4Department of Defense of United States of America. Global Positioning System Standard Positioning Service Performance Standard. 4th Edition September 2008 [S/OL]. http://pnt, gov/public/docs/ 2008/spsps2008. pdf, 2009.
5DEPARTMENT OF DEFENSE OF UNITED STATES OF AMERICA.Global Positioning System Standard Positioning Service Performance Standard[S/OL].4th Edition September 2008.http://pnt.gov/public /docs/2008/spsps2008.pdf.