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GPS devices are wonderfully capable but also complicated for the new user. GPS Primer provides GPS info explained in simple, everyday terms to get you started on the right foot in your travels.

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How GPS Works FAQ

Why does my GPS only work outside?
Orbiting above our heads, 12000 miles up, are 24 satellites. They were initially placed for military use, but some years ago they were opened up for civilian and commercial uses. They are positioned in their orbits such that at any given time, at least four are visible in the sky at any point on the globe. The visibility part is important. Even though they send signals by radio wave, the signal needs a clear line to reach you. Once the satellite drops behind the horizon, or a building, or even heavy cloud cover, the radio signal may not reach you.

What do the satellites do?
Each satellite is broadcasting the time. But not just any time...atomically accurate time. Your GPS receiver listens to this broadcast. It has an atomically-accurate clock in it, too. A GPS SatelliteBy comparing the difference between the time given by the satellite and the time in your GPS receiver, the GPS can calculate the distance between you and the satellite.

How is time turned into distance?
Well, say you are travelling in a car at precisely 60 miles per hour. You travel for 1 hour. How far have you gone? 60 miles! Now, imagine you are riding a radio wave transmitted from a GPS satellite. Radio waves travel at the speed of light, 186,000 miles per second. If it takes you .06 seconds to get from the satellite to the GPS, how far have you gone? 11600 miles. The GPS receiver in the hand of the human on the surface of the earth is 11600 miles from the satellite.

Why does the accuracy vary on my GPS as I am using it?
The more satellites your receiver can "see," and the more clearly it can get the satellites' signals, the more accurately it can figure your position. It needs to see at least four satellites to be useful. Here's why. A GPS satellite sends out its signal in all directions at once, like a balloon expanding from the center. When your GPS receiver calculates that you are 12000 miles from one satellite, as far as it can tell you could be anywhere on the outside edge of that balloon of signal! That's not very useful. But by communicating with three satellites, the unit calculates where all three balloons of signal intersect--and there you are! In a perfect world, just three satellites are all you need. But the fourth is needed to correct for atmosperic conditions and other interference. When the GPS receiver is getting intermittent signals from one more satellite due to atmospheric interference, or an obstruction you are passing by, the accuracy drops. Today's GPS receivers use WAAS, an advanced error-correction algorithm that uses signals from special WAAS-enabled satellites for greater accuracy.

How does the GPS satellite know where it is?
It has its location stored inside it electronically in the form of calculated tables. But satellites can wobble and get off course minutely over time. To correct this, the satellite communicates with fixed reference stations on the ground around the globe. Each time it touches base with the ground stations, the satellite adjusts its internal location tables. The satellite cannot adjust its actual course--it has no engines aboard. It can only adjust its record of where it should be at a given point in time.

Does a GPS receiver send information back to the satellite?
No. Leave your tinfoil hat at home; your GPS unit can't do that. It simply does not have the power to send a signal 12000 miles, period. Many newer cell phones offer location services for 911 emergency calls. But this is not true GPS. The cell phone is determining its position from cell phone towers (which, in effect are acting locally like fixed GPS satellites), not the GPS satellites in orbit. Some GPS receivers have built-in 2-way radios, such as the Garmin Rino line, but those radios do not communicate with the satellite either.