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What is a GPS? at Cabela's

What is a GPS?

Author: Garmin

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather condition, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

How it works
Satellite Circling the Earth

GPS satellites circle the Earth twice a day in a very precise orbit and transmit signal information to Earth. GPS receivers take this information and use triangulation to calculate the user’s exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user’s position and display it on the unit’s electronic map.


A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2-D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user’s 3-D position (latitude, longitude and altitude). Once the user’s position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

GPS screens

How accurate is GPS?
Today’s GPS receivers are extremely accurate, thanks to their parallel multi-channel design. Models with 12-parallel channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks, even in dense foliage or urban settings with tall buildings. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers.


GPS signal flow


The GPS satellite system
The satellites that make up the GPS space segment orbit 12,000 miles above the earth. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are traveling at speeds of roughly 7,000 mph. GPS satellites are powered by solar energy. They have backup batteries on board to keep them running in the event of a solar eclipse, when there’s no solar power. Small rocket boosters on each satellite keep them flying in the correct path. Here are some other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS):

  • The first GPS satellite was launched in 1978.
  • A full constellation of 24 satellites was achieved in 1994. Today a full constellation exceeds 30 satellites.
  • Transmitter power is only 50 watts or less.


Satellite System graphic

What’s the signal?

A GPS signal contains three different bits of information — a pseudorandom code, ephemeris data and almanac data. The pseudorandom code is simply an ID code that identifies which satellite is transmitting information. You can view this number on your unit’s satellite page, as it identifies which satellites it’s receiving.


The almanac data tells the GPS receiver where each GPS satellite should be anytime throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.

Sources of GPS signal errors

Locked Signal


  • Receiver clock errors – A receiver’s built-in clock is not as accurate as the atomic clocks on board the GPS satellites. Therefore, it may have slight timing errors.
  • Number of satellites visible – The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.

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