What is GPS:- 
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.
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.
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.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.
Differential GPS:
In this section you will see how a simple concept can increase the accuracy of GPS to almost unbelievable limits.
Why we need Differential GPS :
In this section you will see how a simple concept can increase the accuracy of GPS to almost unbelievable limits.
Why we need Differential GPS :
Differential GPS or "DGPS" can yield measurements good to a couple of meters in moving applications and even better in stationary situations.
How Differential GPS works :
How Differential GPS works :
Differential GPS involves the cooperation of two receivers, one that's stationary and another that's roving around making position measurements.
Where to get Differential Corrections :
Where to get Differential Corrections :
Many new GPS receivers are being designed to accept corrections, and some are even equipped with built-in radio receivers.
Other ways to work with Differential GPS :
Other ways to work with Differential GPS :
Not all DGPS applications are created equal. Some don't need the radio link because they don't need precise positioning immediately.
Advanced Concepts :
Advanced Concepts :
Imagine the possibilities. Automatic construction equipment could translate CAD drawings into finished roads without any manual measurements.
