21-08-2014, 04:07 PM
The Global Positioning System (GPS) is a satellite based navigation system that was developed by the U.S. Department of Defense (DoD) in the early 1970s. GPS consists of three segments: the space segment, the control segment, and the user segment. The space segment consists of the 24 satellite constellation. Each GPS satellite transmits a signal, which has a number of components: two sine waves (also known as carrier frequencies), two digital codes, and a navigation message. GPS pseudorange and carrier phase measurements are both affected by several types of random errors and biases (systematic errors). These errors may be classified as those originating at the satellites, those originating at the receiver, and those that are due to signal propagation (atmospheric refraction). The errors originating at the satellites include ephemeris, or orbital, errors, satellite clock errors, and the effect of selective availability. The errors originating at the receiver include receiver clock errors, multipath error, receiver noise, and antenna phase center variations. The signal propagation errors include the delays of the GPS signal as it passes through the ionospheric and tropospheric layers of the atmosphere. GPS has been available for civil and military use for more than two decades. That period of time has witnessed the creation of numerous new GPS applications. Because it provides high accuracy positioning in a cost effective manner, GPS has found its way into many industrial applications, replacing conventional methods in most cases such as vehicle navigation, land and marine seismic surveying, precision farming, civil engineering, monitoring structural deformations, forestry and natural resource monitoring, airborne mapping, retail industry, space applications etc.