Since earliest times, humans have had to know precisely where they were at any given time in order to determine where they wanted to go. Early explorers relied on the stars for accurate assessment of their position. Today, a constellation of navigation satellites orbit our planet, providing us with real-time, highly accurate information about our position on land, sea, or air.

Satellites help travelers calculate their latitude, longitude, altitude, speed, and direction of travel with an accuracy to within 300 feet. They rely on the same principle as a sextant to triangulate on heavenly bodies to plot one's position. A GPS (Global Positioning System) triangulates on 3-4 satellites out of 24 available satellites in the system, so a traveler can find out where he/she is anywhere on or above the Earth.

### Find the lost hunter

1. Explain that the class has to locate a hunter lost in the woods who is trying to relay information about his location by radio to his rescuers. Ask for a volunteer to play the lost hunter.
2. Ask for a second volunteer to play the role of a mountain. Position the mountain about 10 feet from the lost hunter. Assume that the lost hunter can only see the mountain.
3. Ask for a group of students to play the roles of the "Search and Rescue Team." They ask the hunter to describe his position to them. (Probable answer: "I can only see a mountain.")
• (The Team may find it easier to put a symbol for the mountain on a piece of paper and discuss where the hunter might be in relation to the mountain on the map.)
• Will this information help the Search and Rescue Team?
4. Add a second landmark (another volunteer): a television transmission tower. Position the TV tower about 8 feet away from the lost hunter, so that the hunter, TV tower, and mountain form a triangle.
5. The lost hunter now has two points of reference for the Search and Rescue Team when asked to describe his location. (as, "I see a mountain off to my right, a TV tower to my left.").
• Is the Team able to narrow down his location better?
6. Add a third point of reference: a giant redwood tree 6 feet away from the hunter, so that the mountain, TV tower, and redwood tree form a triangle around the lost hunter.
7. Now have the Team ask the hunter for his probable location. ("I can see a mountain to my right, a TV tower ahead and to the left, and a giant redwood tree behind me.")
• Using 3 points, the Team can now triangulate the position of the lost hunter.
8. Explain to students that this is how GPS (Global Positioning System) works. Instead of mountains, TV towers, and other landmarks, it uses highly-accurate satellites as points of reference. Instead of relying on vision to estimate one's location and distance, handheld satellite receivers communicate constantly with the orbiting satellites, triangulating on at least three of them. This enables the receiver to display one's position to within 100 meters.

### Materials

• pacing and measuring tape
• paper
• pencils