Each GPS satellite continuously broadcasts two radio signals on separate L band frequencies.
Positioning system based coarse/acquisition (C/A) code of L1 signal is known as Standard
Positioning System (SPS) and civilian users can use only this SPS (Pfost, 1998). After selective
availability (SA) turned off, the dominant source of positioning error is the distorting effects of
the earth's ionosphere on the signals from space. These effects will be minimized and the
accuracy of the system further enhanced by the GPS modernization initiative, which will add
another two civil signals at 1227 and 1176 MHz. Differential GPS (DGPS) can rectify the
problem of digital noise with some limitations. But this differential mode of operation increases
cost. Generally 12 channel receiver with phase smoothed pseudo range positioning system is
used in PA. Sophisticated enough commercial receivers are now available at, a reduced price
(hand held, 12 channel GPS receiver's price is less than £ 100) (Stafford 2000). All the position
data should be stored and distributed from only one system, installed at a central vehicle (e.g.
the tractor), for any task (Mondal and Tewari, 2007). The basic advantage of central system is
that position data are calculated in accordance with the application and transferred directly to
the point at which they will be used (Speckmann, 2000).