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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
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.
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.
WAAS (Wide Area Augmentation System) capability can improve accuracy to less than 3 meters on average. No additional equipment or fees are required to take advantage of WAAS. Users can also get better accuracy with Differential GPS (DGPS), which corrects GPS signals to within an average of 3 to 5 meters. The U.S. Coast Guard operates the most common DGPS correction service. This system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.
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):
What’s the signal?
GPS satellites transmit two low-power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.
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.
Ephemeris data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.
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
Factors that can degrade the GPS signal and thus affect accuracy include the following: