2. GLOBE NY METRO Modified with permission from AMSTI-GLOBE (www.amsti.org) HOW DOES GPS WORK??

3. Imagine you are somewhere in your town but aren’t sure where. You ask someone for help and they say, “You are 5 miles from the fire station.” 5 miles This is nice, but it really doesn’t help because you could be anywhere on a circle 5 miles around the fire station!

4. A second helpful person tells you that you are 7 miles from the library. If you are 5 miles from the fire station and 7 miles from the library you can only be at one of these two points 7 mi 5 mi Fire Station Library Combine this information with the fire station information, and you have two circles that cross.

5. 4 mi 7 mi 5 mi Fire Station Library A third person tells you that you are 4 miles from home. You must be at the only place where all three circles cross. This circle will cross the other two circles at only one point.

6. GPS satellites work the same way to find the location of a plane, ship, car, or a person lost in the city.

7. Since 1994, there have been 24 functioning GPS satellites orbiting Earth at all times and as many as four spares The more satellites your GPS can lock onto, the more accurately your position can be determined. Your GPS can receive up to 12 satellites at the same time. Since 1994, there have been 24 functioning GPS satellites orbiting Earth at all times and as many as four spares The more satellites your GPS can lock onto, the more accurately your position can be determined. Your GPS can receive up to 12 satellites at the same time. NASA/JPL

8. This illustration shows how the GPS receiver can see the satellites as they pass overhead. NASA/JPL A purple line connecting satellite and Earth means the station on the ground can receive the satellite signal. The line disappears when the satellite is out of direct line of sight.

9. By collecting the information from several satellites, a GPS unit can determine its exact location on Earth. A minimum of three satellites is needed to locate a point (just like the three intersecting circles). This is called triangulation By collecting the information from several satellites, a GPS unit can determine its exact location on Earth. A minimum of three satellites is needed to locate a point (just like the three intersecting circles). This is called triangulation

10. 0.06 http://www.pbs.org/wgbh/nova/shackletonexped/navigate/gps/gps02.html “Doing the Math” with GPS Locating with GPS GPS satellites send a signal at exactly the same time as your GPS receiver. Even at the speed of light (186,000 miles per second) it takes that signal a few hundredths of a second to reach your GPS receiver. In this example, the signal arrives 0.06 seconds after your GPS signal

11. http://www.pbs.org/wgbh/nova/shackletonexped/navigate/gps/gps04.html Learning math with GPS Calculate the distance from your location to the satellite: The satellite microwave signal travels at 186,000 miles per second It took the signal 0.06 seconds to get to your GPS Velocity x time = Distance, so 186,000 miles/second x 0.06 seconds = 11,160 miles You must be 11,160 miles from the satellite, but..

12. that could be anywhere on the surface of a sphere with a radius of 11,160 miles As in the example of your town, you need two more readings to find out exactly where on this sphere (not a circle this time)

13. http://www.pbs.org/wgbh/nova/shackletonexped/navigate/gps/gps07.html Data from satellite 2 0.07 It took this signal 0.07 seconds to get to your GPS REMEMBER: velocity x time = distance 186,000 miles/second x 0.07 seconds = 13,020 miles You are 11,160 miles from Satellite 1, and 13,020 miles from Satellite 2, so you are somewhere where the two spheres intersect.

14. http://www.pbs.org/wgbh/nova/shackletonexped/navigate/gps/gps09.html Two circles intersect at two points. The intersection of two spheres is a curved surface. But where on that surface are you? Data from Satellite 3 will tell.

15. It took this signal 0.05 seconds to get to your GPS REMEMBER: velocity x time = distance 186,000 miles/second x 0.05 seconds = 9,300 miles You now know that you are 11,160 miles from Satellite 1, 13,020 miles from Satellite 2, and 9,300 miles from Satellite 3. http://www.pbs.org/wgbh/nova/shackletonexped/navigate/gps/gps07.html Data from satellite 3 0.05

16. The third sphere intersects the surface at two points. You are at one of them, but which?

17. http://www.pbs.org/wgbh/nova/shackletonexped/navigate/gps/gps13b.html One of the two points is always on Earth’s surface, the other more than 100 miles above it The GPS makes the easy choice for you! One of the two points is always on Earth’s surface, the other more than 100 miles above it The GPS makes the easy choice for you! You are here… …not up here!