Where on Earth? A Look at GPS Click on the GPS receiver to see what each button is for. Photo OM Stars and Navigation Imagine waking up in the middle of nowhere, all alone. How would you know where you were? If it were nighttime and the sky was clear, you could look up at the stars. If you could measure precisely what angle various stars were from you at precise hours, you could calculate your position. That is what sailors did for many centuries, using a special tool called a sextant. Many birds that migrate at nighttime use the stars for navigation, too. They don't use a sextant. They simply migrate toward or away from the North Star. GPS: Navigating Made Easier The problem with depending on the stars to set a direction or find out where you are is that stars are only visible at nighttime, and only when clouds don't cover them up. That's why the Global Positioning System (GPS) was started. Instead of stars, this system uses satellites—"human-made stars." GPS is a worldwide radio-navigation system based on 24 satellites and their five ground stations. Each of the satellites sends out a unique radio signal. A GPS receiver (click the photo to enlarge) works like a radio--buy instead of getting signals from radio stations, it gets its signals from the satellites. As long as it can get signals from at least three or four satellites, a GPS receiver can calculate precisely where on Earth it is. The calculations are based on the time it takes for signals from the satellites to reach the receiver. GPS can be used to figure out where you are no matter where on the planet you happen to be, no matter what time of day, in any weather. Moving Right Along: GPS Helps! A GPS receiver can do more than calculate where it is. It can also keep a constant record of how fast it is moving and what its altitude is. For example, ultralight pilots rely on their GPS receivers to tell them how far and how long they must fly before they get to their stopover sites.This lets them know if the birds can make it without tiring and needing to land. If a crane gets tired and drops out, the pilot can radio the location to ground trackers, who can then locate the "missing" bird. Manatee and eagle scientists also use GPS tracking to follow the migrations of those species. Try this! Discussion and Journaling Look at the longitude and latitude on the GPS above. Where was this GPS receiver photo taken? Since the Ultralight airplanes flying with the cranes have a GPS receiver, why do you think they also have an altimeter and a speedometer? Would the GPS receiver show the plane's ground speed or air speed? Why? Now GPS receivers are on airplanes, buses, and even many cars. What are some situations in which a GPS could be helpful for travelers? Copyright 2003 Journey North. All Rights Reserved. Please send all questions, comments, and suggestions to our feedback form