GPS Explained
What is GPS?
History of GPS
How GPS works
When people talk about "a GPS," they usually mean a GPS receiver. The Global Positioning System (GPS) is actually a constellation of 31 Earth-orbiting satellites (28 in operation and three extras in case one fails) providing users worldwide with twenty-four hour a day precise position in three dimensions and precise time traceable to global time standards. GPS works in any whether conditions and anywhere in the world. There are no subscription fees or setup charges to use GPS.
GPS is operated by the United States Air Force under the direction of the Department of Defense (DoD) and was designed for, and remains under the control of, the United States military. While there are now millions of commercial and recreational civil users worldwide, DoD control still impacts many aspects of GPS planning, operation, and use.
Primarily designed as a land, marine, and aviation navigation system, GPS applications have expanded to include surveying, space navigation, automatic vehicle monitoring, emergency services dispatching, mapping, and geographic information system geo-referencing. Because the dissemination of precise time is an integral part of GPS, a large community of precise time, time interval, and frequency standard users has come to depend on GPS as a primary source of control traceable through the United States Naval Observatory to global time and frequency standards.
Click here for GPS History Snapshots.
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Interesting facts about GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS):
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A GPS receiver's job is to locate four or more of the 24 Earth-orbiting satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle call Trilateration (a method of determining the relative positions of objects using the geometry of triangles in a similar fashion as triangulation).
Each of the 3,000- to 4,000-pound solar-powered satellites circles the globe at an altitude of about 12,000 miles (19,300 km) and at roughly a speed of 7,000 miles per hour, making two complete rotations of the earth every day in the very same exact orbit and transmits signal information to earth. The orbits are arranged so that at any time, anywhere on Earth, there are a least four satellites "visible" in the sky.
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 t he 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 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D 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 multichannel design. 12 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 receivers are accurate to within 49 feet (15 meters) on average. For those applications such as agriculture this is not accurate enough, so the signals from the satellites must be "corrected". This is known as differential correction (a method used to reduce the effects of atmospheric error and other sources of GPS positioning error).
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 I.D. code that identifies which satellite is transmitting information.
Ephemeris data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits ephemeris data showing the orbital information for that satellite and for every other satellite in the system.
Almanac 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.
Sources of GPS signal errors
Factors that can degrade the GPS signal and thus affect accuracy include the below.
- Ionosphere and troposphere delays:
The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error. - Signal multipath
This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors. - Receiver clock errors
A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors. - Orbital errors
Also known as ephemeris errors, these are inaccuracies of the satellite's reported location. - 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. - Satellite geometry/shading
This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping. - Intentional degradation of the satellite signal
Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.
For more detailed information about GPS and how it works, check out:
http://en.wikipedia.org/wiki/GPS
http://www.montana.edu/gps/understd.html#What_is_GPS
