Early explorers longed for an accurate method of navigation. Once people learned to navigate by the stars, time became a major component of celestial navigation. John Harrison developed the spring wound clock and made it small enough to be portable. Soon, Harrison’s clock, the early watch, was a necessary tool for seafaring. Over time, this watch got smaller, until it would fit in a pocket or on a wrist.
Just as the pendulum clock was a grandfather to the watch, so was the ground radio-navigation of World War II the predecessor of GPS. Time is important to GPS, as the satellite signal and the on the ground receiver are constantly moving, as the planet spins through space. Time, as it is understood through Einstein’s Theory of Relativity, and the Doppler Effect are used to triangulate a location.
Harrison’s marine watch has been replaced by atomic clocks, as accurate time is so important to GPS. The satellite sends off its location and a time. The user picks up that satellite and at least two others, and the handheld user device triangulates the position based on how long it took the signals to reach the device from the satellites.
Three segments are needed to run a GPS system: space, user and control. The space component is the satellites in orbit, with their ranges covering the whole planet. The user is the person requesting the location, with a handheld GPS device or one mounted in a vehicle or smartphone. The control segment is the system of receiving antennas mounted around the Earth to amplify the signals from space.
Fact 1: GPS is the American version, but other countries are working on their own systems. Russia and India partnered on GLONASS, the European Union is developing Galileo and China has Compass.
Fact 2: Each of the satellites in the GPS system orbits the planet at 11,000 nautical miles, 12,500 miles, or about 20,000 km.
Fact 3: If the satellites’ clocks don’t run ahead of ground clocks by 38 microseconds per day, GPS would not work. The reading received by the ground unit would be wrong after the satellite was in orbit only two minutes. This is called the General Relativistic Effect.
Fact 4: The satellites are spread out so that any point on Earth has at least six satellites in range at all times. Four are used to determine location, though three will do in a pinch, and the others are for confirmation of the location.
Fact 5: Selective Availability was put in civilian use GPS units until May 1, 2000. This kept civilian GPS from being as accurate as military units; until it was determined this feature was more dangerous than helpful. The U.S. Military has the ability to reactivate selective availability in case of national emergency.
Fact 6: Many GPS units do not have the ability to reference their physical location to a map, as the GPS in a car does. These are mainly used to track animals. Deer, sharks and other wildlife are tracked to learn about their habits, while pets equipped with GPS trackers can be found if lost.
Fact 7: The U.S. Department of Defense run GPS satellites are free to use; most people pay for the handheld device that links to company provided maps. It is possible to make and use a GPS device at home.
Fact 8: A GPS satellite is built to last for ten years is 17 feet across when the solar panels are extended, and weights around 2,000 pounds.
Fact 9: A satellite signal takes 65 to 85 milliseconds to reach an earthbound receiver.
Fact 10: Geocaching is a sport invented to make use of the readily available GPS systems and a love of treasure hunting.