What is GPS(www.track7.net)

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What is Global Positioning System

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. But now 31 satellites working for this purpose.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.

How it works

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.

What is a GPS Signal & Degradation

GPS signal degradation. These include objects that do allow radio waves to pass through, weather, and more.

To get around with this problem, GNSS sends signals at two frequencies along with time stamps for the signals so that the GPS ground stations can relate the signals to help your GPS device compute the data with maximum accuracy. Still, there are times when your GPS device cannot receive the GPS signals – resulting in GPS Blackouts.

Also, the frequencies for military are different from those for civilian uses. This means that GNSS transmits GPS data at two levels: L2 for military purposes and L1 for civilian uses. The L2 signals for military use high encryption so that normal devices cannot decrypt the signals.

As the GPS signals travel miles to reach your GPS device, they may become weak and distorted due to atmospheric disturbances, urban blockages, and other factors. If the signals become weak, your GPS device will give you incorrect results. The GPS devices need minimum signal strength to offer you accurate readings. Please read our article on How GPS Satellite Detection Sensitivity Affects GPS Accuracy to understand how the GPS devices function once the GPS receiver obtains the signals.

Plenty of methods exist that help to increase the accuracy of GPS when the signals are weak. One of such methods formerly used in aviation is now being used in normal GPS systems too.

Sources of errors

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.

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