Global Positioning System - socalled as GSP

GPS is the U.S. Global Navigation Satellite System (GNSS). A network of 24 satellites continuously transmits high-frequency radio signals, containing time and distance data that can be picked up by any GPS receiver, allowing the user to pinpoint their position anywhere on Earth.

Originally designated NAVSTAR (NAVigation System with Timing And Ranging), development of GPS began in 1973. In 1978, the U.S. Department of Defense launched the first GPS satellite, imposing SA (Selective Availability); the intentional degradation of GPS signals to prevent military adversaries from using the highly accurate positioning data. SA limited GPS to 100-meter accuracy for non-U.S. military users. Magellan® introduced the first handheld receiver in 1989, making GPS available and practical for many new industrial and recreational applications. The network required to efficiently cover the Earth was completed with the launch of the 24th satellite in 1994. Replacement satellites continue to be launched, each having a life span of about 10 years.

In 2000, Selective Availability was turned off by presidential order, giving all GPS receivers the potential accuracy of 15 meters without the use of signal correction. The signals are available 24 hours a day in any weather condition, everywhere around the world. When used with WAAS or EGNOS receivers, GPS accuracy can be improved to 3 meters.

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 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 conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

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 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 multi-channel design. Garmin's 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. Garmin® GPS receivers are accurate to within 15 meters on average.

Newer Garmin GPS receivers with WAAS (Wide Area Augmentation System) capability can improve accuracy to less than three 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 three to five 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 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 7,000 miles an hour.

PS satellites are powered by solar energy. They have backup batteries onboard 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.

The first GPS satellite was launched in 1978.

A full constellation of 24 satellites was achieved in 1994.

Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.

A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.

Transmitter power is only 50 watts or less.

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. You can view this number on your Garmin GPS 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 at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.

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Factors that can degrade the GPS signal and thus affect accuracy include the following:

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.

Global navigation satellites continuously transmit time and distance information as they orbit the earth in a precise formation. Navigation satellite receivers use this information to calculate an exact location through triangulation. Every point on Earth is identified by two sets of numbers called coordinates. These coordinates represent the exact point where a horizontal line, known as latitude, crosses a vertical line, known as longitude. The receiver locks on to at least three satellites and uses the information received to determine the coordinates of the device.

By comparing the time the signals were transmitted from the satellites and the time they were recorded, the receiver calculates how far away each satellite is. The distance of the receiver from three or more satellites reveals its position on the surface of the planet. With these distance measurements, the receiver might also calculate speed, bearing, trip time, distance to destination, altitude and more.
The satellite navigation device may display its position as longitude/latitude, Universal Transverse Mercator (UTM), Military Grid (MG) or simply as a point on an electronic map. Many Thales Navigation receivers provide comprehensive mapping data, making satellite navigation an easy tool to enhance your recreational and industrial activities.

Line of Sight: Satellite navigation receivers operate by line of sight with global positioning satellites. This means that at least three satellites must be in "view" of a receiver in order to calculate longitude and latitude. A fourth satellite must also be within line of sight to calculate altitude. On average, eight satellites are continuously within line of sight of every position on Earth; the more satellites in view, the more accurate the positioning.
Though the radio signals of navigation satellites will pass through clouds, glass, plastic and other lightweight materials, satellite navigation receivers will not work underground or in other enclosed spaces.

Precision: On average, a satellite navigation receiver is accurate to within 15 meters. Thales Navigation employs several technologies to increase the accuracy of their professional and Magellan®-branded consumer receivers. An accuracy of 3 meters or better is achieved using correction signals from satellite navigation augmentation systems. In the U.S., an accuracy of 3 meters is achieved using signal corrections from a network of ground stations and fixed position satellites known as WAAS (Wide Area Augmentation System). Throughout Europe a similar system provides the same accuracy; EGNOS (European Geostationary Navigation Overlay System). In Asia, satellite navigation signal correction is provided by MSAS (Multifunctional Transport Satellite-based Augmentation System). Other ways to increase the accuracy of satellite navigation include the use of DGPS (Differential Global Positioning System); ground relay stations, set at known positions, that transmit corrected satellite navigation signals. Various methods and applications of DGPS can increase satellite navigation accuracy from a few meters to within a few millimeters. Using DGPS requires a differential beacon receiver and antennae in addition to a satellite navigation device. Accuracy can also be increased using an RTK (Real-Time Kinematic) satellite navigation system. This is a receiver capable of transmitting a phase-corrected signal from a known position to one or more rover receivers.
A number of positioning errors can occur, limiting accuracy to within 15 to 25 meters. These errors are monitored and compensated for in a number of ways:
Orbiting errors - Occasionally a satellite's reported position does not match its actual trajectory. In the U.S., the Department of Defense continuously monitors each satellite, making orbital corrections with onboard booster rockets.
Poor geometry - If all the satellites within line of site of a receiver are clustered closely together, or lined up relative to the position of the receiver, the geometric calculations necessary for triangulating a position become difficult and less reliable. The use of differential correction signals from satellite-based augmentation systems or DGPS can compensate for both orbital errors and poor geometry.
Multi-path signals - Signals may be reflected off tall buildings or other obstructions before reaching the receiver, increasing the distance a signal travels, reducing accuracy.

Who uses GPS? GPS has a variety of applications on land, at sea and in the air. Basically, GPS is usable everywhere except where it's impossible to receive the signal such as inside most buildings, in caves and other subterranean locations, and underwater. The most common airborne applications are for navigation by general aviation and commercial aircraft. At sea, GPS is also typically used for navigation by recreational boaters, commercial fishermen, and professional mariners. Land-based applications are more diverse. The scientific community uses GPS for its precision timing capability and position information.

Surveyors use GPS for an increasing portion of their work. GPS offers cost savings by drastically reducing setup time at the survey site and providing incredible accuracy. Basic survey units, costing thousands of dollars, can offer accuracies down to one meter. More expensive systems are available that can provide accuracies to within a centimeter.

Recreational uses of GPS are almost as varied as the number of recreational sports available. GPS is popular among hikers, hunters, snowmobilers, mountain bikers, and cross-country skiers, just to name a few. Anyone who needs to keep track of where he or she is, to find his or her way to a specified location, or know what direction and how fast he or she is going can utilize the benefits of the global positioning system.

GPS is now commonplace in automobiles as well. Some basic systems are in place and provide emergency roadside assistance at the push of a button (by transmitting your current position to a dispatch center). More sophisticated systems that show your position on a street map are also available. Currently these systems allow a driver to keep track of where he or she is and suggest the best route to follow to reach a designated location.

A short GPS Guide for Beginners

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Using a Garmin® GPS with Paper Maps

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