GPS
The evolution of GPS has revolutionised the way we measure length, area, height, time, and velocity and this has created new opportunities through out the interdisciplinary world. The GPS system can be divided in three segments – space, control and user segments. The space segment consists of constellation of 24 satellites in six evenly spaced orbital planes, placed in circular 12-hour orbits inclined at fifty-five degree to equatorial plane. The control segment consists of a system of tracking stations located around the world. These stations measure signals from Space Vehicles (SVs), compute orbital data, upload data to the SVs, and then SVs send data to user segment GPS receivers over radio signals. This constellation provides the user with between five to eight SVs visible from any point on the earth. On clear visibility of four satellites, it provides three-dimensional position; X, Y, Z and T (time) on a 24-hour-per-day basis all around the world.
The NAVSTAR GPS is funded by and controlled by the U.S. Department of Defence (DOD), initially to support military applications and later also for civilian uses with certain restrictions (Logsdon, 1992). In addition to NAVSTAR GPS, Global Navigation Satellite System (GLONASS) from Russia and European Global Navigation Satellite System (GNSS) also provides worldwide GPS services (Zarraoa et al., 1998 and Gallileo’s World, 2000). During the discussion in this paper NAVSTAR GPS is referred as GPS unless and until specified. Recently, civilian use of GPS has stretched the limits of technology to explore applications in georeferencing, positioning, navigation, and for time and frequency control using GPS receivers. GPS satellite signals cannot pass through buildings, metal, mountains, or trees. Therefore, in locations where at least four satellite signals with good geometry cannot be tracked with sufficient accuracy, GPS is unusable.
GPS can provide Precise Positioning Services (PPS) and Standard Positioning Services (SPS). The PSS is limited to military uses with a 22-meter horizontal and a 27.7-meter vertical accuracy, whereas the SPS for civilian use can provide 100 meter horizontal and a 156-meter vertical accuracy. Prior to 2nd May 2000, accuracy of PPS was degraded for civilian uses by the use of Selective Availability (SA). With Selective Availability in operation, accuracy in point positioning is only 10 to 20 meters, which is not suitable for most mapping purpose. The removal of SA gives much higher accuracy for standard positioning, as good as 20 meters horizontal and 30 meters vertical (Dana, 1997). The issues regarding removal of SA represents different views, some users believe that now it is possible to achieve sub meter accuracy by averaging readings, however, Wadhwani (2002) argued that there is possibilities of multiplying the embedded errors in inaccurate readings (for further reading see Bennett, 1991 and Jacobson 1991).
GPS errors are a combination of noise (instrumental deformation), bias (SA, clock error, signal delays, multipath signals), and blunders (hardware and human error). The error due to noise could be around 1 meter, whereas bias error can amount from 1-10 meters and above all these, blunder error can cause positional difference of hundreds of kilometres (Dana, 1997). Furthermore, it is important to note that the radio signals from SVs travel at the speed of light, 186000 miles per second, and the delay of even a 1/100th second due to any of these errors can put the accuracy off by 1860 miles (Leick, 1992). After removal of SA the cumulative effect of these errors can be expected to be approximately 10-meter, significantly better than the 100-meter under SA (Harrington, 2000). The effects of bias errors can be overcome with innovative developments of Differential GPS (DGPS). In DGPS an additional reference receiver is used to compute corrections in real-time or later as post-processing, which can provide batter precision.
