkie-size instrument called a receiver to his handlebars. Eighteen thousand kilometers (11,185 miles) above his helmet, three GPS satellites made two complete loops around Earth each day. As they orbited and the Earth rotated, the satellites passed over every inch of the globe. The satellites beamed radio waves--a type of invisible energy called electromagnetic radiation (EM)--toward Earth.

Whenever Birnbach pressed a button to determine his location, his receiver "locked on" to the three GPS satellites that transmitted the clearest radio signal. Microchips inside Birnbach's receiver measured the time it took the radio signal to zip from satellites to the receiver. Since all EM waves travel at 299,337 km (186,000 mi) per second, the receiver used the wave's speed and travel time to calculate the distance from the satellite to Bimbach's receiver. (It's like this: If a car traveling at 80 kph takes two hours to reach you, you know the car is 160 km away.)

Since GPS satellites orbit in a predictable path around Earth, Birnbach's receiver was always aware of its location in relation to the satellites. The receiver displayed his location on its small screen as latitude and longitude (distance measured in degrees on maps) or as a symbol on a computerized map of the area. To calculate Birnbach's velocity (speed and direction), the receiver simply took new readings constantly and updated his position.

Use the diagram at left to find out why three satellites are necessary to accurately pinpoint one spot on Earth. Then learn how scientists, police officers, and everyday people are making GPS their eyes in the sky!

IT TAKES THREE

GPS triangular your position quicker than you can say "Find me!"

STEP 1 Your receiver measures your distance from Satellite 1: 18,000 km (11,185 mi). The computer determines all locations on Earth that are 18,000 km from the satellite. That's a lot of places--in fact, every point at which the blue cone above hits Earth. The blue lines represent four possible locations.

STEP 2 To narrow down the possibilities, your receiver takes a reading from a second satellite, shown in red. Data: your receiver is 20,000 km (12,428 mi) from Satellite 2. So out of the four locations in Step 1, only two of them are both 18,000 km from Satellite 1 and 20,000 Km from Satellite 2.

STEP 3 Final reading: Satellite 3. You're 18,800 km (11,682 mi) from this one. Bingo! Only one of the two locations from Step 2 is also 18,800 km from satellite 3. You've found yourself!

Sometimes GPS uses 4 or more satellites for even more precise measurements, or to calculate your altitude as well, says GPS engineer Kathy Anderson. GPS receivers have measured the heights of Mt. Everest and the Washington Monument!

* Want to try triangulation yourself? Ask your Teacher for the "Find Yourself!" handout. (See page 5 in the Teacher's Edition.)

HOW GPS IS USED

Big Brother? Would you want a dime-size computer chip implanted under your skin that uses GPS to track your travels? Engineers are working on a prototype of this technology, called Digital Angel[TM]. It could help find escaped criminals, soldiers in combat, or your wandering pet. The body heat-powered chip can also monitor the wearer's heartbeat and other vital signs. If your grandmother had a medical emergency, Digital Angel[TM] would alert a medic instantly. But imagine if this technology fell into the wrong hands....

High-tech Sports

"Sometimes when I go for a walk, I measure the distance using a handheld GPS receiver" says GPS software engineer Kathy Anderson. Skiers like the one at left can keep a receiver switched to "on" all day. Later, the receiver combines all the information to create one map, showing where he or she skied that day, how far, and how fast. Golfers use GPS to tell how far their cart is from the next hole.

Animal Trackers In Kenya, scientists strap GPS-receiver collars to wandering elephants. The receivers alert rangers if the animals stray too close to poachers' shooting grounds. Workers exploring Alaska for new oilfields place similar GPS collars on grizzly bears to learn where their dens are--and then keep a safe distance.

Spotting Disasters Near earth-quake-prone Los Angeles, GPS receivers monitor the movement of tectonic plates--the massive rock slabs that form Earth's crust. Also, the National Hurricane Center drops "disposable" GPS receivers into severe storms from aircraft to track wind speed and direction.

Eco Info GPS receivers attached to weather balloons over the North and South Poles measure areas of thinning ozone and report their locations. Receivers also bob on ocean buoys, monitoring the movement of spilled tanker oil on the ocean's surface.

Future Forestcast Anderson predicts that GPS receivers will be standard features on car dashboards, cell phones, and wristwatches. The sky's the limit for GPS--what would you use it for?

RELATED ARTICLE: Cross-Curricular Connection

English: Write a science-fiction story that incorporates GPS. Stretch your imagination!

[ILLUSTRATION OMITTED]

Did You Know?

* GPS was originally designed by the U.S. Defense Department to pinpoint nuclear missile accuracy.

* Last May, President Clinton ordered the military to stop scrambling GPS satellite signals used by civilians. This made civilian GPS ten times more accurate than before.

* GPS is now used commercially in air, road, rail, and marine navigation, mining and oil exploration, telecommunications, construction, recreation, and emergency response systems.

National Science Education Standards 

Grades 5-8: motions and forces * properties and changes of properties in matter * science and technology in society

Grades 9-12: motions and forces * interactions of energy and matter * abilities of technological design

Resources

"18 Wheels, GPS and Radar," The New York Times, 4/4/99, p. G1

"They Can Find You," New Scientist, 1/8/00, p. 7

For more GPS information and photos, go to: www.trimble.com/gps and www.lowe.com

Directions: Read "Eyes in the Sky" and then fill in the blanks.

1. GPS stands for --.

2. GPS works with a network of 24 -- in the sky and on-Earth --.

3. Technology called -- could someday produce a GPS computer chip under your skin.

4. Scientists track animals in the wild by outfitting them with --.

5. In earthquake zones, GPS monitors the movement of --.

ANSWERS

1. global positioning system

2. satellites, receivers

3. Digital Angel

4. GPS receiver collars

5. tectonic plates