Abstract: This paper focuses on modeling the errors that typically degrade the accuracy of the Global Poisoning System (GPS). GPS performance is mainly affected by ionospheric errors. The SiRF Star III single-frequency receiver is used to collect and project data in the form of World Geodetic System 1984 (WGS-84) coordinates. To project the ellipsoidal model onto a map model, conversion of the datum from WGS-84 format to UTM (Universal Transverse Mercator) format is required. The conversion introduces errors into the data. Variation in errors can be observed from day to night, from area to area and also due to seasonal changes. The data is collected from the Ameerpet region of Hyderabad, which is a densely populated area with heavy traffic and tall buildings. The datum variation was observed from afternoon to evening.Keywords: GPS, WGS-84, UTM, Ionospheric Errors, Northing and Easting I. INTRODUCTION The space-based global satellite navigation system that provides reliable location and time information in any atmospheric condition, at any time and anywhere on or near the Earth with sub-meter accuracy is called Global Positioning System (GPS) [1]. It consists of a constellation of 28 satellites in six different orbits that provide information on the user's position with sub-meter precision [3]. If there are four or more GPS satellites in unobstructed line of sight to the receiver, precise spatial coordinates can be obtained [2]. The data obtained from the satellites contain position and timing information by calculating the elements of the Keplerian orbit. Position information from GPS is based on the choice of coordinate system and datum [3]. The most commonly used coordinate systems in GPS are Wor...... half of the document ......ned due to the rounding of values ​​in the algorithm. Works Cited[1] Bradford w. Parkinson. “Global positioning system: theory and applications – vol. I and II", American Institute of Aeronautics and Astronautics, Washington, 1996.[2] http://edu-observatory.org/gps/gps_accuracy.html, accessed 12/01/2010.[3] http:// www. elsevierdirect.com/companions/ 9780126836301/ appendices/ Chapter_2_ECI_ Cartesian_Coordinates_to_Kepler_Orbit_Elements_Conversion.pdf[4] http://www.squidoo.com/GPS-coordinate-systems-datums[5] Iyiade Adeniyi, “Ionospheric Error in GPS Applications” ( www. ursi.org/proceedings /procGA05/pdf/GP1 46(01062).pdf, accessed 30/10/10.[6] Ordnance Survey, “A Guide to Coordinate System in Great Britain”, (http:/ /www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate_Systems_in_Great_Britain.pdf), visited on05/01/11.